clay chapter by ries

8/12/2019 Clay Chapter by Ries

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CL Y

HE term clay is applied usually to certain earthy rocks whoseT m o s t prominent property is that of plasticity when wet. This per-mits them to be molded into almost any shape, which they retain

when dry. Furthermore, they harden under fire. Clays contain hydrousaluminum silicates-the clay minerals-in appreciable amounts, butaside from this a number of other mineral grains, particularly quartz,may be present. Teliturally clays are fine grained, and the so-called trueclay particles are usually under i n diameter.

I

Certain hydrous aluminum silicates that predominate in many claysare known as the clay minerals 8.82 nd are grouped by Ken4' as follows:

KaoliniteDickiteNacriteHalloy siteAnauxiteAllophaneMontmorilloniteBeidelliteNontroniteSaponiteMetabentoniteClay mica*

This includes illite.

A1,O3.2SiO,.2H2OA1203.2Si0,.2H20A1,0,.2Si02.2H20Al2O3.3SiO2.2H20A1,03.3Si0,.2H,0Al,03.nSi0,.nH20MgCa) 0.A1203.5Si02.pH20

A1203.3Si0,.nH20A1Fe)03.3Si0,nH,0

2Mg0.3Si0,.nH20K20-Mg0-A120,-Si02-H20in variable amounts

The kaolin group has been found to consist of a number of min-erals with .the same chemical composition but different crystalline

Kaolinite is commonly formed by the weathering of other mineralparticles, particularly feldspar, but some deposits, like those in Corn-wall, England,36 and others in the Washington-Idaho district, arethought by some to be of hydrothermal origin.

Dickite and nacrite are thought to be due usually to waters of

Professor of Geology, Emeritus, Cornell University, Ithaca, New York.2 7


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2 8 INDUSTRI L MINER LS ND ROCKS

hydrothermal origin, the first being formed probably at a lowert e m p e r a t ~ r e . ~ ~ . ' ~ ~

Halloysite is crystalline, as shown by the X-ray, and has a numberof lines in common with kaolinite. It may have originated in different

ways. I t is very fine grained, and may occur as bedlike deposits orcrusts. In texture it is ~owdery, massive or crustified. Two varietieshave been recognized, a hydrous form, sometimes termed endellite, anda less hydrated one, the halloysite, which is derived from the former onheating.5 It may be associated with k a ~ l i n i t e . ~ ~

Allophane commonly is glassy and amorphous. Ross and Kens4consider it to be a solid solution of silica, alumina and water.

Montmorillonite is a common alteration product of glassy particlesof volcanic ash in the clays known as bentonites, but it may also havebeen formed in marine sediments, soils, in gouge clays, and even asa weathering p r o d ~ c t . ~ ~ , ~ ~eidellite and nontronite are less common.

Illite, formerly referred to as a sericite-like ~ n i n e r a l , ~ ~ . ~ ~ s commonin sedimentary clays and shales.

Many other minerals have been identified in clays but few of themoccur in quantity.73 Those that may be present, sometimes in appreci-able amounts, are: quartz, usually in grains of variable size; calcite,usually in fine-grained, perhaps colloidal form, but sometimes as con-

cretions; limonite, often finely distributed as a coating on grains, some-times as concretions or crusts; gypsum, in grains, selenite plates,crystals or rosettes; siderite, sometimes finely distributed, or occasionallyas concretions in some clays and shales; pyrite as grains and concre-tionary lumps; muscovite, widely distributed, and commonly in verysmall flakes; rutile, almost universally present but only in scatteredgrains of microscopic size.

While the clay minerals may form in place from the other mineralsin residual or even some sedimentary clays, in some instances un-

doubtedly they originate under other conditions, as through the com-bination of colloidal alumina, silica and water. Thus Ross and Kerrdescribe long grains or worms of kaolinite noted in some Coastal Plainsands, which could not have been transported and did not originatefrom the quartz grains.82 There is also good evidence that kaolinite orother clay minerals may replace quartz as in the indianaite deposits ofIndiana75.83 nd other places.84

Identification of la y MineralsWhile the spectrographic microscope has been of great help in

studying clay minerals, the perfection of the X-ray method for de-termining crystal structure has been of tremendous assistance in thiswork.

Differential thermal analysis is another aid that has been used to


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CLAY 209

an increasing extent in recent years. By recording the endothermic andexothermic reaction Fig 1 ) that take place when the clay is heated to1000C, we obtain a characteristic curve for each clay mineral, andeven associated

fJect of lectro lytes

When thoroughly broken down or dispersed, the particles makingup a clay cover a wide range of sizes. Some settle from a dilute suspen-

FIG 1-THERMAL CURVES O TYPICAL CLAY MINERALS.After Cuthbert.22

K kaolinite; I illite; M montmorillonite.

sion in a few minutes but others, on account of their very small size,remain in suspension for a long time. These clay particles are nega-

tively charged and may be affected by small amounts of electrolytes,which exert an important influence on their settling properties. Sometend to increase the charge on the clay particles and help to keep themin suspension or dispersed. Such electrolytes are known as dispersing,deflocculating or peptizing agents. Other electrolytes have the oppositeeffect, in that they tend to reduce the charge on the clay particles, sothat they unite, flocculate or coagulate. These may be called coagulatingagents, and they reach their maximum effect when the negative chargeon the clay particle is reduced to zero.

Clays do not all respond in the same manner to acids or alkalies,because each one may show its own acidity or alkalinity. Furthermore,the presence of soluble salts may exert a modifying effect. Dispersingelectrolytes include sodium silicate, sodium hydroxide, sodium car-bonate, sodium oxalate, sodium phosphate, and others. Coagulantsinclude acids, sodium chloride, calcium chloride, aluminum chloride,


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2 0 INDUSTRIAL MINERALS AND ROCKS

and so forth. Some electrolytes, like sodium carbonate, may causedeflocculation when added to the clay in small amounts, and coagula-tion when larger amounts are added.

ase ExchangeThe process of base exchange, representing the alteration in cation

composition of a solid when treated with a salt solution, is probablyoi much importance in clays. According to the definition, base ex-change may operate with either colloidal particles or larger grains ofmineral matter; in other words, it may operate in the weathering ofrocks or in fine-grained sediments. Opinions seem to differ as to whetherbase exchange involves a change in crystal structure. It is probable,however, that it may go on in clays either during or after deposition,and Ross and Kerrs4 assert that it may take place without any break-down of the clay molecule as a whole. Th e former suggests that certainclay minerals and zeolites possess so open a crystal lattice that certaincations can be displaced and other bases substituted i n their place with-out disruption of the primary space lattice. Marshalls3 believes tha t thisis the type of base exchange shown by most clays. He has measuredthe double refraction of clay particles as small as 50 mp mp 0.00001mrn) and finds that it varies according to the cation present. Such meas-

urement is possible in clays whose particles show oriented coagulationTAELE -Analyses of Clays

iO 46 .3 45 .78 57 .62 59 .9268 .6282 .45 54 .6438 . 07 47 .92120 3 9 . 8 36 .46 24 .00 27 .5614 .981 0 .9214 . 62 9 .46 14 .40

Fe2Oa. 0 . 2 8 1 . 9 0 1 . 0 3 4 . 1 6 1 .0 8 5 . 6 9 2 . 7 0 3 . 6 0FeO. 1 .08 1 .20CaO. 0 . 5 0 0 . 7 0 tr 1 .48 0 .22 5 .1615 .8 4 12 .30

Moisture. 9 . 0 5 2 . 7 1 . 1 2 2 . 7 8c,

1 : ~ ~ 1 1 0 . 4 6 9 . 5 0O3 0 .35 , 1 .44

Also 1.94 organic matter.1. Kaolinite. 8. Brick clay Milwaukee Wis.2. Washed kaolin Webster N C 9. Shale clay Ferris Tex.3. Plastic fire clay St. Louis Mo. 10. Bentonite Otay Calif.4. Flint fire clay Salineville Ohio 11. Potash-bearing bentonite High Bridge.5. Loess Guthrie Center Ia. KY6. Siliceous clay Rusk Tex. 14. Sandy brick clay Colmesneil Tex.7. Brick shale Mason City Ia.


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CL Y 21

and works accurately for particles smaller than 500 mp. He notes alsothat clays with high base-exchangi. capacity have a greater capacity fororiented coagulation.

Sodium chloride solution in contact with an ordinary clay results

in a displacement of some of the calcium or magnesium in it. Con-versely, calcium or magnesium may displace sodium. Only clay min-erals with easily replaceable bases, such as calcium, magnesium, potas-sium or sodium, exhibit much base exchange. The base-exchange ca-pacity is different in different clays. It is highest in montmorillonite,with illite next and kaolinite last.30

hemical omposit ion

Clays vary widely in their chemical composition, from those closeto kaolinite to others that show a high percentage of impurities (Table1 ) . A chemical analysis of clay is not usually carried out in detail,so that the TiO, is included with the Ai2O3, nstead of being determinedseparately. All volatile matter is commonly expressed as loss on igni-tion. All iron is usually determined as Fe,03. Other unsatisfactory fea-tures of the chemical analysis are that it gives little information re-garding the physical properties of the clay, or the distribution of theconstituents in the samples analyzed.73 Moreover, it is unsafe to at-

tempt to calculate the mineral composition from the bulk analysis. Amodification of the ordinary quantitative analysis, known as the ra-tional analysis, attempts to determine the compounds present. It wasfirst applied to kaolins, which were assumed to consist of quartz,feldspar and kaolinite (clay substance). The method is not reliable,however, and no satisfactory way of making this type of analysis hasbeen devised.73

PROP RTI S OF CL Y

Plast ici ty

The property of plasticity, already defined, is the outstandingcharacteristic of clays. They vary from those of high plasticity, or fatones, like the ball clays and bonding clays, to those of low plasticity,termed lean, and represented by some very sandy ones, The plas-ticity may be affected by the amount and character of colloidal ma-terial, the quantity and proportions of nonplastic particles, the amountof water, as well as salts, bases, acids and organic matter.

The cause of plasticity has been much d i s c ~ s s e d ~ ~ , ~ ~ nd has been

variously assigned to hydrous aluminum silicates, shape and size ofgrains, colloidal content, and other attributes. The present general viewregarding plasticity is well expressed by Norton, who says: It is un-doubtedly due to an active particle surface, which has the property ofattracting to it a stable water film. This attractive force both holds the


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CLAY 213

and Table 2 gives such a series. By using a supercentrifuge, M a r ~ h a l l ~ ~has separated the smallest particles of clays, as shown in Table 3. Theseclays, after removal of organic matter, soluble salts and exchangeablebases, were dispersed after bringing to pH9 with NaOH.

T BLE Mechanical Analyses of Clays by Centrifuge Method

Approximate values obtained by interpolation in the distribution curve.

Water in Clay

Two kinds of water usually are recognized in clay: (1 mechanicallyheld water, and (2) chemically combined water.

When a clay dries from its plastic condition to a constant weightat room temperature, the water that evaporates until air shrinkageceases is known as shrinkage water. That which is still left in theintergrain spaces is termed pore water, and may be driven off at100C. The pore water and shrinkage water together are known as thewater of plasticity. There may also be moisture retained on the sur-

face of the clay particles as a film of molecular dimensions, which istenaciously held and which is referred to as hygroscopic or micellarwater. It may not all pass off until the clay is heated to 200C. Table4 gives the range of water of plasticity as recorded for a number oftypes of clay.13

500200 mp

7 . 0

1 6 . 0

1 1 . 8

18 .7

T BLE Water o Plasticity o ClaysClay Pct Clay Pct

Crude kaolin. 3 6 . 3 9 4 . 7 8 Plastic fire clay. . . . . . . . . . . 13.00-37.00Washed kao lin .. 44.48-47.50 Flint c lay . . . . . . . . . . . . . . . . 9.00-19.00White sedimentary kaolin. 28.60-56.25 Brick clay. . . . . . . . . . . . . . . . 13.0CW1.00Ball clay . 25.00-53.00 Sewer-pipe clay . 11.00-36.00Crucible clay.. . . . . . . . . . . . 27.00-51.00

Under50 mp

200

100 mplay

Kaolin.. . . . . . . . . . . . .. . . . . . . .entonitea..

Putnamclay. . . . . . .othamsted..

high percentage of shrinkage water is rather characteristic offine-grained clays that dry to a strong body and they are likely to showexcessive plasticity, high shrinkage, warping and cracking. highcontent of pore water characterizes a clay with a porous structure. Theratio of shrinkage water to pore water is said to be important in claysused in the manufacture of crucibles and glass pots62 and the best ones

1 0

50 mp

Concen-tration,

Pct

0 . 2

1 . 0

0 . 5

0 . 5

2 p 1

6 6 . 0

2 . 0

7 . 8

1 5 .2

6 . 01 2 . 0

1 1 . 6

14 .3

10.5

1 O

31 O

6 . 61 2 .1

9 . 0

1 . 3 4 0 . 9

1 0 . 3 2 9 . 4


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214 INDUSTRI L MINER LS ND ROCKS

are said to show a ratio of I I between shrinkage water and pore water.Table 5 gives the properties of several clays with respect to water andshrinkage.46

T BLE Water and Shr inkage i n lays

Clay

Ratio PoreWater toShrinkage

Water

Volume

Kentucky ballS P G b a l 1English ballGeorgia kaolin

Missouri H ercules

Clay is very hygroscopic and when dry absorbs moisture from theatmosphere, some absorbing as much as 10 pct of its dry weight. Thechemically combined water is chiefly that held in combined form as apart of the hydrous aluminum silicates, and passes off mostly at a tem-perature of 450 to 600C.

hrinkage

ShrinkageWater

Water ofPlasticity

5 4 . 84 8 . 24 0 9

3 3 . 5

SO

Clays exhibit two kinds of shrinkage, air and fire. Air shrinkage oc-curs as the clay dries and continues until the particles are all in con-tact. It depends in part on the water content and character of the clay,being high in very plastic clays and low in sandy ones. An excessive airshrinkage tends to cause cracking but a low air shrinkage is usuallycharacteristic of clays that dry to a weak and porous body.

Open porous clays are easier to dry than dense, highly plastic ones,for in the latter water evaporates from the surface more rapidly thanit can be drawn from the interior, and this develops stresses, which

cause cracking. Very plastic clays therefore require slow drying.Air shrinkage may be recorded in terms of the length or volumeof the dry clay, the two being called, respectively, linear and volumeshrinkage. Both linear and volume shrinkage may be measured seeTests, later in the chapter), but the former can also be calculated fromthe latter.13

PoreWaterrying

Shrinkage

trength

The strength of clay in its dried condition is an important property,as it enables it to withstand shocks in handling of the dried ware; also,a clay of high strength is capable of carrying a larger amount of non-plastic material, without too great deterioration of strength.

Strength of a dried clay may be determined by tension, compression,or transverse tests. The first was formerly much used but is now dis-carded, the second finds little favor while the third is the one commonly


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CLAY 215

TABLE Air hrinkage of Clays

ClayLinear

Pct

Crude kaolin.Washed kaolin.Georgia kaolin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .all clay s..FlintclaysSagger clays.Paving-brick clays..Sewer-pipe cl ay s. .

VolumePct

employed. It is expressed in terms of the modulus of ruptu re and iscarried out on the clay dried at llOC, ranging in different clays froma few pounds per square inch to, in extreme cases, over 1000 lb. Thefigures in Table 7 give some idea of the range of transverse strength indifferent types of clays.73

T BLE Range of Transverse S t reng th in ClaysClay Psi Clay Psi

ashed kaolin.. 75 200 Sewer-pipe cla ys .. . . . . . . . . . . . 190 589Georgia sedimentary kaolin.. 150 166 Sagger clays. . 46 474

. . . . . . . . . . . . . . . . . .all clays. 25 600 Brick clays. ; 50 1500Glass-pot clays. 173 1068

The transverse strength of most clays usually decreases when sandor ground flint is added but occasionally it may increase; the clay alonedevelops minute cracks in drying, which weaken it, but the addition offlint avoids ' this and the true strength of the clay manifests i t ~ e l f . ~ ~ . ~ ~It has-also been shown that the transverse strength of a clay may in-crease with an increase in its base-exchange capacity.31

The bonding strength of a clay refers to its power to hold together

particles of nonplastic materials, such as standard sand, potter's flint,or grog (crushed brick) . The property is important in the use of pot-tery, glass-pot, and crucible clays,12 as they have a n appreciable amountof nonplastic material mixed with them.

Color

Iron is the commonest coloring agent of raw clays, giving yellow,pink, reds and browns, depending on the amount present and the stateof oxidation. Greensand usually gives a green color. Organic matter may

color a clay gray or black, sometimes even pink. Clays free from thesecoloring agents are usually white. Fired clays may owe their color toiron compounds, titanium oxide, or lime reacting with iron, but ironis the usual cause of 'the

The best white-burning clays have less than 1 pct Fe20,; those con-


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2 6 INDUSTRIAL MINERALS AND ROCKS

taining 2 pct tend to develop a light cream tint. An exact re di tion re-garding the color-burning qualities cannot be made from the chemicalanalysis par tly because it does not show whether the iron is evenlydistributed. Buff-burning clays may vary from to 5 pct Fe203 but

just why this color may be obtained from such a wide iron range isdifficult to explain. Most of them have 3 to 4 pct Fe203. Red-burningclays have 5 pct or more Fe203. Such clays a t a low temperature tend toburn pale red or salmon but as the temperature increases they turn toa darker red and finally to purplish or even greenish purple. The moresiliceous clays usually develop a brighter shade of red. Much dependson the kiln atmosphere for if this is reducing iron gives a bluish orbluish black color. Some white cream or buff-burning clays becomebluish gray at certain temperatures owing possibly to the formation offerrous silicate. This is called bluestoning. Lime in excess of ironoxide if the two are evenly distributed gives a cream-colored productunless overfired when the clay tu rn s greenish or greenish yellow.Before the iron and lime begin to react the clay may be pink if enoughiron is present. Titanium oxide to the extent of perhaps 2 pct causes acreamy tint. This may explain why some clays very low in iron oxidedo not burn white.

Porosity

The porosity of a clay refers to its volume of pore space expressed interms of its total volume. In raw clay the pores are all open but ofvariable size. In fired clays the pores may be of variable size but areof two types open and closed the latter being formed by the expansionof gases during fusion. Porosity in the raw clay influences its drylngqualities in that large pores permit the water to escape more rapidly.In fired clays the shape and size of the pores affect the properties ofthe ware such as strength behavior as an absorbent resistance toweathering shock abrasion corrosion discoloring agents efflorescence

destructive action by fungus growths dielectric strength and so forth.62The temperature-porosity relations in firing serve to show the man-

ner and progress of vitrification. Thus a clay in which porosity de-creases rapidly because of sudden fluxing action is one that vitrifiesquickly and when a rise of the porosity curve quickly follows the dropin porosity it indicates a short fir ing range such as would be character-istic of highly calcareous clays.

I t is not uncommon to determine the absorption of a fired clay forwater instead of its porosity since the curve of the former in a generalway follows the latter but is always lower.

Specific Gravity

The specific gravity of a clay may be expressed in three differentways: 3 7 3


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C L AY 217

1. True specific gravity, or the ratio of solid material, exclusive ofclosed or open pores, to an equal volume of water. This is determined ona powdered sample.

2. Apparent specific gravity, or the ratio of the volume of solids(plus the volume of any closed pores) to an equal volume of water.

3. Bulk specific gravity, or the ratio of the ent ire volume of material,including solids, closed and open pores, to an equal volume of water.

True specific gravity is affected by the minerals present in the ra wclay, and in the fired clay by silica inversion, chemical reactions, fusionand crystallization. Th e apparent and bulk specific gravity a re affectedby a ll of these factors as well as by the porosity. I n firing, the true andapparent specific gravity should theoretically decrease, while the bulk

specific gravity increases. Change in specific gravity during f-iring in-dicates progress of vitrification. Table 8 gives the change in porosityand specific gravity of a Maryland clay. O Table 9 gives another seriesof somewhat complete determinations made on Kentucky ball clay.

If the sample tested is a manufactured product, 'the amount ofwater and pressure used in molding may influence the results. There-fore there will be a difference in the bulk gravity, depending on whetherthe ware is slip-cast, hand-molded, plastic, or dry-pressed, if the samematerial is

Firing Changes

TABLE Changes i n Spec @ Gramty and arody during Firing

When a clay is fired it undergoes various 'changes in color, hardness,specific gravity, and other properties, Some of these have al-ready been referred to; others are mentioned in the following para-graphs. These changes may begin at a relatively low temperature rangeor they may not be completed until higher temperatures are reached.

Loss of Volat ile Products

Temperature degPorosity pct..Volumetric shrinkage p c t . .

Apparent specilic gravity..Bulk specific gr avi ty ..

The volatile products liberated during the firing of clay consist of:(1) chemically combined water; (2) organic matter, either carbon orbituminous matter ; (3) oxides of sulphur or carbon given off whencarbonates, sulphates or sulphides are dissociated. Dehydration ofhydrous aluminum silicates' takes place chiefly between 450 and600C, there being a slight variation in the different ones.62.73 Billsiteshows from 25 to 30 pct loss at 310C and diaspore, 12 to 14 pct loss at

13256 . 3

1 6 . 1

2 . 2 92 . 1 4

11502 9 . 8

8 . 0

2 . 5 21 . 9 4

13501 . 4

1 9 . 3

2 . 2 42 . 2 2

13750 . 7

1 0 . 6

2 . 2 52 . 2 3

12002 2 . 0

8 . 9

2 . 5 11 . 9 6

14000 . 4

1 9 . 5

2 . 2 5. a 4

13006 . 6

1 7 . 6

2 . 3 22 . 1 7

1250l o . 11 9 . 1

2 . 4 22 . 1 8

12759 . 6

2 0 . 8

2 . 4 42 . 2 1


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21 8 I N D U S T R I A L M I N E R A L S A N D R O C K S

TABLE WDdermiyt ions on Kentucky Ball Claya

Cone 2 8 1 0 1 2 I 1 4

540C. Hydrous iron oxides generally decompose at 150 to 250C,although some show .the change between 250 and 300C. Hydrocar-bons, which may give trouble in firing, may be completely oxidizedand removed by heating a t between 800 and 900C. Calcium carbonateat normal atmospheric pressure decomposes a little below 900C, al-though the change may begin earlier Siderite decomposes at 800C butin presence of clay this may begin a t 425C.62 Pyrite begins to decom-pose at 350C while gypsum loses three fourths of its water content

between 250 and 400F and al l of its volatile matter by llOOC.The two following analyses, representing, (1) an impure clay

from Ferris, Texas, and (2) brick from that clay, are interesting asshowing the loss of volatile products in firing.R9

Volume fire shrinkage pct..Apparent porosity p c t . .To tal percentage of pores based on

bulk volume of piece.Closed pores..Specific gr avity :

B u l k . .Tru e . .Apparent..

1 2 1 2

SiOz 49 45 56 6 Na2O 0 21 1 4AllOs. 17 1 1 20 4 TiOa 0 17 with A1103F e O s . . 3 45 6 2 H I O . . 4 84 0 5CaO 12 67 11 7 CO 7 10

M g O . .1 77 1 4

SOa2 00

K O 0 13 1 599 43 99 7

usibility

Made by Parmelee and McVay.

31 6

91 3

22 4

1 3

4 09

9 69

2 65

When exposed to a rising temperature, clays do not fuse suddenly;on the contrary, they soften slowly until the entire clay becomes a vis-cous mass. With impure clays this may occur at a relatively low tem-perature but with those approaching kaolinite in composition it takesplace at a much higher one.

Following dehydration, the clay is porous but after a temperature in-terval it begins to compact, then the more easily fusible minerals beginto melt, with the formation of glass. With further temperature risethe fluid portion attacks the mineral grains not yet fused, and finallya solution of molten glass is formed.

34 9

15 8

18 4

3 9

2 18

9 679

9 57

36 7

9 5

38 7

2 9

8 9

6 9

2 32

2 541

4 38

39 4

1 1

6 1

4 0

9 39

2 515

2 41

42 9

1 9

4 1

3 8

2 47

9 587

9 49


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CLAY 219

In the first stages of firing, although the clay may become corn

pacted into a hard mass like common brick, it is said by some that thereis no evidence of softening of the p a r t i ~ l e s . ~ ~ his ~ e r i o d as also beencalled incipient vitrification, but by some that term is used to referto the stage a t which enough glass has been developed to bind the masstogether. Complete vitrification would represent the stage reached inthe fusion of a clay when sufficient glass has been developed to closeall the pores. As the temperature rises and the fluidity of the glass de-veloped increases, the clay mass or object no longer holds its shape, andreaches a condition referred to as viscosity. The temperature at whichthe development of glass begins, as well as its amount and viscosity,exerts an influence on the behavior of the clay during vitrification. I n

some clays, glass has been found to develop at a temperature as low as700C. Th e term vitrification as applied to clay wares does not al-ways mean the same thing. Paving brick and sewer pipe, for example,are said to be vitrified, but technical difficulties prevent the completeattainment of that condition. Electrical porcelain must approach i tvery closely.

The stages of incipient vitrification, vitrification and viscosity mergeinto each other, but the temperature interval between the first pointand overfiring is variable. Clays having a long vitrification range are,

in general, the safest to use for vitrified wares, as most commercial kilnscannot be controlled within a range of a few degrees of temperature,and there is less danger of the ware becoming overfired and ruined.Clays that begin to overfire as soon as they have reached a condition ofvitrification are said to have a short firing range.

The curves of porosity and fire shrinkage shown in Figs 2 and 3illustrate well the behavior in firing of the two types of clay mentionedabove.

Most fire-clay bodies when examined in thin section under thepetrographic microscope show a variable amount of glass, dependingon the amount of fusion that has taken place during firing, whilescattered through this there will be noticed mineral grains that havenot yet been affected. Some of these, however, may exhibit a certainamount of corrosion.

Relatively few new minerals will have been found to have crystal-lized out from the fused material on cooling. Small rods of hematitehave been observed in the glassy matrix of some ferruginous clays, butthe mineral that has been most often noticed, and which has attractedconsiderable attention, consists of needlelike grains, usually colorless.For some years these were identified as sillimanite, but later it wasdiscovered that they were the mineral mullite (3A120,.2Si02). Thetemperature a t which this develops does not se& to be the same al-ways, but it has been observed to form at as low as 900C. Mullite is


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220 INDUSTRIAL MINERALS AND ROCKS

a rare mineral, having been first observed in some igneous rocks on theisland of Mull. Commercial deposits are found of the minerals sillima-nite, kyanite and andalusite, all of which have the composition A12Si05.When these are heated they all change to mullite at temperatures be-tween 1400 and 1550C. It has been found that the development ofmulli te in the firing of certain ceramic bodies has great importance,because it imparts to the ware high tensile strength, superior dielectricproperties, and low thermal expansion, properties that are particularlyvaluable in spark plugs. I t is possible to use the proper amount of the

FIG POROSITY ND SHRINKAGE FIG 3-POROSITY AND SHRINKAGECURVES O F A TENNESSEE ALL CLAY WITH CURVES OF CL4Y WITH SHORT FI RI N GI D N G F I R I N G R A N G E RANGE.Bleininger and Loomis: Trans Amer. Brown: Jnl Amer. Ceramic Soc. (1918) 1

Ceramic Soc. (1917) 19.

minerals mentioned in the clay mixture and have them change to mul-lite in firing.

Th e fusion point of a clay is usually expressed in te rms of Segerconesr3 and may range from as low as perhaps cone 1 1150C) tocone 35 1 785OC).

Soluble Salts

Many clays contain at least a small percentage of water-soluble in-organic compounds, which are brought to the surface in the dryingof the ware, remaining there as a coating, usually white. These solublesalts are mostly sulphates of lime and magnesia but sometimes thereare others. Vanadium salts may also cause a stain.?l Soluble salts may

be in the clay when it is taken from the ground or they may form asthe result of weathering, as when pyri te is present. Th ey may also beintroduced in the water used for mixing the clay. Others may be formedduring firing, by the kiln gases carrying oxides of sulphur from thefuel, which, coming in contact with carbonates in the clay in the pres-ence of moisture, convert them in to s ~ l p h a t e s , ~ ~ , ~ ~

Soluble salts in clays range f rom zero to 1.5 or 2.0 p ~ t . ~ ~ hey may


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CLAY

be the cause of technical troubles because of: (1) the unsightly coatingthat they form on some wares, (2) interference with the adherence ofa glaze, and (3) disintegration of the product caused by the crystalliza-tion of these salts in its pores. Barium compounds sometimes are added

to the clay to render the obnoxious salts insoluble;

ORI GI N OF CL Y

Clays may originate from different kinds of rocks, either by the ordi-nary processes of surface weathering or by the action of solutions,which may be of igneous origin or indirectly of surface origin. Inboth cases the al ternation product is of residual character, and the ma-terial may be called a residual clay.

The removal of the clays so formed by various agents of erosion andtransportation and deposition elsewhere gives rise to a great group oftransported clays.

The several ways in which clay deposits originate are brought outby the following classification:

lassifica tion of la ysA Residual clays.

Formed by weathering of rocks n situ or by rising solutions of magmatic ormeteoric origin.

I. Kaolins, white and firing white or light cream.a. Deposits roughly tabular in form, as when derived from pegmatites, or

hydrothermal alteration along fractures.b. Blanket deposits from areas of igneous or metamorphic rocks.c Replacement deposits, as Indianaite.d. Bedded deposits from feldspathic sandstones.

11 Red-burning residuals, derived fr om different kinds of rocks.B Colluvial clays, practically landslide masses.C Transported clays.

I. Deposited in water.

a. Marine clays or shales. Deposits often of great extent.,White- burning clays; ball clays and sediment ary kaolins; refractoryclays or shales; buff burning

Calcareous.Impure clays and shales

Noncalcareous.

b. Lacustrine clays deposited in lakes or swamps.Fire clays or shales.Impure clays or shales, red burning.Calcareous clays, usually surface deposits.

c. Flood-plain clays. Recent ones usually impure and somewhat sandy.

d. Estuarine clays (deposited in estuaries), mostly impure and finelylaminated.

e. Delta deposits. Variable purity, often lenticular.11 Glacial clays found in the drift. Often stony and lacking stratification. May

be red or cream burning (calcareous).111. Wind-formed deposits (some loess).

D. Chemical deposits (doubtful origin).


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4 INDUSTRIAL MINERAIS AND ROCKS

Foremost among the clays of this group are those found in theCarboniferous of Pennsyl~ania, ~ hio.loO K e n t ~ c k y , ~ ~ i s s o ~ r i , ~ ~ . ~Indiana,l14 I l l i n ~ i s , ~ ~ . ~ ~nd Maryland.66~11 The lays occur at a num-ber of different horizons, and may be associated with coal. Most of the

material is plastic fire clay but with i t there may be a hard type knownas flint clay, which develops little plasticity and which, because ofits texture, structure and appearance, has received this special name.It is found particularly in Pennsylvania, Ohio, Maryland, and Ken-tucky. Many of these fire-clay deposits are worked as open pits butothers are reached by drift or shaft. These clays are widely used forfirebrick and other refractory wares.

The Lower Cretaceous formations of New Jersey have for yearssupplied material for the refractory products industry.77 They are all

plastic clays and are unassociated with coal.Important deposits of Tert iary refractory clays are worked in

western Kentucky and Tenne~see.~~. '~~ n Texas68 and Mi s~ is si pp i,~ ' .~ ~the Tertiary formations supply refractory clays. Those from the latterstate have found favor because of their high bonding qualities, ofvalue in the manufacture of glass pots and crucibles. Other Tert iaryclays have been developed in Cali f~rn ia , ,~ ashington,l17 and Colo-rado.18 A unique type of refractory clay is that found in the north cen-tral Ozark region of M i s s o ~ r i , ~ here there is a series of basin-shapeddeposits carrying flint clay, and some run rather high in diaspore.These diaspore clays, which Allen thinks have been formed by thealteration of the flint clay by the action of percolating solutions con-taining CO,, are of highly refractory character. Clays intermediate inalumina content between flint clay and diaspore are called burlyclays in the Missouri district. The No. grade is said to carry 60 to70 pct alumina.11s

In Canada, fire clays have not been as widely developed, nor are

they as widely distributed, as in the United States. The most importantdeposits are those south of Moosejaw and in southern Saskatchewan.Extensive deposits are known to occur and have been worked aroundClayburn, in southwestern British Columbia. A curious series of de-posits is known to have been preserved under a heavy cover of glacialdrift on the Mattagami and Missinabi R i ~ e r s * ~ . ~ ~ f Ontario. Th e claysare of Mesozoic age and associated with sand deposits. Although theyare reported to be of high quality, their remote location may hindertheir commercial development.

aolins

In their washed condition, kaolins are used largely in the manu-facture of high-grade products, such as white earthenware, porcelain ofall kinds as well as fillers for paper and rubber, and other products.They are rather limited in their distribution.

Kaolins resulting from the weathering of pegmatite and granite


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associated with the Carboniferous fme clays of the Central States, aswell as in the Tertiary and Cretaceous formations of the East, South andWest.

Pauing brick and Sewer pipe Clays and Shales.

As a rule shales rather than clays are employed for paving brickand sewer pipes. There is a large production from the Carboniferousshale formations of the Central States. Some of the Tertiary clays ofTexas, California, and Washington have also been used for this pur-pose. Georgia, Alabama and Mississippi clays and shales have also beenused for sewer pipe. Some Carboniferous shale of Nova Scotia hasbeen used for sewer pipe, and the clays of southwestern British Co-lumbia have been similarly employed.

Brick and Tile ClaysAlmost all the states have clays or shales that can be used in the

manufacture o common b r i C k . * - ~ ~ ~ 6 , ~ 7 ~ 2 3 . 2 1 , 3 2 , 3 3 , 4 6 , ~ 5 - 7 0 0 7 ~ Q he ma-terials commonly used are red-burning surface clays of transportedor residual types, and the clay sometimes selected is not of the best,especially if it is very sandy, as it makes a porous and weak brick.Probably the largest brickmaking district in the United States is that ofthe Hudson River Valley. Cream-burning calcareous clays are some-

times employed, not so much from choice as because they happen to bethe common type of material in areas where such bricks are made, asaround Milwaukee, Wisconsin,BS and some parts of Michigan.17 Shalesof Paleozoic age have been used to some extent in the Eastern andCentral States.

Bentonite

Bentonite is a sedimentary clay in which the mineral montmoril-lonite is an important constituent. It has high plasticity and bondingpower. It is widely used as a bonding agent in synthetic foundry sands,in drilling muds, and for other purposes. Bentonite is a widely dis-tributed material, and important deposits of it a re found in Wyoming,s4South Dakota,s6 Mississippi, Arizona and Texas. I t is also worked inwestern Canada, and deposits are known in a number of other coun-tries. This clay is treated more in detail in Chapters 5 and 6.

Canada and Europe

In Canada,3Q.40.76 omewhat the same types of clay are used as in

the United States. The surface clays are mostly red burning, butaround Winnipeg. Manitoba, cream-burning clays are common,76therefore e selected. Silurian shales are used at Toronto and Creta-ceous shales in parts of Saskatchewan.

The most important clays of Europe17 are the kaolin deposits. Those


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CL Y 7

at Cornwall, England, which have been worked for years and todepths of several hundred feet are widely ~sed,~ ot only at home butabroad. They are the largest deposits of their type that have been de-veloped anywhere. Also important are the kaolins of Karlsbad, Czecho-slovakia, and those worked at several localities irl Germany.gs The Eng-lish ball clays are well known to potters, and, like the Cornwall kaolins,have been exported to the United States in quantity. Belgium, Ger-many and England24.10G ll contain' excellent deposits of fire clay. Tha tfrom Gross Almerode, Germany, has been exported to the UnitedStates for glass-pot manufacture.

POLITICAL AND COMMERCIAL CONTROL PRODUCTIONAND CONSUMPTION

I t can hardly be said that control is much of a ~roblem n the clayindustry, because the United States is relatively independent of othercountries for its supplies of raw clays. Only certain special kinds ofhigh-grade clays have been imported and these have been graduallyreplaced by domestic materials, except perhaps the highest grade ofpaper-coating clays.loB

For many years there has been a somewhat steady importation ofEnglish kaolin or china clay for use in the manufacture of china, paper,

paint, etc. The strong hold this ~ r o d u c t as had on the American mar-ket for years has been due in part probably to its more uniform char-acter. Before the war more than two thirds of the china clay con-sumed in the United States was imported from England, and as late as1925, according to Bureau of Mines statistics, the imports were morethan the domestic production. The replacement of foreign clays in thepottery and paper industry continued during the depression, and in1934 the china-clay imports formed less than 20 pct of the domesticconsumption. This increase in the use of domestic materials has been

due in part to their more careful preparation for the market.In former years, much English ball clay was imported, the potters

in this country preferring it to the American material, which did notvitrify at quite as low a temperature; indeed, when the American ballclays were first developed, some of the potters used to the English prod-uct were loath to class the domestic product as ball clay. I n recent years,however, the domestic clay has replaced the imported ball clays andglass-pot clays.

Since the large majority of the clay-working plants in the UnitedStates obtain their supply of raw material from their own deposits,the production of this clay is not listed separately. According to Tyler,roughly 35 million tons of clay was produced in the United States in1999, but its chief use was in the manufacture of heavy clay products.

Table 10 shows the production of clay that is sold to factories or


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firms on a royalty or tonnage basis as reported by the U. S. Bureauof Mines. I t consists mostly of high-grade clay used in the manufactureof whiteware refractories or for filler in paper paint rubber or fab-rics and in some years is said not to have represented more than 10 pct

of al l the clay mined. The figures of production do not include clayused in manufacture of portland cement. They do include bentonitewhich is used in par t for bleaching as well as a variety of otherpurposes but not by itself for the manufacture of clay products.Even the fire clay reported does not include all of that type as manymanufacturers of refractories operate thei r own deposits.

I t is interesting to note the rank of the five leading states for pro-duction of several types of clay as shown in Table 11

Table 12 shows the salient statistics of the clay industry i n theUnited States for 1944 and 1945 as given in Minerals Yearbook of

Year

Data supplied by the U S. Bureau of Mines.

T . ~ L E 0-Clay Sold by Producers i n the United States@

TABLE 1-Clay-producing States i n 1946, Arranged i n Ordm of Rank

Fire Clayand

Stone-ware

ShortTons

2,765,2474,167,5674,839,3334,701.1446,344,3836,090,4117,907,9749,074,923

Fuller sEarth,ShortTons

146,568907,446204,244247,258294,737296,368298,752~ 2 9 , 0 6 8

Kaolin orChina

Clay andPaper

ShortTons

833,4501,087,848

946,588929,437873,056939,988

1,322,3031,425,106

Rank

12345

Clay,ShortTons

40,707198,445162,293147,785155,667174,594243,145e69,oao

Ben-tonite,ShortTons

251,033354,028374,967480,202546,768573,995601,438763,889

TotalValue

a.Pa.OhioMo.Calif.

Miscel-laneous,

ShortTons

710,5151,210,1681,019,663

850,8972,080,717

10,848,68616,092,01421,737,437

Ton-nage

OhioPa.111.Mc.Ga.

BallClay

Tenn.Ky.S J

Md.

Total

Kaolin andPaper Clay

Ga.S. CFla.N C.Pa.

Fire Clayand Stone-ware Clay

OhioPa.Mo.Ky.Calif.

Sbo.tTons

4,847,5497,295,1727,547,0877,380,632

17,296,32818,923,97530,563,94633,599,473

Miscella-neous,

IncludingSlip Clayand Shale

Ill.Calif.Pa.OhioTexas

Benton-ite

Wyo.S. Dak.Miss.Ariz.Texas

Value

19,633,56827,037,39126,662,69727,654,73236,855,97543,259,29860,863,30874,872,487

Fuller sEarth

exasGa.Fla.111.Tenn.


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CLAY 229

TABLE 12-Salient Statisticsof the Clay In du slr y i n the Un tted States, 1944-1947'S ORT TONS

Use 1944 1945 1946 1947

ottery and stoneware.Tile, high grade.

aggers, pins, stilts, wads.Architectural terra cotta..Paper.

ubber..Linoleum and oilcloth..Paints.Cement manufacture..Refractories..IIeavy clay products..Miscellaneous.

Grand total. 17,295,928 18,929,978 30,563,946 33,599,473Value 96,855 ,275 49.259.228 60 ,863,908 74,272.487

I

Data supplied by U S. Bureau of Mines.

TABLE 9-Imports of Clay nto the U nited Statesa

Data supplied by T . Bureau of Mines.

U S Bureau Mines. If we compare the tonnage for 1945 with theaverage of 1931 to 1934 as given in the first edition of this book (In-dustrial Minerals and Rocks. AIME 1937 we find that it is about ninetimes greater.

In the years 1940 to 1947, all of the clays listed in Table 13 haveshown an increased production but bentonite and fuller s earth havemore than doubled their output.

Year

995-39 avg1940 . .1941.1942 . .1949 . .1944 . .1945 . .1 9 4 6 . .1 9 4 7 . .

Totalaolin orChina

( layShort

Tons

199,232105.56785,14169,27855,56545,89057,49789,29382,648

ShortTons

188,818140,449112,46289,52471,47866,50777,921

116,359112,400

Other( lay,ShortTons

16,9222,267

154160247

2,7121,6964,2499,768

CommonBlue andGross

Almerode.ShortTons

27,80832,14126,82519,79015,50917,59717,85222,68925,849

Value

1,608,3951,159,7901,151,915

862,907230.219661,129

1,069,8201,796,9891,719,169

Fuller sEarth,ShortTons

2,256474942287157908996194155


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CL Y 23 1

A drive pipe drill with a split core barrel can also be employed. Aspecial core barrel that prevents soft wet material from running outwhen the pipe is withdrawn is sometimes employed.

If large samples are desired test pits can be sunk in the deposit.Clay deposits are sometimes prospected by tunnels and shafts.

ampling

The Committee on Standards of the American Ceramic Society3recommends that for preliminary testing the body of clay shall bestripped of loose or foreign materials and a series of straight paralleltrenches cut entirely across the outcrop. If the deposit is stratified andthe beds dip the trenches shall be cut across the dip. Each trench shouldbe 12 in. wide and deep enough to yield 100 lb of material. If naturaloutcrops are not available preliminary trials may be made with ahand auger and test pits can be dug for further sampling. Shouldthe deposit show beds that are visibly different from each other theyshould be sampled separately. If preliminary tests show the deposit tobe satisfactory the deposit as a whole can be bored or drilled throughoutits extent spacing the holes not more than 100 ft apart.

The samples collected from the different trenches are to be re-duced to lumps not over 2 in. in diameter mixed together and reducedby quartering to 100 lb.

Too much stress cannot be laid on the fact that all samples collectedshould represent the average of the deposit or the bed. Furthermorethe clay should be carefully tested before a plant is erected. The writerhas known of several plants erected on sites of improperly tested de-posits; the result being failure of the enterprise.

MININ

Most clay deposits are worked as open pits the method of excava-tion depending on the character of the clay thickness and extent ofbeds and character of the overburden. Where the deposit is small orconsists of beds of different quality which it is desirable to separatethe clay may be dug by spades or mattocks. Selective mining of di fferent beds is practiced in New Jersey western Kentucky and Ten-nessee and also in part of California. Linton50 states that in someCalifornia deposits more than 20 ft thick beds as thin as 2 ft may beseparated in selective mining. With thicker beds and a working facenot more than 20 ft high a power shovel may be employed.

At some clay pits the face of clay if not too high may be under-cut at the base while wedges are driven in at the top thus causing aslice of the bank to fall and break up the clay so that i t can be morereadily handled. If the clay deposit has a horizontal surface of some


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232 INDU STRI L MINER LS N D ROCKS

extent, it may be loosened by plows and the clay then gathered upby wheel or drag scrapers.

Shale banks often are blasted and the material that accumulates atthe bottom is gathered up by power shovels. Occasionally shale is

worked by a planer, consisting of a steel s t r u c t ~ x - e ~ ~ . ~ ~ arrying a power-driven steel chain provided with teeth, which scrape off pieces of shale.These pieces fall to the base of the face and from there are carried byan endless belt to hoppers or vehicles. Such planers can cut a verticalor steeply sloping face. Depending on their construction, they canwork on a straight line or cut a circular swath through an angle of180. A similar machine provided with small buckets has been usedin soft clay.

For removal of overburden, the methods used may be similar tothose employed for excavating clay. In some places drag scrapers oncables are employed, or if the overburden is sandy or gravelly it canbe removed by hydraulicking. At one shale bank near Seattle, Wash-ington, where the stripping was 50 to 75 ft thick, this method was used.The sale of the sand and gravel for concrete and other work paid forthe cost of stripping. The amount of overburden that can be strippedeconomically depends on the thickness of the clay and its market value.Linton5 escribing clay mining in California, says that the ratio of

overburden thickness to that of clay varies from 1 1.2 to 1 1.75. In Ten-nessee as much as 35 ft of overburden has been removed to dig 10 ftof ball clay.

In North Carolina, where the Kaolin mines were in narrow pegrna-tite dikes, the material was worked by circular pits 15 to 25 ft in di-ameter, lined with cribbing. As the more recently worked deposits areof greater extent, the kaolin usually is worked with a power shovel,although at one time hydraulicking was used. The latter method hasbeen used also in Cornwall, England, where the pits may be 200 ft deep

or more.In Florida, the clays have been excavated with a clamshell dredge

floating in the pit. The material is dumped onto barges or pumpedashore through large pipes to the washing plant.

Underground mining is often used for extracting the higher gradesof bedded clay, and usually is carried out by the room-and-pillarmethod. It has been extensively used in some of the fire-clay deposits ofthe central and Appalachian states, as well as in parts of California.

In Colorado, along the foothills, where the clay beds dip steeply, andmay be interbedded with sandstones, the beds are worked down thedip and along the strike, leaving pillars of hard clay to support thewalls. Timbers also may be used to support the hanging wall.

The clay is hauled by different methods from the bank or mine tothe shipping bins or manufacturing plant. At piis close to the plant,


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CL Y

in which t, percentage pore water,T percentage of water of plasticity,t, percentage shrinkage water.

S L K I N G N D T R N S VE R SE S T R E N G T H

Slaking Test-A 1-in. cube, consisting of a mixture of 500 pct clayground to pass a 30-mesh sieve and 50 pct potter's flint, is cut from aslab of the plastic materia l. This is dried in room temperature, then at64O to 76OC, and finally at 1 10C. It is cooled in a dessicator and thenimmersed in water on a -in. sieve. The time for it to disintegratecompletely is noted.

Transverse Strength-The plastic clay is molded into bars 7 in. longand 1 in. square. These are first carefully dried at room temperature,then a t 6d0 to 76O and then at llOC. They are cooled in a dessicatorand broken in a machine in which they rest on supports 5 in. apart, theload being applied at the rate of about 100 Ib per minute. Th e modulusof rupture is calculated by the formula

when MR modulus of rupture, psi,W breaking load, lb,

l distance between knife-edges, in.,breadth of bar, in.,

h height of bar, in.Since the clay shrinks in drying, the .breadth and height of the

bar must be measured before testing. Ten bars should be broken, andvary by not more than 15 pct from the average.

F IRING T E S T S

Test pieces are fired over a range of temperature that depends onthe type of clay, a t a rate of 45OC per hour up to a little below thepoint of drawing trials, and after that a t a rate of 20C per hour. Thefired trials are examined for color, hardne~s,~shrinka~e, orosity, andother properties. The fusibility is determined by grinding the clay to ,pass a No. 60 sieve, molding it into tetrahedra 30 mm high and 7 mmon a side at the base, and testing these in comparison with standardcones of similar size in a furnace with neut ral or oxidizing atmosphere.

S P E CIF IC GR VIT Y

The true specific gravity is determined in the usual manner in apycnometer. The apparent specific gravity of a fired clay is determinedby the formula


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236 I N D U S T R I L M I N E R L S N D R OC KS

a Wfv (sf w jand the bulk specific gravity by

in which W f weight of fired piece, grams,Vf volume of fired piece, cc,Sf weight of saturated fired piece, grams.

B S O R P T I O N N D P P R E N T P O R O SI TY

For fired pieces absorption is determined by weighing the dry piece,soaking in water for 24 hr then weighing again. The increase in weightis calculated in percentage terms of the dry weight.

Apparent porosity is calculated by means of the following formula:

Saturation is obtained by soaking the test piece in distilled water at20C for 100 h r and boiling for 1 h r during the first, twenty-fifth, forty-ninth and seventy-third hours.

Specifications

While no standard specifications have been recommended, it isalways possible for the consumer to prepare them himself, indicatingwhat properties the raw material shall possess.

M RKETING AND USES

Marketing

Clays may be sold under some name or number or under the nameof the use to which they are put. This might mean that one clay mightbe sold under two different names.

Producers of the cheaper types of clay products, such as buildingbrick, drain tile and even common red earthenware, with hardly anyexceptions, obtain their clay from their own deposits. Only the kaolins,slip. clays, paper clays, many refractory clays, bentonite and fuller'searth (see chapters on bentonite and bleaching clay), as a rule, aremined by separate companies or individuals, and sold to consumers by

the ton.Some clays and shales are sold on a royalty basis, this being

quotedlo8 as 5 a ton fo r common clays and shales, 10 for moderate-duty fire clay, and 10 to 25 for high-grade fire clay. The white-burn-ing clays bring the highest prices on the market but for any one type of


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CL Y 23 7

clay the price may vary, depending on the amount of preparation it goesthrough before being shipped.

Refractory clays are rarely shipped long distances to market, un-less by water, but china clays and paper clays, because of their morerestricted distribution, will stand a longer haul. The two great white-ware potting centers of the United States are Trenton, New Jersey, andEast Liverpool, Ohio, but the nearest important domestic sources ofwhite clay a re western North Carolina, the Georgia-South Carolina belt,and Florida. The chief markets for these clays are in the central andnortheastern states, and this means an appreciable freight rate.

China clay from South Carolina and Georgia,* No. 1, air-floatedbulk, sold for $8 to $9 per ton and North Carolina ceramic grade was$18 to $22 early i n 1949.

According to the Bureau of Mines, domestic ball clay sold for $3to $18.25, depending on the quality and degree of preparation. Fireclays averaged $2.52. Sagger clays sold for as little as $2.50 but the proc-essed types sold for $6 to $25, or even higher. Bentonite crude in 1945brought $2 to $13 per ton but prepared ranged from $7.50 to $16.

ses

Clays may be used in their unfired or fired condition. The properties

that govern the use of clays in their raw condition are color, texture,bonding strength and absorptive qualities. The uses to which unfiredclays are put include:

Paper clays, usually kaolins of residual or sedimentary character.Whiteness, fineness and uniformity of texture are of prime importance.Somewhat similar clays are used as fillers in fabrics and in manufac-ture of ultramarine. This group may be designated as fillers. Therequisite properties of fillers as specified by different consumers, eitheras groups or individuals, are somewhat conflicting, but whiteness and

freedom from grit seem to be the essentials.ll1Bentonite, aside from its use as a bleaching agent, is used as a drill-

ing mud in petroleum industry and as a bonding agent in syntheticfoundry sands, which represent two important uses. It is also employedfor stopping leakage in soils, dams and rock fractures underground, aswell as in the manufacture of plastics.

Fire clays are employed in their raw condition as a bond for syn-thetic foundry sands.

Common clays may be used for making stabilized roads, in whichis employed a properly proportioned mixture of gravel or crushedstone, sand and clay. The clay should be tested for its cohesiveness,which commonly is done by the Atterberg test.13

In its fired condition, clay may be used for a variety of purposes,

ngineering and Mining Journal 1949) 150, 85


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prominent among which are structural, refractory, decorative andhousehold. The deciding factor is the physical behavior of the clay inboth its raw and its fired condition. While different types of clay maybe designated by certain names, such as china, brick, stoneware, terra

cotta, such terms refer only in a general way to their properties and donot indicate restricted uses. Thus a clay suitable for brick might alsobe used for drain tile, or a paving-brick clay could also be used forsewer pipes. Furthermore, many clay products other than the cheapestare often made of a mixture of clays, in order to get a material withsuitable properties. Architectural terra cotta and chemical stonewareare made usually from a mixture of clays. Whiteware contains kaolin,ball clay, ground quartz and p ound feldspar. Any reference to the usesof clay, therefore, might better be expressed in terms of physical prop-erties than in terms of names. The following classification, given byPamelee,B3 brings out these points:

CERAM IC USE CLASSIFICATION O F CLAYS

I. Clay burning white or cream, not calcareous.A Open-burning clays (i.e., still dis tinc tly porous) a t cone 15 (2606'F).

Uses: I f of good color or of good strength, is used for pottery. If of goodor high degree of refractoriness, used for various refractories; if also of good

color, used for special refractories (e.g., pots for melting optical glass).I. Low strength. Type: residual kaolins of North Carolina.2. Medium and high strength. Type: secondary kaolins of Florida and

Georgia.B Dense burning (i.e., becoming nearly or completely nonporous) between

cones 10 and 15 (2426 and 2606F). Medium to high strength, mediumshrinkage.a Nonrefractory clays.

3. Good color. Uses: pottery, including certain whiteware, porcelains,stoneware.

4. Poor color. Uses: stoneware, terra cotta, abrasive wheels, zinc retorts,face brick, saggers.

b Refractory clays.5. Good color. Uses: refractories , especially for glass if they do not over-

bum seriously for five cones (about 1800F) higher. Also uses stated in 3.C Dense buming between cones 5 and 10 (2246 to 2426'F) and do not over-

bum seriously at five cones (about 1800F) higher than the temperature atwhich minimum porosity is reached.a Nonrefractory clays, medium to high strength, medium shrinkage.

6. Good color; usually reach minimum porosity between cones 5 and 8(2246 to 2354'F). Type: ball clays. Uses: pottery, whiteware, porce-

lain and stoneware.7 Poor color. Uses: stoneware, terra cotta, abrasive wheels, zinc retorts,

face brick, saggers.b Refractory clays.

8. Dense buming at cone 5 (2246F); do not seriously overburn for 12cones (about 432F) higher; highly refractory; softening point at cone


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CLAY

31 (3182F) or higher; bonding strength minimum, 325 psi. Use:graphite crucibles for melting brass.

9. Dense burning at cone 8 (2354'F); not overfiring at cones 13 or 14(2550F). Strength and softening point as in 8.Use: steel crucibles.

10. Dense burning at cone 8 (2354F) ; not overfiring at cone 15 (2606'F).Bonding strength, 250 psi or higher. Softening point, cone 29 or higher.Use: glass pots.

11 Buff-burning clay.A Refractory clays.

a Open burning (5 pct porosity or more) at cone 15 (2606F) or above.Indurated. Nonplastic or slightly plastic (unless weathered). Type: flintclays.

11. Alumina 40 pct or less. Use: refractories.12. High alumina (over 40 pct). Type: diaspore clays. Uses: refractories,

abrasives.b. Open burning (5 pct porosity or more) at cone 15 (2606F) but plastic.

13. Silica 65 pct or less. Uses: firebrick and other refractories, terra cotta,sanitary ware, glazed and enameled brick.

14. High silica (over 65 pct). Type: many New Jersey fire clays. Uses:firebrick and other refractories.

c Dense burning (porosity under 5 pct) between cones 10 and 15 (2426 to2606F).

15. Medium to high strength, not overburning for five cones (about 1800F)higher than point of min imum porosity. Uses: glass pots, firebrick, sag-

gers, and other refractories; architectural terra cotta, sanitary ware,enameled and face brick.d. Dense burn ing (porosity under 5 pct) at cone 10 (2426F) or lower.

16. (See 8.) Uses: zinc retorts, firebrick saggers and miscellaneous refrac-17. (See 9.) tories, architectural terra cotta, sanitary ware, enameled and18. (See 10.) face bricks.

B Nonrefractory clays.a Open burning (5 pct porosity or more) at cones lower than 10 (2A26 F).

19. High or medium strength. Uses: architectural te rra cotta, -stoneware,yellow ware, face brick, sanitary ware.

20. Low strength. Use: brick.b. Dense burning (porosity under 5 pct) a t cones lower than 10 (2426F).

21. High or medium strength. Uses: architectural terra cotta, stoneware,abrasive wheels, sanitary ware, face brick, paving brick.

111 Clays burning red, brown, or other dark colors.A Open burning (do not attain low porosity at any temperature short of actual

fusion).22. Medium or high strength. Uses: brick, drain tile, hollow blocks, flower

pots, pencil clays, ballast.23. Low strength. Use: brick.

B Dense-burning clays.a Having a long vitrification range (5 cones or about 1800F).

24 High or medium strength. Uses: conduits, sewer pi:e, paving bric kfloor tiles or quarries, electrical porcelain, cooking ware, silo block, art-ware, face brick, architectural terra cotta, roofing tile.

25. Low strength. Uses: as dust body in manufacture of electrical porcelain,floor tile, building brick.


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b Having a short vitrification range:26. High or medium strength. Uses: building brick face brick hollow block

flower pots.c Highly fusible forming a glass at about cone 5 2246F).

27. Slip clays.

IV. Clays burning light gray or light cream.28. Containing calcium or magnesium carbonate or both. Never attain low

porosity. Very short heat range. Use: common brick.

BIBLIOGRAPHY

I. Anon: Ceramic Properties of Some White-burning Clays of Eastern UnitedStates. Nat. Bur. Stds. Circ 325 1927).

2. Anon: Fire Clay Brick Manufacture Properties Uses and Specifications. Nat.Bur. Stds. Circ 282 1926).

3. Anon: Report of Committee on Standards Amer. Ceramic Soc. 1921-22);also Inl Amer. Ceramic Soc. 1928) 11 44.2.

4. Anon: Report of Committee on Geological Surveys Clay Definitions. h e r .Ceramic Soc. 1939) 18 213.

5. L. L. Alexander G. T. Faust S. B. Hendricks H. Insley and H. F. McMurdie:Relationship of the Clay Minerals Halloysite and Endelite. Amer Min 1 943)28 No. 1.

6. V. T. Allen: Mineral Composition and Origin of Missouri Flint and DiasporeClays. Missouri Geol. Survey and Water Res. Append. IV 58th Bien. Rept.

7. C. R. Austin: Surface Clays and Shales of Ohio. Ohio State Univ. Eng. Expt.Sta. Bull 81 1934).

8. 0 G. Bell: Preliminary Report on Clay of Florida. Florida Geol. Survey 15thAnn Rept. 1924).

9. S. W. Beyer J. B. Weems and I. A. Willams: The Clays of Iowa. Iowa Geol.Sunrey 1904) 14.

10. S. W. Blatchley: Clays and Clay Industries of Indiana. Indiana Dept. Geol. Nat.Res. 20th Ann. Rept. 1896) and 29th Ann. Rept. 1904).

11. A. V. Bleininger: Effect of Preliminary Heating Treatment upon the Dryingof Clays. Nat. Bur. Stds. Tech Paper 1 1911).

12. A. V. Bleininger: Properties of American Bond Clays and Their Use in Graph-ite Crucibles and Glassports. Nat. Bur. Stds. Tech Paper 144 1920).

13. A. V. Bleininger and G. H. Brown: Testing Clay Refractories with Special Ref-erence to Their Load-carrying Capacity at Furnace Temperatures. Nat. Bur.Stds. Tech Paper 7 1911).

14. A. V. Bleininger and E. T. Montgomery: Effect o Overfiring on Structure ofClays. Nat. Bur. Stds. Tec h Paper 22 1913).

15. G. J Bouyoucas: Hydrometer Method for Studying Soils. Soil Science 1928)25 365.

16. J C. Branner: The Clays of Arkansas. U. S. Geol. Survey Bull 351 1918).17. G. G. Brown: Clays and Shales of Michigan and Their Uses. Michigan Dept.

Conserv. Geol. Survey Div. Pub 36, Geol. Ser. 30 1924).18. G. H. Brown and G. A. Murray : The Function of T ime in the Vitrification

of Clays. Nat. Bur. Stds. Tech Paper 17 1913).19. G. M. Butler: Clays of Eastern Colorado. Colorado Geol. Survey Bull 8 1914).20. J R. Chelikowsky: Geologic Distribution of Fire Clays in the United States.

l n l Amer. Ceramic Soc. 1935) 18 367.el. A. R. Crozier: Refractory Clay Deposits on the Missinabi River. Ontario Dept.

Mines Ann Rept. 1933) 2 pt. 3, 88.


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INDUSTRIAL MINERALS AND ROCKS

M . J Lintner: Miningsand Grinding Methods and Costs a t the Dominion SewerPipe Co., Dominion, Ohio. U. S. Bur. Mines I. C 6921.

a J. Lintner: Mining and Grinding Methods and Costs at the Evans PipeCompany Clay Mine, Uhrichsville, Ohio. U. S. Bur. Mines I. C. 6929.

50. R. Linton: Clay Mining in California. M in and M et 1936) 198.

51. W. N. Logan: Clays of Mississippi. Mississippi Geol. Survey Bull 2 1907) andPottery Clays, Bull 6 1914).

52. G. F. Loughlin: Clays and Clay Industries of Connecticut. Connecticut Geol.Nat. Hist. Survey Bull 4 1905).

53. C. E. Marshall: Clays as Minerals and Colloids. Ceramic Soc. ( ~ n ~ l a n d ) 1931)30, 81.

54. F. H. McLearn and J. F. McMahon: Buff and White-burning Clays of South-ern Saskatchewan. Can. Geol. Survey 1933) Sum. Rept., p t . ~ , 2.

55. H. S. McQueen: Fire Clay Districts of East Central Missouri. Missouri Geol.Survey and Water Res. [2] 1943) 28.

56. W. W. Meyer: Colloidal Nature and Related Properties of Clays. Nat. Bur.Stds. Res Paper 7C6 1934).

57. A. C. Munyan: Supplement to Sedimentary Kaolins of Georgia. Georgia Geol.Survey Bull 4 - A 1938).

58. F. H. Norton: Critical Study of Differential Thermal Method for Identificationof Clay Minerals. In l Amer. Ceramic Soc. 1939) 22, No. 2.

59. P. G. Nutting: Absorbent Clays. U. S. Bur. Mines Bull 9284 1943).60. J. M. Parker, 3rd: Residual Kaolin Deposits of the Spmce Pine District, Ndrth

Carolina. North Carolina Dept. Consem., Div. Min. Resources, Bull 48 1946).61. C. W. Parmelee: Ball Clays of West Tennessee. Tennessee Geol. Survey Res.

Tenn. 1919) 9.62. C. W. Parmelee: Clays and Other Ceramic Materials, I and 11. Ann Arbor,

Mich., 1935.63. C. W. Parmelee and C. R. Schroyer: Fu rther Investigations of Illinois Fire

Clays. Illinois Geol. Survey Bull 38 1921).64. H. Ries: Clay Deposits of North Carolina. North Carolina Geol. Survey Bull

13 1897).65. H. Ries: Clays of New York. New York State Mus. Bull 35 1900).66. H. Ries: Report on the Clays of Maryland. Maryland Geol. Survey 1902) 4,

pt. 3.67. H. Ries: Clgys of Virginia Coastal Plain. Virginia Geol. Survey Bull 2 1906).68.

H. Ries: Clays of Wisconsin. Wisconsin Geol. and Nat. Hist. Survey Bull15

1906).69. H. Ries: The Clays of Texas. Univ. Texas Bull 102 1908).70. H. Ries: Clays of Kentucky. Kentucky Geol. Survey 1922) 6) 8.71. H. Ries: Bibliography of Clay Deposits (World). Bull Amer. Ceramic Soc.

1925) 4.72. H. Ries: Clays. In t. Crit. Tables 1925) 2 56-64..73. H. Ries: Clays, Occurrence, Properties and Uses, 3d Ed. 1927).74. H. Ries: Geology and Clay Research. Bull h e r . Ceramic Soc. 1935) 14.75. H. Ries, W. S. Bayley and others: High-grade Clays of the United States. U. S.

Geol. Survey Bull 708 1922).76. H. Ries and J. Keele: Clays and Shales of the Western Provinces. Can. Geol.

Survey M e m 24-E, 25,47, and 65.77. H. Ries, H. B. Kummel and G. N. Knapp: The Clays and Clay Industry of New

Jersey. New Jersey Geol. Survey Final Rept. 1 9 ~ ) .78. H. Ries and E. A. Smith: Preliminary Report on the Clays of M ~ -

bama Geol. Survey Bull 6 1900).


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CLAY 2443

79. H Ries and R. E. Somers: Clays of Piedmont Province. Virginia Geol. SurveyBull. 13 1917).

80. H Ries and R. E. Somers: Clays and Shales West of the Blue Ridge. VirginiaGeol. Survey Bull. 20 1920).

81. C. W. Rolfe R. C. Purdy A. N. Talbot and I. 0 . Baker: Paving Brick and Pav-ing-brick Clays of Illinois. Illinois Geol. Survey Bull. 9 1908).

82. C. S. Ross and P. F. Kerr: The Clay Minerals and their Identity. Jnl. Sed. Petr.1931) 1, 55; also Jnl. Amer. Ceramic ~ b c 1933) 16 57.

83. C. S. Ross and P. F. Kerr: The Kaolin Minerals. U. S. Geol. Survey Prof. Paper165-E 1931).

84. C. S. Ross and P. F. Kerr: Halloysite and Allophane. U. S. Geol. Survey Prof.Paper 185-G 1934).

85. C. S. Ross and S. B. Hendricks: Minerals of the Montmorillonite Group. U. S.Geol. Survey Prof. Paper 205-B, 1945), also Ross C. S.: Clays and Soils inRelation to Geologic Processes Jnl. Wash. Acad. Sci. 1943) 53 No. 8.

86. E. P. Rothrock: Geology of South Dakota pt. 3, North Dakota Geol. Survey Bull.15 1944).87. G. A. Schroter and I. Campbell: Geological Features of Some Deposits of Bleach-

ing Clay. AIME Tech. Pub. 1139 1940); Tmns. 1942) 148 178.88. H G. Schurecht: Properties of Stoneware Clays. U. S. Bur. Mines Tech. Paper

233 1920).89. A. B. Searle: Chemistry and Physics of Clays and Other Ceramic Materials.

London 1924.90. H Seger: Collected Writings. Translation by Amer. Ceramic Soc. 1902.91. J. B. Shaw: Fire Clays of Pennsylvania. Pennsylvania Geol. Survey Bull. M - 1 0

[4] 1928).92. F. H Skeels: Preliminary Report on the Clays of Idaho. Idaho Bur. Mines and

Geol. Bull. 2 1920).93. R. W. Smith: Shales and Brick Clays of Georgia. Geol. Survey Georgia Bull. 45

1930).94. R. W. Smith: Sedimentary Kaolins of the Coastal Plain of Georgia. Geol. Survey

Georgia Bull. e 1929).95. I . C. Snider: Prel iminary Report on the Clays and Clay Industries of Okla-

homa. Oklahoma Geol. Survey Bull. 7 1911).96. H H Sortwell: American and English Ball Clays. Nat. Bur. Stds. Tech. Paper

227 1923).

97. S. Speil L. H. Berkelhamer J. A. Pask and B. Davies: Differential ThermalAnalysis. U. S. Bur. Mines Tech. Paper 664. 1945).

98. A. Stahl: Die Verbreitung der Kaolinlagerstatten in Deutschland. Archiv Lagerstattenforschung 1912) 12.

99. S. St. Clair: Clay Deposits near Mountain Glen Ill . Illinois Geol. Survey Bull.36 1920).

100. W. Stout R. T. Stull W. J. McCaughey E. J. Demorest: Coal Formation Claysof Ohio. Ohio Geol. Survey Bull. 26 [4] 1923).

101. W. Stout M. C. Shaw G. A. Bole and D. Schaaf: The Lawrence Clay of Law-rence County Geol. Survey Ohio Bull. 36 [4] 1931).

102.J

L. Stuckey: Kaolins of North Carolina. AIME Tech. Pub. 2219 1947).103. R. T. Stull and G. A. Bole: Beneficiation and Utilization of Georgia Clays. U. S.Bur. Mines Bull. 252 1926).

104. C. 0 Swanson: The Origin of Laterite. Jnl. Amer. Ceramic Soc. 1923) 6 1248.105. W. A. Tar r and W. D. Keller: Dickite in Missouri. Amer. Min. 1936) 21

No. 2.


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106. H. H. Thomas and A, F. Hallimond: Refractory Materials. Mern Brit. Geol.Survey 1920) 16.

107. M. H. Thornberry: Treatise on Missouri Clays. Missouri School of Mines andMet. Bull 2 1925) 8.

108. P. M. Tyler: Clay. U. S. Bur. Mines I. C. 6155 new ed.), 935.

109. A. S. Watts: Mining and Treatment of Feldspar and Kaolin in the Southern Ap-palachian Region. U. S. Bur. Mines Bull 53 1913).110. A. S. Watts and others: ~ i r e c l a ~ s f Maryland. Maryland Geol. Survey 1922)

11.Ill. W. M. Weigel: Georgia and Alabama Clays as Fillers. U. S. Bur. Mines Tech.

aper 343 1925).112. W. M. Weigel: The VVhite Clay Indust ry in the Vicinity of Langley, South

Carolina. U. S. Bur. Mines R 1 2832 1922).113. H. . Wheeler: Clay Deposits of Missouri. Missouri Geol. Survey 1896) 11.114. G. I Whitlatch: Commercial Underclays of Indiana. I n l Amer. Ceramic Soc.

1933) 16 45.115. G. I. Whitlatch: The Clays of West Tennessee. Tennessee Dept. Conserv. Div.

Geol. Bull 49 1940).116. H. Wilson: Ceramics, in Clay Technology. New York, 1927.117. H. Wilson: Clays and Shales of Washington. Univ. Wash. Expt. Sta. Bull 18

1923).118. H. Wilson and J A. Cunliffe: Refdng Pacific Northwest Kaolins by Air Flota-

tion. I n l Amer. Ceramic Soc. 1933) 16 154.119. H. Wilson and G. E Goodspeed: Kaolins and China Clays of the Pacific North-

west, Univ. Wash. Expt. Sta. Bull 76 1934).

clay chapter by ries

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