a glaze
A glaze is the thin coat of glass fused onto the surface of a fired clay object — the smooth, shiny, coloured skin on a mug, tile, or vase. Bare fired clay is often dull, rough, and porous enough to soak up coffee or bacteria; a glaze seals it with a glassy layer that is waterproof, easy to clean, and can be any colour or finish you like, from mirror-gloss to soft matte. It is, quite literally, a purpose-made glass painted on as a slurry and melted in place during firing.
Here is how it works. The glaze is applied as a watery suspension of finely ground glass-forming minerals — silica (SiO2) as the network former, plus fluxes such as feldspar, lime, or lead/boron compounds to lower the melting point, plus opacifiers or metal-oxide colourants. When the coated piece is fired, the coating melts to a runny glass at the peak temperature and flows into a continuous, pore-free film, then freezes to glass on cooling. The single most important design rule is thermal-expansion matching: the glaze and the clay body underneath must shrink at nearly the same rate as they cool. If the glaze contracts more than the body it is stretched and cracks into a fine web (crazing); if it contracts less it is squeezed and can flake off (shivering). Potters deliberately tune this — a slightly compressed glaze is actually stronger, because compression fights the tension cracks a ceramic fears.
Glazes carry most of what we love and use about ceramic surfaces. Colour comes from dissolved or suspended metal oxides — cobalt for blue, copper for green or red, iron for browns and celadon greens, tin oxide for an opaque white. Texture ranges from high-gloss to crystalline to matte, set by composition and cooling rate. And function follows: the glaze makes tableware food-safe and cleanable, makes sanitaryware hygienic, and protects the body from weather. One honest warning from history — many bright old glazes were fluxed with lead, which can leach into acidic food and drink, so modern food-contact glazes are formulated and fired to be lead-safe or lead-free.
A classic celadon glaze is fired in a reducing (oxygen-starved) kiln so a small amount of iron oxide turns from yellow-brown Fe3+ to green Fe2+, giving the soft grey-green of Song-dynasty ware — and the fine crackle prized in some celadons is crazing, a thermal-expansion mismatch turned into a deliberate ornament.
A glaze is a tailored glass melted onto the body; its colour comes from metal oxides and its integrity from matching the body's thermal expansion.
A glaze is glass, so it obeys glass rules: matching its thermal expansion to the body is everything. Get it wrong and the piece self-destructs on cooling — crazing if the glaze is too high-expansion, shivering or peeling if too low.