|Monthly Tech-Tip |
During drying clays and porcelains shrink as they become rigid. When this occurs unevenly, cracks are the result.
Cracks form to relieve stresses. Plenty of stresses occur while ceramic clays dry. This is because they shrink as they transform from soft-plastic to dry-hard. 6-7% is common for plastic throwing bodies. Casting bodies, by contrast, can shrink less than 2%. In the vast majority of cases, drying cracks can be eliminated by understanding simple concepts and making improvements in the drying process. Admittedly, it is often practical to make improvements to the clay body also (although such improvements can other things). Conceptually, any shape, made out of any clay, of any size, can be dried successfully if done so even enough and slow enough ("slow enough" can mean weeks or even months in some circumstances!). That being said, evenness is the more important than drying speed, in fact, fast-yet-even drying results in fewer cracks than slow-but-uneven drying.
When ceramic dries it shrinks. The majority of shrinkage occurs as the first water is released. Unfortunately, that phase is also where gradients within ware can be introduced. A 'gradient' is where one part of an item (e.g. the lip of a vessel) is further along in drying than another part (e.g. the base). That means it is stiffer, possibly having shrunk the full amount to rigidity. If an adjacent section is still soft, it needs to shrink, the only way for the stresses between the two sections to be released is by the formation of cracks.
Since plastic bodies shrink more, it follows they crack more. Plastic clays have many more fine particles, these provide much more surface area between which to trap water. That means better dry strength but more shrinkage. Plastic bodies can have triple the shrinkage of casting bodies, even though the later are formed at a higher water content. The extra dry strength can, on one hand, help prevent cracks from starting. But, on the other, it can create more spectacular ones if they do start! Plastic bodies also dry slower. That enables more time for the stage of drying to differ across a piece.
Different types of clays (e.g. kaolin, ball clay, bentonite, smectite) have different characteristics (e.g. shrinkage rates, shrinkage curves, drying speed, dry strength and ability to withstand the stress, ability to terminate micro-cracks at pores and particles, etc). The most common of drying cracks is called the 'S-crack', it is a classic signal that ware is being dried unevenly. Almost always this is because the rim of the piece was allow to get ahead of the base.
Drying cracks can also occur in grogged bodies. Grog is added to reduce body shrinkage, provide more channels for water to escape and provide points-of-termination for beginning micro-cracks. That being said, the grog can also provide points-of-weakness, especially when ware features abrupt contours, big differences in wall thickness, sharp unfinished corners or rims, rolled edges with splits, etc. Cracks in such clay can start and zig-zag their way in.
This cup is being force dried with a heat gun. The speed of the drying is not the problem. I can dry this mug in minutes as long as I apply the heat evenly to all surfaces. But in this case I have dried the side walls first and the base is far behind. The first surprise is where the crack starts: It is actually the meeting of two, they each start at the outside edge (that is where the stress encounters clay stiff enough to crack). Another surprise is when: Near the end of the process the cracks suddenly grow (these photos span only about a minute). The way it which the two cracks find each other at the center produces the characteristic S-crack.
Half of these Plainsman Polar Ice mugs cracked. But I know exactly why it happened! After throwing them I put them on a slowly rotating wheelhead in front of a fan to stiffen them enough so I could attach the handles quickly. Of course, I forgot them and they got quite stiff on the lip (while the bottom was still wet). I quickly attached the handles and then covered them with cloth and plastic and let them sit for two days to let them even out. Notwithstanding that, that early gradient sealed their destiny. The lesson: At no time in the drying process should any part of a piece be significantly ahead of another part.
Handles expose all sides to the air and dry (and therefore shrink) much more quickly than the walls. Anything you can do to slow them down will produce a more even drying process.
The center portion was covered and so it lagged behind during drying, setting up stresses that caused the disk to crack. This test is such that most pottery clays will exhibit a crack. The severity of the crack becomes a way to compare drying performances. Notice the test also shows soluble salts concentrating around the outer perimeter, they migrated there from the center section because it was not exposed to the air.
It is important that during all stages of drying gradients (sections of different stiffnesses) do not develop in pieces. Thus I like to attach handles as soon after throwing as possible. An unavoidable gradient develops anyway because the rims need to be stiff enough to attach the handles without going out of shape too much. Now how can I stiffen these mugs for trimming and even them out at the same time? The first key is to put them on a plaster bat (as I have done here). Then I cover them with a fabric (arnel fabric works well because it flows). Then I put the whole thing into a large garbage plastic bag folded underneath to seal it. The plaster stiffens the bases and absorbs moisture in the air to stiffen the walls also. The next day every part of the piece is an even leather hard.
Because they dry and stiffen much faster. When the handles are glued on with slip about an hour later they are about the same stiffness as the mugs. The handled mugs, which are sitting on plaster batts, will then be covered with cloth and plastic over night. The next morning, the bases will have stiffened and all sections of the mugs will be about the same stiffness, ready for trimming. After that they will be decorated, then placed on smooth wooden batts and wrapped with a cloth for drying.
The key is avoidance of methods that result in one part of the piece being stiffer at any stage of drying (not vinegar in the water, compressing the bottoms, etc.). Throw mugs with walls of even thickness. Put them on a plaster bat (it dewaters the base). Make the handles a while after you have made the mugs (they stiffen quicker). Apply them as soon as the rims are stiff enough to maintain shape (in my climate, two hours). Use a join method that enables application of lots of pressure (better than scoring). Use only enough slip (of thick cream consistency) to make the join (no excessive squirting out at the perimeter). Pack all the mugs closely on bats, rims up, cover with flowing cloth (e.g. arnel). Put them inside big bags or wrap plastic around and tuck it under. Trim the bases the next day (to the same thickness as the walls). Place rims down (with handles at the center) on smooth batts (not plaster) and cover them with large fabrics that can wrap under leaving no holes exposed to the outside air (in our dry climate two days dries them).
The plastic porcelain has 6% drying shrinkage, the coarse stoneware has 7%. They dried side-by-side. The latter has no cracking, the former has some cracking on all handles or bases (the lower handle is completely separated from the base on this one). Why: The range of particle sizes in the stoneware impart green strength. The particles and pores also terminate micro-cracks.
The center portion was protected while the perimeter dried and shrank first (reshaping the central section). No cracks. But as the central area hardened it reached a point where it was stiff enough to impose forces that forced two cracks to start from the outer edge (opposite each other), these grew inward and found each other. Then the gap widened to dissipate more of the stresses (the width of this gap relates to the drying shrinkage of the clay). But the accelerated pace in the top disk left more stresses, they were relieved by the other hairline cracks from the outer edge, these happened at the very end.The lesson: The stage was set for cracking on both samples very early in the drying process. But the actual cracks occurred very late. Accelerating the process only created small extra edge cracks (on top disk).
Drying cracks are opportunistic. They will initiate at sharp, convex angles. By doing this procedure after trimming you will deny a crack a place to start. Of course even drying is still important. In the pictures on the right, round-ended tools are being used to compress and round the angle at which the handle meets the wall of the mug.
The foot ring on these hard mugs has already been trimmed. At the stiff-leather-hard stage an engobe was applied to the inside. This rewet the bodies of the mugs, almost to the same point as freshly-thrown. But the handles did not get rewetted. To re-dry these mugs to the point of being able to turn them over will take 4-6 more hours. But by that time the handles will be bone dry. To prevent that I waxed them after trimming. That slows their drying down enough to keep them even with the body of the mug. To dry ware successfully the key is to keep all parts of a piece of the same water content throughout the process.
This clay normally dries well, but not this time. Strangely, this crack is not at the handle join, it is penetrating into the mug wall. Actually, this is not a crack, it is a split. Excessive slip around the join was not removed, that is bad when a body has larger particles, they permit water left on the surface to penetrate inward and begin a split. An aggravating factor was that the handle was allowed to dry faster than the mug itself, pulling at this join and opening the split even more.
Drying crack on the inside of a mug at the handle join. Why?
This is an exchange I had on Facebook on this topic. Many people believe cracks are caused by high water content, high shrinkage clay, not compressing the base when throwing, throwing off the hump or stretching the clay excessively when throwing. But I break some or all of these rules every time I make mugs and have almost no cracks. Why? Because the reason pieces crack is unevenness in wall thickness and drying. And I make sure both are even.
Its shape, growth during the firing and penetration of glaze down into the crack demonstrates it preexisted firing (happened during the drying).
These are Plainsman Coffee clay. They, and the handles, were made on the wheel about half an hour ago, then stiffened enough in front of the fan to enable handle attachment. Coffee clay is plastic and will crack if pieces are not dried evenly. But if they are dried evenly, there is no problem. The handles were waxed after they were attached (leaving only a thin section on the inside where some water could escape). This slowed them down, otherwise they would have dried far ahead of the body. They went in the kiln and were ready for glazing the next morning.
Two mugs have dried. The clay on the left shrinks 7.5% on drying, the one on the right only 6%. Yet it consistently cracks less! Not the slightest hairline crack, not even at the handle joins. Why? Green or dry strength. If the dry clay matrix has the strength it can resist cracking even if there are stresses from uneven drying. The clay on the right is made using Kentucky ball clay, which has good plasticity but fairly low drying strength. The clay on the left is a native terra cotta, very plastic and very strong in the green state (likely double or triple the white clay). To demonstrate further: If paper fiber were added to the white clay, it would not crack. Why? Not because it would shrink less with the added fiber, no, the shrinkage would stay the same. Increased strength imparted by the fiber would give it the power to resist cracking.
Examples of various sized grogs from CE Minerals, Christy Minerals, Plainsman Clays. Grogs are added to clays, especially those used for sculpture, hand building and industrial products like brick and pipe (to improve drying properties). The grog reduces the drying shrinkage and individual particles terminate micro-cracks as they develop (larger grogs are more effective at the latter, smaller at the former). Grogs having a narrower range of particle sizes (vs. ones with a wide range of sizes) are often the most effective additions. Grogs having a thermal expansion close to that of the fired body, a low porosity, lighter color and minimal iron contamination are the most sought after (and the most expensive).
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