All ceramic materials when heated expand and, on cooling, contract to varying degrees. It is the different expansion and contraction between the ceramic body and it's glaze coating which gives rise to crazing.

Glaze Fit

The degree to which a glaze and the underlying claybody are in harmony (no crazing) is referred to as glaze fit. If a glaze expands more than the claybody during firing up then it will contract more than the body during cooling and will cause crazing. If, on the other hand, the body expands and contracts more than the glaze then the glaze is said to be in compression and it will not craze.

Coefficients of Expansion for the Oxides

Over the last 100 years ceramic scientist have experimented to determine the different rates (coefficients) of expansion for the individual oxide components of ceramic materials so that the expansion of that material as a whole may be calculated.

Different tables of coefficients of expansion for the oxides are available which vary according to the way in which the coefficients were obtained. The tables are often named after the scientists who developed them e.g. English & Turner, Winkelmann & Schott, Mayer & Havas etc.

Most of these tables are incomplete in that they do not contain expansion values for a complete list of the base and colourant oxides which may be found in a glaze. It is common, therefore, to combine coefficient factors from more than one set of tables. Although this practice is scientifically questionable an exact expansion factor for a glazes is of little value in itself. Potters are more interested in values which enable them to compare glazes and to determine trends when attempting to influence the degree to which a glaze crazes.

Calculating the Coefficient of Expansion for a Glaze

When a glaze is converted to an oxide % analysis values for each oxide in the glaze are multiplied by the coefficient of expansion for that oxide to calculate the expansion of that oxide in the glaze. The expansion values thus calculated are then totalled to give an overall coefficient of expansion for the glaze.

Matrix uses the abbreviated term Expan. to point to a glaze expansion value. This value is placed at the foot of the formula display panels together with values for the Al2O3 : SiO2 ratio and the Surface Tension (S.T.) for the glaze. Each time a displayed formula is changed the coefficient of expansion is automatically recalculated.

Which Set of Expansion Factors Should be Used?

Matrix offers a number of sets of expansion factors which may be used to calculate the Coefficient of Expansion for a glaze. In deciding which factors to use it is important to remember that the values are comparative values only i.e. if you consistently use one set of factors in your glaze research you will develop bench mark expansion values for the clay bodies you use. Over time, in your research environment the factors will become meaningful. It is important then to be consistent and to only use expansion sets with which you or others have developed some experience.

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