In February we opened the kiln to find that roughly two thirds of the load had come out wrong. The oatmeal glaze, which we have mixed the same way for years, had gone thin and patchy over every rim.
The obvious suspects
Our first assumption was the glaze bucket. A glaze that has sat too long separates, and if you do not stir it properly from the bottom you end up dipping into something much thinner than you think.
But we had stirred it. We double-checked the glaze mix afterwards and it was exactly as it should be.
What it actually was
It was the first firing. We had run it slightly hotter than usual because we were sharing the kiln with some thicker pieces that needed longer.
A hotter first firing makes the clay less porous. Less porous clay absorbs less water from the glaze when you dip it, so a much thinner layer of glaze stays on the surface. Same glaze, same dip time, half the thickness.
The variable that ruins a firing is almost never the one you changed on purpose.
The two changes
First, we no longer mix thin and thick work in the same first firing. It costs us an extra firing a month and it is worth it.
Second, we now test-dip a single piece from every load and check the dried glaze thickness with a needle before committing the whole batch. It takes ten minutes and would have saved us thirty pieces.
The failed pieces went into the shard bucket, which is now nearly full. At some point we will crush it and work it back into future clay — nothing here really goes to waste, it just takes a long detour.