How can unity formulae be manipulated to control crazing in a glaze?

Unit 7 Topic 19
Using Unity Formula

When a glaze recipe is converted to a unity formula the coefficient of expansion is calculated. The oxide content in the formula can then be changed so that the coefficient of expansion is lowered thus reducing the tendency for the glaze to craze.
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The list of individual expansion factors shows that some oxides expand more than others in the firing.

Crazing can be controlled by reducing the levels of high expansion oxides in the formula and / or increasing the levels of oxides with low expansion factors. The target would be to bring the coefficient of expansion for the whole glaze down lower than the benchmark set for the clay body being used.

Steps in the process would be:

  • convert the glaze to a unity formula
  • change the formula to lower the coefficient of expansion for the glaze.
  • convert the unity formula back again to a recipe.
  • test fire the glaze.

Changing a unity formula is easy in Matrix.


Oxide
Mol. Parts
Change
New Formula
KNO
CaO
MgO
0.30
0.55
0.15
-0.01

+0.01
0.29
0.55
0.16
Al2O3
0.31
SiO2
3.20
+0.4
3.60
Expan.
10.58
9.82

In this example 0.01 mols of the high expansion KNO flux group is removed and the lower expansion oxide MgO is increased by the same amount (maintaining unity).

The level of SiO2 in the glaze is also increased by 0.4 mols. These changes lower the overall expansion of the glaze from 10.58 to 9.82.

Have a look at >>>>>>>>>> Matrix's Change Formula Window.
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