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: kiln wash mistake - talc

updated sun 9 mar 03


Ron Roy on sat 8 mar 03

Talc is not a low fire flux - it acts in a catalytic way to produce
cristobalite in low fire bodies to help avoid crazing - and also to reduce
rehydration in low fired bodies.

Talc decomposes at 900C - giving MgO and SiO2

Starts fluxing at 1170C and is used for both glazes and clays.

A very useful flux because it has such a low coefficient of expansion.

The action of MgO is quite strong from 1191C to 1230C after which it
settles down and behaves like other mid range fluxes.

All this from Hamer and much more - ie - high surface tension and increased
fluidity. High surface tension is very good for making nicely rounded rims
on cups and mugs.

Usual sources are talc, dolomite and magnesium carbonate and some frits.

Warning - glazes with high amounts can have a low enough CTE to make them
pot busters and can promote shivering.


>Would you please explain the chemistry on this one. The inference is that
>Talc will melt. I have always understood Talc to be a heat resistant
>material which could only be fused with great difficulty. Hamer says, in 1
>st Ed, that it decomposes to become Magnesia and Silica. Someone might
>confirm this from an independent source tell us what he says in the latest
>Best regards,
>Ivor Lewis

Ron Roy
15084 Little Lake Road
Brighton, Ontario
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513