What is a Glaze?

Copyright © 1998 by Ian Currie

A glaze is a layer of glass on the surface of a piece of ceramic. It consists of the BASE GLAZE with or without colourants and opacifiers.

The base glaze consists of three types of ingredient:

-- Glass former, e.g. silica. This is what makes the glass. It has a very high melting point, so we need to add something to bring the melting point down to the temperature we are using. This is done by adding...

-- Flux e.g. feldspar, frits, zinc oxide, dolomite... there are many of them. See the LIST OF FLUX MATERIALS. We can make a glaze that will melt in our kiln just using silica plus fluxes, but we will probably find that it is very runny - as soon as it melts, the glaze runs right off the pot. To solve this problem, we add...

-- Stiffener, usually alumina. Alumina is provided by a number of ingredients, especially clays, such as china clay or ball clay. Some of the fluxes also contain alumina, e.g. feldspar and many frits. We can add pure alumina as well. As we add more alumina, from one source or another, to the glaze, we will find that we get a bigger melting range i.e. the temperature range increases between when the glaze melts and when it runs off a vertical surface. Ideally the range should be enough so that we can put it in our kiln and know that within our normal firing variations, the glaze will mature but not run off the piece.

Base Glaze:

Fluxes Alumina Silica
(Lower melting point) (Makes less runny) (Makes the glass)

Glaze Recipe = Base Glaze + Colouring oxides and/or Opacifiers

A base glaze will usually (not always!) be colourless. If we want colour, we usually add a colourant. If we wish to add a colourant to our tests we will usually be using a colouring oxide (or carbonate).

A base glaze may be transparent or opaque to varying degrees. We can increase opacity by adding an opacifier such as tin oxide, or a zirconium opacifier. This will usually make the glaze white.

So far I've tended to present a simple picture.. this is the glass former, this is the stiffener, this is the opacifier etc. For some of our experiments this will in deed be the case, but we need to know that the boundaries are far from neat. In fact the boundaries are almost always blurred. For example, some flux materials contain alumina and silica as well. In the experiments we will be doing, our source of variable alumina is kaolin which contains some silica as well. Some colouring oxides act as fluxes. One glass former (boric oxide) is also a flux! Some materials behave quite differently in oxidation and reduction.