Avoiding Problems in Ceramic Firing: Tips for High and Glaze Firings
Here are some simple tips to help you avoid problems in your firings:
In high firings (glaze firings), pieces must not touch each other or the kiln walls. Ideally, they should have a small gap between them (for example, about a finger’s width). That way, if there’s a bubble, a run or a drip, it won’t affect the piece next to it.
(Important: remember what we said in the post about bisque firing. In bisque firings, the pieces can touch each other, so you can maximise how many pieces you fit into the kiln.)
The base of your pieces must be completely free of glaze. Make sure the feet are well cleaned so they don’t stick.
If they still tend to stick, you can try a couple of tricks:
1) The first is to “paint” the kiln shelves with a mixture of kaolin, alumina (in equal parts) and water. Once you’ve mixed it, you can brush it onto your kiln shelves. This coating usually lasts a long time. Over the weeks you’ll see it start to flake off or stick in small bits to the bottoms of your pots, but these chips come off easily and it’s not a problem.
2) A second option is to sprinkle a thin layer of silica sand on the shelves. (Careful: make sure the sand doesn’t accumulate in the corners or fall onto other pieces, as it can affect their glaze.)
Place your work on flat, stable shelves. This will help prevent pieces from tipping over or warping at high temperatures.
Some clays deform more than others. The most extreme cases can often be improved by adding grog or sand to the clay body.
Uneven wall thickness within the same piece can cause cracking. You can absolutely make pieces with thinner or thicker walls, but ideally you avoid very big changes in thickness.
Ceramics has a “memory”. When you’re making a piece, the clay particles tend to align in a certain way. During firing, as the clay starts to shrink, the piece can deform, tending to return to the internal structure it “learned” while you were forming it.
We hope you enjoyed this post. Thanks for reading :)