What is the significance of the Al2O3 : SiO2 ratio?

Unit 7 Topic 4
Using Unity Formula

The alumina : silica ratio (Al : Si) represents the proportional relationship between the alumina and silica values in a unity formula.
.


This value is calculated by dividing the SiO2 value by the Al2O3 value in the unity formula.

SiO2
divided by
Al2O3
=
alumina : silica ratio
e.g.for Nicki's Blue Celedon
3.20
divided by
0.31
=
10.32

 

The ratio may be expressed as a single value e.g. 10.32 or in the form 1 : 10.32 meaning that theoretically for every one molecule of Al2O3 there is 10.32 molecules of SiO2.

At stoneware temperatures the Al2O3 : SiO2 ratio for clear glazes is in the vicinity of 1 : 9.

Nicki's Blue Celedon may therefore be close to a clear glaze with perhaps a slight tendency to be opacified by the small excess of SiO2 in relation to the alumina.

Nicky's Blue Celedon

Oxide
Mol. Parts
KNO
CaO
MgO
0.30
0.55
0.15
Al2O3
0.31
SiO2
3.20
Al : Si
Expan.
10.32
10.58

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