Ron Roy on wed 18 nov 98
This was sent by a clayart friend - he had been using it on functional ware
and discovered it was changing colour when in contact with certain acidic
foods - like tomatoes -and recalled the ware.
What is significant about this glaze - it has enough alumina and silica
according to the limits I use with cone 6 glazes with boron included in
unity with the fluxes - not much more then the lower limits.
Cone 6 limits for cone 6 glazes with boron included in unity with the fluxes.
*CaO 0 - 0.55
*MgO 0 - .325
*KNaO 0 - 0.375
*ZnO 0 - 0.3
*BaO 0 - 0.4
*B2O3 0 - 0.35
Al2O3 0.275 to 0.65 (this glaze has 0.28)
SiO2 2.40 to 4.70 (this glaze has 2.52)
The reason for the colour change must be the copper carb - thats a lot at 6%.
If someone is interested it would be useful to many of us to see at what %
the copper would start coming out. It would be a fairly simple experiment
to add 1% copper carb, in increments to this glaze and conduct some acid
leach tests.
I would expect this glaze to be stable when not oversupplied with oxides to
the point where they can no longer be held in the glaze solution during
cooling.
Cone 6 unstable glaze - not good when in contact with food.
I have used OM#4 but the original called for Tenn #5 - they are very close.
The copper is not included in my calculation.
-----------------
DOLOMITE............ 4.78
Gers Borate 9/97.... 15.93
WHITING............. 12.00
CUSTER SPAR......... 35.85
OM-4................ 1.59
SILICA.............. 25.00
EPK................. 10.00
coper carb.......... 6.00
----------
Total 111.15 < this is the total the glaze arrived with.
FORMULA & ANALYSIS
------------------
*CaO........ .53 12.42%
*MgO........ .11 1.89%
*K2O........ .10 4.09%
*Na2O....... .08 1.96%
Fe2O3...... .00 .30%
TIO2....... .00 .05%
*B2O3....... .17 4.92%
AL2O3...... .28 11.68%
SiO2....... 2.52 62.67%
P2O5....... .00 .01%
RATIO 9.12
EXPAN 471.69
WEIGHT 241.46
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
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