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.
Follow
these links for more information about:
