process ceramic

CERAMIC PROCESSING

CERAMIC DEFECT SERIES 1-10

by

JOHN T. JONES, PH.D.

All Rights Reservedcopyright©2007 John Taylor Jones, Ph.D.

John T. Jones, Ph.D.213 3rd Avenue North

Buhl, ID 83316email: [email protected]

208-543-4053 (phone or fax)www.tjbooks.com

www.ceramicdefects.comwww.potterydefects.comwww.chinadefects.com

www.brokenceramic.com

of 82

ABOUT THE AUTHOR

John T. Jones served in the 17th Infantry Regimental CombatTeam during the Korean war: http://www.tjbooks.com/koreapics.htm

After the war he entered the University of Utah, spent onesummer at the Norton Company, and graduated in CeramicEngineering. He then worked for Coors Porcelain Company inGolden, Colorado. After five years of service at Coors, hereturned to the University of Utah in 1962 and received thePh.D. degree in Ceramic Engineering and Metallurgy on1965. He then joined Vesuvius Crucible Company inPittsburgh followed by eight years teaching at Iowa StateUniversity. There he and Mike Berard wrote Ceramics:Industrial Processing and Testing:http://www.tjbooks.com/ceram.htm

Leaving the university, he joined Interpace Corporation, thenPfaltzgraff Company, finishing his career after 17 years atLenox China Company where he was Vice President of R &D. Jones then became editor of Ceramic Industry Magazine. He is a fellow of theAmerican Ceramic Society and past president of the New Jersey Ceramic Association.He was Man of the year for that association, the Philadelphia Section of the AmericanCeramic Society, and for the University of Utah Ceramic Engineering AlumniAssociation. Jones served for many years on the Government Liaison Committee ofthe American Ceramic Society and served as the committee’s chair.

of 82

How These Articles Came About

After I retired I wrote two detective novels and two western novels. I didn’t want tospend the rest of my life writing novels so I started to write shorter articles forwww.ezinearticles.com

Those articles cover a multitude of topics but I let ceramics slip in there a few timesjust from habit. I was writing several articles each day but a sore neck ended thatprocess. Now I write articles only occasionally. You can read the title and abstractsof my hundreds of articles at www.tjbooks.com

After writing the ceramic articles I decided to write this book on ceramic defects.Most of my experience is in ceramic whitewares. My early experience in technicalceramics and refractories is probably obsolete so if you are looking for information

in those areas, you may not find what you are looking for here.Well, I did add some information from my experience and fromthe literature.

Much of the information on defects is in table form. For thatreason I placed a Table Index right after the Table of Contents.You can use your search function to search the book.

You can contact me if you like using the information on the title page. Since I’m oldand senile I would like you to point out errors and useful additions.

I do not reject criticism. When we first published Ceramics: Industrial Processingand Testing our publisher was very angry about a book review published by theAmerican Ceramic Society. The reason was that the book had been reviewed byseveral distinguished and very knowledgeable members of the ceramic communitybefore publication and the person reviewing the book seemed to not have the propercredentials.

I simply contacted the writer of the review and asked if I could have his review notes.

He provided me much more information than was in the review because he hadstudied the book with great interest. We made corrections where justified in thesecond printing. Many of his suggestions were not made because he preferred using

of 82

English rather than American terminology because he was English. He wastechnically wrong here and there. The first edition had over ten printings before thesecond edition was published. He made it a better book.

The gentlemen who wrote the review and I became very good friends. He was veryhappy with my response and said that the book was very good except for the list ofcorrections needed.

He could have said that in the review!

John

John T. Jones, Ph.D.January 9, 2007

of 82

The Ceramic and Pottery Defect Series 1-10

Abstract

1. Selected Definitions and Ceramic Definition References

2. Raw Materials and Batching of Bodies and Glazes

3. Forming

4. Drying Operations

5. Bisk Firing

6. Glost Firing and Glaze Formulation Notes

7. Decaling Operations

8. Enameling Operations

9. Gilding Operations

10. Process Control

TABLE INDEX

References

of 82

TABLE INDEX

2:1 Particle Size Defects

2:2 Particle Size and Surface Area Measurement Methods

2:3 Raw Material Contaminants

2:4 Glass and Glaze Constituents & Composition Range in Glazes

2:5 Typical Molecular Glaze Formula

2:6 Glaze Classifications

2:7 Properties Imparted to Glazes by Atomic Constituents

2:8 Ceramic Body Constituents

2:9 Ceramic Body Classifications

2:10 Materials for Advanced Applications

3:1 Forming Defects

3.2 Capacitor Defects (multiple thin film)

3.3 Glass Making Defects

3.4 Injection Molding Defects (See also Table 4.2)

3.5 Spray Drying Criteria

4:1 Drying Defects

of 82

4.2 Injection Molding Drying Defects (See also Table 3.4)

5:1 Bisk Firing Defects

6:1 Glazing and Glost Firing Defects

6.2 Customer Complaints Glazed Ware

6:3 Thermal Shock and Autoclave Testing

6:4 Special Crazing Problems on Firing & Dunting

7:1 Decaling Operation Defects

8:1 Enameling Operation Defects

9:1 Gilding Operation Defects

TOC

of 82

Abstract

Defects in ceramics cost money; money that is often lost without compensation.

Some defective ware can be reclaimed, but there is a cost to do this.

Some ware can be sold as is which avoids a complete loss. In fact, selling defectiveware can have almost the same profit margin as good ware except for the cost ofselling the ware. This occurs, for example, when the defective ware is retailed at thesame discount that commercial customers receive.

Defective ware should never be ignored. Noticing a defect and not taking action toeliminate it can result in major loss as the occurrence of the defect increases from dayto day (or from hour to hour for that matter).

The cause of a defect is not always easy to determine. A defect in the final stages ofdecorating may have been caused by an impurity in a body raw material. For thatreason, close contact with suppliers is always proper. To ignore the source of thematerial, such as a particular mining pit containing a known particular contaminant,can delay resolution.

STP (Statistical Process Control) can help eliminate defects, For that reasons,references are given to implant STP for those not having already done so.

I’ve tried to use charts as much as possible to help identify and eliminate defects. Westart with the raw materials, then proceed through batching, forming, drying, bisk-firing, glazing and glost-firing, decaling, enameling, and gilding.

Reference Text: The reference text for this e-book is Ceramics: Industrial Processingand Testing by John Taylor Jones and M. F. Berard. The book has two editions. Thepublisher has asked that a third edition be prepared but it is not available now.

There is a list of other references at the end of the book.

of 82

CERAMIC AND POTTERY DEFECTS

1. DEFINITIONS

Defects in ceramics are of interest to potters and ceramic manufacturers because theyare a major cause of financial loss. They are of interest to collectors of ceramicsbecause they may (or may not) reduce the value of an item. They are of interest tousers especially if they can cause damage or injury in use. Because the language ofceramics is rich, I list here sources for ceramic definitions:

REFERENCES FOR CERAMIC DEFINITIONS:

International PorcelainFloor Facts (Tile)Astbury (Pottery)Turner Pottery (Pottery)Keramverband (Technical Ceramics)U.S. CensusCeramic Industry Magazine (definitions and suppliers, etc.)

DEFINITIONS FOR PROCESSES USED IN THIS E-BOOK

For those of you who are not familiar with ceramic processing here is a listing of theceramic processes we will discuss in this series of articles on ceramic defects:

Formulating and Batching:

Selecting a composition for the ceramic and then choosing raw materials for thebatch.

Forming or Making:

Shaping the part by slip casting, pressing, hand forming, injection molding, rollforming, jiggering, or any other of a number of ways available.

of 82

Drying:

Controlled heating to dry the ware before firing or just leaving it on a shelf inArizona.

Bisque Firing (or Bisk Firing) and Single Firing:

A low-temperature firing followed by a high-temperature glost or gloss firing inthe Porcelain Process.

A high-temperature firing followed by a low temperature glost or gloss firing inthe China Process.

A single high-temperature fire in the (true) Stoneware Process where the body andglaze mature together. The body porosity is less than 0.5 percent.

A single lower-temperature firing in the Semi-vitreous Process resulting in aporous body and mature glaze. The body porosity is usually between 4 and 8percent.

Glost Firing:

Firing of glazed ware (see also above).

Decal Firing:

Firing a decal or decals on ware.

Enamel Firing:

Firing an enamel usually on decaled ware.

Gild (Precious Metal) Firing:

Firing gold or other precious metal decorations. (Now days, the decal may includeenamel and gold so that a single firing can be used.)

of 82

Figure 5

TOC


2: DEFECTS FROM RAW MATERIALS ANDBATCHING ERRORS

Formulating and Batching

Ceramic bodies are formulated according to the requirements of the final product. Animportant consideration is often the mineralogical composition of the final product.This often yields a chemical composition required in the initial batch. Materials areselected to satisfy the chemical and final mineralogical composition. Some bodieshave a single component.

In the case of glazes the composition is often strictly chemical and mineralogicalconsiderations are not as important except where a particular mineral may be addedto the batch because of its particular composition. The final glaze is usually, but notalways, completely vitreous.

If you are not familiar with formulating and batching you might want to study thereference.

Reference Text: Ceramics: Industrial Processing and Testing by John Taylor Jones

and M. F. Berard. CHAPTER 9 BATCH CALCULATIONS: Oxide Formulas and

Formula Weights, Loss on Ignition and

Moisture, Empirical Formula and Formula

Weight, The Batch Recipe

Many ceramic raw materials are already refined

by the supplier. If you can keep them clean

between the source and your factory, they will

not be a source of defects. That is usually but

not always the case. (The pic to the right is courtesy

of 82

Figure 6

of ECC International. The operator is controlling the blending of kaolin clay slurriesby computer.)

Clay Materials

Brick Clays and ShalesSome folks use materials right out of the ground to make ceramics. A high-volumeexample of this is the brick manufacturer up the road. (If you don’t have clay or shalewhere you live, there is no brick manufacturer up the road.)

Brick manufacturers and clay suppliers usually mine clay by the open pit method.That means that they usually don’t tunnel for the clay. They carefully remove theoverburden (the dirt, weeds, trees, old cars, and what-have-you on top) leaving aclean clay or shale face. Then they mine the clay or shale and deliver it to theprocessing plant or directly to the factory.

(When the clay is depleted, the lawusually requires that the terrain berestored to its original condition.However, a lake may replace themining pit. Figure 6: Pit reclaimedcourtesy of KT Clay. See aninteresting article on clay blendingfor the sanitary ware industry at:http://www.k-tclay.com/FKBLend.asp)

At the factory, it is crushed or ground as required, water is mixed in, and the clay isextruded by means of an auger into a very long rectangular cross section that is cutinto brick as it moves along the conveyer belt.

Now days everything is automatic in most plants including the loading and unloadingof bricks into and out of the kiln. (See Ceramics: Industrial Processing & Testing,John T. Jones & M. F. Berard, Iowa State University Press)

Brick and sewer pipe manufacturers, etc., add barium carbonate or some otherchemical to tie up the sulfates in the clay or shale because barium forms insoluble

of 82

sulfates which can cause blisters and scumming, etc.. These materials are notpremixed into the clay. They are just sprinkled on the clay and the extruder augerdoes sufficient mixing.

Some other manufacturers may add barium carbonate such as the sanitary waremanufacturers that manufacture your bathroom fixtures. Some potters add it too.Better safe than sorry.

Vanadium compounds cause scumming in otherwise pretty white bricks. This is atough problem. How do you tie up vanadium. If you know how, you are in demandas a consultant. The defect is often called efflorescence. How to prevent stains andhow to clean brick having such manganese and vanadium stains, etc., can be foundat: http://www.bia.org/BIA/technotes/TN23.pdfhttp://www.upchurchkimbrough.com/technotes/efflor1.html

A useful source on materials found as impurities or used in ceramics and theireffects is: http://www.kickwheel.com/chemical.names.and.uses.html

Primary Clays

Primary clays are in situ. That means they are mined from the spot where they wereformed by hydrothermal activity. The composition of the clay depends on thecomposition of the parent rock, often granite.

Granite is a coarse-grained igneous rock composed chiefly of orthoclase and albitefeldspars and of quartz, usually with lesser amounts of one or more other minerals,such as mica, hornblende, or augite.

There may be iron compounds such as hematite and magnetite, titanium compoundssuch as rutile or anatase, and manganese compounds such as manganese dioxidecalled pyrolusite. These minor compounds can cause trouble if they are not removedby the supplier.

Primary kaolins are often mined by using high-pressure water. I learned in Englandthat I was not a very good washer and was causing unnecessary segregation in theslurry. Well, my grandfathers used dynamite, picks, and shovels to mine.

of 82

I will say this: If you get a chance to blast the clay from the pit in England, do it. Itis great fun!

Primary clays may be processed in slurry form or by air floating. Individual slurriesfrom different pits can be characterized as to their chemical, mineralogical, andceramic properties. Then several slurries can be blended to give a slurry with theproper properties specified. (See Figure 5 above.)

Even after wet grinding and screening and magnetic separation a mineral such aspyrolusite may survive. It will decompose on firing. If the decomposition is completeduring a preliminary bisk firing, there may not be problems. However, if preliminaryfirings do not finish the decomposition you can have bubbles form even in the laststages of decorating.

For example, MnO2 does not decompose until 535oC. If it does not completelydecompose during bisk firing, it may show up as blisters in gloss and decorating firesas the oxygen is released. We had this problem. In one factory with a high-temperature bisk, there was no problem. The pyrolusite contaminant completelydecomposed.

In the other factory with a lower temperature bisk, we got blisters in glost anddecorating using the same contaminated clay. This event never should have occurredas we were not to receive clay from the contaminated pit. The loses were high and thesuppliers’s insurance company had to pay.

Secondary Clays

Secondary clays were washed down from primary clay deposits. This is a naturalrefining process in a way because the fines are removed leaving the coarser materialsincluding heavy oxides behind. The material were collected in natural pools whichcontain other materials washed down with the clay. These materials may be organicsuch as tree limbs, etc., which has changed to lignite. Some kaolins and all ball clayswere formed by sedimentation.

Ball clays may contain excessive lignite and sulphate. If you walk around a ball claypit you may find an old palm tree.

of 82

Ball clays may be air floated but they are generally ground and aged. The sulfatesbreak down as the clays age and sulphate control is required by the supplier. (You canspell sulfate as sulphate. It is okay. I generally just mix them up.)

Fire Clays

Some clays form a natural fire brick refractory, often a single-component body.Diaspore in fire clay increases the alumina content giving a more refractorycomposition than kaolin alone.

Fluxing Minerals

Feldspars are a product of hard-rock mining. The granitic rock is blasted from thequarry face, trucked to the processing plant, crushed, and ground. Screening andmagnetic separations is important in these operations.

Finally, the feldspar is floated off by chemical means, leaving the quartz behind.Rutile is a secondary float product.

After the final grinding of the feldspar it is checked for color and fusibility. This isdone by making small truncated cones and firing them to a specified schedule in thelaboratory.

Note: Particle size distribution affects fusibility.

Silica

Silica for the ceramic and glass industry are from pure sandstone. Grinding andscreening are the main operations. Impurities may be removed by magnetic separationas with all ceramic raw materials. The particle size distribution can be important inceramic operations and must be controlled.

Refractory Oxides

Alumina is usually formed from bauxite by the Bayer process which involvescrushing and grinding of high-alumina hydrates followed by crystallization andcalcination. Tabular and fused aluminas are formed at high temperatures. Magnesia

of 82

can be formed from dead-burning magnesite from sea-water brine. Other oxides arelime from limestone and chrome-ore refractories from chromite, a complex oxide.

Miscellaneous Materials

There are many materials used in ceramics that do not fall into the above categories.They are formed by a variety of process. It is important to understand the processesused to create the materials you use in your operations.

Stay in Contact with your Raw Material Suppliers

It is also very important to stay in contact with your suppliers and to tell them aboutany problems that could be material related.

When I was in the industry, I often asked suppliers what problems their othercustomers were having. I didn’t always get a good answer but trust between theceramic manufacturer and the material supplier is essential.

When I suspected that something was not right in the material supplier’s processingand I was not getting decent answers to my questions, I would hop a plane and fly tothe source. Then management would get on the band wagon and I would learn whatwas going on.

Once we had a problem with the particle size of incoming nepheline syenite. Mylaboratory manager could not get the information he needed. I flew to the mine inCanada and asked, “What is going on?” I finally go the answer by talking to thegrinding plant manager. He told me that he had increased production by changing themedia distribution in the ball mills. That had changed the particle size distribution.When we complained, he changed the mills back. So we had a range of product thatwas always changing in particle size distribution.

I told him that he would not have had to change the mills back if management hassimply told me what was going on by telephone before the media distribution waschanged. All we needed to know was the truth. We could have easily adjusted ourmilling operations.

So the company lost the advantage of the change and they faced a very embarrassing

of 82

Figure 7

Figure 8

situation. I had warned them on other trips that they were to inform me of any changein production procedures or product. You should tell your suppliers the same.

Impurity Defects

Defects caused by raw materials are usually related to impurities or particle size. Thedefect can turn up at any time in the process. Here are some more examples of defectscaused by impurities:

Lignite

Lignite is common in many clays. It can be hard anthracite or softer bituminous coalor lignite which is softer than the first two.

Screening can reduce lignite from slurries even after ball milling.(Ball mill and media pics courtesy of Paul O. Abby.)

Soda ash can reactwith soft lignite andturn it into a usefulcolloid that will keep

the slurry dispersed.

In other words, some lignite can bebeneficial in slip casting operations andother operations that use slurry. But thehard anthracite and other coal varieties arenot good. They can cause blisters and pitsduring bisk- and glost firing operations.

Clay companies have some control over the size and amount of lignite in their clays(often by blending).

of 82

Figure 9

You can run a screen analysis on the material and see how much lignite remains onthe screens. If it is higher than the previous shipments, call the clay company and saythese words: “What’s with all this lignite!” (Figure 9: Vibratory Screen image thanksto Sweco®.)

That should get theirattention. Once I said, "Whatis all this plastic?" (The filterbags had melted in the airfloatation equipment.)

Make sure you blame themfor all defects generated forthe next six months. (Hey,you think I’m kidding! Makethat the next two months orwhenever that batch of clay isused up.)

For a description of claysused in the ceramic industry go to:http://www.hamgil.com/html/claytype.html

Grit

We had a recurring blistering problem in fine china where I once worked and foryears nobody could figure out what was causing it.

I was new with the company but not inexperienced in solving material problems.

I called each of our suppliers and said that we had blisters and it was their fault.

The representative from a kaolin company asked, “What’s the grit?”

I wasn’t sure what he was talking about because I had not used that particular claybefore. Rather than tell him that I didn’t know what he was talking about, I said,“Hold on a minute.”

of 82

I asked the quality control manager, “What’s the grit in that stuff?”

He had a questioning look on his face despite his 30 years of service. He pulled outthe specification sheet for the last shipment, found that there was such a thing as"grit," and gave me a number.

The number represented the amount of coarse material that showed up during a screenanalysis of the clay. For this particular clay, the grit was in the form of mica. (Micacan cause blisters and discoloration. A close look at the blisters in our bisk warerevealed a slight discoloration. We knew we had found the mica.)

Hearing the number, the representative said, “That is too high! I’ll call the mine!”

We set a control standard and never had that problem again.

I‘d paid my first years salary in five minutes a few days after joining the company.

Learn how to evaluate clay materials at:http://www.ceramicindustry.com/CDA/Articles/Feature_Article/fcd8eaeeba25d010VgnVCM100000f932a8c0____

Iron and Manganese Compounds, Silicon Carbide, Soy Beans and Salts

Sometimes clay manufacturers ship clays to storage areas by rail, ship, or barge.Dockside raw material storage is always dangerous for contamination. The reason isthat these facilities ship iron ores, ferro-silicon, silicon carbide and they are not verycareful about cleaning out a shed of ferro-silicon or other contaminant before loadingit with a shipload of clay. (We changed from bulk to Super Sack® shipments.)

Some of these materials give off oxygen when they decompose. That occurs at somecharacteristic temperature.

We used the same British china clay in two different plants.

In one plant we had blisters in our decorated ware, the worst possible condition.

The other factory using the same clay didn’t. Why? The bisk temperature was much

of 82

higher in the not affected plant. The contaminant, a manganese compound,decomposed before glazing and decoration.

The affected plant had both lower bisk and glost temperatures. The material did notfully decompose at these temperatures and bubbles were still appearing at thedecorating temperatures.

The problem was complicated because we had to prove that the supplier caused aheavy loss that their insurance company should pay.

I’ll not go into the details because they proved we were right and did pay.

The reason for reimbursement was that I had requested them to never ship us materialfrom a certain pit (mine). It had caused the company loses for years and I wanted nomore of it.

They shipped from that pit rather than shipping us the processed clay that I specified.

I know I told you the above story before. I don’t want you to forget it.

Identifying Impurities

Identification of impurities can be tricky without a Scanning Electron Microscope(SEM).

Heavy liquids can be used, but that is a nuisance. Microscopes are good for those whoknow how to use them. You can look at the contaminants from the heavy liquidseparation.

Sometimes you must use an outside lab if you don’t have the correct equipment.

To isolate a contaminant before you send samples out, elutriate clays and screen nonclays. Wet screening is best.

To elutriate a clay, take about ten pounds of clay and keep washing it down until onlythe contaminant is left. Put the contaminant in your body or glaze and reproduce thedefect you are seeing in production. Some clays can be wet screened but not at 325

of 82

mesh in most instances. Still, before you elutriate, wash the clay though a screen asfine as possible. Contaminants can be very fine. Common contaminates are quitedense and settle readily from slurry.

Chloride can be detected from salts by washing the clay with de-ionized water andtesting with a soluble silver nitrate solution. A milky precipitate is silver chloride.Salts cause blisters!

We had soy beans once in one of the plants where I worked. The clay picked up thebeans from a hopper car. They make big holes in your ware but they are easy toscreen out if you have screening in your operation, which we didn’t!

There is a funny story that goes with the soy beans. We were putting up silos for claystorage in an automatic batching system. I wanted to set raw material specificationsto avoid problems. I decided to call my cronies in the ceramic industry who wereworking with silo systems.

I called my friend, Fred (not real name), out in Ohio and I said, “Fred, we are puttingin a clay storage system. Have you had any problems with your clay from railroadcars?”

“Nope!”

“Fred, you never had problems with iron oxide, manganese, or ferro-silicon?”

“Well, we had some iron oxide problems (from taconite, low-grade iron ore) but thatwas from the barge, not the railroad.”

I said, “I see, you get some of your materials by barge. Was the iron contaminant aproblem?”

“Hell, yes! It made ugly brown holes in our ware.”

“So, Fred. What did you do with the clay?”

“We had to take it out of the silo. We had to use a wheelbarrow.”

of 82

“Who did that?”

“Me and my technician.”

“Fred, what did you do with the contaminated clay?”

“Our pond was leaking so we put it in there.”

I asked Fred, “What about soy beans?”

“Well, the railroad brought them.”

“Didn’t they cause you a problem, Fred?”

“Hell, yes! They put big holes in the ware. We could see them coming down our clayconveyor belt.”

“What did you do with that clay, Fred?”

“Same thing! The pond didn’t really need it, but we dumped it in anyway.”

Particle Size

One of the most serious formulating errors is in not controlling particle size in thebatch recipe.

For example, if you use too much of a certain clay having a very fine particle size,you will have problems. Let me say this, you must choose the correct particle sizedistribution for your process.

Slip casting requires a coarser particle size distribution than a plastic forming processfor the same formulation. What does all this nonsense mean? You must use somecoarser grained clay(s) in your casting process. If you don't you will still be trying tocast a piece when the next millennium comes in.

If you want to have control of your casting process, then try using a coarse-grainedkaolin plus a fine-grained kaolin, and a coarse-grained ball clay plus a fine-grained

of 82

ball clay. Changing the ratio of fine to coarse clays will give you control. You mustmaintain the total amount of kaolin and the total amount of ball clay to preserve colorof the product.

Look at the particle size distribution of the clays you are using in your process. Thisinformation should be provided to you with each shipment.

Look at the particle size portion that says <0.5 microns. (If that information is notthere, the <2 micron information usually is. Use it but it is not as good for control.)

The <0.5 microns is called the colloidal fraction. Keep the colloidal fraction thesame in each casting batch. (Send me an email if you can’t figure out how to do this.Hint: if you have 30% Clay A in your batch and the colloidal fraction of Clay A is20%, the colloidal fraction added to the batch is 6%.)

You control your shrinkage by controlling particle size (and water content). Too finea formulation will cause excessive shrinkage and you will get distortion and crackingproblems.

Section 2 Tables

Table 2:1 Particle Size Defects

Wrong Distribution Effects plasticity in

plastic forming. Effects

powder flow in pressing

operations. Effects

casting properties.

Effects shrinkage.

Supplier may have

changed

milling/grinding

procedures. Changed

grinding media type,

size, or media size

distribution.

of 82

Too Fine Increases shrinkage.

Slows casting rate.

Changes plasticity.

Supplier may have

changed material

source. For clays,

supplier may have

moved to another part

of the mine.

Too Course Decreases shrinkage.

Increases casting rate.

Changes plasticity.

Supplier may have

changed material

source. (See above.)

Supplier may have

underground the

material.

Table 2:2 Particle Size Measurement Methods (Chapter 11 in

Ceramics: Ind. Proc. & Test.)

Sedimentation Stokes Law

Light Scattering Often LASER techniques

Screen Analysis ASTM Series Screens

Microscopic Analysis Optical or SEM techniques, etc.

Surface Area Absorption Techniques

Table 2:3 Raw Material Contaminants

Silicon carbide, ferro-silicon, etc. Source is storage areas such as

ports as these are man-made

materials. Cause blisters in bisk

and/or glost firings.

of 82

Manganese oxides, iron oxides,

titanium oxides.

Source can be storage areas or

such oxides are sometimes found

in mining deposits. Cause blisters

and discoloration. Problems with

these oxides may be related to

bisk firing temperatures.

Manganese oxides may

decompose at high bisk

temperatures and may not cause

trouble in glost and decorating

fires. For low bisk temperatures,

the oxides may not completely

decompose and bubbles and

blisters may appear in glost and

decorating firings.

Soluble salts like NaCl and KCl Can be from sea water during

shipping or from deicing

operations at ports, etc. Detect

chloride with silver nitrate.

Plastic, lint, paper, etc. Can come from out-of-control

drying operations (melted bag

filters, etc.)

Sulfates Inherent in ball clays, shales, etc.

Can be controlled with barium

carbonate which forms barium

sulphate which is insoluble. High

sulfates yield a very “mushy” cast

in slip casting operations.

of 82

Vanadium, titanium, iron, etc. These compounds are tough to

deal with in brick manufacturing

because they cause scumming.

Vanadium scumming on fine

white brick is particularly bad.

See the text for references on how

to clean such brick.

Lignite Inherent in clays. Lignite may be

bituminous or anthracite coals or

soft lignins. Causes blisters, pits,

and possible discoloration. Can be

controlled by screening or by

material selection by supplier.

Soda ash is helpful during

grinding because it forms

beneficial compounds when it

breaks down the lignite and

improves dispersion.

Mica Mica is common in clays and

comes as biotite or muscovite in

most cases. It can cause pinholes

and / or coloration. Often

measured as “grit” by clay

suppliers.

Organic Binders Carbonization of organic binders

can result in bubbles and blisters

and discoloration, especially in

glazes. The kiln must be kept

oxidizing with good air flow

during the binder burnout period

in bisk and glost firing.

of 82

Table 2:4 Glass and Glaze Ionic Constituents & Composition Range in Glazes.Compiled from reference 2. (LF = lead free)

Cation Valence Radius IonicPotential(Valence /Radius)

Role in Glass % CompositionRange / Typical asOxide in Glazes

Li 1 0.68 1.5 Modifier 0.0-1.0 / 0.0

Na 1 0.98 1.0 Modifier 0.0-8.0 / 3.0-5.0

K 1 1.33 0.75 Modifier 0.0-10.7 / 5.0-8.0

Be 2 0.31 6.5 Toxic: Donot use in anyform.

Modifier no information

Mg 2 0.71 2.8 Modifier 0.0-2.7 / matt 6.4

Ca 2 0.99 2.0 Modifier 0.0-33.6 / 5.0-8.0

Sr 2 1.13 1.8 Modifier see Ba

Ba 2 1.35 1.4 Modifier 0.0-48.4 / 10.0 LF

Zn 2 0.74 2.7 Modifier 0.0-6.0 / 0.0

Pb 2 1.21 1.17 Toxic:Limit use.

Modifier 11.0-50.0 / 20-40

B 3 0.20 15.0 Glass Former 0.0-42 / 6.0-10.0

Al 3 0.50 6.0 Intermediate 0.0-25.3 / 8.0-15.0

Si 4 0.41 9.8 Glass Former 0.0-79.2 / 20-50

Ti 4 0.68 5.9 Intermediate no information

Zr 4 0.80 5.0 Intermediate 4.0-6.0 opaqueglazes

Sn 4 0.71 5.6 Intermediate 8-13 matt &opaque glazes

of 82

Pb 4 0.84 4.8 Intermediate 0.0-61.4 / 20-30

P 5 0.34 14.7 Glass Former no information

As 5 0.47 10.6 Glass Former no information

Table 2:5 Typical Molecular Glaze Formula (Earthenware Type Glaze

Reference: 2 For glazes but not bodies the basic oxides total is set to 1

mole and the other oxides adjusted accordingly.)*

Basic Oxides Amphoteric Oxides Acid Oxides

0.078 K2O 0.261 Al2O3 2.905 SiO2

0.221 Na2O 0.370 B2O3

0.378 CaO

0.333 PbO (added as

frit)

*See Ref. 1 Chapter 9 to learn how to batch bodies and glazes.

Table 2:6 Glaze Classifications (I call this the Jones Classification)

Single-Component Raw Glazes Some materials form a natural

one-component glaze. Albany slip

clay forms such a glaze and can be

used to protect graphite

refractories from oxidation. It can

also be used on pottery, etc. Salt

glaze is a single-component glaze

formed when salt vapors react

with the surface of the ceramic

substrate.

of 82

High-Lead Raw Glazes White lead is a very good agent to

have in a dipping tank. However,

the material is toxic and should be

avoided.

High-Lead Fritted Glazes Usually contain lead bisilicate frit.

The density of the frit must be

controlled by the supplier. Frit

batches can be blended by specific

gravity or refractive index.

Low-Lead Fritted Glazes The lead is combined in the frit

with other glaze ingredients such

as the alkalies and alkaline earths,

Zn, Al, Si, etc. Ba may be added

to improve brilliance.

Non-Lead Raw Glazes Everything is in the batch but

there is no frit or ground glass.

Not too common.

Opaque Glazes Contain opacifier of course.

Crystallized Glazes Crystal formation glazes not

opaque.

All other glazes in the universe. Excludes the above. Now where

does the snake skin glaze go?

of 82

Table 2:7 Properties Imparted to Glazes by Atomic Constituents

(Partly extracted from Reference 3 and other references.)

Na Increases expansion, decreases tensile strength and elasticity,

increases fusibility, lowers viscosity. Should be used with K or

other alkalies to optimize properties such as electrical resistivity.

K With Na increases cost, may increase gloss, lowers fluidity,

increases scratch hardness, increases chemical resistance,

decreases expansion.

B Strong flux and solvent for other glaze constituents, stabilizes

colors, lowers viscosity and surface tension, heals defects

caused by air bubbles or liquid non-miscibility, reduces surface

friction to improve knife marking / cutting resistance. Greatly

improves chemical durability as glass former. Increases gloss /

brilliance, Excess can form pinholes and blisters.

Li Reduces bulk needed for same number of equivalents of alkali

because of its low molecular weight. Reduces expansion and

greatly increases chemical and mechanical durability. Small

amounts lowers softening temperature. Large amounts can cause

crystallization. Improves mechanical properties.

Ca Increases the mechanical and chemical durability of a glaze.

Lowers expansion. Increases fusibility at higher temperatures.

Mg Increases fluidity at higher temperatures, lowers expansion more

than other bases, improves colors, excess causes opacity,

improves chemical and mechanical properties.

Sr Strong flux, slowly increases expansion, best added to frit,

lowers solubility, improves mechanical and chemical durability.

Be TOXIC: DO NOT USE. Use Zn or Li instead for improved

properties.

of 82

Ba Strong flux. Increases brilliance as it is a big atom with a heavy

electron cloud. For that person, substitute for Pb to lower Pb

content of glaze. Incorporate in frit because of slight toxicity.

Lowers elasticity. Does not reduce in kiln atmosphere like Pb or

Zn (as seen in kiln wrecks).

Zn Very important in color stability. A cure all for many glaze

defects but not quite as good as Pb in that regard. Improves

melting and gloss. Excess causes opacity. Lowers expansion.

Improves mechanical and chemical durability. Every glaze

should have some in my opinion. Well, maybe not every glaze.

Pb The best glaze cure all but chemical leaching can be a problem.

Should be used in lower amounts and always in frit form to

protect workers. Gives high brilliance, low expansion, increases

elasticity, lowers viscosity, wide maturing range, lowers surface

tension and improves smoothness.

Al Improves melting characteristics. Improves chemical and

mechanical durability. Can stop crazing. Important for some

colors. Common glaze ingredient.

Si Main glass former in glazes. Increases durability. Greatly lowers

expansion in glazes (raises expansion in crystalline bodies).

Eliminates crazing but may raise required firing temperatures.

Sb,

Ti

Opacifier and color former.

P Not used much in glazes. Opacifier and increases brilliance.

As TOXIC COMPOUNDS rarely used in glazes. Opacifier. Color

former.

Sn Opacifier. Color former. Expensive.

of 82

Zr Opacifier and color former. Minute amounts can improve

mechanical and chemical durability without causing excessive

opacity.

Bi Like lead but not as good. May need an oxidizer like As to

prevent grayness. I was always disappointed with Bi glazes.

Cd Like Zn but not nearly as good. Color former.

Cr Color former

Ce Expensive opacifier required in large amounts. But could be

useful in lead-free glazes in low amounts

W Small amounts in frit can be helpful but expensive. See Mo.

Cu,

Co,

Au

Color formers.

Mn,

Fe

Can be strong fluxes. Color formers.

Mo Lowers surface tension of glaze. Larger amounts add opacity.

Best in frit.

Ni,

V

Color formers

of 82

Table 2:8 Ceramic Body Constituents (See Reference 1 and Table 2:9

Body Classifications)

Alumina,

Beryllia, MgO

High-temperature bodies which have good

chemical, electrical, and mechanical properties.

May be transparent to radar. Used in many

electrical, mechanical, and chemical processes.

Can be bonded to metals.

Refractory

Oxides, Silica,

Graphite, SiC,

Zirconia, Zircon,

Chromite Ore,

Magnesia

Resistant to slag and molten metal attack at high

temperatures.

Complex Oxides,

etc.

Used in many magnetic and electronic

applications such as magnets and capacitors.

Clays Kaolins and ball clays are used in many ceramic

products including whitewares and structural clay

products. Kaolins fire white. Ball clays may fire

off-white but add plasticity to ceramic bodiers.

New Zealand halloysite can be used for very

white, high-strength, high translucency bodies

with good plastic properties .

Fluxing Minerals Talcs and feldspars used for tile and whitewares

and other ceramics. Talc used to make steatite.

Alumina Rich

Minerals

Kyanite, sillimanite, and andalucite are used for

refractory materials.

of 82

Table 2:9 Ceramic Body Classifications (See Reference 1)

Near Pure

Compounds

Alumina, beryllia, graphite, and other essentially

one-component systems. Okay, I’ll allow up to

1% non-organic binding systems such as clay,

magnesia, chromia, etc.

Near-Pure Systems Above with up to 10% clay, magnesia, calcia,

chromia, etc., additions.

Mixed Oxide /

Silicate Systems

Ferrites, capacitor compositions, optical or

electronic device compositions, etc.

Clay / Talc Based

Systems

Brick, tile, sewer pipe, floor tile, etc. Also

steatite having superior electrical properties.

Triaxial Systems China, stoneware, porcelain, pottery, and such

made form feldspar, clays, and silica.

Fused Silica Low-expansion, chemically durable glass.

Borate Glasses High chemical resistance.

Structural Glasses Plate glass, automobile glass, glass blocks, etc.

Container Glass You know, like Coke® bottles.

Specialty Glasses Optical glass and other glasses with special

properties.

Glass Ceramics Dinnerware and technical glass ceramics usually

based on Li compounds.

All other bodies in

the universe.

Those bodies of which I don’t have a clue.

of 82

Table 2:10 Materials for Advanced Applications (see ref. 1)

High Purity Aluminas Purity beyond that of Bayer Process Alumina.

Sodium vapor lamps. TTA is zirconia

transformation toughened alumina. Titanium

aluminate for automobile parts, etc.

Barium Titanate, etc. Capacitors and transducers. PZT and PLZT

also—incorporating lead, zirconia, and

lanthanum.

BeO Good electrical and nuclear properties. High

thermal conductivity is advantage for heat

dissipation from electronic devices such as

klystron tubes. Powder is toxic and AEC

standards should be used to prevent injury.

Boron and Titanium

Carbides and Titanium

Boride

Grinding tools, etc. Very hard and strong.

Ceramic armor.

Boron Nitride Good electrical properties. Good for specialty

refractories, etc. Like graphite in some ways

except it is an electrical insulator. See ref.

Rare Earth Oxides Used in many diverse applications. See ref.

Iron and Other Oxides Used in ceramic magnetic materials includes

MgO, ZnO, BaO, SrO, etc.

Silicon Nitride Refractories and automotive engine parts.

Antifriction applications.

TOC

of 82


3: DEFECTS FROM FORMING OPERATIONS

Common defects from forming operations are discussed in this 3rd article in theCeramic Defect Series. Dry forming methods generally have defects related to particlesize distribution, binders, and pressing parameters. Wet processing methods havedefects generated by the state of flocculation of the slip and moisture control.

Forming methods of ceramics are sometimes classified as wet or dry. Dry formingrefers to pressing operations from dry or perhaps damp powders. Wet formingincludes slip casting and plastic forming methods.

For a review of industrial forming methods see Ceramics: Industrial Processing andTesting by John T. Jones and M. F. Berard, Iowa State University Press.

Dry pressing requires that a shape be dimensionally stable after firing. That will occurif the pressing operations are in control and the firing is specified. If a pressed partis oversize after firing, it can be ground to size, but that is an extra operation usuallynot included in the costing of the part. If the part is undersize after firing, the part isscrap.

Important factors in pressing are the die size, the particle size distribution of thepowder, the binder system, the pressing pressure and pressing cycle. Problems arepowder sticking to the die, powder not flowing into the die freely, and incorrect firedshrinkage.

In isostatic pressing an additional problem can occur due to the incompressibility ofair. This can sometimes be relieved by flooding the tooling cavity with propane whichis compressible and on release will evaporate into the air slowly rather than expandinstantly like air expands which can crack the part. In isostatic pressing the partusually must be machined after forming.

Laminations in pressed parts can be caused by the powder not flowing easily duringdie filling or during pressing. These laminations will not usually heal during firingand must be eliminated in the pressing operations. Experiments with binders such as

of 82

paraffin, gums, and starches, etc., must be conducted to determine the correct bindertype and amount. Binders with lubricating properties can help. Sometimes a lubricantis needed to prevent sticking of parts to the die. See the reference for moreinformation including spray drying of powders.

Plastic forming by machine such as plate rollers and jigger machines usually requirethe clay to be in the flocculated state. Small amounts of plaster added to the bodywhen in slurry form before filter pressing and extrusion will provide the desiredplastic properties. Cast jiggering is an exception but even then the clay slip is placedon a plaster mold providing what is needed to flocculate the slip as it builds up on themold before jiggering.

Clay for plastic forming must have the correct moisture content or the dimensions ofthe dried product before firing will be incorrect. Firing will not improve this situation.Most other problems occurring during plastic forming can be resolved by keeping theclay flocculated.

Nepheline syenite and alumina tend to deflocculate plastic bodies before forming soit may be essential not to delay forming in these cases only. Generally, aging ofplastic clay improves forming operations especially for bodies with low inherentplasticity. Aging not only distributes moisture but also breaks down sulphates,etc.

Information on clays used in plastic forming can be found at these sites:http://www.walkerceramics.com.au/practical%20information.htmhttp://cavemanchemistry.com/oldcave/projects/pottery/basic_clays.htmlhttp://web.clas.ufl.edu/users/sassaman/pages/classes/Ceramics/Clays.htmhttp://www.basic-stuff.com/hobbies/ceramics/

Slip casting requires that the clay slurry be dispersed. The slip specific gravity andviscosity must be controlled. The particle size distribution is critical and must becontrolled by formulation as well as processing. See the referenced text for details oftesting and slip control.

Whether from plastic forming or slip casting, leather hard ware must be handledcarefully during finishing operations. Clay has a memory and distortions in greenware can reappear during firing.

of 82

See the reference for other forming methods and problems.

Section 3 Tables

Table 3.1 Forming Defects

Shrinkage Problems (see raw materials above)

Bubbles and Blisters May be caused by air entrapment. Thevacuum chamber of extrusionequipment must be maintained byauger replacement or repair, etc.Vacuum lines must be open.Sometimes too plastic clay will plugvacuum lines when it is alreadydifficult to de-air. Pressing operationscan also entrap air (see Laminations).

Laminations In pressing operations caused by poormaterial flow or air entrapment.Particle size distribution and particlefineness are to be suspected. Clay maynot “mend” during plastic forming.

Cracks (see also drying) Cracks are oftenrelated to the pressing cycle. Inisostatic pressing, the expansion ofentrapped air can cause cracking.Propane is compressible and may beused to fill die cavities or isostaticpressing fixtures to displace the air.Air is not compressible.

Cracks and Distortion During Firing (see also firing) Cracks and distortionsduring firing may come from formingoperations. Check pressing cycles,particle size distributions, etc.

of 82

Setter and Support Design Errors (see also firing) Setters need to bedesigned to “pull” the piece in duringfiring and to support dimensioncontrolling setters during firing. Seetext and illustration of setter design.

Pits and Blisters Binder preparation may have left smalllumps of organic material notdestroyed during pressing. Checkbinder preparation methods andpossible contaminants in materialpreparation areas.

Table 3.2 Capacitor Defects (multiple thin film) See:http://www.ceramicindustry.com/CDA/Archives/cde08fabca9c7010VgnVCM100000f932a8c0____

Swiss Cheese or Other Void Structures Excessive binder. Solid particles nottouching. Solid volume should beabout 60%.

Pin Holes Too little binder. Air is trapped.

Surface Craters Too little binder. Air is trapped.

Table 3.3 Glass Making Defects

Stones, Cords, Knots Glass tank refractories

Bubbles Improper melting and refining.Trapped gases.

Un-melted Material Grains Improper melting and refining.

of 82

Table 3.4 Injection Molding Defects (See also Table 4.2)

Slumping and Warping Improper cooling after injection.Binders too soft. Support not provided.

Air Pockets and Pinholes Particle size distribution. Airentrapped during extrusion. Usevacuum to evacuate chamber beforemelting.

Poor Microstructure Microstructure depends on the closepacking of particles during forming.Slip casting gives a good structurebecause of good particle packing. Thestructure is not as good in pressedparts because often more and stifferbinders are used. In injection molding,there is an excess of binder that mustbe removed. Therefore the structure issometimes inferior to other formingmethods.

Table 3.5 Spray Drying Criteria

Inlet and Outlet Temperature This temperature difference combinedwith the airflow volume supplies theheat required to heat the slurry as it isfed into the chamber and to evaporatethe water from the batch. More air isdesirable over too-high an inlettemperature to control particle sizedistribution. Moisture control dictatesair control parameters.

of 82

Moisture Content and Angle of Repose A SiC bomb is a quick way todetermine the moisture. Weighing anddrying a small sample also works. Toomuch moisture will degrade the flowproperties. The angle of repose shouldbe determined at the same time themoisture content is being determined.An angle of repose of 32 degrees orless is favored. Angle of repose abovethis mean that the powder may notflow properly in isostatic pressing anddry pressing operations.

Spray Nozzles Versus Spray Disc Disc are usually desirable for

large chambers, nozzles for small

chambers. Small dryers are often

less desirable than larger driers.

Binders Paraffin can be emulsified and

blended with the slurry. This is

often more desirable for isostatic

pressing than using wax

emulsions. A 0.1% addition of

gum arabic can reduce excessive

fines. Dextrin and other binders

and lubricants such as stearates (a

salt or ester of stearic acid,

containing the group

C17H35COO), may improve dry

pressing operations but not always

isostatic pressing. Alkaline

binders containing Na, K, etc.,

may degrade electrical properties.

of 82

Particle Size Segregation Large dryers cause segregation

but blending can be obtained in

air- blending chambers.

TOC

of 82


4: DEFECTS GENERATED DURING DRYINGOPERATIONS

In this 4th in the series on ceramic defects, drying of ceramic parts is discussed. Atfirst, water is easily removed allowing rapid drying, but then diffusion takes over andthe water must be removed slowly or the ware will crack or distort. Finally, the lasttightly-held water must be removed. The process is moderated by temperature,humidity, and air-flow control.

Drying operations relate to plastic forming operations and casting operations. Forceddrying in controlled driers expedites production and guarantees continual controlledproduction flow.

Driers are usually built into automatic casting machines and roll forming machines.The drier is often designed to accommodate the different stages of drying. Air flowis adjustable throughout the drier.

When a piece of ceramic ware is first formed, the particles are separated by a waterlayer which can be easily and safely removed. For that reason, excessive heat can besupplied at this stage of drying.

Once the particles touch, the process becomes diffusion controlled and the watermolecules must move slowly through the body matrix.

Finally, the last tightly held water must be removed.

These last two steps require careful heat and humidity control in the drier. SeeCeramics: Industrial Processing and Testing by John T. Jones and M. F. Berard.

For automatic plastic forming operations incorporating built-in driers, the slip mustbe controlled as to particle size, specific gravity, and viscosity. The slip must be inthe flocculated state during filter pressing and subsequent operations. If notcontrolled, the ware may not dry properly after forming and the operations can bedrastically slowed down. Drier losses due to cracking or distortion can be dramatic.

of 82

Drying of cast ware is less critical except for automatic casting machines. Slip controlis again the most important function to prevent loss.

See the reference for more information on the drying process, slip control, andindustrial driers.

To obtain a psychrometric chart go to:http://www.engineeringtoolbox.com/psychrometric-chart-mollier-d_27.html

Section 4 Tables

Table 4.1 Defects from Drying Operations

Cracks Often caused by improper dryingcycles. Plastic formed ware may bedried rapidly at high temperaturesinitially until the water layer betweenparticles is removed. Then thingschange when the particles touch eachother. Now the removal of water iscontrolled by the diffusion rate of thewater to the surface. This stage ofdrying must be carefully controlled byhigher humidity and lowertemperatures. The very last water ishard to remove and is often notremoved until firing. See Chapter 5 inCeramics: Industrial Processing andTesting.

Distortion Plastic formed ware such as dinnerplates may have to be supported duringdrying operations. Special setters areavailable.

of 82

Laminations Laminations may appear duringdrying. They may be related toforming.

Table 4.2 Injection Molding Drying Defects

Slumping and Warping in

Injection Molding Dryers

Thermal plastic effect. Include a

thermal setting resin in mix. Use

range of thermoplastic binders

burning out over different

temperature ranges. Control

binder burnout stages removing

lowest-temperature binders before

softening of other binders.

Voids, Air Pockets, Blisters, Etc. Control binder particle size

distribution. Remove air before

injection by vacuum techniques.

TOC

of 82


5: DEFECTS GENERATED DURING BISK FIRINGOPERATIONS

In this part of the series on Ceramic Defects we discuss the bisk (or bisque) firing offine china, porcelain, and industrial ceramics. Drag, shrinkage, and slump that occurduring firing must often be accounted for by using special setters and fixtures.

The purpose of bisk firing is (1) to completely vitrify or densify a ceramic body in thecase of bone china, fine china, and industrial ceramics or (2) to partially vitrify abody in the case of porcelain.

Taking porcelain first, Case (2), the body is taken to a temperature to which enoughstrength is developed to be automatically handled in glazing operations (in modernfactories) but porous enough to be easily glazed by dipping methods.

Porcelain ware does not have to be supported during bisk firing and defects generatedare usually chips or breakage from handling.

Kiln dirt can cause defects here, but rarely. If they are formed, they often can beremoved by grinding methods.

Sometimes sand, perhaps alumina, is placed between dinner plates so they can bestacked without any chance of sticking. This sand is easily removed by subsequentoperations. If a speck of sand is left on the wear during glazing, it will cause a glost-ware defect as described in the nest article.

In the first case, Case (1), the body is heated on a temperature / time cycle where fullstrength is developed (usually through sintering) in non-glazed industrial ceramics.

For fine china and bone china, the body develops maximum translucency throughvitrification.

The ware often has to be supported during firing.

of 82

Figure 10 Shrinkage Block

Three major problems relate to the high-temperature firings of Case (1). The firstproblem is shrinkage. The second problem is slump. The third problem is drag. Thesefactors play a major part in the distortion of large parts during firing.

For fine china and bone china, the dinner plates and similar items are fired inrefractory saggers. The ware settles into the shape of the sagger as it softens duringfiring. Therefore, the ware is fully supported. The saggers are covered so there isusually little or no kiln contamination. Operations may be required to remove anyparticles from the ware before glazing.

Small parts are not a problem for industrial ceramics. Larger parts are a differentmatter.

The shrinkage and slump and drag factors must be known.

Shrinkage is caused as the particles move together during sintering or vitrification.

Slump is due to the weight of the part on itself.

Drag is due to the friction between the part and its setter or the kiln car deck.

A shrinkage block of a body can be formed into atwo-diameter cylinder about 3 inches high with a tophalf diameter of perhaps 1-3/4 inches and a bottomdiameter of 2 inches. The drag shrinkage isdetermined by the diameter of the block touching thekiln car deck or setter, the slump is determined bymeasuring the total and shoulder height of the block,and the normal shrinkage is determined bymeasuring diameters not affected by drag or slump.

An example of how large cylindrical shapes are fired will illustrate what sometimesmust be done.

A setter is machined from the same material as the cylinder. This base setter is coatedwith a solution of PVA (polyvinyl alcohol) and sprinkled with sand (alumina). Forsmaller cylinders this step can be omitted.

of 82

Figure 11 Ceramic cylinder firing setup.

Figure 12 Ceramic Cylinder Firing Setup

On top of this is placed a step setter. The step is about 3/8 inch.

The ceramic cylinder has its ends coated with a solution of PVA (polyvinyl alcohol),dipped in setting sand (perhaps alumina), and is placed on the step setter.

The upper step is the diameter ofthe ceramic cylinder. (Well leavea few thousandths.)

The step setter maintains thelower diameter of the part. Asthe base setter shrinks, the basesetter (and step setter) willshrink, pulling the cylinder inwith it.

But we are not firing yet.

A hollow cylinder (perhaps 2inches in diameter) made of the same material as the part is placed on the step setterdead center.

A top step setter, treated as above, is placed on top of the ceramic part, its minordiameter fitting into the cylinder and resting on top of the inner support cylinder.

The support cylinder will decrease slump caused by the weight of the top step setter.

The top step setter maintains the diameter of the part.

After firing, the cylinder will not be perfect, but if there is enough grind stock and theupper and lower diameter havebeen maintained during firing,the part can be machined tothe specified dimensions.

I was not able to illustrate thisin the original article. I hope

of 82

with the figure you see it in your mind: A larger cylinder resting on a step setter witha step setter on top, the steps maintaining diameter as they fit into the cylinder. Aninner cylinder supports the top step setter. Got it!

For some shapes and very large cylinders, a refractory solid support cylinder is used.In this case, the support maybe placed right on the kiln deck. A hole is cut in the baseand bottom step setters to accommodate the refractory shape.

To protect shapes from shrinkage, drag, and slump, you must design the firingfixtures for every affected part. Sometimes bubble alumina comes in handy.Aluminum silicate wool such as Fiberfrax® can be useful for support at fine chinabisk temperatures and lower.

For a discussion of ceramic kilns go to Ceramics: Industrial Processing and Testing,John T. Jones and M. F. Berard.

Section 5 Tables

Table 5.1 Defects from Bisk Firing Operations

Distortion Plastic formed ware may have to besupported during low or high biskfirings. Refractory setters serve thispurpose.

Drag Foot distortion caused by a partdragging on kiln furniture orrefractories. Bubble alumina, aluminasand, etc., can reduce drag but it mayrequire a setter between the ware andthe kiln deck.

Slump Usually caused when the firing is nearthe maturing temperature for the bodyespecially with vitreous bodies.Support setters are required (see text).

of 82

Flame Impingement Flame impingement can cause cracks,blisters, and distortion. Sometimes asimple burner adjustment will alleviatethe problem. More air to the burner cancool the burner temperature.

Scumming Caused by impurities such asvanadium. See raw materials tableabove.

Bloating In late stages of firing after the body issealed, clay or other materials are stilldecomposing forming internal bubbles.Keep the firing oxidizing and controltime / temperature stages. Over-firingalso can cause bloating.

Black Coring Oxidize carbon compounds forremoval before the final stages offiring. Can be exasperated by ironcompounds causing early melting.Provide good air circulation aroundparts. Thick sections take longer firingtimes if carbon is present.

Lime Pop Too large limestone particles can causeblow out. A white particle is often seenin the “moon-shaped” crater. Screenout large particles and tell supplier ofyour problem.

Delayed Crazing (See Next Section) Hard to overcome by just glazeadjustment. The body must be sealedfrom moisture absorption by reducingporosity and closing porosity withglassy phase. Moisture expands thebody placing the glaze in tension.Time delay reaction. May take years.

TOC

of 82


6: DEFECTS GENERATED DURING GLAZING ANDGLOST FIRING OPERATIONS

In this segment of Ceramic Defects we discuss glazing operations and how to preventcertain glaze defects by formulation before they can happen. We also describe thecommon glazing operation defects and what can be done to remove or eliminate them.

A glaze is a specially formulated glass applied to ceramics. In the case of electricalinsulators the glaze must maintain specific electrical properties even in inclementweather. For chemical porcelain it must have high chemical durability. For finechina, porcelain, and semi-vitreous china, it must have high gloss and be resistant todish washing chemicals. In all cases it must be resistant to thermal shock. For mynotes on glaze formulation see below: Notes on Glaze Formulation and Firing.

The properties of a glaze are determined by its chemical composition. Potassiumfeldspar is a single-component high-temperature glaze for chemical porcelain. Leadis a component of fine china glazes because it “fixes” many of the application andflow problems of glazes and adds high gloss. Lead is not used as it once was, in thewhite lead form, which was desired in dipping glazes. All most all lead is nowcontained in frits.

A frit is a special glass used in compounding glazes. It ties up toxic and solublematerials and sometimes coloring oxides. Clays and insoluble oxides and orcarbonates may be added to the frit to form the glaze composition. To learn how toformulate a glaze, frit, or ceramic body, see Ceramics: Industrial Processing andTesting, John T. Jones and M. F. Berard, Iowa State University Press.

While not all ceramic bodies are ground, all glazes are ground to a specific particlesize. Over grinding can cause crawling of a glaze. The crawling of the glaze from theceramic substrate usually does not show up until firing. Firing does not always fixthis particular application problem. Sometimes binder additions like PVC or dextrincan reduce crawling. Read on for more information on crawling.

The glaze is sprayed on to the ware in most cases. Many shapes are dipped. Have you

of 82

ever wondered how the innards of a toilet bowl can be glazed? A Nerf Ball® issoaked in glaze and sucked through the trap.

In the porcelain process the body is porous after a low-temperature bisk fire.Therefore it is easy to dip. This process is automated for dinner plates and such inmodern factories.

In the china process the ware is vitrified and it must be heated during the glazingprocess. This is usually done by burners in the first section of the glazing tunnel.

Glazes can also be applied in powder form by electrostatic spraying. This is a goodway to put enamel on household appliances. I have little experience in this area butI have seen such glazing machines.

I might mention that in the zillion tile factories I have toured in Italy (and one inColumbia, S.A.) a water fall works well for glazing. The tile industry has many cleverways of glazing and decorating the tile as it moves down the line. I’ve learned thatpeople are generally not required in modern tile factories. Everything including lunchbreak is automated.

Typically most glazes have a small amount of montmorillonite. This can be a refinedsynthetic white material in glazes for fine china or Wyoming bentonite.Methocellulose, dextrin, and gum Arabic are also used as binders. Crawling is aproblem that can be prevented by using a proper binder system and avoiding over-ground glazes and overly-thick glaze application. Crawling usually appears duringthe firing process but it can occur on drying of the glaze.

Some glazes tend to craze after firing. This can occur during decorating operationsor sooner or much later with time. One form is delayed crazing that occurs when aporous body (semi-vitreous ware) absorbs water which can expand the body, stretchthe glaze, and crack the glaze. Sometimes adding nepheline syenite or talc to formmore glass in the body, adding silica to raise the body expansion, or other bodyadjustments can prevent this.

Crazing during decorating operations indicates that the glaze needs reformulating.This may be done by simply adding a small portion of silica or low-expansion frit tothe batch. Changing the cooling cycle during the decorating process can help. I’ve

of 82

seen large shapes crack on cooling during glost firing, sometimes crazing the glaze.This is called dunting.The solution was always in controlling the cooling cycleespecially though the silica conversions. (See the reference.)

It is interesting that silica can raise the thermal expansion of a body and lower thethermal expansion of the glaze. Can you figure out why? [Answer: the silica is incrystalline form in the body and in the vitreous form (or very low thermal expansionform) in the glaze.]

For all practical purposes the thermal expansion coefficient of the glaze must melower than that of the body. This keeps the glaze in compression after firing. Glazesalways fail in tension. Keeping the compressive forces higher than the tensile forcesis the key.

Silica and boron compounds can lower the expansion coefficient of the glaze.Replacing alkalis with alkaline earths can help. There is a thing I call multiplicity inglaze formulation: The more different materials used in compounding a glaze, thebetter. It minimizes the negative effects of a particular material while retaining thegood effects. (Anyway, that is my theory and I’m sticking to it.)

Kiln contamination can be a problem in some operations. Pits, pin holes, and pockscaused by impurities dropping on the glaze during firing can be removed by grinding.The ware is resprayed with a thin coat of glaze and then refired. The thickness of aglaze before firing is always a concern so you don’t want to over do it.

Keep your kilns clean. This should be a routine for the kiln loaders.

Pinholes can be caused by body contaminants. If body impurities are still releasinggases on refiring, your problem may not be resolved until you fix the body problem.

There is a discussion of glaze defects at: http://www.ceramicstoday.com/articles/122000.htm.

One of the defects discussed there is shivering caused by too low a thermal expansioncoefficient for a glaze. In all my days I’ve seldom seen this except in the laboratory.The problem is almost always the opposite (crazing). Anyway, what can be easierthan raising the thermal expansion coefficient of a glaze?

of 82

Notes on Glaze Formulation and Firing

I hesitate to write an article on glaze formulation when there is so much on

the Internet. Still, there are a few generalities that might be useful.

Some years ago I categorized many glaze formulations according to firing

temperature and surface finish. I did this for lead and non-lead glazes both

glossy and matt. I was amazed how easy it was to correlate composition

with firing temperature (cone or Buller ring).

You might run the same type of exercise if you are looking at many glaze

compositions trying to come up with just one. You can start your

formulation activities using the averages of the ranges of composition for

each oxide addition for the temperature range you are shooting for.

Also my associates formulated hundreds of glazes over the years for

earthenware and fine china. Much of this work was done in a joint effort

with frit suppliers as existing frits don’t always do the trick in large

production operations.

Here are some factors that I found to be important:

Glaze Composition

Lead is magic in glazes. It must be used in fritted form and in the minimum

amount possible to maintain the glaze flow in the molten state. The glazed

ware must pass all FDA and other restrictive testing. If lead is allowed,

your problems are minimized.

Your workers must be monitored for lead in the blood if lead air levels

exceed OSHA standards. Borate frits can help reduce lead content but may

increase solubility.

of 82

Complexity of formula is important. Every element you add to a glaze

impacts the final-glaze properties. For example, too much alkali content

will increase the solubility of a glaze, raise its thermal expansion, and in

general raise havoc. A minimum amount of alkali content will give the

glaze fusibility. Also, individual alkalis act differently in extent. Therefore

one would use more than one alkali and the minimum amount of each that

yields the best balance in properties.

The same is true of the alkaline earths, the glaze modifiers, and the glass

formers.

You should consider the entire periodic table of the elements when

formulating a glaze.

Remember that a very small addition of a particular element might give you

a property you want, but increasing the amount only slightly can ruin your

progress.

One other thing: You should use the minimum amount of binder.

Sometimes it’s best to use several binders in small amounts rather than just

one binder. Binders can be purified clays or organic compositions like

gums or resins. Like I say, a mixture is usually best. None would be better.

To learn how to formulate gazes from scratch or using frits you can find it

in my book Ceramics: Industrial Processing and Testing. Use

http://tinyurl.com/gbxbz to obtain a new or used copy at Amazon.com. You

don’t need the second edition if you are only interested in glaze

formulation.

Firing Conditions

The firing curve naturally has heat up and cool down periods.

of 82

In between these slopes is a flat or modified soaking time/temperature.

During heat up, the binders are removed from the glaze.

They must be completely removed and they must not be reduced to carbon

during heat up (preheat).

This does not mean that you can’t approach reducing conditions. Some

compounds like MnO and FeO can greatly improve melting although they

are often in the glaze only in tiny amounts as impurities. These compounds

do not form under oxidizing conditions.

Anyway, do not reduce the binders to carbon. It’s near impossible to

remove for many glazes during the rest of the firing.

There are a number of instruments to determine the nature of the burnout

for a particular binder. Thermal gravimetric analysis (TGA) and

Differential Thermal Analysis (DTA) come to this old head.

The preheat or heat up is usually a production standard for tunnel kilns.

Therefore if you are not getting what you are looking for, you must adjust

the burners or heating input in this zone. You don’t want bubbles from

binder forming all during the soak period. You have enough bubble

problems without that.

During the soak phase and at all higher temperatures the glaze is changing

composition. That is, the glaze is losing volatile elements because of the

high temperatures. This can be complex as when one component of a glaze

is removed it may increase the volatility of other components.

For you chemical engineers, this would be somewhat like steam distillation.

When you lose volatile elements you also increase glaze viscosity in the

of 82

molten state. For this reason, and others I suppose, fast firing is often better

than a long firing cycle. In fact, I’ve seen glazes that required fast firing.

Cooling is important to both the glaze and ceramic ware. You can cool

quickly to just above the silica phase transition and then cool slowly

through the transitions. This may not be good for the glaze which is trying

to get rid of the bubbles created after the glaze melted. This can put you

between a rock and a hard place. You have to be able to control the whole

cooling zone of a tunnel kiln from the minute the ware comes out of the hot

zone.

Section 6 Tables

Table 6.1 Defects from Glazing and Glost Firing Operations

Rough Foot-unglazed If the glazed ware is fired on

refractory setters glaze may cause

sticking during firing. The

solution is to clean the foot

carefully after glazing, often using

a sponge belt. The foot can be

waxed prior to glazing. This can

be done by melting paraffin in a

flat electric frying pan with a

sponge bottom. Waxing belts are

also available. After firing, the

foot should be checked for

roughness and smoothed using a

burnishing stone or a foot-

burnishing machine.

of 82

Rough Foot-glazed If the foot is glazed, pin-firing is

required. Small refractory spheres

are often used instead of ceramic

or metal pins. After firing, the foot

should be checked for roughness

and smoothed using a burnishing

stone or a foot-burnishing

machine.

Pinholes Pinholes can be caused by

overfiring, impurities in the glaze,

or contaminants in the body.

Closed setters can be used to

protect the ware from flame

impingement during firing. (See

blisters.)

Spit Out Occurs in decorating operations. It

is often due to moisture trapped in

earthenware or in fine china if the

bisk ware was under-fired. Pits

left from spit out can collect dirt.

Some refire earthenware before

decorating or a small hole is

drilled in the back of the ware to

allow moisture to escape. Aged

earthenware is more prone to this

default and should be re-fired.

Decorators are advised to

purchase vitrified china rather

than earthenware blanks for

decorating or refire may be

required.

of 82

Cut Glaze (also see crawling) Often from dipping operations.

The glaze is very thin or

nonexistent on some areas of the

ware because the ware failed to

take up the glaze. It can be caused

by dirt, grease, nonuniform

vitrification due to soluble salts,

oil, perspiring fingers, or water.

Over-grinding of the glaze or

mechanical handling of the ware

causing knock-off may result in

cut glaze. Binder additions can

help reduce cut glaze.

Starved Glaze Loss of shine due to overfiring or

too thin glaze. Can be caused by

loss of glaze ingredients during

firing.

Sucked Ware Dullness of glaze near cranks or

setters. The porous refractory is

sucking up the glaze constituents.

Use vitrified cranks or coat them

with a suitable surface sealing

wash.

of 82

Droppers (Kiln Drip) Glaze drip from the roof of tunnel

kilns. (We had a similar problem

using a kiln that had previously

been used to calcine fluorite or

calcium floride.) Kiln ventilation

may help. Refractory coatings to

increase the viscosity of the glaze

so it does not drop may help.

Protecting the ware with setters or

cranks is a sure but inefficient

way to protect the ware.

Sulfuring, Staring, Feathering Surface patterns cause by

combustion products at the end of

firing forming calcium and or lead

sulfates that do not dissolve in the

glaze but crystallize on cooling

into needle-like shapes. Avoid

high sulfur fuels and keep good

circulation in the kiln. Aired ware

is similar but caused by the

formation of calcium or zinc

silicates. It occurs during the

crystallization period of the glaze

typically 700-850oC. Almost

always caused by accidental kiln

firing cycle disruption. I’ve never

seen it.

of 82

Stuck Ware I hate it when this happens. The

glazed ware sticks to the kiln

setters or kiln furniture. The cause

is glaze on the foot or glaze

flowing to the foot area. This can

be caused by overfiring, too-thick

glaze application, or too fluid

glaze. Setter washes can reduce

the trauma. Ugh!

Specking Another irritating fault. Cobalt

dust floating in the air can put tiny

blue spots on glazed (or bisk)

ware. Color contaminants can

occur in decorating operations.

Possible contaminants must be

controlled and confined.

Chromium oxide is another

contaminant to watch for. A little

dust goes a long way. Reference 2

says that even birds in the beams

can drop dust (and you know

what) onto the ware. A blast of air

on the ware before glazing or

decorating can remove unwanted

dust. Plant as well as kiln

cleanliness is absolutely essential

in ceramic operations. Vacuuming

should be done with care and

absolute filters should be used

where warranted. And don’t for

get to maintain the filters on your

air lines.

of 82

Delayed Crazing

(Moisture Expansion of Body)

See Crazing below.

Hard to overcome by just glaze

adjustment. The body must be

sealed from moisture absorption

by reducing porosity and closing

porosity with glassy phase.

Moisture expands the body

placing the glaze in tension. Time

delay reaction. May take years to

occur.

Blisters (see also Egg Shell) Too fast firing or incomplete

firing. Non oxidation of carbon

compounds from organic binders

due to too little heat in the preheat

zone and or non-oxidizing

conditions. Impurities such as

limestone or SiC from grinding or

clay storage operations.

Hardening of the glaze may help.

of 82

Bubbles (in general) Bubbles in glazes degrade the

properties of the glaze and cause

many glaze defects. Knife

marking and knife cutting

problems are enhanced by

bubbles. Electrical, mechanical,

and chemical properties suffer.

Modification of the body

(including the removal of

impurities) and / or the glaze may

help. The grinding and application

of the glaze are important. Do you

have soluble salts migrating to the

surface? Add barium carbonate or

chloride to the slip. You might

also want to check your molds in

slip casting operations. Are they

too dry? Is there oil on the surface

of your ware or in your glaze.

Clean air filters and add oil traps.

What about fermentation? Does

your glaze tank stink? Is

something decomposing? Do you

see bubbles forming in the slip?

Egg Shell (see also Blisters) Failure of blisters to heal due to

underfiring or too-hard glaze.

of 82

Crazing Cause by a glaze / body mismatch

where the coefficient of expansion

of the glaze is higher than that of

the body. Addition of silica or a

low-expansion frit usually

corrects the problem. An addition

to the body of silica (at the

expense of plastic clays and or

feldspar) of 5-10% will also help

correct the problem. (Silica has

low expansion in glaze because if

is fused. It has a higher expansion

in the body because it is

crystalline.) Increase ball clay at

the expense of china clay.

Underfiring of earthenware or

underground flint reducing the

formation of crystobalite can be a

cause of crazing.

Shivering or Peeling Caused by the opposite of the

above. The glaze is in so much

compression that it shivers off.

Curved or sharp surfaces enhance

the effect. This is not a common

default and is the easiest fault to

resolve simply by increasing the

thermal expansion coefficient of

the glaze. Simply lowering the

flint content (or grinding the flint

coarser) is the common cure. A

more fluid glaze also can reduce

peeling.

of 82

Pinholes, Blisters, and

Crowfooting in Engobes

May appear before firing. Check

the composition and properties of

the slip, slip applications, and

firing procedures. SCROLL DOWN

of 82

Crawling Crawling of the glaze from the

substrate can occur before or

during firing. During firing

viscous glaze can pool and

overcome surface tension forces

binding the glaze to the ware.

Over-grinding can increase the

tendency to crawl due to the

formation of colloidal factions

that tend to shrink more cracking

the glaze on drying. The glaze

formula may have to be changed

to reduce or increase the fired

viscosity of the glaze. Too-fluid

glaze may extract silica and

alumina from the body during

firing changing the glaze

composition. Fine clays can cause

excessive drying shrinkage.

Opacifiers and some underglaze

colors can cause crawling. Ware

should be clean before glazing.

The glaze thickness must be

controlled to about 10 thousands

of an inch or so. Too-thick glaze

crawls. Binder additions can help

crawling. About one-tenth percent

of dextrin often does the trick.

PVA can also be used.

Methocellulose in small amounts

may also help but it dries slowly

and may cause trouble in

of 82

Kiln Dirt, Spits, etc. Dirt from refractories and setters

can be removed by using small

hand grinding tools. The ware can

be touched-up and re-glazed.

Table 6.2 Customer Complaints on Glazed Dinnerware

Rough Foot (see above) Suggest the use of doilies between

plates during storage. Use a

polishing stone to remove

roughness. Replace ware if

required to keep customer happy.

Send a brochure on handling

ware. Rubbing one plate over

another can scratch the glaze. See

also Knife Cutting.

Knife Marking Rough Glaze marks easily,

especially those with ZrO2

opacifier. Sometimes the markings

can be removed with Soft Scrub®

from the Dial® Corporation.

Improving glaze flow during

firing can smooth the surface.

Knife Cutting Steak knives are very hard metal

compositions and can easily cut

into soft glazes. Hardening the

glaze usually means higher

temperature glazes. Fine china

manufacturers should provide a

brochure on the care and handling

of fine china.

of 82

Gild Rub Off The ware should be regilded if

possible and returned to the

customer.

Fussy Complaints Some customers are more picky

than others. Inform your customer

as to your production control

procedures and control limits.

What size speck to you allow?

What size pinhole do you allow,

etc? If the ware is out of your

control ranges, repair or replace

the ware.

Table 6:3 Thermal Shock and Autoclave Testing

Thermal Shock

Testing

Determine the ambient temperature of the

water placed in a tub. Maintain this ambient

temperature throughout the test. Place the

items to be tested (usually 6) in an oven and

heat them long enough to be uniform in

temperature. The first temperature should be

about 50 degrees F above the water

temperature. Using the same procedure every

time, slide each piece into the water. Check for

craze lines. Check for cracks in the piece. Use

ink or die penetrant if required to find the

cracks. If no cracks, raise the oven temperature

50 degrees F. Repeat until failure by crazing or

cracking. If your ware can pass a test of 300oF

to 80oF water, you may be okay in the field.

400oF would be better.

of 82

Autoclave Testing Dinnerware should pass a 150 psi controlled-

test autoclave pressure. Tile usually passes 200

psi. The heating and cooling to and from

pressure should be gradual. If your ware does

not pass the test, you may have a delayed

crazing problem due to moisture expansion.

Sometimes just soaking ware in hot water will

cause crazing due to moisture expansion of the

body. Earthenware bodies are particularly

prone. Earthenware will sometimes craze

during decorating operations. In that case,

changing the firing cycle doesn’t always help.

Hot Water Test Sometimes ware will craze when soaked in hot

or even cold water for 24 hours. In this case,

the body porosity needs to be reduced and the

glaze expansion may have to be lowered.

Sometimes a small talc addition to the body

will help seal off the porosity.

Table 6:4 Special Crazing Problems on Firing & Dunting

Crazing During Glost

& Decorating Fires

(See Dunting below)

This sometimes occurs, especially with

earthenware and the body may crack too. The

silica transition points should be put in

suspicion. Quartz converts abruptly at 573oC

with a density change of 2.65 " to 2.30$. Kiln

control may help but not always from my

experience.

of 82

Dunting Cracking of ware caused by stress due to silica

phase transitions during the firing and cooling

process at slightly over 400 F and again at

slightly over 1000 F. Glaze often crazes during

this process. Dunting is not always evident

immediately upon removal from the kiln. It

sometimes occurs as much as a month or more

later but it should not be confused with cracks

caused by "thermal shock". Glaze fit can be a

factor.

TOC

of 82


7: DEFECTS GENERATED DURING DECALINGOPERATIONS

In this section of Ceramic Defects we discuss decaling. Two internet sources describethe current processes. The loss due to decaling is not high in the ceramic industryhowever the decal can often not be repaired after firing. Some decals incorporateenamels and precious metals eliminating further firings.

Ceramic decals have been used for decades to enhance the value of ceramic products.Hand painting using colored glazes (enamels) is more expensive and the productionrates are much slower.

Decals can be produced by silk screening or by lithography, the former being moreexpensive.

The process to make a decal is to add colored oxides to drying solvents and bindersto form an ink. Low-melting frits may be incorporated in the inks to assure that thedecal will sink into the glaze on firing. The colors are printed or silk screened eachcolor individually onto a special decal paper. A cover coat is applied to the printeddecal sheet before the final drying of the decal sheets.

Go to http://www.beldecal.com/create.cfm to learn the actual details of the process.(It’s a pretty stinky process. Take a sniff while you are reading.)

Several color separations are used. For fine china, seven passes through the silkscreening process are often required to obtain the color detail desired. Three-colorseparations are okay for many other products.

Now I am going to send you away again. Richard Wasowski, a long-time friend ofmine, (don’t call him “Dick,” please) wrote an article for Ceramic IndustryMagazine (I was once the editor of this magazine) on the digital process fordecaling. Just click on the Ceramic Industry link. You will need to register withthe site for free.

of 82

This newer process will probably someday completely replace the older methods. (You can also see the speed and utility of the digital process at: http://www.designpoint.com/custom_digital_ceramic_decals.htm)

Oh, you came back again!

Decals are typically printed on large sheets. A worker cuts the sheets into individualdecals and places them into water (usually with a wetting agent) where they soak untilanother worker slides the decal off the paper and places it on the ceramic ware. Heor she then squeegees out the water making sure that the decal is placed correctly onthe ware and that there are no bubbles trapped under the decal. The decal is dried,usually in open air, and then fired on a 1-3 hour cycle. I’ve seen decals fired in 15minutes or less in England.

Decaling is a low-loss process. If there is a flaw in the decal after firing the wareusually must be scrapped. It may not be profitable to continue processing a piece ofware that you know will be sold as seconds.

Now days, enamel and or precious metal are often incorporated in decals. This savestwo extra firings if both enamel and precious metal are added. If precious metal isadded separately, it still saves one firing. This can be very cost effective. For onething, you are not wasting gold or enamel nor do you have to worry so much aboutenvironmental considerations. There are usually problems getting these decalsworking in your factory but close liaison with the decal manufacturer should resolveproblems with test decals before making a production run of decals.

SCROLL DOWN TO TABLE

of 82

Section 7 Tables

Table 7.1 Defects from Decaling Operations

Improper Decal Application Trained workers must apply

decals to ceramic ware. The must

be supplied with clean water

containing a wetting agent and

squeegees to remove the air

pockets from under the decal.

Excessive Decal Sink during

firing on overglaze decals.

The problem can be the glaze or

the decal. Soft glaze, too-fluid at

the decaling kiln temperatures,

will allow the decals to sink too

much. Too much frit in the decal

can cause this problem. Glaze

control is very important and the

viscosity of the glass must be

controlled. The decal supplier can

solve the other problem. This

should be done with test decals

before production starts.

Too Little Decal Sink Opposite problem. The decal is

not down into the glaze and the

situation is easily recognized by

moving your fingers over the

rough decal. See above for the

solution. You can soften the glaze

or add more flux to the decal.

of 82

Color Problems Some times you will see a change

in color as you go through a batch

of decals. Call your supplier.

Remember that firing conditions

can effect the color.

Deco Kiln Atmosphere An addition of about 4% moisture

to the deco kiln atmosphere can

improve decal sink and reduce the

emission of lead and cadmium

from the decal and glaze.

TOC

of 82

CERAMIC AND POTTERY DEFECTS 8: DEFECTSGENERATED DURING ENAMELING OPERATIONS

In this segment on Ceramic Defects we discuss decorating processes using stains,enamels, or colored glazes. Some processes have been automated which means thatthe control standards for enamels must be maintained.

Color is what sales ceramics.

Okay, I lied. Shape is more important.

But color is very important and is used under-glaze, over-glaze and in-glaze. If thecolor sinks deep into the glaze on firing, the process is called in-glaze.

Many manufacturers use pad printing or silk screening to apply colors and enamelsto bisk or glazed ware. Stamping, bulbing, and hand painting are also used. Go to:http://www.ceramicindustry.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2710,8117,00.html for more information on specific decorating processes.

Some stains are more reactive than others. Iron and manganese compounds tend tobe so. Sometimes a frit is added to the stain to increase the reaction with the body orglaze. Stains may be encapsulated. This is true of cadmium colors, for example, thatare tied up in zirconium silicates. This prevents leaching of the cadmium from foodceramics. Grinding can destroy the encapsulation.

If the stain is made into a paint or paint, as in the normal method, it is called enamel.

Stains can also be applied in powder form. With tile, a binder pattern is silk screenedonto the ceramic. Then stain or colored frit in powder form is dusted onto the tile andshaken off. When the ceramic is fired, the binder pattern is reproduced in color.

Hand painting is another method for applying color. It can be automated. Rememberthat bond signers on Wall Street in New York sign fifty or more bonds at a time. Themovement of a painters hand can be recorded and then transmitted repeatedly to servomotors that drive the brushes that do the actual painting. They can even wipe the

of 82

brush.

Bulbing can also be easily automated. (Well, maybe not that easy. It took someengineering for us to do it in two of the companies I worked.)

The Inkjet process can be used to apply colors but there are restrictions as to theamount of color that can be deposit. At least, that is what I last heard. The BritishCeramic Research Association has put a lot of effort into this area.

The many variations in color formulations, and also in the application processes, canlead to defect generation. That’s what manufacturers don’t want. However, once aproblem is resolved in a process, it usually doesn’t come up again.

For example, say that a formulation error in the color causes crawling, faint or offcolor, running, etc. Correcting the formula eliminates the problem forever (usually).

Close control parameters must be adhered to by both the color supplier and thecustomer. Color meters are not always the complete answer. I preferred to usestandard color standards. After instituting this, the design department could just giveus the color number which we could match without argument. Most people have eyesthat are more color discriminatory than any color meter. That’s the reason why somescientists believe in God.

Colors can be purchased in enamel form so that no preparation is needed at thefactory. Often, the pigments are purchased and then frits and binders are added asneeded at the factory. The particle size of the prepared enamel must be controlled asmust the specific gravity and the viscosity. A hand painter can adjust for variationsin these properties but a machine can not.

Drying is the usual process before firing. However, some enamel can be cured muchfaster by using ultraviolet light. You must purchase the u.v. light-sensitive resins.Infrared lights can dry regular enamel. More heat might be required in automatedenameling and glazing lines.

Enameling on decaled ware requires skilled workers or precise machinery. Errors areeasily corrected before firing. The enamel can be removed and you can start over.After firing, forget it!

of 82

Enamels are usually fired on decal or glazed ware in a 1-3 hour cycle.

I didn’t mention that stains can be applied to green ware by stamping, bulbing, orhand painting.

Well, I forgot.

Section 8 Tables

Table 8.1 Defects from Enameling Operations

Enamel Batch Related Problems Colors must be ball-milled, roll-milled,screened, and the proper amount of binderadded according to the enameling operations.Banding machines, bulbing machines, hand-decorating operations, pad printing machines,etc., may each need a particular viscosity andspecific gravity control.

Color Variations The use of ceramic stains rather than rawoxides can help this problem. Firingconditions are also factors.

Complex Decals Colors are often printed on decals along withprecious metals. This eliminates one or twofirings. Testing of such decals is absolutelyessential before purchasing for production.

TOC

of 82

CERAMIC AND POTTERY DEFECTS 9: DEFECTSGENERATED DURING GILDING OPERATIONS

In this last segment of the Ceramic Decorating Defects series we touch on theproblems of gilding. The fusibility of the glaze is critical. Too soft glaze means hardgold. Too hard glaze means soft gold. Soft gold rubs off. Hard gold will not burnishproperly. The secret is glaze control because the gold emulsion supplier is not goingto let you know how he formulates his gold emulsions. However, he will work withyou to help resolve problems.

You are technically helpless in some ways when it comes to gelding. You are at themercy of your gold supplier. Preparations of gold emulsions used in ceramicdecorating are prepared in secret. Any technical problems relating to the emulsionsused must be resolved by the gold vender. Also, the emulsion suppliers often adjustemulsions to resolve problems not caused by regularly used emulsion formulations.

Gold and other precious metal emulsions are used in the ceramic industry to addvalue to a product. This is usually in the form of banding or hand painting. Bandingcan be by hand or by machine. On firing, the gold must sink to the proper extent. Itis called soft if it easily abrades off with a pencil eraser. If it sinks too far intothe glaze and looses its ability to be buffed to a fine polish, it is called hard.

Hard gold can be caused by soft glaze. That is the glaze is more fusible than usual.The decorating supervisor tells the lab that the glaze is soft, lowers his decorating kilntemperature, and hopes the next lot of glazed ware will not cause him more problems.

Soft gold from hard glaze causes him to raise his decorating kiln temperature. Thismay cause blistering of the gold or other problems. The gold emulsion supplier canmake the emulsion more suitable for higher firing.

So the problem is glaze control. Why do you have variation in the hardness of theglaze? Remember that we are talking about fusibility here. Most fine china glazes arelead based. Lead bisilicate is a lead frit commonly used. It is make in large batchesand the specific gravity of each batch is different. Therefore the laboratory at thefactory must calculate the lead content of a glaze made with a particular frit batch. Ifthe lead content is too high, then the lead bisilicate must be cut back. If the calculatedlead content is too low, it must be increased.

of 82

The best control for the glaze itself is the FIRED specific gravity. Fire a cup of glaze,break out a chunk, and measure the specific gravity. Use this as a control procedure.

To maintain better control, blend frit batches before glaze formulation to even out thecomposition. Work with your suppliers to standardize their materials. Blend glazebatches to get the right glaze hardness.

Section 9 Tables

Table 9.1 Defects from Gilding Operations

Too Much Sink The control of the glaze is

essential in gilding operations. If

the glaze is too soft, the gilding

will be sink too much. You can

see this but check it out with a

rubber eraser. The gold, or other

precious metal, will be very hard.

The precious metal solutions can

be adjusted by the supplier or in-

house by using a harder frit in the

metal solution or by reducing the

amount of the softer frit.

Too Little Sink Hard glaze can be the problem.

The precious metal will easily

come off with an eraser. Soften

the glaze or harden the precious

metal solution.

of 82

Complex Decals Complex decals containing color

and precious metal should be

tested and problems resolved

before purchase for production

operation.

TOC

of 82

CERAMIC AND POTTERY DEFECTS 10: STATISTICALPROCESS CONTROL (STP)

The object of Statistical Process Control is to refine and re-engineer

processes to prevent losses and to keep processes in control within

specified statistical limits. The cost of SPC must be weighed against

benefits. I have made a list of books describing the benefits and

management of SPC program. Click on the link below to see the list

available at Amazon.com.

SPC Book List

STP or Statistical Process Control is now common throughout the ceramic

industry. Workers and management contribute to this effort. If you are not

familiar with STP you can take a course online at:

www.flexstudy.com:

http://www.flexstudy.com/catalog/index.cfm?location=des&coursenum=

9561a

The course outline states that you will:

Learn How To:

! Apply statistical thinking to quality improvement

! Use statistical tools to analyze a wide variety of daily work problems

! Select appropriate control charts for different applications

! Interpret variability in work processes

! Construct a histogram of set data

! Develop and analyze run charts and control charts for variables and

attributes data

! Quantify process capability through data analysis and control charts

! Compute and interpret process capability indexes

of 82

References

1. Ceramics: Industrial Processing and Testing, John T. Jones & M. F. Berard,Iowa State University Press, First Edition 1972, Second Edition 1993

2. Ceramic Glazes, Felix Singer & W. L. German, Borax Consolidated

Limited, London, 1964

3. Ceramic Glazes, Cullen W. Parmalee & Cameron G. Harmon,

Cahners Publishing Company, Inc, Boston, Third Edition 1973

END OF DOCUMENT

TOC

Copyright©2007 John Taylor Jones, PhD: All International Rights

Reserved. This document can not be copied or reproduced in any

electronic or other form without the express permission of the

author. (Please do not email this document. It is only for your

personal use.)

process ceramic

Documents