|Monthly Tech-Tip |
The body is Plainsman L215. Both were thinly applied and fired using the 04DSDH schedule. The glaze has 2% iron oxide added and sieved to 80 mesh, this reddens the color and acts as a fining agent to reduce micro-bubble population. The one fired to cone 03 (left) is considerably stronger, better surviving the stress of successive impacts with a hammer. However, it has minute dimples in the surface, likely because it is having to clear bubbles originating from decomposition occurring in the body below (as it fires well above bisque temperature). The mug on the right fired about 40F cooler, between cone 04 and 05, only slightly above bisque. The glaze surface is much better, almost crystal clear. Since the glaze fits well the mug has surprising strength, better even than a stoneware piece with a poorly fitted glaze. This one survived about ten whacks before a piece broke out! A big advantage of cone 04 and cooler is that ware can be fired on stilts, meaning you can glaze the whole thing, leaving no bare clay exposed.
These terra cotta clays were bisque fired at cone 04 and glaze fired to 04 using the 04DSDH schedule. The glaze is G1916Q, an expansion-adjustable cone 04 clear. That schedule alone is often enough to get transparent, defect free glazes in many situations. But not in this case. The solution was to add a fining agent. In this case we added 2% red iron oxide (to the top glaze). The particles of iron floating in the melt acted as a congregating points for bubbles, helping them to escape. And we got a bonus: a more interesting aesthetic. A 1% addition also worked, but not as well (we have settled on 2% iron and screening the glazes to 100 mesh). Screening out the larger particles slightly degraded the fining performance (so we have to accept the tiny specks). Iron does not always workin other situations. Other fining agents we have used at cone 6 do not work in this situation (e.g. 2% Zircopax, Alumina). Of course, this glaze will fire amber on a white body.
G1916Q - Low Fire Highly-Expansion-Adjustable Transparent
An expansion-adjustable cone 04 transparent glaze made using three common Ferro frits (low and high expansion), it produces an easy-to-use slurry.