Potters & Sculptors - Making Rock from Mud
Thank you guys for your help, I have much to learn and your wisdom is greatly appreciated.
Norm Stuart said:
Joseph - It's true that the the COE of commercial clays tend to increase as the maturity cone declines. This is an artifact of cost control as sodium and potassium are the least costly way to reduce the cone maturity.
Using lithium as a flux, you can make extremely low COE Cone 6 "flameware clay" - far lower than the COE of any Cone 10 clay. Flameware is a clay body which can be heated over the burner on a stove because the difference in expansion between the hot parts of the ware and the cold part is not enough to fracture to pot. Flameware requires an engobe as most glaze can't withstand thermal shock.
Boron, a flux in Cone 6 and Cone 06 glaze, does not increase COE and also helps glaze and clay bodies with a COE mismatch fit properly. Ceramic research papers show that boron allows the glaze to have one structure where it meets the clay body and a different structure further from the clay body. This could be a reason Cone 6 and Cone 06 glazes fit Cone 10 clays with very low COEs.
But this doesn't explain Cone 10 glazes without boron and seeming excessive COE mismatches working out well, which leaves me wondering when does a COE mismatch create problems? It's seemingly more than just the numbers alone. Perhaps glaze strength is the missing factor.
One example is Pete Pinnell's Cone 10 Weathered Bronze Green with a COE of 8.7 which seems a bad match for a Cone 10 clay with a COE of 3.46 like Laguna 373 Dark Brown yet the glaze fits well.
One of Pete's students discovered this glaze is wide-firing enough to also work at Cone 6, though I find it's more reliable at Cone 6 after adding 20% Ferro Frit 3269 (shown below). So it's become a popular Cone 6 glaze as well.