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bisque range cone 5-6. and a general guide for the appraisal of

updated sun 16 sep 01


iandol on sat 15 sep 01

Bisque Temperatures

Dear Mia,

You state <<6 or 9 to 10.>>>.

I have the feeling that this is not necessarily so, though as a general =
statement it does seem to be true. Bisque temperature ranges for =
compounded clays are determined by their compositions and the =
proportions of the various ingredients. To achieve vitrification or =
acceptable mechanical properties, fluxing agents are needed which ensure =
the required physical and chemical properties at the chosen maturing =
temperature. I believe for cone 8 and above it is usual to incorporate =
Potash felspar, but for lower cone values it is not unusual to sacrifice =
Potash and replace a proportion of it with a compound containing Sodium =
oxide such as Nepheline Syenite or Soda felspar. Alternatively a high =
melting point frit can be used.=20

The temperature of the onset of sintering, which results in the growth =
of bridges between particles that give strength to biscuit clay, can be =
determined using the Tamman Formula. The upper temperature for bisque is =
set by the point at which enough flux has melted to become a solvent for =
the other ingredients. This leads to the rapid densification and =
eventual vitrification of the clay body. The lowest temperature for a =
bisque firing is set by Tamman Temperature of the most refractory =
materials which must reach their sintering points. These refractories =
are Mullite and Silica derived from the decomposition and restructuring =
of Kaolinite. There is a window of opportunity peculiar to each clay =
body. If you do now know the nature of the clay the maker will tell you =
of this temperature range. If you compound your own clays you work out =
the temperature for yourself.

Best regards,

Ivor Lewis.