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question - discolouring glaze

updated tue 30 sep 97

 

ZALT@aol.com on wed 27 aug 97

I have been using an altered Shaner Red Glaze for a while and recently had a
report of discolouring of the glaze when the pot was used to store acid
foods. I henceforth placed strawberry jam and vinigar in one of the pots
and left it in the refrigerator for approximately 14 days. The result was a
bleaching to a whitish stain on the surface of the glaze.

What is happening?
Is the process creating toxins?
Can the colour be returned through refiring?

The recipe is as follows:

Bone ash 9
EPK 22
G200 47 (the original doc uses Custer Spar)
Talc 3
Whiting 19

addition:
RIO 4

fired to cone 9 flat, reduction

Terrance Frank Lazaroff
St Hubert, Quebec, Canada!!!!!!!!

Ron Roy on fri 29 aug 97

A classic example of a glaze short of silica - It's a matte glaze (right?)
and it's crazing as well (right.) The fortunate part - no toxic materials
are being leached out because there are none present - unless ferritin is a
problem as it is for me.

The acid in foods have a tough time getting into a glaze with enough silica
present for the maturing temperature of that glaze. To put it another way -
if you want your glass to have resistance to acids - make sure there is
enough silican in it.

The generalization that matt glazes should not be where they can come in
contact with food in confirmed here - especially colored matte glazes.

Here is the same glaze with enough silica to be stable - it is no longer
matte however - but a better solution for a cleanable liner glaze. The
expansion is still a little on the high side but it may not craze on some
bodies.

Bone ash 7
EPK 17
G200 36 (the original doc uses Custer Spar)
Talc 3
Whiting 14
Silica 23
addition:
RIO 4
Total 104.0
Ratio 7.36 (original was 4.34)

>----------------------------Original message----------------------------
>I have been using an altered Shaner Red Glaze for a while and recently had a
>report of discolouring of the glaze when the pot was used to store acid
>foods. I henceforth placed strawberry jam and vinigar in one of the pots
>and left it in the refrigerator for approximately 14 days. The result was a
>bleaching to a whitish stain on the surface of the glaze.
>
>What is happening?
>Is the process creating toxins?
>Can the colour be returned through refiring?
>
>The recipe is as follows:
>
>Bone ash 9
>EPK 22
>G200 47 (the original doc uses Custer Spar)
>Talc 3
>Whiting 19
>
>addition:
>RIO 4
>
>fired to cone 9 flat, reduction
>
>Terrance Frank Lazaroff
>St Hubert, Quebec, Canada!!!!!!!!

Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm

Carol Ratliff.clayart.CLAYART.MAILING LIST on mon 1 sep 97

Hey guys...I thought we weren't going to post formulas with out the firing
temp/ cone. So where is this one firing at?

In a message dated 97-08-31 22:29:45 EDT, you write:

<<
Here is the same glaze with enough silica to be stable - it is no longer
matte however - but a better solution for a cleanable liner glaze. The
expansion is still a little on the high side but it may not craze on some
bodies.

Bone ash 7
EPK 17
G200 36 (the original doc uses Custer Spar)
Talc 3
Whiting 14
Silica 23
addition:
RIO 4
Total 104.0
Ratio 7.36 (original was 4.34)

>>

Ron Roy on tue 2 sep 97

Excuse me - Terrance says he fired the original to cone 9 flat - I guess
that means 9.5 at least. This is an untested glaze at any temperature but
it will be a balanced glaze at cone 9 -10



>----------------------------Original message----------------------------
>Hey guys...I thought we weren't going to post formulas with out the firing
>temp/ cone. So where is this one firing at?
>
>In a message dated 97-08-31 22:29:45 EDT, you write:
>
><<
> Here is the same glaze with enough silica to be stable - it is no longer
> matte however - but a better solution for a cleanable liner glaze. The
> expansion is still a little on the high side but it may not craze on some
> bodies.
>
> Bone ash 7
> EPK 17
> G200 36 (the original doc uses Custer Spar)
> Talc 3
> Whiting 14
> Silica 23
> addition:
> RIO 4
> Total 104.0
> Ratio 7.36 (original was 4.34)
>
> >>

Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/people/ronroy.htm