Donna Kat on sat 28 oct 06
This is a studio glaze which is crazing. I have been asked to adjust it
and have been adding increments of 100gms of Silica. From my calculations
it seems that the Silica should actually measure 3500 grams rather than
the 2200 of the recipe. Any comments on this?
Bate's Clear,
Clear,
Glossy,
6,
,
,
Works nicely over other glazes,
,
Feldspar--Kona F4 ,3500,
Flint ,2200,
Gerstley Borate--1999 ,1800,
Kaolin--EPK ,1000,
Strontium Carbonate ,800,
Whiting ,800,
Timothy Joko-Veltman on sat 28 oct 06
On 10/28/06, Donna Kat wrote:
> This is a studio glaze which is crazing. I have been asked to adjust it
> and have been adding increments of 100gms of Silica. From my calculations
> it seems that the Silica should actually measure 3500 grams rather than
> the 2200 of the recipe. Any comments on this?
>
>
> Bate's Clear,
> Clear,
> Glossy,
> 6,
> ,
> ,
> Works nicely over other glazes,
> ,
> Feldspar--Kona F4 ,3500,
> Flint ,2200,
> Gerstley Borate--1999 ,1800,
> Kaolin--EPK ,1000,
> Strontium Carbonate ,800,
> Whiting ,800,
>
Donna,
It depends a lot on what body you're using. Insight gives the
expansion of this glaze as: 7.59. On the clay bodies I use, this
would definitely craze. To fix this recipe for my bodies, and ONLY by
adding silica, I'd have to more more or less triple the amount of
Flint. But then ... it's not matte, anymore, is it? Magnesium can
really help reduce thermal expansion (ie., crazing). Maybe some line
tests with dolomite and/or talc would be helpful.
Cheers,
Tim
Daniel Semler on sat 28 oct 06
Hi Donna,
I assume that you've found from your additions of silica a glaze
that no longer crazes ? You said calculations am I'm not sure if you
did calculations and know what expansion you are shooting for or if
you arrived at 3500 gms silica by testing.
As to comments. I would not have immediately picked this for a high
gloss with its silica alumina ratio in the original, so it must be the
boron helping out. If the glaze with all the extra silica is still
melting well and giving a defect free result then great. Its pretty
much at the high end of limits for much of the elements so I assume
that you're firing a hard cone 6. You've certainly lowered the
expansion - about 11% as far as I can tell.
If you haven't tested it I assume that you already know what you
are shooting for in the expansion calculations. It should still be
glossy but its temp will be a little higher.
Thanx
D
Ron Roy on sat 28 oct 06
Hi Donna,
Looks like you wind up with too much silica that way - you may not get
enough of a melt - and the expansion may not be low enough.
I would sub in spodumene for half the spar and add 5% talc to get the
alumina back down.
That lowers the expansion a lot - then do a 5 part line blend with the
original as #1 and the spod variation as #5
RR
>This is a studio glaze which is crazing. I have been asked to adjust it
>and have been adding increments of 100gms of Silica. From my calculations
>it seems that the Silica should actually measure 3500 grams rather than
>the 2200 of the recipe. Any comments on this?
>
>
>Bate's Clear,
>Clear,
>Glossy,
>6,
>,
>,
>Works nicely over other glazes,
>,
>Feldspar--Kona F4 ,3500,
>Flint ,2200,
>Gerstley Borate--1999 ,1800,
>Kaolin--EPK ,1000,
>Strontium Carbonate ,800,
>Whiting ,800,
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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