iandol on fri 25 jul 03
Dear Wesley Derrick,
Once a glaze has been formed from its original ingredients the melting =
point changes. Those Dilatometer plots Ron Roy talks about illustrate =
that the melting point of the mature glaze is much lower than that of =
any of the original ingredients. This is called the "Glass Transition =
Temperature", the point at which a glaze changes from rigid to plastic =
and so is deformable. For most of the work produced by potters this may =
be in the range of 650 C to 750 C.
Hope that helps.
Ivor Lewis. Redhill, South Australia
Ron Roy on thu 31 jul 03
Actually the range is much greater - I have dilatometered cone 10 glazes
that start to melt only at 850C and there are some high boron cone 6 and 04
glazes that are pyroplastic below 550C.
Note that if glazes are still pyroplastic below 573C then the quartz
inversion in the body - which helps to keep glazes in compression (stops
crazing) no longer works. That is - when the body gets suddenly smaller at
573C - the glaze being still pyroplastic - adjusts to the decrease in size.
In order for the body quartz inversion to help prevent crazing the glaze
needs to be rigid (frozen.)
This helps explain why it is so difficult to stop crazing in low fire
glazes - because so many of them are not "frozen at 573C
It was a great surprise to me at how low the freezing temperature of glazes
actually was. In all the reading I had done - up to the time I actually
started doing the work on glazes - it was never once mentioned in any of
the texts I had read - I'm not sure it is not mentioned by Cardew but - it
was a question which I have never had answered. Perhaps the authors had
never seen a dilatometer chart of a glaze - possible. If so it is a comment
- again - on the lack of understanding so prevalent in so many of our
Ti be fair - there is a chart of a glaze and a very good explanation in
Ceramic Glaze Technology - by J. R. Taylor and A. C. Bull (Pergamon Press)
- and a lengthy explanation on the subject of how all this fits together.
It is out of print but there are second hand copies around.
>Once a glaze has been formed from its original ingredients the melting
>point changes. Those Dilatometer plots Ron Roy talks about illustrate that
>the melting point of the mature glaze is much lower than that of any of
>the original ingredients. This is called the "Glass Transition
>Temperature", the point at which a glaze changes from rigid to plastic and
>so is deformable. For most of the work produced by potters this may be in
>the range of 650 C to 750 C.
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