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how can you tell if a glaze is food safe?

updated sun 24 sep 06

 

Steve Slatin on thu 21 sep 06


Jared --

You really can't tell in general, but there are some guidelines
that are good places to start. Mastering Cone Six Glazes
has a good list. The problem is even some glazes that
don't meet the guidelines are stable, and even some that
do are unstable. Most that meet the guidelines are
stable, though.

Glossy Mossy Flossy pushes a few of the guidelines -- it's
a little low on COE (though it fits well on several of my clays)
and I recently discovered that it breaks one of John H's rules
on contents, being just a little above his suggested max on
alumina. (It gets its texture from being low on Si:Al ratio, but
it does have sufficient Si to have a crack at stability.

The way to be sure a glaze is food-safe is to test it for
stability under acid attack (easy) and alkali durability
(harder). There are really good guidelines for testing in
Mastering Cone Six Glazes.

I've tested GMF with cobalt, copper, and extra iron in
the glaze, and it does hold up over the clays I use (Sea
Mix 5, OH-6, Vashon gray, Goldstone) but it might not
with other clays. It is very different in surface texture
and color over different clays, and there may be some
combination that wouldn't work.

Test, test, test.

Best wishes -- Steve Slatin



Jared Stutesman wrote:
I am wondering about these glaze I came across on the internet:

Recipe Name: Glossy Mossy Flossy

Cone: 6 Color: White, brown specks
Firing: Oxidation Surface: Not quite glossy

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Jared Stutesman on thu 21 sep 06


I am wondering about these glaze I came across on the internet:

Recipe Name: Glossy Mossy Flossy

Cone: 6 Color: White, brown specks
Firing: Oxidation Surface: Not quite glossy

Amount Ingredient
400 Nepheline Syenite
300 Silica
100 Whiting
300 Gillespie Borate
200 Kaolin--EPK
100 Spodumene--Gwalia

1400 Total

Additives
100 Tin Oxide
11 Iron Oxide--Red
(found this on on clayart archives)

and :

Purple Icing (Cone 6 Oxidation)

Gerstley Borate 6.2%
Lithium Carbonate 2.0%
Magnesium Carbonate 6.1%
Strontium Carbonate 12.6%
Whiting 12.1%
Nepheline Syenite 21.0%
Flint 40.0%
Add: Tin Oxide 3.0%
Cobalt Carbonate 1.0%
Rutile 3.6%
Bentonite 2.0%

if anyone knows anything about these two glazes or a way that i can
tell in general i would love to know.

Jared Stutesman

Ron Roy on sat 23 sep 06


Hi Jared,

I agree with Steve about the first glaze but you would need to have it
tested in a lab to find out if it was really a stable glaze - it does
contain a lot of alumina so it may not be well melted - one of the
criteria for a stable glaze.

The second glaze is not stable - way low in alumina and also short of
silica. It also has no clay in it - relying on the GB to float it in
the bucket - also a lot of Neph Sy so it may deflocculate eventually.
This glaze may run of pots so test it with something to catch drips.

Maybe a mixture of the two would be stable.

Let me know if you have any questions - RR


On 21-Sep-06, at 8:40 AM, Jared Stutesman wrote:

> I am wondering about these glaze I came across on the internet:
>
> Recipe Name: Glossy Mossy Flossy
>
> Cone: 6 Color: White, brown specks
> Firing: Oxidation Surface: Not quite glossy
>
> Amount Ingredient
> 400 Nepheline Syenite
> 300 Silica
> 100 Whiting
> 300 Gillespie Borate
> 200 Kaolin--EPK
> 100 Spodumene--Gwalia
>
> 1400 Total
>
> Additives
> 100 Tin Oxide
> 11 Iron Oxide--Red
> (found this on on clayart archives)
>
> and :
>
> Purple Icing (Cone 6 Oxidation)
>
> Gerstley Borate 6.2%
> Lithium Carbonate 2.0%
> Magnesium Carbonate 6.1%
> Strontium Carbonate 12.6%
> Whiting 12.1%
> Nepheline Syenite 21.0%
> Flint 40.0%
> Add: Tin Oxide 3.0%
> Cobalt Carbonate 1.0%
> Rutile 3.6%
> Bentonite 2.0%
>
> if anyone knows anything about these two glazes or a way that i can
> tell in general i would love to know.
>
> Jared Stutesman