Monthly Tech-Tip | No tracking! No ads! |
The body is Plainsman L215 and the glaze is G1916Q. Both were thinly applied and fired using the 04DSDH schedule. The glaze has 2% iron oxide added and sieved to 80 mesh. The iron reddens the color and its particles act 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 surface dimples, likely from decomposition beginning in the body. The mug on the right fired to cone 04, only slightly above the 05 bisque. The glaze surface is much better, almost crystal clear. A big advantage of cone 04 and cooler is that ware can be fired on stilts (enabling glazing the bottoms).
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.
The same mug fired at increasing temperature declouds the glaze and darkens the color of the terra cotta body. Two coats of the G1916QL1 transparent glaze, prepared as a brushing glaze, were initially applied to this Plainsman L215 terra cotta. The extra cone, and the 60F degrees that go with it, greatly helped to clear the bubble clouding by cone 05 (center). But 60F more degrees, to cone 04, both darkened the terra cotta clay and further cleared the bubbles.
Recipes |
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. |
---|---|
Glossary |
Fining Agent
|
Buy me a coffee and we can talk