A great question cam up on the Pottery Forum recently about raw glazing (glazing greenware for single-firing as opposed to glazing bisqueware). Can you use a commercial-bought glaze on a bagged clay body if you want to raw glaze?
A: Sure!! I just unloaded a kiln of nothing but raw glazed pieces done with commercial glazes. There are some things to consider, however!
- Glazing on raw ware can change the surface of the glaze. Glazes may come out more matte than if glazing was done on bisqueware.
- I do not recommend raw glazing earthenware or low-fire clays. The glazes can become a barrier too quickly for carbon and other gases to escape the clay body, causing problems such as bubbling glazes and bloated clay bodies.
- Commercial glazes are not specifically formulated for raw glazing, Because of this, glaze defects can more easily occur. I've had best results with brushing commercial glazes on bone-dry ware. Also, instead of the usual 2-3 coats, I tend to see better results with 3-4 coats, depending on the specific glaze.
- Always do a preheat (under 200 degree F) --- I prefer 4-8 hours, depending on humidity, but some potters do well with a 2 hour preheat. The firing ramp should be fairly slow, as well, especially at the beginning.