iandol on tue 3 jun 03
Dear Jim Murphy,
My Friend, I think you are picking up the vibes. Your summation seems in =
tune with my thinking.
The notion of a cone 6 with Calcium oxide as a sole representative of =
the R2O group would not work even if you hit on the right mixture of =
prime ingredients. The melting range is too narrow and overfiring might =
increase the fluidity to the extent where it all ran from the pot onto =
the shelf. Remember, Most glazes are compromises combining a selection =
of complementary, contrasting and even opposing qualities.
The Calcium Felspar, Anorthite, has a listed melting point of 1550 or =
thereabouts and does not contain Sodium or Potassium. If it did it would =
be a different material, some sort of Rock, like Nepheline Syenite. I =
suggested it as the third entity in the triumvirate which encompasses =
the 1170 Eutectic Point.
IF Calcium oxide is not dissolved into the melt it will be available for =
attack by water or acids. If it is in solution in a glass it will not be =
open to attack at the same rate. If it reacts on cooling to precipitate =
Wollastonite or Anorthite (depending on the original chemistry" it will =
be insoluble to water and, I suspect, many acids.
Lime, AKA CaO, does not promote Fusibility. But it has a major effect on =
Viscosity. It also affects that other quality of importance, Surface =
Tension. Perhaps the commentators you speak of confuse an exceptional =
degree of Fluidity with Fusibility. It is well known that Felspars can =
"Fuse" and be taken to very high temperatures without becoming =
"Fluid"(see Singer and Singer)
Care should be taken when considering the qualities of some materials. =
Volcanics by mature and definition are fast cooled materials. I believe =
both Ash and Pumice are Glassy by nature. I would treat them as being =
Prime Melting Agents, as Natural Frits. This would confirm your opinion =
that they might be useful agents in Cone Six Glazes.
Best Regards,
Ivor Lewis.
Louis Katz on wed 4 jun 03
just a bit of data that might be missed.
Calcium Carbonate makes useful wadding for cone ten. Work it up to a
claylike consitancy with water. use it as wads. Leaves a small bit of
darkened clay on light stoneware. Releases from glazes easily. It may
be Very hazardous as it can react quickly with water release copious
amounts of heat. Water turns to steam, wad of calcium oxide blows up
sending hot caustic fragments into eyes.
But if the whiting is pure enough it does not eat into the clay as many
other "fluxes" would. Seems to need intimate mixing.
Louis
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