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This is a white engobe (L3954B) drying on two dark burning cone 6 stoneware leather-hard mugs (Plainsman M390). Those lumps are on the left cannot be screened out, they are agglomerates. That slip has excessive flocculant (powdered Epsom salts are added to gel it so that it stays put on the piece after dipping). About 4 drops of Darvan were added to one gallon of the slurry, this immediately made it smooth and a perfect consistency for application. It remains stable on ware (without runs). Engobes require tight control to have the right viscosity and thixotropy (which can be achieved over a range of specific gravities (about 1.45-1.6). When they are right they are a joy to use, when they are not ware is ruined.
The clay is Plainsman M390. The inside glaze is G2926B cone 6 base transparent. The outside glaze is the same recipe but with 4% added red iron oxide (screened to 80 mesh). A white engobe (L3954B) has been applied inside and outside down to the midway point (done by pour-filling the leather-hard piece, then pouring it out and pressing it lip-down into the engobe). The incised design was done after the piece had stiffened (after the engobe application). The inside clear glaze was poured in and out, then the lip dipped. Wax resist was applied to the top inch inside an up over the lip. A sharp knife was then used to cut away the glaze from the outside of the lip (at a 45 degree angle) and remaining glaze from there down the outside was scraped off with a fettling knife. The mug lip was then dipped into the outside glaze, the mug quickly turn over and pressing down into the same glaze. Finally, any drips of the glaze on the wax were sponged off and the foot ring cleaned off.
L3954B - Cone 6 Engobe (for M340)
Dry and firing shrinkage fitted to Plainsman M390, M340