|Monthly Tech-Tip |
A recommended flameware recipe from a respected website (equal parts of 35 mesh grog, talc and ball clay). It looks good on paper but mix it up for a surprise! The texture is ridiculously coarse. Recipes like this often employ fire clays and ball clays, but these have high quartz contents. In flame test like this a ball clay vessel could easily fail in 5 seconds. But this one is surviving still at the 90-second mark. Or is it? While porcelain pieces fail with a spectacular pop of flying shards, these open-porous bodies fail quietly (note the crack coming up to the rim from the flame). There was a potential to create cordierite crystals (the reason for the talc), but what potter can fire to cone 14 to make that happen? But the porosity of 12.5% would be difficult to deal with. On the positive side, you could likely continue using this vessel despite the crack, the coarse texture would make the crack seem like a minor inconvenience. The previous being said, heating here is asymmetrical, creating exponential stresses from hot to cold face. Ceramic can absorb differentials if the heat source were more evenly distributed radially from the bottom distributing centre-outward symmetrically.
Thermal Shock Failure
A simple test any potter can do by making and firing square tiles and using a plumbing torch to see how long before they fracture.
When sudden changes in temperature cause dimensional changes ceramics often fail because of their brittle nature. Yet some ceramics are highly resistant.
Flameware is ceramic that can withstand sudden temperature changes without cracking. The low thermal expansion of true flameware makes craze-free glazes very difficult.
Grog is a term used in ceramics to describe crushed brick (or other fired ceramic) aggregate that is added to sculpture and structural clays to improve drying properties.