Crack Pot
Everyone who has done any kind of work with clay has eventually had a pot crack. Often times this happens sooner rather than later in the potter's learning time. Many different factors contribute to the problem of pots cracking. Often it has to do with not enough compression or uneven drying.
Compression happens on the wheel when throwing. It's a good idea to use a rib on the bottom of the pot, before raising the sides, to give the bottom some solid compression. I hold the rib at a low angle to the floor of the pot and give it a firm but gentle push down. Later, when the pot has walls I try to remove any water in the bottom of the pot right away and I compress with the sponge while I'm soaking up the swamp. Water sitting in the bottom of the pot for long will weaken even the best compressed pot.
After the pot is thrown you have to deal with uneven drying issues. I often put my pots on a wire rack to dry, this way the top and the bottom both get plenty of air. However, the pots will still dry out unevenly on the rack so as soon as the rim is dry enough to support the pot I flip it over so the bottoms get maximum air and the top gets less. Sometimes I will even place some plastic around the rims while they are on the ware board to slow drying. Of course trimming a good foot also takes off some of the excess clay from the bottom of the pot and helps it to dry evenly, a little compression while trimming can also help minimize cracking.
If none of these steps work before the pot is bisque fired and I discover a crack I simply toss it into the recycle bin. It's more time consuming to try to fix a cracked pot (and success is not guaranteed) than to recreate the piece.
What do you to to prevent cracking?
Check out the gallery page - Future Relics Gallery by Lori Buff
Compression happens on the wheel when throwing. It's a good idea to use a rib on the bottom of the pot, before raising the sides, to give the bottom some solid compression. I hold the rib at a low angle to the floor of the pot and give it a firm but gentle push down. Later, when the pot has walls I try to remove any water in the bottom of the pot right away and I compress with the sponge while I'm soaking up the swamp. Water sitting in the bottom of the pot for long will weaken even the best compressed pot.
 |
| Pots Drying Upside Down |
If none of these steps work before the pot is bisque fired and I discover a crack I simply toss it into the recycle bin. It's more time consuming to try to fix a cracked pot (and success is not guaranteed) than to recreate the piece.
What do you to to prevent cracking?
Check out the gallery page - Future Relics Gallery by Lori Buff
I do the same, compress and compress again with my fingers, dry most items upside down too. It also helps to have a clay body that seems nearly indestructible!
ReplyDeleteYou're right Gary, a great clay body can really help.
DeleteI dry my pots on a board, often flipping them upside down on their rims with a sheet of plastic on the board to prevent the rims from drying too quickly. Like you, i often wrap the rims with plastic.
ReplyDeleteWe sometimes need to hurry things through and have many times put not so dry pots (or even wet ones!) in the bisque kiln. We will keep the temp at just under 200 degrees for a few hours, then shut it off overnight. The next day we will fire, going very slow at the beginning.
Michèle, isn't it funny how we are alway chasing the perfect level of moisture in a pot. It's so often too wet or too dry and when it's just right we are off at a show or something. Hehehe.
DeleteI cone 3 times and compress well. I live in dry Nevada, so it really helps to have a drying cabinet (mine is a diy wire rack with plastic covering).
ReplyDeleteI have noticed that when I use a wiggle wire cut off, I sometimes get a tiny crack on the bottom middle, but it doesn't show up until after bisque.
Here in the humid south east a drying cabinet would consist of heat lamps and a dehumidifier. Funny how different yet similar these things can be.
DeleteI hand build all of my work and I dry on a wire rack with sheet rock and cover lightly with plastic. I notice if I have two wall boards a piece may not dry so the open shelf helps. If I have a flat piece I may move it gently to several other pieces of wall board to aid in drying the flat bottom. At the college I had a problem with some severe cracks from a draft because they were placed near a door and someone removed my plastic. All of my work is in closets in my home which hopefully prevents my heat or air from drying them too quickly. the closets have louvers facing down. I've never had success with fixing any cracks and it's too time consuming.
ReplyDeleteI agree with Gary I tried working with Frost porcelain and I couldn't avoid cracks. On cone 10 clay on the West coast I used Windsor porcelain which I loved and never had problems with it. I've also used half and half for cone 10 and 6 with little to no problems.
Slow drying and lots of compression are important in handbuilding too as you've discovered Linda. Thanks for the comment, I didn't mean to leave hand-builders out.
DeleteCompress! Compress! Compress! Also keeping walls and bottoms the same thickness. I used to use a clay that had frit in it (Miller#25, which doesn't seem to be available now) that was amazingly tough. I've had to adjust a few things since switching bodies but I'm getting there. If we have a very humid summer I've had to resort to heat lamps. But the best quick drying area is a rack under my wood stove. I don't bother with keeping cracked greenware. If cracks show up in the bisque I use them for tests.
ReplyDelete