Jan Mandrake on tue 10 feb 98
I have used the Zakin recipe for Mexico Point Green for a while. This is a
cone 6 glaze, recipe: Soda spar 34, Spodumene 14, EPK 6, Dolomite 14,
Gerstley borate 12, Zircopax 20, copper carbonate 2.5. I fire to a high cone
6 and the glaze is a nice greenish turquoise. Problem--I used it on a brie
baker and when the brie was baked in it the glaze faded to a lighter turquoise
without the greenish wherever the brie touched. I wondered if it was the
acidity in the cheese (Or milk products in general) so I put some yogurt in it
(having dumped the brie out) and the same thing happened--wherever the yogurt
touched, the glaze faded to the point it had with the brie. I guess this is
not a food safe glaze?? Since I saw the Mexico Point Green posted in clayart
I thought I would mention this problem and also see if anyone else had run
across the same problem with this glaze--or if these is a way of correcting
it? Someone suggested it was caused by the iron in the clay--I use a buff
clay. Any thoughts?
Ron Roy on sun 15 feb 98
I use two sets of limits to assess glaze durability - the limits which come
with Insight say a durable glaze fired to cone 3-7 should have 2.5 to 3.5
mols of SiO2
The other set - from British texts (B2O3 included with fluxes in unity)
says a cone 6 glaze should have 2.4 to 4.7 mols of SiO2
This glaze has 2.26 of silica (B2O3 not included in unity) and 1.96 of
silica (B2O3 included in unity with fluxes.
The point is - in both cases the limits I am using confirm the glaze is
short of silica and therefore I would have predicted solubility problems
with this glaze.
The problem is even more acute when I look at the glaze without the
Zircopax - Zircopax is part zirconium and part silica - this silica is not
available as a glaze material because it is tied up in the Zircopax - which
does not melt.
In case one (B2O3 out of unity) SiO2 is only 1.90.
In case two (B2O3 in unity) SiO2 is only 1.65 - and it should have at least
2.4 SiO2.
The problem was predictable - and with matte glazes it usually is.
The addition of copper to the glaze probably added to the problem.
----------------------------Original message----------------------------
>I have used the Zakin recipe for Mexico Point Green for a while. This is a
>Gerstley borate 12, Zircopax 20, copper carbonate 2.5. I fire to a high cone
>6 and the glaze is a nice greenish turquoise. Problem--I used it on a brie
>baker and when the brie was baked in it the glaze faded to a lighter turquoise
>without the greenish wherever the brie touched. I wondered if it was the
>acidity in the cheese (Or milk products in general) so I put some yogurt in it
>(having dumped the brie out) and the same thing happened--wherever the yogurt
>touched, the glaze faded to the point it had with the brie. I guess this is
>not a food safe glaze?? Since I saw the Mexico Point Green posted in clayart
>I thought I would mention this problem and also see if anyone else had run
>across the same problem with this glaze--or if these is a way of correcting
>it? Someone suggested it was caused by the iron in the clay--I use a buff
>clay. Any thoughts?
Ron Roy
93 Pegasus trail
Scarborough Otario
Canada M1G 3N8
Phone: 416-439-2621
Fax: 416-438-7849
Web page: Home page http://digitalfire.com/education/people/ronroy.htm
Unruly JuliE on mon 16 feb 98
Ron,
what would be your recalculated glaze formula?? If you don't
mind me asking.
JuliE
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