A Lesson On Shrinkage
Clay shrinks as it dries because the water molecules between the clay particles evaporates and the clay particles move closer together.
The inside diameter of the lids, when freshly made is 2 3/4″. When the lid is bone dry it will measure appx. 2 5/8″ and when it is fired it will shrink again because it achieves a certain molecular density and it will measure about 2 9/16″.
Clay also grows, like a sponge, when it has dried and re-absorbs moisture from recycling; like I’ve been doing, or when it absorbs atmospheric moisture like we had recently. Clay also grows in the firing so if the pot measured 2″ when the firing started, it might measure 2 1/8″ when it’s at maximum temperature and then it may come out of the kiln measuring 1 7/8″! Clay is in constant motion much in the same way concrete is when it expands and contracts due to heat, cold, and moisture.
So, it’s not a great problem that I had 4 lids crack as that’s natural. All potters have a ‘casualty rate’ and I can expect to lose 5% to cracking. However; when I start blowing up pots, that’s a problem, a me problem because I tried to hurry along a firing. I’ll reload the kiln this afternoon and go again!