Lynn Goodman Porcelain Pottery on sat 30 sep 06
A mason stain in a clear, stable glaze would do it. You can probably
mix in a little copper (2% or so) to add depth to the green stain. You
can also mix a couple of different stains together for some variation.
Lynn
On Sep 30, 2006, at 11:35 AM, Jane Murray-Smith wrote:
> I have a cone 6 transparent dark emerald green glaze that has 5% copper
> and failed the vinegar test.
> My favorite thing to glaze with this is a platter with an imprinted
> pattern (the glaze pools beautifully)
> Is there such a thing as a glaze with these properties that is food
> safe?...I also fire to cone 10, so am open to any suggestions..
> Mericfully I made very few pieces in this glaze before a fellow potter
> did
> the vinegar test as she had the same glaze..(given as functional
> glaze by
> an experienced production potter at a workshop!....)
> Jane
>
Lynn Goodman
Fine Porcelain Pottery
Cell 347-526-9805
www.lynngoodmanporcelain.com
Don Goodrich on sat 30 sep 06
Okay Jane,
The troublesome recipe is a tad high in alumina. Its silica:alumina
ratio is 7.71:1,
with calculated expansion of 77.9 .
By reducing the kaolin and increasing the silica, the ratio can be
increased without reducing the expansion much
(so the glaze should still fit okay).
How about trying the following:
Dark Transparent Green, revised
Kaolin 4
Gerstley borate 10
Whiting 16
Nepheline syenite 37
Silica 32
Zinc oxide 1
Copper carbonate 5
Na2O 0.20 Al2O3 0.29 SiO2 2.69
K2O 0.05 B2O3 0.12
MgO 0.03
CaO 0.57
CuO 0.11
ZnO 0.03
Alumina:Silica ratio is 1.00:9.39
Neutral:Acid ratio is 1.00:6.65
Alk:Neut:Acid ratio is 1.00:0.41:2.69
Expansion: 76.5 x 10e-7 per degree C
Take precautions when testing this, in case it's runnier than the original.
You mentioned that you also fire to cone 10. The original recipe is within
limits for a ^9-10 gloss glaze, but could take a lot more silica and still
be within those limits. You could try the recipe at ^10, with kaolin at 8
but with silica increased from 27 to 40 or more. This should make it more
likely to pass the vinegar test but will reduce the expansion. It shouldn't
take too much tinkering to get what you're looking for.
Happy testing,
Don Goodrich
Jane Murray-Smith on sat 30 sep 06
I have a cone 6 transparent dark emerald green glaze that has 5% copper
and failed the vinegar test.
My favorite thing to glaze with this is a platter with an imprinted
pattern (the glaze pools beautifully)
Is there such a thing as a glaze with these properties that is food
safe?...I also fire to cone 10, so am open to any suggestions..
Mericfully I made very few pieces in this glaze before a fellow potter did
the vinegar test as she had the same glaze..(given as functional glaze by
an experienced production potter at a workshop!....)
Jane
Don Goodrich on sat 30 sep 06
Hi Jane,
It's possible that your dark green glaze might become able to pass the
vinegar test with some tinkering. Increasing the silica could help.
I have a light green that, although very pleasing, would change color if
vinegar or lemon was left in it for several hours. A slight adjustment
made it stable enough that this is no longer a concern.
If you're willing to let the list see the recipe, you're bound to get
some helpful suggestions that you can test.
Don Goodrich
goodrichdn@aol.com
http://dongoodrichpottery.com/
>I have a cone 6 transparent dark emerald green glaze that has 5% copper
>and failed the vinegar test.
>My favorite thing to glaze with this is a platter with an imprinted
>pattern (the glaze pools beautifully)
>Is there such a thing as a glaze with these properties that is food
>safe?...I also fire to cone 10, so am open to any suggestions..
>Mericfully I made very few pieces in this glaze before a fellow potter did
>the vinegar test as she had the same glaze..(given as functional glaze by
>an experienced production potter at a workshop!....)
>Jane
Jane Murray-Smith on sat 30 sep 06
So heres the troublesome recipe
Kaolin 8
Gerstley Borate 10.5
Whiting 16
Neph sye 37.5
Silica 27
Zinc 1
copper carb 5
Thanks, Jane
----- Original Message -----
From: "Don Goodrich"
To:
Sent: Saturday, September 30, 2006 2:52 PM
Subject: Re: food safe dark transparent green glaze?
> Hi Jane,
> It's possible that your dark green glaze might become able to pass the
> vinegar test with some tinkering. Increasing the silica could help.
> I have a light green that, although very pleasing, would change color if
> vinegar or lemon was left in it for several hours. A slight adjustment
> made it stable enough that this is no longer a concern.
>
> If you're willing to let the list see the recipe, you're bound to get
> some helpful suggestions that you can test.
>
> Don Goodrich
> goodrichdn@aol.com
>
> http://dongoodrichpottery.com/
>
>>I have a cone 6 transparent dark emerald green glaze that has 5% copper
>>and failed the vinegar test.
>>My favorite thing to glaze with this is a platter with an imprinted
>>pattern (the glaze pools beautifully)
>>Is there such a thing as a glaze with these properties that is food
>>safe?...I also fire to cone 10, so am open to any suggestions..
>>Mericfully I made very few pieces in this glaze before a fellow potter did
>>the vinegar test as she had the same glaze..(given as functional glaze by
>>an experienced production potter at a workshop!....)
>>Jane
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.
>
Ron Roy on sun 1 oct 06
Hi Jane,
Because the GB amount is low for a cone 6 glaze I suspect this glaze is
relying on the high amount of copper to help melt it - but it also might me
not fully melted.
I would suggest a line blend adding some frit 3134 and more silica. Try
adding 2% at a time up to a total of 12% of each.
Remember also that the vinegar test only shows grossly unstable glazes -
that glaze - because of the high amount of copper - will probably always
leach copper into acidic food. It will change the taste of orange juice in
a short time for instance.
Keeping that amount of copper in a glaze is very difficult - even with high
levels of silica and alumina - increasing the silica is a good idea but
decreasing the copper will help even more.
I would like to know which experienced potter said it was good for food -
I'm not surprised to hear that though.
RR
>So heres the troublesome recipe
>
>Kaolin 8
>Gerstley Borate 10.5
>Whiting 16
>Neph sye 37.5
>Silica 27
>Zinc 1
>copper carb 5
>
>Thanks, Jane
>
>
>
>----- Original Message -----
>From: "Don Goodrich"
>To:
>Sent: Saturday, September 30, 2006 2:52 PM
>Subject: Re: food safe dark transparent green glaze?
>
>
>> Hi Jane,
>> It's possible that your dark green glaze might become able to pass the
>> vinegar test with some tinkering. Increasing the silica could help.
>> I have a light green that, although very pleasing, would change color if
>> vinegar or lemon was left in it for several hours. A slight adjustment
>> made it stable enough that this is no longer a concern.
>>
>> If you're willing to let the list see the recipe, you're bound to get
>> some helpful suggestions that you can test.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Ivor and Olive Lewis on mon 2 oct 06
Dear Lynn Goodman,
I agree that using a proprietary green stain in a durable glaze would =
solve the problem as far as an approximate colour match is concerned but =
are there two other points that might be considered ?
The first is that to get the colour intensity needed the quantity of =
stain required may cause some degree of opacity to a transparent glaze.
The second point seems to be that neither Copper oxide nor Copper =
carbonate may never have been reported or recorded as entering into a =
chemical reaction in a silicate fusion to give a stable unreactive =
compound. This could be the reason Copper bearing glazes leach that =
element regardless of the percentage added to a glaze.
Best regards,
Ivor Lewis.
Redhill,
South Australia.
Lynn Goodman Porcelain Pottery on mon 2 oct 06
True; I was going only on the basis of color.
I use a lot of stains, and I get translucency, not transparency. If a
stain gives me an opaque result, I don't use it. I do mix stains and
oxides--that gives me better transparency, and better depth of color.
As far as the copper being in the original calculation, I suspect
you're right. While I account for additions now, I was taught (back
when the dinosaurs roamed) to do testing only on the base glaze.
Lynn
On Oct 2, 2006, at 2:22 AM, Ivor and Olive Lewis wrote:
> Dear Lynn Goodman,
>
> I agree that using a proprietary green stain in a durable glaze would
> solve the problem as far as an approximate colour match is concerned
> but are there two other points that might be considered ?
>
> The first is that to get the colour intensity needed the quantity of
> stain required may cause some degree of opacity to a transparent
> glaze.
>
> The second point seems to be that neither Copper oxide nor Copper
> carbonate may never have been reported or recorded as entering into a
> chemical reaction in a silicate fusion to give a stable unreactive
> compound. This could be the reason Copper bearing glazes leach that
> element regardless of the percentage added to a glaze.
>
> Best regards,
>
> Ivor Lewis.
> Redhill,
> South Australia.
>
> _______________________________________________________________________
> _______
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at
> melpots@pclink.com.
>
>
Lynn Goodman
Fine Porcelain Pottery
Cell 347-526-9805
www.lynngoodmanporcelain.com
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