There is a recipe going around about “No Knead” Bread. A very simple recipe that just takes minutes to prepare. It apparently originally ran in the New York Times, and around here ran in the Minneapolis Tribune on Feb. 15.
In the recipe the bread is baked at 450F. One of our good customers called after trying the bread and said there were craze lines (very fine lines running randomly through the glaze) in the bowl. She wondered both, what happened and was the bowl safe to continue to use.
First answer, yes, the bowl is safe to use. Either from a food safety angle as well as structural.
Why did it craze? This gets a bit more technical. Here is a note I sent to Rick Nelson the Star-Trib’s author:
We’re potters out here in Hutchinson, and had a customer call about using one of our bowls for the bread recipe you published on the 15th. I noted in the recipe that the baking dish needs to be heated to 450 degF. and that ceramic bowls were one of the listed utensils. There is some risk in using ceramic bowls for this as it can cause the glaze to craze after it cools.
To give you the long part of this, normal highfire ceramics can go easily in the oven (not on the stovetop). The reason for this is that most of a clay pot is silica (glass) which expands and contracts on heating and cooling. The glaze on the surface of a pot is also silica, but of a different percent and formula than the clay.
When we fire pots, the goal is to have the shrinkage of the melted glaze to be the same as the shrinkage of the clay after cooling. You can then heat and cool this clay/glaze system any number of times without risk.
The problem is that one of the silica crystals that is formed during firing is cristobalite. This crystal undergoes a very rapid expansion/contraction of about 2.5% at 226 degC (about 425 degF). This expansion can cause the overlying glaze to craze/crackle since glazes typically don’t have the cristobalite form of silica and don’t do their major expansion until over 1000 degF.
It won’t ruin the pot for future use as crazed/crackled glazes are not unusual, but a user will see a change. The solution is to keep the temperature under 425 degF. when using ceramic pots.
Hope this helps.
If you’d like any additional information we’re at 320-587-2599.
I have since had conversation with Paula Wolfert and she said she had run the recipe at 425 degF and it baked up just fine. I’m going to try it at 400 which would be even safer to the pot.
More to come.