What are limit formuae?

Unit 5 Topic 4
The Unity Formula

The table of Limit Formulae suggest upper and lower levels of particular oxides in the unity formula in relation to firing temperatures.
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This concept originated with the work of Hermann Seger in devising recipes for cones which soften within narrow temperature ranges. Since that time exhaustive testing by various researchers has established oxide parameters for stable glazes across a range of temperatures.

Flux oxides are given upper limits while alumina and silica are given lower and upper limits for each temperature.

In the example for cone 12 lead oxide is given a nil value because it vapourises at high temperatures and is lost from the glaze.

Some glaze software such as MATRIX show these limits as bar graphs on which the glaze being studied is also displayed.
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KNO + Li2O
CaO
MgO
BaO
ZnO
PbO
0.335
0.625
0.340
0.440
0.340
nil
Al2O3 - from
to
0.360
0.740
B2O3 - with PbO
leadless
nil
0.260
SiO2 - from
to
2.8
5.4
Suggested oxide limits for
C.12 1306 deg.C

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