I just unloaded two kilns of mostly thrown functional work. I threw about 50 pieces and lost about 7 to s-cracks in the floor before firing. I suppose this means I have about a 14% loss rate for my thrown work. Mostly I think the cracks came from thick floors or floors that dried too slowly while I was away in Michigan last week.
|thrown work, fresh out of the kiln|
S-cracks are probably my biggest problem with thrown work. I tend to get more S-cracks at school, but I've generally attributed this to forgetting to finish my work when I am helping students with theirs.
I've never devoted a great deal of energy to my own thrown work, so I have also never devoted much energy to eliminating problems like s-cracks. From what I have read, they show up for a whole host of reasons, most of which are easy to fix.
In most simple thrown forms, floors cracks because:
- floors are too thick, when compared to walls
- floors aren't compressed well enough
- water sits too long in the floor during throwing or drying
- floors are still attached to the bat during drying (especially for plates)
- floors dry more slowly than walls
|most of my cracked work from this batch|
In more complex forms, ones with attachments, carving or modeling, cracks can also happen because:
- wet clay is attached to drier clay
- different parts of the clay dry at different rates
- the walls of the pot were stressed (by pressing, paddling, carving too deeply, etc) while the piece was being made or drying
|more thrown work out of the kiln|
Cracks caused by any of these mistakes can also show up during the firing process as well. Lucky for me, all my work seems to have made it through the bisque firing without more cracks showing up. Now I just have to decide what to bring with me to the workshop in Montana.
|a lady slipper I saw near the lake in Michigan|