What is a Glaze?

Q: What is a glaze?

A: In plain terms, a glaze is a very thin layer of glass formed on a clay pot during the firing processes, that is, during one of the times that the fragile clay vessel is heated to a high temperature (above 1800° F, 1000° C).

On a typical piece of pottery the glass layer is 1-2 mm (0.04-0.08″) thick on average. However, some pots undergo several “glost firings” (ie, glaze-forming firings) and these pots may end up with glass layers 3 mm thick. Most pots are fired twice, a few five or six times.

The glaze layer usually starts out as a glaze “recipe”, also referred to as a “glaze” in short-hand talk. This recipe is a mix of “chemicals” and minerals, all as fine powders (pulverized). A typical glaze recipe reads as follows:

Satin White Glaze, Cone 4-7 Oxidation

by Michelle Lowe

  • 26.5 Nepheline syenite (mineral, source of “soda”)
  • 15.8 Custer feldspar (a “potash” feldspar)
  • 21.0 Dolomite (mineral, 50/50 calcium carbonate, magnesium carbonate)
  • 21.0 Bell Dark ball clay
  • 5.2 Gerstley borate (mineral, source of boric oxide)
  • 10.5 Flint (or silica or quartz)

To this mix, which totals 100 weight units, Michelle Lowe adds 6 weight units of Tin oxide. She then combines the mixed dry powders with an appropriate amount of water to form a “slurry”, and dips a bisqued pot in the slurry. When dry again, the pot (with others) is placed in a kiln and heated to “Cone 6” (1230° C, 2250° F).

She says this glaze produces a “very smooth even white, really nice with stains on top, or with other glazes splashed on for decoration.”

Glaze recipes are often given in books and magazines, and sometimes a particular recipe will yield a pleasing result on a particular claybody, and sometimes the result is disappointing. To be able to predict a given result requires that the potter study the properties of ceramic raw materials and their behavior when heated in a kiln.

Because glazes are more complex than simple glasses, chemists tend to stress fine structure to explain performance. Atomic and molecular notions are used to interpret the results of experiments in the fields of crystallography and spectroscopy. Such work can give us a picture of glass on a microscopic level, but such fine detail is beyond the needs of a glaze designer.

by Tom Buck