I enjoy using slab construction to form trays, but I have problems with serious warping during the cone 6 glaze firing. In my most recent tray disaster I used KPS 222 clay, a brown specked stoneware rated cone 4-8. My slab was just under 3/8" thick when the 17" long tray was finished. I tried not to over handle the slab, forming it in a wooden slump mold. When it was leather hard, I placed 4 decorative feet under it, taking care to space the feet evenly. Then I added folded slab handles on each end, propping them until the clay dried completely.
The piece was bisque fired to cone 05. I used a RIO wash on the textured handles, the rim, and the underside, including the feet. The interior surface was glazed with a cone 6 commercial product from Coyote.
The cone 6 glaze firing left the tray body sagging; the handles really sag --one started to pull off the pot. The clay body is fully vitrified and it is actually quite beautiful where I used the RIO. The glazed area has some bloating.
I figure it could be any or all of the following:
- the RIO acted as a flux, especially in combination with the specks of manganese in the clay body
- the piece should have been bisque fired to cone 04 instead of 05
- the clay is more suitable in slab form to a cone 4
- the piece should have been propped under the handles during the glaze firing
I know I could switch to a lower fired clay and glaze, but I want to know if it is possible to make this project using mid range clay and glazes. What do other slab workers think?
Thanks,
Fern