What Is the Shiny Layer on a Pot? Glass Made to Fit

A glaze is glass, meaning a material that melts and cools without crystallising, and the ingredients fall into three functional groups. The glass former, almost always silica, is what actually becomes glass, and by itself it melts far above any practical kiln temperature. Fluxes lower that melting point into the achievable range, and different fluxes work at different temperatures and affect the colours that develop, with lead, sodium, potassium, calcium, magnesium and boron all used. Stabilisers, principally alumina, raise viscosity so the molten glaze stays on the pot instead of running onto the kiln shelf. Every glaze balances those three, and a recipe is essentially a statement of that balance with colourants and opacifiers added.

The characteristic faults each have a specific cause:

• •Crazing, a network of fine cracks, caused by the glaze contracting more than the body on cooling
• •Shivering, where the glaze flakes off, caused by the opposite mismatch
• •Crawling, where the glaze pulls away leaving bare patches, caused by a dusty or greasy surface
• •Pinholing and blistering, caused by gases escaping from the body after the glaze has sealed
• •Running, caused by too much flux or too little alumina
• •Dryness, caused by insufficient flux for the temperature reached

A small number of metal oxides supply nearly all ceramic colour, and each behaves differently according to the other ingredients and the kiln atmosphere. Iron gives yellows, browns and blacks when oxidised and greens and blue-greens when reduced, which is the basis of celadon. Copper gives greens when oxidised and reds when reduced, which is why copper red glazes are difficult and prized. Cobalt gives blue reliably in almost any conditions, which is why blue and white ware exists in so many traditions. Manganese gives purples and browns, chromium greens, and tin and zirconium produce opacity rather than colour by remaining suspended as particles. Modern stains are prepared pigments that behave predictably and are correspondingly less interesting to many potters.

Getting an even layer of the right thickness onto a porous object is a practical skill with several methods. Dipping the piece into a bucket of glaze suspension is fastest, with the porous body drawing water out and leaving the solids behind, and the thickness depends on how long it is held and how absorbent the body is. Pouring suits large or awkward pieces. Spraying gives control and requires equipment and extraction. Brushing is slow and suits decoration rather than covering. Thickness matters enormously, since too thin gives a dry patchy surface and too thick runs or crawls, and experienced potters judge it by the feel of the dried coating and by scratching a test line. Raw glazing, applying glaze to unfired clay and firing once, saves a whole firing and is riskier.

The most consequential requirement is that glaze and body expand and contract at compatible rates, since they are bonded together and cool together through hundreds of degrees. If the glaze contracts more, it ends up in tension and cracks, producing crazing, which is a fault in functional ware because the cracks admit liquid and harbour bacteria, and is deliberately induced as a decorative effect in some traditions by staining the cracks. If the body contracts more, the glaze ends up compressed and may flake off, which is worse. Adjusting fit means altering the glaze recipe, and the standard approach calculates expansion from the proportions of each oxide present, which allows a recipe to be corrected before firing rather than by trial. Potters test with tiles fired alongside the work.

A glaze is glass made from a glass former, fluxes lowering its melting point into kiln range and stabilisers keeping it from running. Iron and copper change colour completely depending on whether the kiln atmosphere has surplus oxygen, while cobalt gives blue regardless. Crazing and flaking both come from glaze and body contracting at different rates on cooling.