CLAY & FIRING DEFECTS
Since most potters use commercially prepared clay bodies, the emphasis here will be on identifying and defining the problem and then on corrective actions relating to the process rather than simply changing clay bodies. When there are problems, it seems the first suspect is always the clay body itself. While this may be the case, more often than not the source is found in the process. Generally, commercial clay bodies are properly formulated by the manufacturer but, as is the nature of our branch of ceramics, the quality control of the naturally occurring raw materials from the mines through the entire processing system is less than ideal. Thus, a standard clay body successfully used for years may change only very slightly and tip the scale in your process from satisfactory to unsatisfactory. Changing clay bodies will usually create more problems than minor changes in the forming, handling, drying, glazing, firing and cooling process. Further, if your glaze fit is so close to being a problem on the present clay body, you can be reasonably sure that the problem will recur on another clay body at some time in the future.
Serious detective work is often required to determine the proper corrective action. The first thing to do is to look critically at the design and shape of the piece to determine whether there is an inherent structural weakness causing or contributing to the problem. Next, identify the problem from the following descriptions and look at your entire process from clay and glaze storage through forming, firing and cooling for any changes. Do not assume that you haven't changed anything. Very subtle changes in your process combined with very slight changes in the clay body may combine to seemingly inexplicably result in problems that never existed before. For example, consider the weather. Did studio temperature or air circulation change and thus did ware dry faster, slower or less evenly? Could wind have affected the gas firing or kiln cooling? Did your fuel source change? Did you start using a new tool? Was there any change in simple things like the foot-rim size, shape or location? Compare new and older pieces carefully. Were peephole plugs or damper changes done any differently? Are the problems more prevalent from one part of the kiln than another? Does the decoration or glaze make a difference? Was the clay more or less wet than usual? Was the glaze mixed before application and of the same consistency as usual? If the problem is in the glaze firing, were all pieces from the same bisque firing? Again, be a critical detective, not assuming that nothing has changed. Finally, contact your supplier(s) of not only the clay body but also of your glazes and glaze materials to determine whether they have had any similar reports from others. Obviously, many problems result from the glaze. Refer to Glaze Defects in that section of the Resource Center.
Warping is caused by uneven shrinkage or mishandling while the piece is still plastic. Uneven shrinkage is caused by uneven drying which can be due to uneven clay thickness around the piece such as the addition of a handle or decoration on one side, drying in a draft or near heat that effects one side more than the other, or simply from uneven exposure to the air such as the piece sitting on a non-ventilating board.
This term refers to bubbles, bulges, swelling or "pimples" protruding from the surface of the clay body after firing, usually after the glaze firing. They are normally hollow inside and are caused by gases produced in the heating process being unable to escape. Broadly, the cause is firing too fast or too hot. The temperature range of 1150 F to 1900F (approximately cone 020 to cone 06) is critical since that is when most of the volatiles burn away. Also important is the final two or three cones of the firing, especially on more vitrified clays such as porcelains since the outer surface will fuse and seal before the heat treats the inner areas and all inner reactions that release gases are complete. Reduction or lack of air flow at the lower cone 016 to cone 09 temperatures, especially in the bisque firing, can also cause bloating by limiting the early oxidation and expulsion of carbon which then combines with oxygen from the iron oxide at the higher temperatures and can not escape. This entrapment of carbon or Black Coring is seen as a dark gray or black coloration of the clay, especially upon breaking the piece and looking inside it. Over-reduction also can produce Black Coring. Bisque firing with gas can contribute to the problem since any inefficiency in fuel burning produces even more carbon in the kiln atmosphere.
This term refers to cracks that go completely through the clay and usually to cracks that result in an entire part of the piece breaking off. Usually this occurs in the glaze firing or even months later. The cause is usually a result of the pieces inability to withstand the expansion and contraction that occurs as the result of what is called the silica inversion or cristobalite and quartz inversions which take place at 439 F and 1063 F. This can occur in firing or cooling through those temperatures. Firing and cooling slowly through those temperatures is an obvious solution. Another cause of this kind of breakage may be a poor glaze fit whereby the glaze shrinkage is so much less than the clay shrinkage and the resulting stress of the glaze on the clay so great that the clay actually breaks (see Glaze Defects-Shivering in Resource Center). Another possibility is that uneven cooling may intensify the stresses on the piece. A classic example of this is a ring crack about one half inch above the bottom, especially on tall bottles or wide bowls, caused by the slower cooling of the bottom because the mass of the kiln shelf holding heat longer. Raising the piece off the shelf by placing it on clay wads or stilts should eliminate this problem. Of course, one must also look at the physical structure of the piece to determine how any inherent structural weakness might contribute to the problem. See Cracking below for additional specifics.
CRACKING IN GENERAL
Cracks are caused by stress, obviously. The fact that the nature of clay is that it shrinks in drying and further expands and contracts with heat means clay must withstand a great deal of stress and the fact that cracking isn't more common is actually quite remarkable. Keep in mind that cracks do not just happen but have a cause. They begin with some defect or extreme stress and grow. It is a structural problem and the crack needs to be investigated carefully to determine its actual cause. Slower, more even drying is always advisable as is avoidance of sudden changes in clay thickness. Cracks usually started where they are widest which gives a clue as to their cause and possible corrections. It is also useful to know when a crack actually occurred. On a glazed piece, if the glaze itself is also cracked with a clean edge, then the crack occurred upon cooling immediately suggesting the possibility of too rapid or uneven cooling. If the glaze edge is rounded and has flowed into the crack, then the crack occurred before that. Most cracking problems start early in the process but don't actually show up until the glaze firing when the final shrinkage is completed, the full cycle of silica inversion is completed and the added stress of the glaze fit is added.
CRACKS AT JOINED PIECES
These are drying cracks caused by insuffient interfacing of the two pieces (inadequate blending or mixing of the two clay pieces where they join) or too great a difference in shrinkage due to differences in clay wetness when joined or differences in thickness of pieces and the resulting difference in shrinkage rates. Besides altering the shape or physical design to minimize the above, better blending of the two pieces, slowing and evening the drying time by covering the entire object with plastic, misting the thinner part to help equalize drying or coating the thicker part with wax resist to slow its drying time will help. The use of a slip of the same clay and the addition of a flocculent such as epsom salt (magnesium sulfate), sodium chloride, an acrylic binder such as APT II or even some added flux (try a few drops of clear glaze) to the joint will also help.
Horizontal or slightly diagonal cracks that follow the forming process are caused by either over-stressing the clay by over-working or stretching it too fast or too unevenly. This can even occur if throwing rings are not removed with sufficient pressure to equalize the compression of the still wet clay. Cracks very close to the bottom may also be caused by over-wetting the bottom (see Bottom S-Cracks below)
Cracks starting from the lip or edge of the piece and directing primarily perpendicularly towards the center of the piece are caused by too rapid/uneven drying whereby the thinner lip dried faster and thus shrunk faster.
These are somewhat 'S' shaped cracks in the bottom of thrown pieces but may be almost straight and are usually caused by inadequate compression (hard ribbing) of the bottom when throwing. Over-wetting of the bottom while throwing by not continually sponging off excess water can also cause this as the excess water displaces the clay, again causing excessive shrinkage. Another less common cause is inadequate wiring off and separating of the piece from the wheel head and/or leaving the piece on the head too long resulting in uneven drying.
FINE, OFTEN CROSSHATCHED SURFACE CRACKS
Fine surface cracks usually occur after drying or bisque firing. They are caused by using too much water in the forming process whereby the outer layer of clay becomes far wetter than the interior clay and thus shrinks more. May also be caused by pulling up soft clay over the more stiff basic form. Minimizing the above and adequately compressing the clay with a rib should eliminate the problem.
These are cracks from the base upward and may be a dunting crack or caused by too thick a glaze layer inside the piece, especially on the bottom inside such as when a glaze runs and puddles excessively.
BOTTOM STAR CRACK
A main crack with others connected at approximately right angles to the larger crack and radiating outwards. Usually caused by overly thick glaze application on the inside bottom (see Vertical Crack above).
CIRCULAR BOTTOM CRACKS
These may be accompanied by one or more perpendicular cracks going outward through the edge of the foot and toward the rim. When inside the foot rim, it is usually caused by differences in bottom and sidewall clay thickness. Overly thick glaze application on the inside bottom may also cause this crack (see Vertical Cracks). On plates, the circular crack is often just outside the foot rim and may be caused by too rapid a bisque firing (see Dunting). Stacking plates rim to rim will assist in uniform cooling and shrinkage to alleviate the problem.
Obviously an extreme "crack" whereby an entire flat, flake-like section will crack off, most often caused by the piece not being dry enough or heated too rapidly the first 400-500 F. If it approximates where clay was added, see Cracks at Joined Pieces above. If it is a small flake and there is a white colored speck in the cavity, it may be from lime in the clay which can be confirmed if it effervesces when hydrochloric acid is added (called Lime Popping).