iandol on tue 23 jul 02
"Please design for me a gloss base glaze for Cone 3. I give you a hint: =
start with the Watts Formula: 0.4 KNaO; 0.3 CaO; 0.3 ZnO; 0.5/0.6 Al2O3; =
3.5-3.55 SiO2."
Dear Tom Buck,
An interesting proposition. I suppose you are asking for a recipe from =
this Formula which can be fritted first then applied to the pot and not =
a mixture which is applied as raw ingredients. (Bet you forgot about the =
Fritting option)
My charts suggest that the ratio of Al2O3/SiO2 should be in the region =
of 0.3/2.6 and, since there are no exclusions, I would like to add a tad =
of Borate of some form or other. I also fancy a touch of Lithium as =
well, possibly from Petalite, to allay any crazing problems and assist =
with the gloss. I would be worried about so much Zinc oxide. Always a =
tendency there to go into the opaque part of the diagram, unless you =
intend that this should be opaque. Using ZnO also excludes the chance of =
doing good reduction colours.
It will be interesting to see how many people slavishly follow your =
Watts Formula to the enth degree and come up with what their computer =
programs propose after they have chosen a few raw materials.
Best regards,
Ivor Lewis.
Tom Buck on wed 24 jul 02
Thanks, Ivor:
you have nicely isolated some problems inherent in designing a
Cone 3 "Bristol" type base clear. Parmelee cited the Watts Formula as one
that could lead to a base clear that would mature at Cone 06 were it
to combine certain raw materials. and as you noted, both lithium and boron
compounds are needed both to get a melt at C3 and to avoid crazing on
cooling.
I found that this target glaze was a project that led me to a good
understanding of the factors involved in glaze design.
til later. Peace. Tom B.
Tom Buck ) -- primary address.
"alias" or secondary address.
tel: 905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street, Hamilton ON L8T 3E1 Canada
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