- THE MAGAZINE
We have all heard the terms “clay body absorption” and “clay body shrinkage.” But what do these terms mean, and how do they affect the quality of our finished pieces?
Clay body absorption is the amount of water that can leach through a fired clay body, regardless of the glaze or coating, while shrinkage is the rate at which the body shrinks during drying and firing. Both of these characteristics are interrelated and are affected by the clay body formulation and by the firing time and temperature. Knowing the absorption and shrinkage rates of your clay can help you create stronger, higher quality pottery.
Compensating for ShrinkageMoist clay begins to shrink as soon as it is taken out of its plastic storage bag and comes into direct contact with the air. Water is drawn out of the clay until it reaches the moisture content of the surrounding atmosphere. Some potters mistakenly believe if they let the clay air dry for months, it will be thoroughly devoid of water; however, mechanical water (moisture in the studio atmosphere) and chemical water (water tied up on a molecular level within the clay) will still remain in the “bone dry” clay. If the clay is heated too quickly, steam will build up within the piece, causing it to crack or explode in the dryer or kiln. Clay also shrinks during the vitrification, or glass phase, process as it reaches higher temperatures in the kiln.
To ensure that water removal and resulting shrinkage occurs slowly, without damaging the ware, the drying and firing processes should be as controlled as possible. Knowing the shrinkage rate of your clay body can help you develop drying and firing cycles that are both efficient and thorough.
Ensuring VitrificationMany potters assume that a glazed surface will “seal” a fired clay body and prevent water from penetrating and eventually moving through it. However, this is not the case.
When a glaze is used on functional pottery, it has two roles—to provide aesthetic appeal and to provide a smooth surface for easy cleaning. The idea that a glaze can keep water from reaching the underlying clay body seems reasonable, since glaze or glass is often used to hold liquids. The fired glaze looks smooth and feels like a solid, impenetrable surface; however, microscopic imperfections that will allow the penetration of liquids can exist in the glazed surface. If the clay body underneath is porous, it can wick or draw moisture to itself. The moisture will eventually move through the cross section of clay and leak onto any exterior surface on which the pottery is placed. The same action can be observed when placing one end of a dry sponge in a bowl of water; in time, the water will move through the whole sponge. The only way to prevent the clay body from absorbing moisture is to ensure that it is fully vitrified or mature.
To know what constitutes a vitrified or non-absorbent clay body, it is important to understand how clay bodies react when heated to increasing temperatures. Mechanical or “free” water is driven off during the initial stages of firing, from 100 to 200 degrees C, and the chemically combined water begins to leave the clay structure in the 450 to 600 degrees C range. As the temperature increases, organic matter is oxidized or driven off, and the clay body begins to sinter or form itself into the first stages of melting.
Vitrification starts when the primary flux in the clay body (frit or feldspar) begins to react with silica, which is generally in the form of flint or is tied up with other materials in the clay body. (For example, many frits and/or feldspars found in clay bodies contain silica.) As the temperature increases, more silica is drawn into the flux, causing increased amounts of glass formation.1 The optimum vitrification temperature depends on the type of clay being fired. For example, most stoneware clay bodies have a maturity range of two or three cones (cone 6 [2232 degrees F] to cone 9 [2300 degrees F]). This means they will be non-absorbent if they are fired within these cones. Porcelain clay bodies, on the other hand, must be fired to cone 9 (2300 degrees F) to achieve full vitrification.
If a clay body does not reach its maturity range during firing, the finished product will be porous and absorbent. Alternatively, if the clay body is fired past its maturity range (or if it contains too much flux), it can warp, stick to the kiln shelf, exhibit a glassy appearance, and eventually slump and deform into a glass puddle.
The glass phase in a clay body is also influenced by the amount of time required to fire the clay to maturity. A clay body that is placed in a kiln and fired to its endpoint temperature in three hours will not be anywhere near as dense, hard and vitreous as the same clay body fired over 12 hours to the same temperature.
The rate at which any given clay body becomes vitreous also depends on several other factors, such as the specific clay body formula, the particle size of the clay body materials (smaller particles tend to create less absorbent bodies), whether the clay is pugged or unpugged (pugged clay can result in slightly lower absorption rates because the clay platelets are physically packed tighter, causing a greater melting action than unpugged clay, which has a looser platelet arrangement in the moist clay structure) and the kiln atmosphere.
There are some applications where an absorbent clay body is desirable. For example, low-fire porous pottery can offer many different aesthetic qualities, ranging from the “soft” painted majolica look to the very bright colors that can sometimes be elusive at higher temperature ranges. However, these pieces are not suitable for use as functional ware since they are prone to leaking and glaze crazing. In some cases, the glaze can look intact after firing but will develop delayed crazing when exposed to moisture in the atmosphere or in the normal everyday cleaning of the pot. Additionally, the permeable clay body provides an ideal medium for growing mold or absorbing stains from food or drink. A porous clay body with a high absorption rate is also not as physically strong as a dense, vitreous clay body with a low absorption rate. As a result, pieces made from absorbent clay can more easily chip or crack under normal use.
Understanding the NumbersBoth the absorption and shrinkage of a fired clay body are expressed in percentages. A direct correlation exists between these two characteristics, both of which are influenced by the firing temperature. For instance, red earthenware pots have average absorption rates of 12 to 14 percent and fired shrinkage rates of 6 to 8 percent when fired to cone 06 (1828 degrees F). Porcelain clay bodies have absorption rates of 0 to 1⁄2 percent and fired shrinkage rates of 14 or 15 percent when fired to cone 9 (2300 degrees F).2
Stoneware clay bodies, which are typically fired between cone 6 (2232 degrees F) and cone 9 (2300 degrees F), fall between the two extremes. Their average fired shrinkage rates are 11 to 13 percent, and their absorption rates are between 1⁄2 to 3 percent. However, if the clay bodies are properly formulated and fired, they can successfully hold liquid and are durable enough for functional use.
Suppliers of moist (premixed) clay publish each clay’s shrinkage and absorption percentages in their catalogs, usually at the rated firing temperature of the particular clay. However, these percentages are typically based on the clay’s performance in the supplier’s test kilns and dryers, and are not completely accurate in determining the exact shrinkage and absorption rate for the clay in your own kiln. The supplier’s kiln might be a different size or might operate under a different firing cycle, which can affect the absorption rate. Additionally, the moisture content of the test bars might have been higher than the moisture content of your clay, which can affect the shrinkage rate.
The test methods, as well as the experience and accuracy of the people performing the tests, can also vary from supplier to supplier, making it difficult to accurately compare the shrinkage and absorption rates of clays from different suppliers.3 For these reasons, it is best to use the published shrinkage and absorption figures simply as rough estimates in comparison with other clays. Add a “fudge factor” of ±1 percent when relating the percentages to your own needs.
You can achieve a more accurate analysis by doing your own testing. One method that can be used to obtain practical information on a clay body’s absorption characteristics is to make several “pinch pots” and place them unglazed throughout your kiln, interspersing them with glazed pottery. Since not all kilns fire evenly, placing several test pots in different sections of the kiln will offer the widest potential range of test results. Once the pots are out of the kiln, fill them with water and let them stand for 24 hours on a non-absorbent surface. If moisture appears under the pots during the 24-hour period, the clay is absorbing water and is leaking. If this happens, try firing the clay to a higher temperature or over a longer time period, or adjusting the clay body formula by adding increased amounts of flux materials, such as feldspar or frit.
To achieve a more exact absorption figure, weigh several fired test bars, then place them in boiling water. After two hours, remove the bars from the water, pat all sides dry of surface water, and weigh the bars on a laboratory scale. The average absorption of all of the bars generally reflects the absorption percentage for the clay.
To determine the dry shrinkage of the clay, mark a 10-cm line on several moist clay test bars, allow the bars to dry, then re-measure the line. Fired shrinkage can be measured by marking a 10-cm line on the moist clay and then re-measuring the line after the clay is fired to maturity. (See “Testing for Success" for more details about shrinkage and absorption testing.)
Knowing Your ClayShrinkage and absorption characteristics are often overlooked when formulating clay bodies or buying premixed clays from a supplier. However, these attributes are important in developing durable, sellable pottery. Pieces can easily crack or explode during drying or firing if these processes are not carefully controlled to compensate for the clay body’s shrinkage. Likewise, if the firing time and temperature are not carefully controlled to ensure maximum vitrification, the resulting clay bodies will have high absorption rates, making them unsuitable for use as functional pottery.
Know your clay’s shrinkage and absorption rates, and perform your own tests on premixed clay to ensure an accurate analysis. With this information in hand, you’ll be able to create high-quality pieces from every batch of clay.
Author's note:Author’s Note: Additional information on absorption test methods can be found in the 1999 Annual Book of ASTM Standards, Volume 15.02, American Society for Testing and Materials (ASTM) Designation C 373–88, ASTM, West Conshohocken, Pa.; and Mastering Cone 6 Glazes, by John Hesselberth & Ron Roy, Glaze Master Press, Brighton, Ontario, 2002, p. 48.
References:1. W.G. Lawrence, Ceramic Science for the Potter, Chilton Book Co., Philadelphia, 1972, p. 115.
2. The Edward Orton Jr. Ceramic Foundation, P.O. Box 270, Westerville, OH, cone chart for a self supporting regular cone fired at 108 degrees F/hr.
3. Daniel Rhodes, Clay & Glazes for the Potter, Third Edition, revised and expanded by Robin Hopper, Krause Publications, p. 331.