"I wonder if re-firing makes the pots more brittle that convention would
I am joining this late
here are my observations:
1. once upon a time long ago, I studied the work of Ron Nagle. His glazed cups were fired significantly many times, one in particular was over 25 times. Yes his clay body was about cone 5 and first fired at about cone 3 and his refiring was more like cone 0. The point: his work was refired many times. Don't know about the brittleness but the cup made it to a gallery show with a large $ tag attached.
2. the "old kind" of kiln shelves are fired many times and do not become more brittle than when new, actually they seem to become less brittle and more crushable; the same for fire bricks; kiln posts seem to last longer than shelves.
3. From my study's of solid materials over the last 15+ years my guess is that the elemental composition of the clay body plus the heat and cooling rates would be major variables. I have seen some items that became very "brittle and weak" after the bisque firing and became stronger after the glaze firing. Glaze movement toward the bottom is highly likely. Layered glazes will begin to mix due to element diffusion differences in the glazes compositions and the "newly created glaze" will have different properties such as thermal expansion and (cooling shrinkage). Glaze running is important possibility. Glaze diffusion into the clay body also occurs.
4. At school, we often refire some cone 10 ware to improve the glaze, especially copper red glazes. I know of no failures due to physical properties of the items -- glaze messes yes! We also get some glaze messes from the first glaze firing when students fail to pay attention to what they are doing!. I do vaguely remember a refired a sculpture having a shape change due to slow bending from weight (creep deformation in engineering jargon).
5. Hank, Mel, and others, are effectively "refiring" when they turn the kiln back on after letting the ware cool to a lower temperature and soaking at that low temperature for hours. Not exactly the same as "refiring" but getting close.
6. Heating and cooling, if done too fast can create local stresses that are above the strength of the material at the point of the stress and something will happen. Same on cooling.
7. At school recent items from a recent cone 10 wood firing were refired to cone 10 reduction in the gas kiln to "improve" the copper red glazes; all were significantly different, some better, some just different, all nice, all interesting, none trashed.
8. If I were really interested in an answer for refiring of my ware in the kiln where my ware is fired, I would make a some of "test" items and place them in the kiln and refire them over and over until I either get bored with the project, or until the test pieces fall apart. Then I would have some data that would be useful to me. If I were to take on such a project, the first question would be what exactly is this question? is it a question about the strength and/or robustness of the fired clay body? or is it about a specific glaze on a specific clay body? or is shape, size, thickness, reduction/oxidation an important concern?
Define the question and identify the assumptions before trying to answering it. (from problem solving 101, class of 1970)