​Why are my pieces warping in the soda kiln?

Ps. I do read your messages, and thought I would answer this readers questions here on my blog so that all of you could learn from it too.
 
Question:
“I would like to hear your thoughts on wadding. For example, how many wads per pot, how large or small the wads should be, what shape they should be, where exactly on the foot should they be placed, etc?”

Typically I use 3 wads for everything. Exception 1: I have a 5-sided piece and I want to echo the pattern by placing a wad underneath each corner. Exception 2: the piece is wide…I then use more wads to support the piece.
 
Pea-sized wads are the size wad I use on almost everything. For heavy pieces they might be more like grape sized wads (mostly so they don’t get squished totally flat under the weight). But it’s also an aesthetic choice…you could use bigger wads or use a cookie cutter to make different shapes…I’m not interested in doing that for my work right now.
 
Wad placement should be under the “foot.” This is more obvious when you have a foot ring. When there is no foot ring think about where there is a change in direction from vertical to horizontal…ie where most of the pots weight would go on a counter top.
 
Question:
“I think wadding ties into my other area of curiosity--warping! I am focusing on making wide and low forms lately, like planters, and I am wondering the best way to wad them. Should they have an inner and outer ring of wadding? Is that overkill, or necessary to inhibit warping? Should I even be firing these wide low forms in a soda kiln if I am trying to avoid warping?”
 
Yes, wadding can contribute to the issue of warping. I recommend placing a wad about every 2-3 inches along the foot ring of really wide forms such as plates and platters. If wads are placed too far apart it can contribute to the problem of the ceramic piece warping. Another thing to consider is when stacking forms, such as plates, wads should be stacked in alignment with wad above wad. Think about loading a kiln. Posts need to line up one above the other to put less strain on the kiln shelves and carry the weight down through the posts to the floor of the kiln. There are many causes for warping that are not related to wad placement.
 
The way you make and dry a ceramic piece is usually the guiltiest culprit for warping and cracking. More compression and even drying in my opinion is the solution to most clay problems. Spend more time with a rib compressing a slab or bottom of a low wide pot. In general, you will have greater success having a shallow curve and foot ring for a bottom of a wide piece than adding a second inner foot ring. Warping could also be related to needing to leave that bottom a little bit thicker. In addition, a piece may have started to warp because of uneven  drying depending on the piece this may not become apparent until the final firing.
 
Another solution is addressing the clay body. Certain clays will warp like crazy when fired slightly above cone 10. Adding a grog could also strengthen the clay and reduce its chance of warping. So you may need to look for a different clay body to use that’s better for low wide forms.
 
All forms are more at risk of failing in a soda kiln over an electric kiln. Don’t be discourage…I fire lots of low wide forms in the soda kiln. To Sum it up it’s a matter of problem solving the combination of these elements: better shape, compression, even drying, clay body, and wad placement.
 
Please keep sharing questions you might have about soda firing. If I can't answer your question I'm sure I can find an artist to interview to answer the question for both of us. Plus it might be a topic in my soda firing educational series just waiting its turn to be release.