How can B2O3 be used to lower the melting range of a glaze?

Unit 7 Topic 7
Using Unity Formula

Additions of boron as a partial replacement for silica will bring down and extend the melting range of the glaze.
.

Boron is unique in its ability to act as both fluxing agent and glass-former. It also behaves as an amphoteric extending the melting range of a glaze i.e. the glaze starts melting at a lower temperature.

Changing Sandy's Clear from C.12 to C.10 by exchanging some boron for some silica.

Small amounts (e.g. 0.1 mols.) will appreciably lower the melting range of the glaze, while reducing the potential for crazing (note the lower expansion) and may improve the surface.

Sandy's Clear

Oxide
Mol. Parts
Changes
New Formula
KNO
CaO
MgO
0.20
0.54
0.26
0.20
0.54
0.26
Al2O3
B2O3
0.50


+ 0.1 =

0.50
0.10
SiO2
4.38
- 0.1 =
4.28
Al : Si
Expan.
8.83
6.86
8.60
6.80

SiO2 value is decreased and the B2O3 value is
increased by the same amount.


Note: the changes in the Al : Si ratio may not reflect the true relationship between alumina and glass-forming oxides (SiO2 & B2O3).

 

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