© 2009 Anthony Foo.
The bane of all ceramic artists - CRACKING! There are many reasons why and when a piece cracks.
1. Crack during the drying stage - greenware
2. Crack after the bisque stage
3. Crack after the high fire (in this case, Cone 10) stage
What to do and how to mitigate the problem. The discussion here pertains only to paperclay, and is based on my personal experiences with the sculpture paperclay from Aardvark Clay Company, Santa Ana. IMCO (from Sacramento) is the manufacturer. I've been using this paperclay since 2004.
1. Crack during drying stage - from leather hard to bone dry greenware.
This is usually due to improper/weak joining, especially when you are doing dry to dry joints. Another reason is unbalanced/uneven drying once the joints are formed. There are 2 kinds of cracks here.
One is a surface crack formed primarily from not using enough moisture in your paperclay slips/slurry. The solution here is to use a thinner paperclay slip (ie more watery slip) and cover/re-coat the cracks. It may take several applications to completely cover the cracks.
The other more serious one is the deep structural crack due to improper technique. I find that the only way to find out if this is a structural crack is to take a knife or similar tool and dig out the crack and see how deep it goes into your piece. If it is not too deep, the good news is that it can be fixed with paperclay. Remove enough of the dry paperclay until you do not see the crack. Make the hole/opening large enough to do a decent repair/patch job. If during this digging out of the crack, your piece/seam falls apart, then you really know you've not done a good joint. Don't worry. Don't panic. Just start over and this time make sure you spend the time making a good, strong joint with good technique. That's one of the advantage of paperclay. You just can't go wrong with it, and even if you did, it's fixable. This advantage alone gives me peace of mind.
Do not be stingy in your "excavation." Spritz the area well or soak the area with a wet paper towel and let stand for some time for the surrounding dry paperclay to soak up this moisture to create a better moisture balance. Use very "mushy" paperclay to fill in the hole you made. Keep the area moist as the surrounding dry paperclay will want to "suck" the moisture from this spot, so keep the water sprayer handy.
2. Crack after the bisque stage.
Sometimes, even with our best care and technique, we still get cracks in our pieces. Sometimes everything looks good in the green stage, but after the bisque fire, you notice the cracks. If it is a hairline crack, it's really tough to do anything about it. If the crack is large/wide enough to patch with paperclay slip, you may try that. I ususally use a very runny paperclay slip as filler. The bisque piece will absorb the water from the paperclay slip very quickly. Several coats may be necessary to fill the crack. It also depends on how bad/serious the crack is. To be sure my patch holds, I usually re-bisque the piece to test my mending. If it survives, it stands a good chance in the high fire; if not, it's better to remake your piece.
3. Crack after high fire.
This happens sometimes as high fire (Cone 10) puts a lot of strain/stress on a piece, especially tall, vertical pieces. As sculptural pieces, there is no reason to take it to Cone 10. A Cone 6 firing will do fine. When you find a crack in your piece after a high fire, it's pretty much "putty/epoxy time." Unless your piece is meant to hold water (for example, an ikebana vase or something similar) I usually don't do much about it. Sometimes the crack adds character to the piece, sometimes it does not. At this point, you can't do much fixing it.
A very appropriate quote from my instructor, Julia Klemek, "Don't fall in love with your piece until it comes out of the kiln (this is Cone 10 fire)."