A Potter's Notes on Thermal Expansion by PETER SOI--INGEN

LIKE just about everything, ceramic materials expand and contract when heated and cooled. The thermal expansion characteristics of these materials affect potters in various - - a n d sometimes devastating--ways. You know the prob- lems: stoneware pots that develop shivering or crazing, a casserole that cracks in use, a raku piece that can't take the process and winds up being part ceramic and part glue. In working through such problems, it helps to have a practical understanding of the general principles in- volved, and toward that end I have accumulated some notes which, though they are a long way from a definitive treatment, might help open discussion of the subject.

Thermal expansion relates to two important problems: glaze fit and heat shock. In the first, if glaze doesn't "fit" the clay body it is bonded to - - tha t is, if the body and the glaze, after being fired, don't have nearly the same rate of expansion and contraction--the ware will show either crazing or its opposite, shivering and shattering. In craz- ing, the glaze contracts more than the body as the pot cools in the kiln. Remember, the body and the glaze are welded tightly together, and as the glaze hardens on cooling it can no longer stretch or flow to fit the body; both must contract at the same rate as they cool, or some- thing has to give. When the glaze contracts more, that puts the glaze in tension and the body in compression. Ceramic materials can take a lot of compression but not much tension, so what happens is that the glaze, unable to stretch, cracks open and produces a network of lines we call crazing. Shivering and cracking (also called peeling and shattering) are brought about by the opposite set of conditions. As the ware cools in the kiln, the body con- tracts significantly more than the glaze. Here the glaze is in compression, which actually strengthens it; the body is in tension, and though it is much thicker than the glaze, it can give way. Commonly it yields at thin edges, and small slivers of body and glaze flake off---that's shivering. Or the glaze might stretch and crack the whole pot, across the middle of a plate, or in a spiral up the wall of a pi tcher-- that 's called shattering. These symptoms fre- quently appear before the kiln is unloaded, but may remain latent for weeks or longer.

In fact, the previous description somewhat oversimpli- fies what happens as body and glaze cool together in the kiln. These materials actually can flex to some small

28 CERAMICS MONTHLY

degree, and the glaze in particular, like other glasses, will continue to "flow" somewhat after it is cooled to apparent rigidity, which helps it continue adjusting to the body's own rate of contraction. But in practical terms, shivering and crazing result from the inability of the ceramic mate- rials to stretch or flow enough, and might be better under- stood that way.

It should further be noted that the best relationship between body and glaze is often one which puts the glaze in some compression. This increases the mechanical strength of the glaze, in much the same way that rein- forcing rods strengthen concrete. In clay products that

cristobalite

1.5

o

1.0

• quartz

0.5

0 200 400 600 Degrees centigrade

Thermal expansion o[ typical ceramic materials. Since cristobalite's inversion hump is within the range of a kitchen oven (220°-275°C), its expansion in clay can cause ware breakage.

800

must have high mechanical strength, such as electrical porcelain, this compressed glaze coating actually makes the entire object stronger--provided that the glaze sur- rounds the object. Since mechanical stress cracks normally start at the surface of the object, and the surface is com- pressed glaze which is extremely resistant to cracking, the object itself resists cracking. This would obviously not be the effect on a partially glazed piece, such as a tile glazed on one side, or a jar glazed inside only. In those cases, a glaze in compression will more likely put the unglazed surface in tension, rendering it more susceptible to cracking.

There is another glaze fit problem called "delayed crazing," a problem mainly with low-temperature ware, which is brought about by the slow moisture expansion of the fired clay body. In this ca~e not only must the glaze be put in some degree of compression so as to accommo- date the later swelling of the body, but the moisture expansion of the body must be minimized--talc added to the body will usually accomplish both. Though it is a problem mostly in porous clay products, delayed crazing can occur in stoneware and even in porcelain. In these cases it is usually enough to alter fit so that the glaze is more in compression.

The second main thermal expansion problem has to do with the heat shock that certain kinds of ware must sus- tain: raku is a dramatic example, but casseroles and teapots too must be constituted to stand some degree of thermal shock. Glaze fit is not the only concern here, though you definitely don't want the glaze to be appre- ciably in compression on casseroles and teapots-- that in itself could contribute to the cracking of a piece when it is used. The primary concern here--and the only one in raku-- is to have a clay body that, because of low thermal expansion and some other properties, can stand being suddenly and unevenly heated.

The main constituent influencing thermal expansion is silica (SiO._,). Without changing its chemical identi ty-- that is, though it remains silica, it can appear in different forms having very different physical and thermal proper- ties. Silica can appear in various crystalline forms-- the most important for potters are quartz and cristobalite. Quartz is the most common; it is a naturally occurring mineral, and if you buy a bag of silica or potter's flint, it is mined and crushed quartz; mason's silica sand is nor- mally quartz too. Clays contain some free silica--some contain quite a lot of i t - -and that is mostly quartz, usually in very fine particle sizes.

Glaze [it is impor tan t /or ware such as this casserole,/or crazing would be functionally unacceptable on the inside. Yet , i / the glaze is appreciably compressed, it will stress the pot and make it more likely to crack in use.

, " " " • - . * . . . . A ~ ~ 7 i , . ~ . . ¢ ~ ~ - ~ z *~

March 1979 29

Cristobalite appears in a clay body as it is fired. At stoneware temperatures the mineral which started out as kaolinite gradually breaks down into mullite and cris- tobalite. Chemically the change is: 3(A12Oa ' 2SIO2)-- ' 3AlzO3 • 2SiOz + 4SIO2 - - that last compound, the freed silica, being cristobalite.

Also at these high temperatures any quartz in the clay body will begin to change to cristobalite. This change is extremely slow, however, and may be negligible as a source of cristobalite in a stoneware body. Like most ceramic reactions, it is encouraged by smaller particle sizes, higher temperatures, and longer soaking periods.

Silica can also exist as glass, sometimes called fused silica. A glass has no crystal structure at all; its atoms are not held in a regular and repeated arrangement, but are in random relation to each other, as molecules in a liquid are. Silica can be turned into a glass by heating it to its melting point (over 1,700°C), or at a lower tem- perature in combination with a flux; when it cools (under normal conditions) it will not return to a crystalline state, but will remain a glass.

And here's the point: quartz and cristobalite show a very high thermal expansion, while silica in the glassy state has an extremely low one. Furthermore, quartz and cristobalite exhibit sudden increases in expansion (and contraction) as they pass through certain temperatures (inversions), whereas silica glass shows a very steady, even rate of expansion at any temperature. The inversion tem- perature of quartz is 573°C, at which it goes through an abrupt cubical expansion (or contraction) of about 2.4 percent. Cristobalite's inversion temperature is around 220-275°C, and it goes through a volume change of approximately 5.6 percent. These "inversion humps" can be even more troublesome than their overall high expan- sion rates, because they are so abrupt and because, in the case of cristobalite, the sudden expansion takes place well within the range of a kitchen oven and can drastically shorten the life of a casserole (see graph).

One more general point: the silica in a glaze is glassy; in a clay body it is largely crystalline, but it can be fluxed and turned into a glass to some extent. Now to get down to practical cases, we can see how these principles can be applied.

Let's say you want to decrease the thermal expansion of a stoneware body. The first thing to try would be to lower the silica content, either by eliminating added flint if present in the recipe, or by changing to lower-silica clays. (A check of the chemical analysis of various clays shows they vary tremendously in the amount of silica present.) The other main way to decrease the body's thermal ex- pansion is to vitrify its silica by increasing the amount of feldspar. Make sure it is a potash feldspar--better in a clay body for several reasons, as well as contributing a significantly lower thermal expansion than soda feldspar. The spar works mainly on the cristobalite formed in the firing, and that's probably the most effective way to fight high-expansion problems. It should be noted that the above methods of lowering a clay body's expansion can be reversed in order to raise it.

In the case of a casserole body, you sometimes want to get the thermal expansion down as low as possible. There are some supplemental tricks too, less commonly used and

30 CERAMICS MONTHLY

not as pronounced in their effect, which can nevertheless be employed in conjunction with the main methods. One is to decrease the amount of iron oxide, because it en- courages the development of cristobalite: you can lower the iron oxide in the recipe, or change to lower-iron clays. Another is to steer clear of clays that contain significant amounts of montmorillonite, such as bentonite and certain ball clays. This mineral has a lot more silica in it, and therefore contributes a lot more cristobalite during mullite formation.

The previous modifications are intended to change thermal expansion of the body; but there are other ways to prevent cracking due to heat shock. You can make the clay body open and porous, underfired, by using very coarse and refractory clays and eliminating any fluxing agents. You can add granular material such as grog, coarse fireclay, even sand (though, since it is composed of high-expansion quartz, it is not ideal). These methods are essentially the same. You are making the clay body open and grainy, filling it with discontinuities. This in itself has little or no effect on thermal expansion. What it seems to do is to prevent cracks from moving through the body-- the cracks tend to be arrested by the grains and pockets in the material.

Some potters seek out especially low-expansion grog-like materials. In Pioneer Pottery, Michael Cardew recom- mends zircon sand, which unfortunately is not commonly ---or cheaply--available in this country. Calcined kyanite, called "mullite" in the trade, is a low-expansion material available in various mesh sizes (raw kyanite expands quite a bit the first time it's fired and may damage the body). As long as the grog material has a thermal expansion suf- ficiently different from the matrix, it will produce micro- cracks. High-expansion particles, on cooling, contract more than the matrix and crack away from it. At room temperature they exist in "pockets," so to speak, so that on heating they do not contribute their own thermal expansion to the body until those surrounding microcracks are closed by the differential expansion. Low-expansion grog particles contract less than the body matrix on cool- ing, putting the surrounding body in tension, which (if adequate) generates microcracks too. In both cases the microcracks formed may impart a degree of elasticity to the body as well as the ability to inhibit the propagation of large destructive cracks. When there is enough grog present to produce considerable direct contact between grog particles, the thermal expansion of the clay body is affected more directly.

My own teapot and casserole body incorporates most of these ideas:

Stoneware Body (Cone 10) G-200 Feldspar (Potash) . . . . . . . . . . . . . . . . . 10.0 parts A.P. Green Fireclay . . . . . . . . . . . . . . . . . . . . . 90.0 Kentucky Ball Clay (OM 4) . . . . . . . . . . . . . . 30.0 Tennessee Ball Clay (SGP 1) . . . . . . . . . . . . . 50.0 Kyanite (Calcined, 35 mesh) . . . . . . . . . . . . . 9.0 Wollastonite (F-1) . . . . . . . . . . . . . . . . . . . . . . 1.0 Grog (20 mesh) . . . . . . . . . . . . . . . . . . . . . . . . 9.0

129.0 parts

Add: Red Iron Oxide . . . . . . . . . . . . . . . . . . . 0.5 parts

For casseroles and other ware subjected to constant heat stress it is desirable to use a clay body with a thermal expansion as low as possible. This may be accomplished by eliminating [lint in the recipe, using low-silica clays, increasing the amount o[ potash/eldspar and lowering the amount o/iron oxide and bentonite in the body.

I 've used this body for several years, and guaranteed my casseroles to survive--straight from refrigerator to pre- heated oven. The only ones that have come back were two I had re-fired; later I realized that cristobalite forma- tion, like most ceramic reactions, is an ongoing process-- the longer you hold a piece at 1,260°C the more cristoba- lite you get, and by firing those casseroles twice I prob- ably got a lot more cristobalite. Never again!

The F-1 wollastonite in the recipe is something of a mystery to me. Wollastonite is a mineral, C a O ' SiO.,, which has needle-like crystals, and the F-1 grade is so much like a mass of microscopic needles that it "pills" in the bag. I use it because it helps knit the clay body together, like the chopped fiber used by many hand- builders today. I t seems to improve both the plastic strength and the heat shock resistance of my raku and teapot bodies.

A casserole body with the features discussed above is not likely to take salt glaze readily. What seems to con- tribute to a bright, rich, salt glaze are high-silica clays,

added flint, and a smooth, vitreous composition. The body recipe given previously not only reverses this, but includes calcined kyanite and wollastonite, both of which have a powerful "drying" effect on a salt glaze. My personal answer has been to keep casseroles and teapots in standard reduction firings. Other solutions are of course possible: use more covering slips and glazes, or perhaps accept a dry rough surface and work with it aesthetically.

Another way to treat the casserole problem is to adopt the traditional low-fired earthenware pieces that are essentially underfired and porous, and usually include a lot of coarse grog, sand, mica or other granular material. They owe their durability not to a low thermal expansion but to their open, coarse and even slightly flexible physical structure.

Though the expansion (and contraction) of ceramic products during firing can be important, it probably is not a critical concern for most potters. Our kilns normally heat up and cool down slowly enough, and our pieces are usually modest enough in size, so that even the inversions

March 1979 31

of quartz and cristobalite are not destructive to the ware - -no t at the moment the inversions take place. Cool- ing can be extremely important in putting the glaze in compression, and that can cause the ware to fail. But it i3 highly unlikely that the volumetric changes accompany- ing quartz and cristobalite inversions will, of themselves, cause even fair-sized sculptural pieces to crack from thermal stress.

Raku, of course, is an exception. The raku process puts pretty stiff demands on a body, especially in larger pieces and delicate shapes. There are four ways to get a greater resistance to heat shock in a raku body: (1) use low-silica clays; (2) make sure they are underfired and porous; (3) use lots of granular material; and (4) use special low- expansion materials where available. Many bodies get by on numbers 2 and 3. Often potters manage with sand and other high-expansion materials, just because of the graini- ness and porosity of the body. But if you want a raku body that will not limit what can be made with i t - - th ink about the thermal behavior of your ingredients. Sand for in- stance: fireclay grog has a much lower thermal expansion, and raw kyanite much lower still. The clays are vital: why use a siliceous clay like Jordan when a low-silica clay can take heat shock better? Of course, there is little or no chance for cristobalite to develop in a typical raku pro- cedure, but quar tz - -wi th its overall high expansion and its inversion--is very likely to affect your work's chance for survival. Lots of raku potters seem to be using talc, and getting away with it, but at raku temperatures it has exactly the opposite effect from what you want. Talc in a low-temperature clay body will harden and strengthen the body, but gives it a high thermal expansion. Our raku body at the Memphis Academy of Arts is:

Raku Clay Body

A.P. Green Fireclay . . . . . . . . . . . . . . . . . . . . . . . 90 parts Tennessee Ball Clay (SGP I) . . . . . . . . . . . . . . . 15 Kyani te (35 mesh) . . . . . . . . . . . . . . . . . . . . . . . 15 Wollastonite (F-I ) . . . . . . . . . . . . . . . . . . . . . . . . 5 G r o g . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

132 parts

When potters talk about the thermal expansion of ceramic materials, the subject of flameproof lithium bodies inevitably comes up. These are high-fired ceramic bodies that are dense and have a very low thermal expansion because of the combination of lithium minerals and kaoli- nite. It is even possible to make a ceramic material that has a zero or even a negative thermal expansion with this combination. Such remarkable ceramic bodies might be composed of 50/50 spodumene and kaolin, and be fired to Cone 14.

But several peculiarities of spodumene must be noted. Firstly, a small amount, up to 10 percent or so, will raise the thermal expansion of the body, because the lithium mineral promotes the development of cristobalite. You have to go above 20 percent to begin to get a lowered expansion. Secondly, beta spodumene should be used, which has been calcined to Cone 11. Raw spodumene swells when fired and makes a very porous body. Thirdly, minerals other than spodumene and kaolin in your compo- sition will change the results enormously. Using less pure clay, or adding fluxes or colorants, can have wildly unpre-

32 CEP.AMmS MONTHLY

dictable results. And finally, temperature is cr i t ical-- bodies that are excellent at Cone 13 might be unusable a cone higher or lower.

All this makes lithium ceramic bodies sound like some- thing for the aerospace engineer rather than the potter. They probably are, especially when you consider the cook- ing properties of clay--excellent in the oven, poor on top of the stove, where you want the fast heat conductivity of iron or copper. If you've ever tried to fry an egg in a ceramic skillet, you know what I mean.

Talc in a clay body deserves some attention all to itself, which may help clear up some points raised earlier. It 's a widely used ingredient, and rightfully so, for it does some important things very well.

In earthenware and whiteware (up to stoneware tem- peratures) talc in combination with clay produces ensta- tite, a mineral with high thermal expansion. This helps prevent crazing, which may be a problem at lower tem- peratures. Since at these temperatures talc makes the ware tighter and stronger, it is the most commonly used non- clay mineral in low-fire bodies.

Talc is also used in the manufacture of cordierite bodies. Cordierite is a low-expansion mineral formed in talc-clay mixtures when fired to around Cone 13. Cordie- rite bodies are very special products, used in items such as electrical insulators. Unfortunately, they have an extremely narrow firing range, and are therefore not practical for potters. It also seems impractical to add a little talc to a stoneware body in hopes of getting just a little cordierite. You are more likely to get enstatite as well as more cristobalite, and a shortened firing range in the bargain.

The main thing to remember about talc, then, is that, except in cordierite bodies, it tends to raise the rate of thermal expansion. It is therefore working against you in a raku body, and probably in a Cone I0 teapot body as well.

In some respects, understanding the thermal expansion of a glaze is much simpler, especially assuming a typical bright glaze in which all the oxides are combined as a glass. In such a glaze, each oxide contributes its own coefficient of expansion, in proportion to the amount of that oxide present. Since all the constituent oxides have been completely melted and are now dissolved in a glass, there are no crystals to worry about, no newly formed minerals with their own thermal expansion rates to take into account. In a clay body, as we have seen, the original ingredients change to new crystalline forms when they are heated, new minerals and glasses appear with increas- ing temperature and time, so that it is possible to add an ingredient to the recipe and get a totally unexpected effect on thermal expansion, or opposite effects at different temperatures. Talc, for instance, in a clay body can pro- duce varying amounts of enstatite, or cordierite, or melt, depending on temperature, time, and other ingredients present. When you add talc to a glaze, on the other hand, no such complications ensue. You are adding known amounts of M g O and SiO.,, and each has a known coeffi- cient of expansion, plus a calculable influence on the thermal expansion of the glaze.

Once all that is stated, it's time to back up a bit, and admit to some complications. Mat t glazes, or any other

glazes which contain crystal development, will of course

be affected by those crystals. At the interface of body and

glaze there is a zone, especially well developed in high-

temperature ware, which is its own combination of crystal

and glaze, and thin glaze coats especially can be affected

by this. (My own favorite matt glaze will, on the wrong

clay body, shiver where it's thin and craze where it's

thick.) Furthermore, when you substitute oxides in a

glaze to alter its thermal expansion, you will also change

its other characteristics--brightness, color, viscosity, and

even maturing point. And finally, though I said that the

coefficients of expansion were known, they are actually

not scientific "facts," but only useful average figures

reflecting how the oxides have acted before, in some

glazes, but not in all possible glazes. Not only have re-

searchers gotten somewhat different results, but different

textbooks record these results differently. Yet that presents

no practical difficulty, since with only one exception

(PbO) the ranking of the oxides, from lowest expansion

to highest, remains the same from researcher to re-

searcher. The most recent findings: B203--0.29 (x 10-7),

MgO--0.30, SIO2--0.37, A1._,O:~--0.61, PbO--0.80, Z n O - -

1.00, BaO--1.30, CaO--1.50, K.,O--3.30, Na20--3.90.

Lithium is inconsistent.

Clearly, if you want to cure a crazing problem with a

glaze you have, you could lower its thermal expansion by

cutting back on the soda and potash, and increasing things

like silica and magnesia.

Just as clearly, such substitutions may change your glaze

in other respects, rendering its color or transparency or

matraring temperature so different that it is no longer the

glaze you want; and in that case you've gained nothing.

That's why the most common method of dealing with

glaze fit problems is to modify the clay body rather than

the glaze. Here are some rules of thumb: To correct

crazing on a stoneware body, raise the body's silica and

lower its feldspar content; to correct shivering, do the

opposite--from 2 to 5 percent may do it. In the case of

a low-temperature body, add talc to cure crazing; decrease

it if you have shivering. The effect on glaze fit will be

pronounced, whereas the color, density, strength and

firing shrinkage should not change excessively. About the

only drawback is that the amount of talc needed for glaze

fit at Cone 05 may appreciably affect the plasticity of

the body.

About the author Peter Sohngen studied at Mills Col-

lege, Oakland, California; and Alfred University, New

York. Since 1969 he has taught at the Memphis Academy

of Arts, Tennessee.

In extreme cases, shattering may result if the body

contracts more than the glaze on cooling. Interior glazes under compression are more likely to

crack the pot when boiling water is poured inside.

March 1979 33