iandol on fri 20 dec 02
Dear Ababi,
As I said in my last note, this seems to be a Design task if a rational =
and logical option of creating such a glaze is taken. We seek a =
transparent glaze with a dull surface.
Well, I put a bit of Dreamtime in on your idea and came up with three =
hypothetical options, two of which might give a positive result at cone =
6 and a better option for the cone 10 firing which might be subject to =
less light scatter.
The following range of values should give mixtures which become molten =
above 1100=B0C. I leave it to you to calculate the weights of materials =
needed
QC 001
K2O 17% - 9%
Al2O3 3% - 10%
SiO2 80% - 81%
The following range of blends should give mixtures which become molten =
near or above 1200=B0C. Again, I leave it to you to select materials and =
calculate weights.
QC 002
K2O 15% - 8%
Al2O3 3% - 8%
SiO2 82% - 84%
For raw materials I suggest Quartz or Ground Silica and Potassium =
Felspar and would select one which came as close to the theoretical =
Molecular formula with the minimum of extraneous elements, especially =
Sodium. Although not an essential ingredient the inclusion of Potassium =
Tetra Silicate would be an advantage since it gives a better low =
temperature melter than the Felspar. It is water soluble but is a better =
option that Potassium Carbonate.
My third suggestion is a high temperature option which would not usually =
be experienced by most potters and ceramists since it avoids using =
common place Felspars or Frits. I think it is a style of glaze which =
remains unexploited, possibly because there seem to be no historic =
precedents which give definitions of its potential aesthetic value. I =
am presently exploring a similar system which is a little more complex =
that has given excellent initial results.
QC 003
73% (SiO2 90% + MgO 10%)
12% (CaO 90% + MgO 10%)
15% (Al2O3 90% + MgO 10%)
If this is made up of the raw oxides or carbonates it will need a very =
high temperature to achieve fusion but constructed from Talc, =
Wollastonite, Silica and Kaolin it should mature in the cone 10-12 =
range. This would almost be guaranteed if Anorthite could be used rather =
than Kaolin to source the Alumina fraction. ( Especially for those who =
chant the Eutectic mantra... 11/70.. 11/70.. 11/70..)
Firing program. To obtain the predicted Quartz Crystals, dissolution of =
all ingredients must be complete. Vitrification must be finished. This =
may entail holding at maturing temperature for a longer period than some =
clay workers would wish. But to get the effect of a clear glaze with a =
Matt finish it will be necessary to cool rapidly to 850=B0Celsius then =
arrest the temperature to allow Isothermal transformation of the glass. =
This has to be done above the Glass transition temperature, but as late =
as possible to prevent excessive crystal density which might cause =
translucency or growth in size which would give a rough surface.
As you will realise these recipe precursors are the products of my =
imagination but they are open to testing by people who are interested in =
such things.
Best regards and good wishes for success with your current work,
Ivor Lewis, Redhill, South Australia.
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