What Is a Glaze? Glass Fused to Clay and Everything That Can Go Wrong
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A glaze is a thin layer of glass formed on the surface of fired clay, which seals it, colours it and gives it a surface. Getting one to fit the clay beneath it is a materials problem that potters have been solving empirically for millennia.
What a glaze is made of
Three components are required and each does a distinct job. A glass former, almost always silica, is the substance that will melt into glass, and on its own it requires temperatures far beyond any kiln. A flux lowers that melting point into a workable range, and the choice of flux, which may be lead, sodium, potassium, calcium, magnesium or others, affects colour, surface and durability substantially. A stabiliser, usually alumina, raises the viscosity of the melt so that the glaze stays where it was put rather than running off the pot, and getting that balance wrong produces a piece welded to the kiln shelf. Colour comes from metal oxides added in small quantities, and the same oxide can produce completely different colours depending on the other ingredients and on the kiln atmosphere, which is why glaze chemistry is complicated rather than a matter of choosing a pigment.
How it goes wrong
The recognised faults each have a cause and most concern the fit between glaze and clay:
- •Crazing, a network of fine cracks appearing when the glaze contracts more than the clay on cooling, which is sometimes a defect and sometimes deliberate
- •Shivering, the opposite case where the glaze contracts less and flakes off, which is dangerous on functional ware
- •Crawling, where the glaze pulls away into islands because the surface was dusty, oily or too thickly applied
- •Pinholing and blistering, from gases escaping through the melting layer
- •Running, from too little stabiliser or too much heat, which can ruin the kiln as well as the piece
- •Dunting, cracking of the piece itself from thermal stress, which is a firing fault rather than a glaze one
The atmosphere in the kiln
Whether the kiln has plenty of air or is starved of it changes the chemistry substantially, and this is the variable that separates traditional wood and gas firing from electric. An oxidising atmosphere, with abundant oxygen, lets metal oxides keep their oxygen and produces one set of colours. A reducing atmosphere, where the fuel consumes more oxygen than is available, pulls oxygen out of the metal oxides in the glaze, and the same copper that gives green in oxidation gives red in reduction, while iron shifts from amber towards the grey-greens of the celadon tradition. That effect is why certain historical wares could only be produced in particular kilns, and why electric kilns, which are oxidising by nature, cannot reproduce them without special measures. Controlling atmosphere deliberately is a skill in its own right and much of the character of East Asian ceramics depends on it.
Putting it on
Application is a separate skill from formulation and most faults begin there. Glaze is usually suspended in water and applied to a piece that has already been fired once, which leaves it porous enough to absorb water and draw the solid onto the surface. Dipping gives the most even coating and requires enough glaze to submerge the piece. Pouring suits interiors and large forms. Brushing needs a formulation adjusted to stay put and generally requires several coats. Spraying gives control over thickness and gradation and needs equipment and ventilation. Thickness is the variable that matters most, since too thin gives a dry patchy surface and too thick runs, crawls or blisters, and judging it comes with practice. The foot of a piece is left bare or wiped clean, because any glaze touching the kiln shelf will fuse the pot to it permanently.
Why it was a technology
Before glazing, fired clay was porous, so a vessel leaked slowly, absorbed what it held and could not be cleaned properly, which limits what it can be used for. Sealing the surface made storage of liquids, safe reuse and hygienic serving possible, which is a substantial practical advance rather than a decorative one. Glazing techniques therefore spread as valuable knowledge and were in several cases closely held, with the European effort to reproduce Chinese porcelain occupying centuries and considerable resources before the composition was worked out. Lead glazes, which melt at low temperatures and are easy to use, were standard for a very long time and are a genuine health hazard where the glaze is poorly formulated or underfired, since lead can leach into acidic food, and their replacement in functional ware is recent enough that antique pieces should be treated with caution.
The takeaway
A glass former, a flux to lower its melting point and a stabiliser to stop it running, with colour from metal oxides whose result depends on everything else. Most faults come from the glaze and clay contracting differently on cooling, producing crazing one way and flaking the other. A kiln starved of oxygen pulls it out of the oxides, which is why copper gives green in one atmosphere and red in another.