|Monthly Tech-Tip |
These mugs were fired at cone 10R. The body is L4168G5, I mixed it myself using 50% Plainsman Saint Rose Red, 40% Plainsman A2, 10% Custer feldspar. The Saint Rose clay contributes the color, the A2 the speckle and plasticity and the feldspar matures the body enough to avoid black coring. The heavy iron specking is being sourced by these very unique clays, both were ground at 42 mesh only. The left glaze is GR10-CW Ravenscrag Talc matte with added Zircopax. The right one has that same glaze on the inside and G2571A bamboo matte on the outside. The unglazed body is a beautiful deep red. These are certainly not porcelain strength but the glazes fit, the mugs are durable and serviceable for normal use. This type of ware is the domain of potters only, no industry would be able or even want to make them.
These are iron reduction test bodies. The L4168G (left) is far stronger yet it has a higher percentage of Fe2O3 and much more black coring. How is that possible? Because it has 10% added feldspar. The black FeO iron is staining the feldspar black (bleeding out from each pyrite particle), helping it do its job of producing a glassy black color is not coming from the kiln atmosphere? Because the buff-burning bodies in the same kiln did not have any of this. On the right, the iron is restricted by its ability to vitrify the body by limited glass development, ending up destabilizing it instead (by increasing body thermal expansion).