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1/16www.ceramicartsdaily.org | Copyright 2011, Ceramic Publications Company | Successful Tips for Buying and Using Pottery Clay | i

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how to select, process,

and test clay bodiesfor better results

successful tips

for buying and using

pottery clay| Second Edition |


2/16www.ceramicartsdaily.org | Copyright 2011, Ceramic Publications Company | Successful Tips for Buying and Using Pottery Clay | 1

Selecting a Clay Body by Bill JonesMost potters use commercially prepared pottery clays, and ceramic suppliers and clay manuacturers want to make sure

youre getting the clay you need. Heres a checklist or selecting the right earthenware, stoneware, or porcelain.

Buying Porcelain by Antoinette BadenhorstThe lure o porcelain can be so irresistible! Working with a pottery clay thats smooth, white, and translucent certainly has

its appeal, but at what cost? Working with porcelain takes a bit o a change in how you work, what you make, and what

your skill level is. Check out what it may take or you to switch; porcelain may be just the thing!

Recycling Pottery Clay in Your Studioby Jonathan KaplanWe all end up with clay scraps when making pottery, and because this happens at various stages in the cycle, we need a

way to bring all o that clay to an even level o moisture and consistency so it can be used again. Here is a simple way to

recycle your pottery clay without a lot o equipment or hassle.

How Much Clay Do You Need? by Robin HopperFor potters its hard to visualize just how much clay you need to last a month or three months or six months. But i you

know that you can get 50 coee mugs rom a 50 lb box o clay, you at least have an idea o what youll need beore you

order again. I you make a chart like Robin Hoppers, youll be able to judge your clay supply with no problem.

How to Wedge Pottery Clay by Michael WendtNo potter really likes to wedge, but its a necessary step in getting clay uniorm or throwing or handbuilding. While spiral

wedging is widely practiced, over time its hard on the hands and wrists. With this stack and slam method o wedging, you

can blend colors, textures, and moisture levels into a smooth blend in just minutes without pain.

Testing Pottery Clay by Paul A. WandlessPottery clay bodies have the potential to be really versatile. The more we know about a clay body, the more we can

understand its limits; how high will it fre, how low can it go, what happens to the color? You may just fnd that you have a

whole lot o choices with your particular clay body that you just didnt know about.

Glossary by Vince PitelkaPottery clay has its own unique language. Youll fnd this glossary helpul when deciphering that language.

Successful Tips for Buying and Using Pottery Clay

How to select, process, and test

clay bodies for better resultsBuying or making the right pottery clay is one o the keys to success in your studio. There are many variables that deter-

mine the right pottery clay or your needs including color, temperature range, the type o pottery you make, and what kindo orming methods you use, just to name a ew. Whether you end up using earthenware, stoneware, or porcelain (or per-

haps all o these) depends on you understanding the properties, benefts, and drawbacks o each type o clay.

Most o the value in a piece o pottery is in the time and eort you invest, and the clay is one o the least expensive ele-

ments, but that does not mean it is the least importantquite the opposite. Becoming amiliar with the types o pottery

clay available will allow you to make smart design decisions and can open up new areas o creative exploration.



M

ost potters use commercially prepared clay,and ceramic suppliers have met their needs byoering a vast, though sometimes bewilder-ing, selection o clay bodies to choose rom. I

youre using the clay someone else just handed you, maybe itsnot the right one or the work youre doing. Every clay bodyis ormulated or a specic use, and nding the right one oryour needs takes a bit o research and experimentation. Thereare several decisions to make when nding a clay body orbodies that meet your skill level, techniques, and aesthetic.

Choosing a Clay SupplierNow why would you choose a supplier rst? They workwith amateurs, proessionals, students, teachers, sculptors,tile makers, etc., and have solved a wide variety o problems.There are three types o suppliers: those that manuacture

their own clay bodies, those that sell clay rom other manu-acturers, and those that do both. And, i you have prob-lems a supplier cant answer, either you or the supplier cancontact the manuacturer.

Handbuilding or Throwing?Clay bodies are ormulated or dierent applications.Throwing tableware requires a clay body that is plastic (mal-leable) and smooth, while constructing a large sculpture mayrequire toothier (coarse-grained) clay. In many cases, aclay body may be used or several applications, but some arehighly specializedraku or ovenware, or example. Make alist o the things you want to do, and you may end up withseveral bodies to achieve the best results.

Firing Stoneware, Earthenware,and PorcelainClay bodies are ormulated to re at specic temperatureswithin three general ranges: low-re (cone 062), mid-range(cone 37) and high-re (cone 810). Within a range, thehigher you re, the greater the shrinkage and the lower theabsorbency o the red clay. Low-re clay bodies, such asearthenware, are easy to work with and re, and there is a

wide range o colorul glazes available. These clay bodies arealso used or raku and pit ring. Mid-range clays are moredurable and include stoneware and some porcelain bodies,but color is more limited. This situation improves each yearas glaze companies improve and expand their oerings.High-re clays are very durable and include stoneware andporcelain. Color palettes vary depending on ring atmo-sphere (oxidation or reduction).

Choosing a Clay Body ColorColor in basic studio clay bodies results mostly rom natu-rally occurring iron and/or iron that has been added. Por-

celain contains no iron, light bu bodies have some iron,and earthenware bodies may have over 10% iron. The bodycolor (as well as the glazes) changes based on the type oring atmosphere you havereduction (rom uel ring) or

oxidation. Reduction ring deepens or darkens an otherwiseneutral-colored clay body. Some clay companies have dupli-cated this eect by adding more colorants to their ormulasmaking it possible to achieve reduction colors rom anelectric kiln. Since the iron and other colorants in a bodycolor aect glaze color, youll need to test your glazes witheach body.

How Much Tooth?Texture can range rom smooth to rough. Smooth bodiescontain very small particles o clay, which tend to shrinkmore. These are best suited or small, ne, and/or detailed

work. Adding grog (ground-up red clay) or sand gives thebody tooth, and the larger the particle size, the less waterthe piece will need (hence less shrinkage). Manuacturers o-er a range o bodies that incorporate ner particles o grogand sand to get a texture between smooth and rough.

Buy What You NeedBuying prepared clay requires a little judgment. First, tryto nd a supplier that is nearby because shipping costs canadd up and sometimes equal or exceed the cost o the claybody itsel. Next, test a sample beore purchasing a largequantity. I youre buying several types o clay bodies, sup-pliers will typically allow you to combine the weights or abetter discount. Finally, buy only enough clay to last you ayear maximum. Clay loses moisture in storage and becomesstier, possibly even unworkable. You can ask your supplierhow long they have stored the clay. A good supplier will onlystock what they can sell within a reasonable amount o time.

Mother Nature vs ClayAll commercially-prepared clay bodies are made rom natu-rally occurring elements scooped rom the earth or industrialusers. Studio potters do not purchase enough materials to be

a major user, so we have to make do with a small portion owhat the industry uses (brick and tile manuacurers, chinacompanies, steel industry, paper mills, pharmaceuticals, etc.).Mother Nature did not use any quality control when shecreated clay deposits, so veins o clay vary rom one spot toanother. And a mammoth ront loader is not a delicate ma-terial-selection tool. The good news is that the industry otenrequires a degree o consistency in their raw materials, so thatclay mines make every eort possible to provide them withthe very best product out there or an intended application.To make sure your clay meets your every needs and yourown quality standards, always test each batch everytime.

Selecting a Clayby Bill Jones



I youve only worked with red, brown or bu clayin the past and youre looking or a change, maybeporcelain is the right clay or you. Planning, research,and evaluation are the best ways to assure any uture

success in making a switch rom one clay body to another.To determine i porcelain is what youre looking or,

youll need to evaluate where you want to go with yourclay work, your skill level, and your vision as a potter.Decide i youre happy with your current work, and iso, consider the eect that work will have i made witha white or porcelain clay body. Not all works in claymaximize the qualities that porcelain has to oer, so iyou have to change your work in order to use porcelain,evaluate whether thats something you want to do.

In my own experience, I had a vision o pots dancing

like ballerinassot gurines moving around in brightcolors against pure white backdrops. I also envisionedtranslucent light and instantly knew what to do, but ittook some time to nd the right porcelain and to developa body o work.

Studio Setup and Working MethodsDo you have the right studio setup or porcelain and areyou able to adjust your current workplace with ease? Canyou work with precision and in a clean studio? Do youwork with other clay bodies that might contaminate por-celain, or are there other potters working with you thatmight not respect a porcelain work station? Which tech-niques do you use most? For instance, i you work mostlywith an extruder with a steel chamber and plunger, youll

Buying Porcelainby Antoinette Badenhorst

Beore making a large investment in porcelain, test several bodies to seewhich one best suits your needs.



need to replace it with a stainless steel or aluminum oneto avoid possible rust contamination.

Skill LevelIts important to know your own abilities and skill level.I youre a beginner who wants to throw 20-inch pots,youll have a lot o diculty achieving your goals andthere will be a whole lot o rustration, time, and moneywasted beore you can reach them. In such a case, its bet-ter to use white stoneware clay and gradually work yourway rst through a semi-porcelain body and then eventu-ally use pure porcelain as your skills improve.

Different PorcelainsI you want to become a porcelain production potter,youll look at a dierent clay body than someone whowants to make one-o-a-kind porcelain pieces, porcelainsculptures or strictly handbuilt orms. Your workingmethods will dier dramatically rom theirs. Maybe you

need a clay body that combines some or all o the abovementioned clay techniques.Once you decide that you want to take on the chal-

lenges that porcelain oer, youll have to nd the claythat suits your newly set goals. There are many dierentporcelain clay bodies available on the market.

I tested several com-mercially available cone6 porcelain bodies andsuggest you do the samebeore settling on one.Each clay had some

special characteristicthat I could use or myown work and couldsee used by anyone else.Commercial porcelainclay bodies meet almostall the needs o thepotter, and there aresome excellent throw-ing, handbuilding, andsculpture bodies avail-able. The producers

and suppliers know which one best suits each purpose,and they are an excellent resource when you are trying togure out what you need.

They develop some bodies to be more plastic andstretchable, but less white and translucent. These bodiescan go urther in height and thinner in walls than someothers that might be pure white and translucent, but maybe a little harder to throw.

I you choose to work with pure white, translucent clay,you can always throw thicker and trim thin aterward. I

you need an all translucent, white, and a non-warping claybody, it might cost a little more, but your ceramic suppliercan recommend the right clay body or your purposes.

Amazingly, you will even nd that some o the semi-porcelaneous clay bodies meet all the characteristics oporcelain and have the added green strength that is otenmissing in true porcelains. Add these qualities to the actthat you can save energy because many o the commercialclays are ormulated or ring at cone 6 electric, and thereare very ew restrictions let that would limit you romworking with this material.

Test several clay bodies or their ability to throw, trim,and to keep their shape when stretched beyond theirlimits. Also test them to see how they stand up to adjust-ments and attachments, then re them to the proper conein an electric kiln. Check them to see i shrinkage cancause problems. Compare the tests or shrinkage, color,and translucency.

Transition CarefullyIts always best to start by buying one bag o clay andtesting it thoroughly. Then, even when you think youresatised with your choice, make the transition to yournew style and clay body slowly and careully. Porcelain isexpensive but i you take a conservative approach, and do

enough testing to makean inormed decision,it will pay to make aninvestment in a largebatch o clay.

Ive seen porcelain

clay bodies improverom one batch toanother. Clay com-panies are constantlydoing research toimprove their clays. Iyou consult your claycompany, theyll knowwhat to recommend toyou only i you under-stand your own needsand what you want. To

us, as potters, thats good news, because it means that iwe admire a specic clay body today, but its not work-ing or our circumstances, its worth discussing that withour clay producer and retesting a body again to see i ithas changed. Maybe your skills improve, perhaps the claycomposition improves, or maybe you and that specicclay body simply get in sync with each other.

Read the literature available online, then talk to a salesrepresentative and theyll be able to recommend the rightclay body or your needs.

Once you decide that you want

to take on the challenges that

porcelain offers, youll haveto fnd the clay that suits your

newly set goals. There are many

different porcelain clay bodies

available on the market.



The work we make generates discardedclayscraps rom trimming and slop romthrowing or handbuilding. Whether youmake your own or buy prepared clay, you

need to decide what to do with used clay. For some,the solution is to throw the scraps in the dumpsterand dump the slop down the drain, which isnt re-ally a solution. Its economically and environmentallywasteul and can wreak havoc on your plumbing,sewer and septic systems.

All discarded clayexcept that which comes incontact with plaster or other contaminantsis recy-clable and reusable. Reclaiming it requires planninga system or your particular studio situation. Al-though theres no one way that works or everyone,some basic guidelines can make this job easier.

The ollowing solutions, submitted in a recentPMI readers online survey on clay reprocessing willgive you a place to start.

Collecting and Storing

1 A trash container o some sort is the key to any

system. Placed on wheels, it can be maneuvered any-where in your studio. Remember that when lled withclay scrap, slop and trimmings, it will be very heavy,so buy a heavy-duty container, and use heavy-dutycasters or the dollie. Pouring the scrap clay through aramed screen that sits on top o the container breaks

up the scraps into smaller pieces and catches tools thatgot lost (fgure 1).

2 When youre done with a days work, dump yourthrowing slop and any scraps into the bucket. I you planto recycle scraps that have allen on the foor aroundyour wheel, make sure theres no non-clay debris onthe foor rst, otherwise you should simply throw thesescraps away as the oreign matter could cause problems.Rather than using a broom, use a metal dustpan andyour hands to collect the clay.

3 As the bucket approaches the top, decant thewater. As long as there is water over the layer o clay inyour recycle bucket, the clay scraps will all break downand become thoroughly saturated. I there are lumps ordry material in the slop, the resulting reclaim will havehard and sot spots. You dont have to mix the clay inthe bucket with a mixer. Let the water do its job.

ReclaimingThe resulting very wet scrap usually needs to be driedout beore reprocessing can continue. Heres a list o themore commonly mentioned and used methods.

1 Spread a thin layer (2-inches or so thick at most)o scrap onto an absorbent surace (fgure 2). Plasterbats, wedging tables and large plaster slabs are goodchoices or this. A material called HardiBacker Boardworks very well. This is a heavy cement board that isnot as eective as plaster, but is strong and will absorb

A screen over the scrap clay barrel keeps out tools andsponges, especially the dreaded needle tools.

Place recycled clay on a porous surace and allow to dry.Flip the clay over occasionally then wedge and store.

1 2

Recycling Clayin Your Studio

by Jonathan Kaplan



moisture rom the clay. Caution: do notuse sheetrock. This is thin and porous,absorbs water very quickly, then the pa-per delaminates rom the surace and the

boards weaken. Flip the clay slab overperiodically (it should pull away romthe surace easily when it is ready to fip)and continue to do this until it is rightor wedging.

2 Store drying bats on a verticalrack or ware cart to save space. Note:the HardiBacker boards need a boardunderneath or support. Some potterspreer to dry their reclaim outdoors.Note: cover the reclaim with a sheet orother breathable abric i you live in awindy area to prevent oreign materi-als rom getting in your clay. A an canalso help dry wet clay more quickly.This will accelerate the drying, so turnthe clay requently.

3 Another practical methodinvolves a rudimentary lter presssystem made rom cotton pillowcases.Simply ll the sacks with clay slopand hang them up, either over a buck-et or outside. Excess water eventually

drips out over time and evaporatesrom the surace.

Reprocessing1 When adding reclaim that has

already been through one o the aboveprocesses to the clay mixer, make sureyou mix the water and the dry clayrst beore adding the reclaim. Alter-nately, add the slaked-down reclaimstraight rom the bucket to the mixer(without drying it out rst). This

works best i youre mixing a largebatch o clay (over 100 pounds). Putthe reclaim in rst and add dry ingre-dients in small batches on top o thereclaim, adding water as needed to

maintain the right consistency. A ratioo23 dry materials to 13 reclaim usuallyworks well.

2Once your reclaimed clay has

achieved the consistency you want,block it, bag it and leave it alone ora ew days to let the moisture contenteven out. It can then be cut into smallpieces or thin slices and re-wedged withyour boxed clay or used as is.

3 A common misconception witha pug mill is that you can combine drymix and wet mix successully and geta decent extruded pug. Not true. Al-though the dry mix sets up the wet mixin the machine, the dry mix itsel willnot be suitably hydrated and, as a result,the clay will be exceedingly short. I youprepare your clay this way, do so wellin advance o using it; bag the clay, let itage, and it will ully hydrate over time.The same is true i you are wedging dryclay into very wet wheelthrown clay tomake it workable again.

4 All plastic bags have a degree oporosity, and release moisture or let in airover time. Mix clay and reprocess scrap

just a bit on the wet side to allow or asmall degree o air inltration through theplastic bag.

5 Finally, dont let the volume oreclaim get out o control. I you keep upwith it on a daily basis, youll always havereclaimed clay ready to go, and not have anoverfowing bucket o scrap. And, i yourejust overwhelmed and dont have the time,space or inclination to delve into clay re-cycling, consider donating your unwantedclay to a local school or art center.

Jonathan Kaplan is a requent contributor toPottery Making Illustrated. He has been actively in-volved in the ceramics feld or 40 years. He lives inDenver, Colorado, where he curates Plinth Gallery.



How Much Clay Do You Need?by Robin Hopper

I

youre doing repetitive throwing or production work, keep a chart o weights and measures showing the amount o clay needed and sizeo objects at the throwing stage. Here is a list o weights and measures o standard items produced in my studio. These are or an average

throwing thickness o /16 in. or smaller objects, and /8 in. or larger objects. Adjust accordingly i throwing thinner or thicker.

Excerpted from Functional Pottery by Robin Hopper and published by The American Ceramic Society.

Weights and Measures for Basic Production ItemsITEM WEIGHT HEIGHT WIDTH

Grams lb./oz. inches cm inches cmDrinking vessels6 oz. coffee mug 275 10 oz. 3 7.5 3 7.5

8 oz. coffee mug 400 14 oz. 5 12.5 3 7.5

14 oz. beer mug 600 1 lb. 5 oz. 7 17.5 3.5 8.5

chalice (cup only) 500 1 lb. 2 oz. 4 10.0 4 10.0

goblet (cup only) 340 12 oz. 5 12.5 3 7.5

cup 300 11 oz. 2.75 7.0 3.75 9.5

saucer 350 13 oz. 1 2.5 5.5 13.5

large dinner plate 1800 4 lb. 1.25 3.0 11.5 29.0

medium dinner plate 1350 3 lb. 1 2.5 10 25.0

side plate 1000 2 lb. 3 oz. 1 2.5 8 20.0

bread and butter 600 1 lb. 5 oz. .75 2.0 6.5 16.0

glutton plate 2300 5 lb. 2 oz. 1.25 3.0 14 35.0

Bowlslarge 2600 5 lb. 12 oz. 6 15.0 12 30.0

medium 1800 4 lb. 4.5 11.0 10 25.0

small 600 1 lb. 6 oz. 3 7.5 6 15.0

onion soup 600 1 lb. 5 oz. 3 7.5 6 15.0

large mixing bowl 1800 4 lb. 4.5 11 10 25.0

Casseroles4 quart 2600 5 lb. 12 oz. 8 20 12 30.0

lid 1600 3 lb. 8 oz. - - - -

2 quart 1800 4 lb. 4.5 11 8.5 21.0

lid 1000 2 lb. 3 oz. - - - -

1 quart 1000 2 lb. 3 oz. 4 10 6.5 16.0

lid 750 1 lb. 12 oz. - - - -

individual 600 1 lb. 6 oz. 3 7.5 5.5 13.5

lid 450 16 oz. - - - -

Pots for pouringcream pitcher 400 14 oz. 5 12.5 3 7.5

1 pint pitcher 675 1 lb. 8 oz. 6.5 16.5 4 10.0

4 pint pitcher 2600 5 lb. 12 oz. 14 35 6 15.0

coffee pot 1800 4 lb. 11 27.5 4.5 11.0

lid 400 14 oz. - - - -

large teapot 2000 4 lb. 6 oz. 8 20 8 20.0

lid 250 9 oz.

medium teapot 1500 3 lb. 6 oz. 6 15 6 15.0

lid 200 7 oz. - - - -

small teapot 1000 2 lb. 3 oz. 4.5 11 5 12.5

lid 150 5 oz. - - - -

1 liter decanter 2000 4 lb. 6 oz. 12 30 6 20.0

small decanter 1200 2 lb. 11 oz. 8 20 5 12.5

liqueur or sake bottle 1000 2 lb. 3 oz. - - - -

Storage containerslarge storage jar 2250 5 lb. 12 30 5 12.5

medium storage jar 1500 3 lb. 6 oz. 10 25 4 10.0

small storage jar 800 1 lb. 12 oz. 7 17.5 3 7.5

jam or honey pot 450 16 oz. 3.5 8.5 4 10.0

Serving disheslarge cooking/serving 2500 5 lb. 8 oz. 3.5 8.5 15 37.5

small cooking/serving 1350 3 lb. 2.25 6 10 25.0

cheese bell 2000 4 lb. 6 oz. 6 15 10 25.0

base 1500 3 lb. 6 oz.

butter dish 600 1 lb. 5 oz. 3 7.5 5.5 13.5

base 600 1 lb. 5 oz.

salt and pepper shakers 400 14 oz. 4.5 11 3 7.5

egg bakers 400 14 oz. 1.25 3 3.5 8.5



How to WedgePottery Clay

by Michael Wendt

Stack-and-slam wire wedging is a method orwedging that is quick, eective, versatile, andeasier on the hands and wrists than any othertype o manual wedging. This method allows you

to uniormly wedge very large pieces o clay or largepots. You also can add water (or soter clay) to pieces oclay that have become too sti, or even mix clays withdierent characteristics such as stoneware and porce-lain. In addition, this method oers a superior way to

get perectly fat slabs or tile work or handbuilding.To illustrate this method, I took two 3-pound ballso clay o dierent colors and spiral wedged themor two minutes. I sliced through the ball to see how

uniorm the mixing had become. Ater two minuteso spiral wedging, there were still pockets o red andwhite clay in the pink mixture that had not beencompletely dispersed. I repeated the exercise with twomore balls o dierent colored clays using the stack-and-slam wire-wedging technique. The bottom photosshow the remarkable change that took place.

Stack and Slam

Choose a comortable amount o clay or the rstattempt. I seldom wedge less than 3 pounds becauseit is too slow to wedge each piece one at a time. Ipreer to wedge enough clay or several pots at one

Clay after spiral wedging for 2 minutes.Two 3 pound lumps of clay.

Stack-and-slam wedge 10 times:1,024 layers.

Stack-and-slam wedge 30 times:1,073,741,824 layers.

Stack-and-slam wedge 20 times:1,048,576 layers.

Do not wedge more clay than you

can use in a throwing session since

clay loses uniformity in storage

and requires wedging again if itsstored for very long. Always wrap

wedged pieces immediately to

avoid the inevitable hardening that

happens if they are exposed to air

for very long.

Tip



time because, with this stack-and-slam wire-wedgingtechnique, it is just as easy to wedge a large amount asit is to wedge a small amount.

First, block the clay into a rough rectangular shape(fgure 1). Next, you will lit the piece by the sides andcut it roughly in hal (fgure 2) passing the let arm un-der the wire as you place the two pieces back onto thewedging table (fgure 3). Make sure the cut is parallelto the ront edge o the table.

Take the piece closest to the table edge and care-ully place on top o the other piece (fgure 4). Besure not to make any dimples in either o the mat-

ing suraces.Roll the joined pieces toward you, fipping themover so the bottom is now on top. Pick up the clayand slam it down with enough orce that it ends upbring the original thickness you started with (fgure 5).

Repeat steps 13 at least 30 times. This will giveyou over a billion layers o clay particles! It is verycritical that you pay attention to the lamination pat-

tern since the nal goal is to layer the clay rather thancross the layers with each other.

Once you have completed the required number ocycles, place your let hand on the top o the piece androll it to the let onto its side. Now your right handcan be placed on the area that was the bottom on thetable surace. The goal is to keep track o these twosuraces while converting the block into a cylinder byrepeatedly tapping it onto the table surace and nallyrolling it round (fgure 6).

Taking this extra step assures you can keep tracko the laminated ace. I have ound that orienting

the laminations parallel to the wheel head mini-mizes cracks on the bottom o all o my pieces, andthat selecting the smoothest end or the top urtherreduces losses.

Changing Softness of ClaySometimes clay is too hard or our liking. Watercan easily be added with wire wedging resulting

1 2 3

4 5 6 Stacked Planes



in the right eel every time. Slice the block irstinto thin sheets. Spread them onto the tablesurace and spray with water. Restack and wedgeas previously explained or mix some soter claywith the harder clay. This lets you reclaim potsthat have ailed rather than drying them out andreusing them later. I some o your pugs are toohard and others are too sot, weigh out dier-ent proportions o sti and sot clay. Record theamounts so that you can gauge how much needsto be added the next time.

ConclusionI youre already happy with the results rom your cur-rent wedging or pugging, theres no reason to change.But i youre struggling with uneven clay and would likea method that gives you more control, try wire wedging.It can create some troubles i done improperly mostnotably, the introduction o air pockets due to poorjoining surace qualitybut this is easy to diagnose andcure. I have used this method or over thirty years andhave no wrist or hand problems.

Wedging Table tipsTables suitable or stack-and-slamwire wedging must be solid witha airly thick top to reduce noise.My table is made with two sheetso -inch plywood eaturing sup-port legs directly under the areawhere the clay hits. It is bolted tothe loor to prevent any kind orebound or side movement. Theheight rom your elbow to the topo the table should be no morethan 46 inches when you arestanding straight. Higher and youmay bang your arms into the rontedge o the table; lower and youhave to reach down too ar. Useheavy canvas or the top becauseclay barely sticks to it and replace-ment is easy. Periodically sprayingthe surace with water keeps thedust down and assures the layerswill bond to each other better.

Wire and FrameI your table is against a wall, it iseasy to screw a hook into a stud ora concrete anchor to attach the topend o the wire. I the table is reestanding, youll need to weld a steelrame out o 11-inch angle iron

and screw it to the table. I added acounter to keep track o the num-ber o times I cut and slam the clay.

Guitar strings make good wiresbecause they are very thin, strong,inexpensive and last a long time.Attach the wire to a turnbuckle,which will allow you to periodi-cally tighten the wire as it loosenswith use. Always keep spare wireson hand.

If you do not have a wall stud to anchorthe wire, youll need to weld an angle-iron bracket to the table.

Constructing a Better Wedging Table



Testing Pottery Clayby Paul Andrew Wandless

At some point we all change clay bodies or one reasonor another. Whether you want a body that shrinks less,has more/less absorption, a lower/higher maturationpoint or just a dierent color, there are hundreds o

commercial clays to choose rom. While most clays have prettygood general catalog descriptions o what they are and what theycan do, once we apply our specic working and ring processesother issues can arise. A combination o tests can give you plentyo inormation that makes choosing and learning about a claybody a little easier.

Why Test Clay?Testing clay bodies provides you with inormation that you canobserve, touch, and eel rst hand in your own environment.While a catalog photo shows what a body may look like redat one or several cones, it may not tell you what it will do at thecone youre ring to. Basic clay bar tests give you inormationmore specic to your needs, and a 25-pound sample is usuallyenough to complete all the tests you need.

What to TestTests should be done at multiple temperatures to yield thewidest range o inormation on the body. You need to under-

stand the same general characteristics at every temperatureyou re to, and even at temperatures you may want to re toin the uture. I test at every potential cone I may re to andkeep records o all the results.

The three important general characteristics are shrinkage,absorption, and warping/slumping. Other important qualitiesto note are color, texture, plasticity, and hardness. Some resultsare determined with visual and touch tests while others requiresimple ormulas. All require consistency o procedure so theresults you achieve are created under the same conditions.

Firing Box and StiltsWhen ring above a recommended temperature either on pur-pose or by mistake, clay bodies start to melt and use or stick tothe shelves. To test clay bars, youll need a shallow ring box toprotect your kiln shelves and make handling and transportationeasier. Youll also need clay stilts or the warping test.

Using a high-re clay, make a simple clay box thats 810inches square with a 1-inch high wall. Make several boxes at thesame time so you can test multiple bodies in the same ring or tohave on hand or later (fgure 1).

Make triangle stilts about the thickness o your pinky and longenough to span the width o the clay bar (fgure 2). All stilts mustbe the same height.

Construct test trays rom a high re clay body.

1

Prepare triangular stilts to support the bars.

2

Four test bars with a measured and marked 10 cm line.

3

Dry trays and bars slowly and evenly.

4



Clay BarsYoull need three clay bars or each body youre testing. Forconsistency and accuracy o results, use the same dimensions orall your clay bars. While some tests yield correct results regard-less o the bar dimensions, i they are consistent, you can alwaysrule out size and dimension as variables that could cause any ir-regularities. I make -inch thick bars measuring 2 inches wide by6 inches long. For thicker work, make bars to match, but dont

exceed a inch in thickness. Ater cutting the bars to size, drawa 10-cm-long line on one o them with a hash mark at each end.This will be used or a shrinkage test later (fgure 3).

For the tests here, Ive selected our bodies to test: two whitebodies rom Standard (#257 Grolleg Porcelain cone 810 and#181 White Stoneware cone 610) and two rom Amaco (#29Brown Stone Earthenware cone 0604 and #77 Terra CottaClay cone 5). All our bodies have dierent characteristics at thetemperatures to which Im interested in ring them, and testingseveral bodies at the same time takes better advantage o eachring. Write the clay number on the back o each tile.

Once all bars are cut and ring boxes are built, let them dry

to bone dry. They can be stacked with newspaper layered inbetween and a board on top or a little weight to keep romwarping. Its important that the ring boxes and clay bars stayfat while drying to assure accurate test results (fgure 4).

Pre-BisqueAt the bone dry stage, visually examine the bars or a colorchange i any. Use a ruler to measure the shrinkage line to see i ithas changed rom its original 10 cm length. Record the results.

For each ring, place the bars side by side in the ring box,and place one o them on the triangle stilts (fgure 5). The stiltsshould be placed about to inch in rom the ends o the bar.

I the stilts are too close, the bar may not warp or slump to itsullest potential. The shrinkage bar and the other regular bar aresimply placed in the bottom o the box.

Firing Clay SamplesWhile the test ring can be done in any kiln, the results aremost applicable i done in the same kiln used or your work.Indicate in your notebook or worksheet i its an electric or gaskiln, oxidation or reduction ring, salt, wood, soda, etc. Forbest results, start rings at the lowest cone temperature andthen progressively rere the bars at higher cones until the high-est desired cone is reached. A sample or progressive test ringsor a cone 10 clay body would be cone 06, cone 01, cone 6 thennally cone 10.

Clay Bar TestsThe ollowing traditional tests give a good range o basic inor-mation that helps you to better understand your clay body. Keepgood records in your notebook or worksheet so youll have theresults or uture reerence. I there are other specic qualities,such as glaze t or color eect, test or these as well.

Place one bar on stilts and two o them fat in the tray.

5

Ater each ring, measure the amount o defection.

6

Measure the 10 cm line to determine shrinkage.

7



Warping/SlumpingVisually examine or any warping or sagging move-ment in the center. Remove the bar rom the stilts,turn it over and place it on fat surace so the gap (iany) can be measured (fgure 6). This test inorms youat what temperature the walls may start to warp or

deorm or a plate may begin to slump. The inormationcan be applied to the sculptural or structural applica-tions o the clay body. An exact percentage or this isnot as important as just knowing when the clay bodystarts to move.

Shrinkage TestMeasure the length o the line in centimeters on theshrinkage bar (fgure 7), and subtract it rom 10(based on the original 10 cm line). For example, 10cm 8.5 cm = 1.5 cm. An easy way to convert this re-sult to a percentage is to move the decimal to the right

one place, so 1.5 cm means 15% shrinkage. Knowingthe shrinkage rate helps in determining which glazeswill t the body and even which two bodies can beused together.

Absorption TestThere are two types o tests that can determine the ab-sorption o a clay body. One is a simple visual test andthe other is a weight calculation. For the visual test,place a ew drops o a liquid (like ink) on the surace othe bar to create a stain. Let it soak in or an hour thenwash o the surace with water. The darker the stain,

the more absorbent the clay body. This is not an exacttest, but it gives a quick and useul general result.

The weight calculation test is more specic. Weighthe red bar on a gram scale and record the result(fgure 8). Soak the bar in water or 24 hours, pat dry,then weigh again and record the result. Subtract therst weight (dry bar) rom the second weight (watersoaked bar) to get the weight o the absorbed water.Divide the weight o the absorbed water by the origi-nal dry weight and move the decimal two places to theright to nd the absorption rate.

Example: Original bar weight o 4.2 grams is sub-tracted rom soaked bar weight o 4.6 grams giving you

an absorbed water weight o .4 grams. Divide .4 by 4.2which equals .095 making absorption 9.5%.

Color TestVisually examine a bar to see i there has been acolor change (fgure 9). The color change can some-times be dramatic depending on the cone it was redto and is important or aesthetic purposes. It can alsohelp you determine the best glazes to work visuallyon the surace.

Surface/TextureVisually examine the bar to determine i the surace haschanged. Run your ngers across the bar to see i itsthe same, smoother or more coarse. This inormationis important aesthetically and helps you determine i itmeets your visual and tactile needs.

HardnessUsing a metal nail or similar tool, see i you can scratchthe surace to see how hard or sot it is at the red tem-perature. This test helps determine the surace durabilityat dierent temperatures.

Soak bars in water and weigh them ater 24 hours.

8

Note the color o the clay ater each ring. Here are ourclays red to cone 04 (top) and cone 7 (bottom).

9



Glossaryby Vince Pitelka

ABSORPTION The ability o a red clay to absorb

water. Used as a gauge o vitrication.

AIR-FLOATED; AIR-FLOATING Industrial methodused in processing o raw clays, where powdered clay is

foated in an airstream to settle out heavier particles.

BALL CLAY Al2O32SiO22H2ODeposited in

marshy areas. Very ne particle size, high plasticity,

high drying shrinkage, high in organic contaminates.

Fires white or o-white.

BASALT BODY; BASALT WARE Clay body with

sucient amounts o dark clays and/or metallic oxides

to re dark brown or black.

BENTONITE Al2O35SiO27H2O

montmorillonitic clay ormed rom decomposition o

airborne volcanic ashnest particle size o all clays

plasticizer (three times as powerul as ball clay), sus-

pension agent, should be used in quantities o no more

than 3% o dry materials weight.

BONE CHINA Translucent porcelain containing

bone ash.

CARBONDALE CLAY Reractory red stoneware

clay, used to obtain rich red and brown colors in high-re clay bodies.

CHINA CLAY See KAOLIN.

CLAY Widely occurring aluminum silicate mineral

resulting rom natural decomposition o eldspar and

granite. Composed o microscopic disk-shaped platelets

that give clay its slippery, plastic quality.

CLAY BODY Clay mixture ormulated o clays and other

ceramic raw materials to give desired working characteristics.

DEAIRING The process o removing the air rom aplastic clay mass, usually accomplished through wedging,

or ar more eectively with a vacuum deairing pugmill.

EARTHENWARE CLAY Natural low-re second-

ary clayfuxed with iron, res porous. Oten called

common clay, ound almost everywhere, matures

below 2000F.

EPK KAOLIN Al2O32SiO22H2Opure white

kaolin, less plastic than Tile-6 kaolin, requently used

in glazes.

FIRE CLAY Highly reractory secondary clays with

minimal fuxes and usually airly coarse particle size

low shrinkage, bu-color, oten nonplastic.FIRE CLAY Very reractory clay, or sculpture and raku

bodies. Tremendous variation among dierent brands.

FLAMEWARE Wares made to withstand stove-top

heat. Explosions rom trapped moisture, and resulting

lawsuits have caused studio fameware to disappear

rom the domestic market.

GOLDART Bu stoneware clay, produced by Ce-

dar Heights Clay Company.

GROG 1 Crushed hard-red claya source o grit

or clay bodiesgraded in sizes rom 15-mesh (very

coarse) to 150M (extra ne). Does not shrink in r-

ing, so medium and coarse grades will show texture

through thin to medium glaze.

GROG 2 Filler or tempering grit ormed by grinding

high-red clay; added to clay bodies to reduce shrink-

age and give structure or throwing or handbuilding.

GROLLEG KAOLIN Al2O32SiO22H2O

English kaolin, more costly than other choices, but

gives whiter porcelain. Less plastic than TILE-6. Bestkaolin or translucent bone china.

HAWTHORN BOND Reractory stoneware clay or

reclay, used in stoneware clay bodies.

HELMER KAOLIN Al2O32SiO22H2OKaolin

that works especially well as fashing slip or salt, soda,

and wood ring, especially since Avery kaolin is no

longer being mined.

HIGH-FIRE High-temperature ring range usually in-

cluding cone 8 to cone 12, or ring stoneware or porcelain.KAOLIN; CHINA CLAY Al2O32SiO22H2O

very reractory white primary clayessential ingredi-

ent o porcelain and whitewareless plastic than most

other clays. See EPK, GROLLEG, HELMER, TILE-6.

KAOLINITE Crystalline clay mineral orming the

basis o most clays we use in ceramics.

LIZELLA CLAY High-iron (4%) orange-red stone-

ware claysubstitute or Ocmulgee which is no longer

being mined.


16/16

LOW MID-RANGE Firing range usually includ-

ing cone 01 to cone 3, under used in studio ceramics,

useul or unctional earthenware, reractory sculpture

bodies, and outdoor terra cotta work.

LOW-FIRE Low-temperature ring range, usually

below cone 02 (2048F), used or most bisque-ring

and or glaze-ring terra cotta and whiteware.

MID-RANGE Firing range usually including cone 4

to cone 7, very popular with electric kilns.

MONTMORILLONITE Clays such as bentonite,

resulting rom the breakdown o airborne volcanic ash.

The nest particle size o all clays. Very high shrinkage,

and generally used only as an additive to clay bodies or

glazes. See BENTONITE.

NEUMAN RED CLAY Reractory red stoneware

clay, used to obtain rich red and red-brown colors in

high-re clay bodies.

OM-4 (Old Mine #4) A well-known Kentucky ball clay.

PAPER CLAY Technique utilizing a clay body or

slip containing paper pulp which reduces shrinkage in

drying stage, and encourages extremely strong joinery,

allowing unconventional joinery such as wet to dry.

PIONEER KAOLIN Al2O32SiO22H2Oplastic

secondary kaolinwider particle distribution gives

greater plasticity, green strength.

PLASTICITY Quality o moldable fexibility in

damp claysuperior plasticity depends on smaller clay

particle size, slight acidity, ewer nonplastic additives,

aging o damp clay body, adequate water content, and/

or addition o accessory plasticizers, such as Veegum T

or Macaloid.

PLASTICIZERS Materials added to some clay bod-

ies, especially those high in kaolins, to increase plastic-

ity and dry strengthincludes bentonite, Macaloid,

Veegum T.

PLATELETS Flat, thin crystals that make up clay.

When wet they become sticky and slippery, creating the

phenomenon we call plasticity.

PORCELAIN High-red vitreous clay body con-

taining kaolin, silica, fuxes and, oten, ball clay to

increase plasticity, with total clay component not more

than 50%. Usually pure white or eggshell in color;

some porcelains may re translucent where thin.

PORCELANEOUS White-ring stoneware clay

bodies closely related to porcelain.

REDART Brick-red earthenware clay, produced by

Cedar Heights Clay Company.

SAND Granular silica (usually)source o grit or

claybodieshigh shrinkage in HT claybodies, but gives

smoother red surace than grog. Toxic in inhalation.

SECONDARY CLAYS; DEPOSITED CLAYS; SEDI-

MENTARY CLAYS Clays that have been transported

away rom their points o geologic origins by wind or

water. Finer particle size gives greater plasticityball

clays, stoneware clays, reclays, etc.

SECONDARY KAOLINS Kaolins that have been

transported some distance rom the parent rock and

are still very pure, but considerably more plastic than

primary kaolins.

SLAKING; SLAKE DOWN The process o return-

ing dry unred clay to a slip by soaking in water.

STONEWARE CLAY Naturally occurring rerac-

tory clays with adequate fuxes to re in stoneware

temperature range.

TERRA COTTA Low temperature, porous earthen-

ware clay body, res red-brown due to high iron con-

tent, which also fuxes clay, making it the most durable

low-red clay ater ring.

THERMAL SHOCK RESISTANCE Resistance to

the damaging eects o thermal shock.

THERMAL SHOCK Eect o sudden temperature

changes during ring or during subsequent heating and

cooling in daily use.

TILE-6 KAOLIN Al2O32SiO22H2Oair-foated

secondary kaolinbroader particle distribution than

primary kaolins, greater plasticity, green strength.

WATER OF PLASTICITY Amount o water re-

quired to bring a dry clay to its state o ideal plasticity.

Common clay test used to determine this amount. The

more water needed, the ner the particle size, the more

plastic the clay, and the greater the drying shrinkage.

XX-SAGGER A plastic reractory stoneware clay,

oten used in sculpture and raku bodies.

Excerpted rom Clay: A Studio Handbook, by Vince

Pitelka, published by The American Ceramic Society, 2001

pottery clay 2

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