Glazes
A glaze is a thin glass layer designed to fit the body beneath it. The exam tests the roles of the oxides, application thickness, and the diagnosis of crazing and shivering.
The oxide roles
- Silica (SiO₂) - glass former; durability and low expansion.
- Alumina (Al₂O₃) - stiffens the melt, stops it running off, improves fit.
- Fluxes - lead, borax/boric oxide, soda, potash, calcium, magnesium, zinc; lower the melting point. Soluble fluxes must be fritted.
- Colourants - iron (brown, tenmoku, celadon), copper (green in oxidation, red in reduction), cobalt (strong blue from 0.5 %), manganese, chrome, nickel.
- Opacifiers - tin oxide, zircon.
Fit: the two failures
- Crazing - the glaze contracts more than the body (glaze expansion too high). Fix by adding silica or boric oxide, reducing flux, or raising body expansion.
- Shivering - the glaze contracts less than the body and flakes off at edges. Opposite correction.
Application
Dipping, pouring, spraying, brushing. Keep glaze specific gravity around 1.30–1.45 and check thickness; 0.3–0.5 mm dry is typical. Wipe the foot ring - glaze on the shelf fuses the pot to the kiln.
Worked example
A stoneware mug crazes after two weeks of use. Diagnosis: glaze expansion higher than body expansion. Remedy: add about 5 parts silica and 2 parts alumina, fire a test tile, and confirm with a boiling-water and ink test rather than by eye.
Traps
- Glazing too thick - crawling, running, shivering.
- Firing above or below the glaze cone; fit changes with heat work.
- Sharing buckets and tools between leaded and lead-free glazes.
- Assuming one test tile proves a whole batch.