iandol on sun 27 jul 03
Dear Tony,
You ask <into a porcelain body to replace felspar. I expected it to drastically =
reduce the firing temperature of the porcelain but it did the opposite. =
How is this possible. Ulexite melts to a crystal clear glass at cone 06, =
felspar does not even do that at cone 10. Then why is felspar a more =
effective body flux?>>
I think it would be a great idea to ask such questions of those people =
who write text books explaining the Chemistry of Ceramics and =
dictionaries for our ceramic community.=20
However, speculatively thinking.... Ulexite will have a peculiar =
structural formula with lots of water of crystallisation where the water =
is more likely to be as Hydroxyl units associated with metal ions and =
coordinated to the Boron ions.=20
As you say, Ulexite melts at a low temperature and cools to a glass =
which will be a Calcium Sodium Borate. So the first reaction is =
Dehydroxylisation. Now it then seems possible that in the molten =
condition it firstly promotes liquid phase sintering with the Kaolin and =
free Silica and that during this process chemical reactions take place =
which result in products which have maturing, or melting temperatures, =
much higher than might be expected in a standard porcelain body. In =
particular I suggest that oxo- compounds, or even oxygen depleted =
compounds, of Aluminium and Boron may form with metal atoms of Ca and Na =
in solid solution. Perhaps rather than creating a Borosilicate glass as =
a cementing phase you are creating Aluminium and Silicon Borides, =
substances which are perhaps immune to wetting. Hence, the rather =
refractory nature of you resultant material.
As I said to begin with, speculation from and artistic imagination =
thinking in metaphors.
Best regards,
Ivor Lewis. Redhill, South Australia
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