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At one time or another, all potters have been faced with a defect in the forming, drying and/or firing stage of their process. Some defects are obviously clay body-related, while others are unmistakably glaze-related. Still other defects are caused by the interaction between the clay body and glaze. Clay and glaze defects can also be caused by any number of other miscalculations, including choosing the wrong clay body or glaze formula for the firing temperature; inferior clay forming techniques; or a general lack of knowledge on how ceramic materials react in the forming, drying and firing stages. Correctly diagnosing the cause of any defect is essential to correcting the problem.
Glaze CrazingOne of the most common defects is glaze crazing, which appears as a fine network of lines in a fired glaze. When the clay body and glaze reach the right temperature in the kiln, everything fits perfectly, and the glaze achieves a “honey-like” viscosity on the rigid underlying clay body. However, once the ware begins to cool, the glaze sometimes contracts more than the clay body and is placed under tension, causing crazing. Some glazes craze while cooling in the kiln or immediately after the kiln is opened, and these primary craze lines can be joined by delayed crazing or secondary crazing that occurs days or even months later. In other cases, a glaze might look intact after firing and then craze when it comes in contact with moisture.
While not technically a clay body defect, crazing can often be corrected by adjusting the clay body or glaze—or both. Glazes are most stable when they are under slightly compressive loads. Adding a low expansion material, such as flint, to the glaze will bring the glaze fit into a slight compression. Adding 5 to 10% flint to the glaze can correct crazing if the craze lines are farther than a 1⁄2 in. apart. However, if the craze lines are closer together, high expansion glaze materials such as feldspar and/or frit will have to be reduced in the glaze formula. Another potential solution is to substitute low expansion oxides (such as zinc, magnesium or barium) for the relatively high expansion oxides in the glaze formula.
If all or most of the glazes are crazing, the solution might be to add 5 to 10% flint to the clay body. The flint will remain a crystalline solid within the clay body rather than converting to glass during firing, producing a more compatible clay/glaze fit. Other glaze crazing defects can be traced to an under- or over-fired clay body—often caused by firing the kiln too quickly through some parts of the firing cycle. Ceramic materials need time to reach a given temperature, as well as their absolute end point temperatures, in order to fully vitrify and mature. Adjusting the firing cycle should correct this problem.
Glaze ShiveringGlaze shivering, which is also caused by a poor fit between the glaze and clay body, is most likely to be observed on the edges or ridges of ceramic forms. This defect occurs when the fired glaze is under too much compression and begins to buckle or flake off in sheets, exposing the underlying clay body. Some ceramic pieces might look stable after firing but exhibit shivering later on. If one or two pieces in a kiln load show this defect, it is a good practice to tap the remaining pieces lightly with a metal object (such as a screwdriver) to see if shivering can be induced.
Statistically, shivering glaze/clay body mismatches are rare compared with the many possible glaze and clay body combinations that are compatible. However, when this defect does occur, adjusting the glaze with a high expansion material, such as frit or feldspar, will typically allow the glaze to shrink to fit the clay body. Additions of as little as 5% feldspar or frit in the glaze can often resolve the problem.
Shivering in a number of different glazes probably indicates a problem with the clay body. In this case, adding 5 to 10% feldspar to the clay body—or switching to another clay body in the same fired color and temperature range—should correct the situation.
Bloating and Black CoringFurther complicating the diagnosis of many clay body defects is the fact that they often have more than one possible cause. For example, a clay body that exhibits bloating (a series of bubbles or voids that appear after firing) might have been over-fired, causing the lower melting materials in the clay to over-flux. Alternatively, the clay body might have been mixed incorrectly, causing the clay to flux past its point of maturity.
Gases trapped in the clay during its vitrification phase can also cause bloating. To overcome this type of defect, a complete oxidation atmosphere in the kiln must be achieved during the 662 to 1292 degrees F range to remove organic material from the clay.
Black coring is another common clay body problem with a number of potential causes. The problem occurs when organic material in an iron-bearing clay turns into a gas at higher temperatures, and this gas reacts with the iron, causing silica to flux in the clay body. The fluxing action stops oxygen from penetrating the clay body during firing, preventing carbon from being removed during the early stages of the firing process. Since the carbon cannot escape the vitrified clay body, it creates a black core in the ware.1
A clay body that has not been properly bisque fired to remove organic material can exhibit black coring. This problem can also occur during the first firing if the temperature increase is too fast in the early part of the firing cycle (400 to 1100 degrees F), preventing the organic material from being burned out of the clay. A densely packed bisque firing that does not allow the circulation of oxygen can also trap unburned organic material in the clay.
The root cause of bloating or black coring can typically be determined by asking a series of questions about the process. Was the kiln fired to the correct temperature? Did the bloating problem start with a new batch of clay (possibly a clay body mixing error)? Did the placement of the ware inside the kiln allow the right amount of air circulation? Was the kiln fired in a complete oxidation atmosphere in the early stages to ensure that any organic material in the clay was completely burned out?
Many potters fire their bisque kiln the same way every time and get good results. However, if a batch of clay is delivered with a higher-than-normal organic content, the standard bisque firing cycle might not completely volatize the organic materials. Additionally, wide-based ceramic forms, such as plates and tiles, can hold carbon during firing if they are not properly placed in the bisque kiln to allow for complete air circulation around the forms. Often a bisque firing cycle that has worked well for functional pottery will not work when firing sculpture, because the larger surface area in contact with the kiln shelves prevents oxygen from reaching the clay. Thicker cross sections of clay also need more time in the bisque firing to allow the carbon to be completely volatilized.
ScummingScumming occurs when soluble salts are released from clay(s) in a clay body formula. It appears as a white crystalline powder on bone-dry and/or fired clay, and most frequently affects red low-temperature, high-iron-content earthenware clay bodies, as well as common red building bricks. Soluble salts can discolor an unglazed fired clay area and can also cause defects on a fired glazed surface by interrupting or over-fluxing the clay/glaze boundary layer.
Adding 1 to 2% barium carbonate during the clay mixing process can neutralize the soluble salts in the clay body. Alternatively, using a clay body additive with a barium component, such as Additive-A2 (a modified calcium lignosulfonate), can add green strength and plasticity to the moist clay while also neutralizing the soluble salts.
Lime PopLimestone in the form of calcium hydroxide particles exists in some clays, and if these particles are large enough they can cause a clay body defect known as “lime pop.” Lime pop occurs when moisture expands the lime (calcium hydroxide) particle, producing a half moon-shaped crack in the fired clay body. If the crack is pried up, the black or white limestone particle can sometimes be seen at the bottom of the conical-shaped hole. If the same amount of limestone were dispersed as a finer powder, the smaller particle size would not cause a problem in the clay body. It is the size of the particles, and not the presence of limestone, that is the problem.
Lime pop can be eliminated by running the contaminated clay through a sieve to remove the large limestone particles; however, this can be an expensive, labor-intensive process. Potters who buy moist clay from a ceramic supplier should ask for a replacement batch of clay if they receive a contaminated batch.
Cooling CracksA unique type of cooling crack sometimes occurs when the glaze and clay body shrink at incompatible rates during cooling. If the glaze puts the clay body under extreme compression, the clay body can be torn apart by a long, sharp, spiral-shaped crack after it has cooled. In some instances, this defect doesn’t emerge until years later.
Glazing only one side of a pot (either the inside or outside) with an incompatible glaze can cause this type of cooling crack. However, this problem is rare—most clay bodies and glazes are compatible, and a glaze in a stable, compatible configuration can successfully be used on only one surface of the pot.
The majority of cooling cracks are caused by the presence of too much free silica (silica not tied up with clays, feldspars, talc or other clay body materials) in the clay body. Many stoneware and high-temperature clay bodies are made of low-quality fireclays that often contain high levels of free silica, typically in the form of fine sand. Since irregular batches of fireclay are random, it is often difficult to determine whether adjusting the clay body will solve the problem. In many instances, the next batch or another type of fireclay will not contain the same high levels of free silica. Potters often blame a given type of fireclay for causing the problem; however, if millions of pounds of all types of fireclays were tracked, they would probably have almost equal rates of failure.
One way to correct this type of cooling crack is to add more flux—commonly in the form of feldspar—to the clay body. The feldspar binds with the free silica, resulting in less contraction in the clay body. Often as little as a 5% addition will bring about a stable clay body and glaze configuration. Changing the clay body or glaze can also correct the problem because it considerably lessens the chance for glaze/body incompatibility.
Firing CracksA fast bisque firing can result in clay body cracks and, in some cases, can even cause the clay body to explode, damaging other ware inside the kiln. Clay contains mechanical and chemical water, both of which have to be driven off slowly. The removal of mechanical or free water is accomplished from 212 to 392 degrees F, followed by the removal of chemically combined water, which takes place from 842 to 1112 degrees F. While other changes are taking place in the clay during the bisque and subsequent glaze firings, a fast heat increase in the early stages of the firing cycle can cause cracking and exploding pottery.
The general rule for bisque firing is that slow firing is better than fast firing. When in doubt, a slow bisque firing can do no harm to the ware. For most functional pottery that is dry to the touch when placed in the bisque kiln (such pottery still contains mechanical and chemical water), a 12-hour total firing time is recommended. Longer bisque firing times are required if the pots are thicker than 1⁄2 in. or taller than 14 in. A longer firing time is also required for bisque firing plates, tiles or wide-based forms.
The Value of InformationThe vast range of clay body defects can be attributed to a number of different causes. Unbalanced clay body formulas, forming irregularities, firing deviations and raw material variables such as chemical composition and particle size can all affect the quality of the finished ware. For potters who don’t have a great deal of experience with such defects, it can be difficult to correctly identify the cause of the problem, much less figure out how to solve it.
The only real defense potters have against the variable nature of ceramic materials is knowledge. It is important to understand the individual characteristics of each raw material and how the materials react with each other in various firing conditions. It is also a good idea to assemble a collection of different faults as a visual guideline for correcting future defects. Never take anything for granted when working with ceramic materials. Keeping accurate records of clay deliveries, clay body formulas and firing logs can help potters more easily reconstruct a clay body failure. Likewise, maintaining careful records of firing times and kiln loads can help focus the investigation on the problem’s origin and its eventual resolution. In addition to keeping detailed records of their own operations, potters should also avail themselves of the many ceramics books that can be used as reference guides in determining what course of action will resolve a specific defect.
Ultimately, a varied and extensive ceramics education will be the best tool in identifying and correcting clay body defects.
References:1. W.G. Lawrence, Ceramic Science for the Potter, Chilton Book Co., New York, 1972, pp. 122, 126.
2. Additive A is produced by LignoTech USA, 721 Route 202/206, Bridgewater, NJ 08807, (908) 429-6660.
Additional Resources:• Frank Hamer, The Potter’s Dictionary of Materials and Techniques, Watson-Guptill Publications, New York, N.Y., 1975.
• Daniel Rhodes and Robin Hopper, Clay and Glazes for the Potter, Third Edition, Krause Publications, Iola, Wis., 2000.