glaze

A glaze is a ceramic coating applied to unfired clay that fuses into a hard, glassy layer during firing. It serves two primary functions: it makes pottery watertight by sealing the porous clay body, and it provides color and surface finish. Glazes are mixtures of silica, fluxes, and other materials suspended in water as a slip, either applied by dipping, spraying, or brushing onto leather-hard or bone-dry clay.

The chemistry hinges on silica (SiO2) as the glass-former, combined with fluxes that lower the melting temperature to a practical range. Common fluxes include feldspar, lead oxide, alkali metals, and calcium. At kiln temperature, typically 1000 to 1300 degrees Celsius depending on the clay body and glaze type, the silica melts and flows across the surface, bonding chemically to the clay beneath. As it cools, it hardens into a vitreous (glass-like) layer, usually 0.5 to 2 millimeters thick.

Raw glazes contain all their components as dry materials and mature during a single firing. Fritted glazes use pre-fused glass (frit) as part of their composition, which reduces solubility issues and improves consistency. Lead glazes, historically common for bright color and smooth finish, are now heavily restricted or banned in food-contact ware due to toxicity. Modern food-safe production relies on lead-free formulations using boron compounds, zinc oxide, or alkaline fluxes.

Common faults include crazing (fine cracks in the glaze from thermal expansion mismatch), crawling (glaze beading up and leaving bare patches), pinholing (tiny craters from gas escape), and running (glaze flowing down the pot during firing). Crazing typically indicates the glaze expands more than the clay body during cooling; crawling often results from dust, grease, or over-application on the bisque surface. Glaze chemistry is finely calibrated: a change in feldspar source or water hardness can shift melting behavior enough to spoil batches.

The term derives from the Middle English and Middle Dutch glasen, related to glass. In industrial production, glazes are batch-mixed and regularly tested for consistency. Quality control involves measuring specific gravity, viscosity, and particle size; test tiles are fired alongside production runs to verify color and surface. Glazes are categorized by firing temperature (low-fire earthenware glazes around 1050°C, stoneware and porcelain glazes reaching 1200°C or higher), by surface character (matt, satin, glossy), and by functional requirements (dinnerware, sanitaryware, decorative tiles, refractories).