Janet Haugen on wed 1 mar 06
I have a recipe for a beautiful green-black satin matt cone 9 reduction glaze:
Green Black Satin Matt, Cone 9 Reduction
Kona F4 Feldspar 41.0
Cornwall Stone 20.0
Whiting 19.0
Kentucky Ball Clay 9.0
Calcined Kaolin 8.0
Zinc Oxide 3.0
Cobalt Oxide .5
Copper Oxide 5.0
Rutile 1.0
I would like to use it on a baking or serving dish, but I do not yet understand enough about glaze chemistry to know if it is food safe.
Can any of you who are knowledgeable about glaze chemistry tell me:
Is it food safe?
Could it be used for baking at around 400 degrees F?
If the answer to these questions is No, could anyone recommend substitutions that would make it functional?
Thank you very much for any help you can give me.
Janet
Ron Roy on thu 2 mar 06
Hi Janet,
Not an easy question - first of all - the zinc is probably gone if it's
fired in reduction - I recommend testing this glaze without it - like a
1000 gram batch.
If I look at the silica and alumina levels with the zinc in as a flux -
there is enough of each to make a durable glaze but just enough silica -
which means it is going to release copper into acidic food - there is just
too much copper. It will probably discolour food and change the taste.
If we look at it without the ZnO - then there is more alumina and more
silica. The increased silica will help the stability of the glaze but the
alumina is now quite high and the glaze may not be maturing properly - well
melted glazes are more durable than glazes that are immature.
Safety is not the issue except for those individuals that cannot tolerate
extra copper.
I would say this glaze is a poor choice for a liner glaze - especially oven
ware.
Try drinking orange juice from a pot glazed with it - you should be able to
taste the copper after the juice has been in it for a while.
Expansion is high by the way - probably crazing.
Would it be OK with out the copper - I would say maybe - but there is still
that high alumina which may affect the durability.
RR
>I have a recipe for a beautiful green-black satin matt cone 9 reduction glaze:
>
> Green Black Satin Matt, Cone 9 Reduction
>
> Kona F4 Feldspar 41.0
>Cornwall Stone 20.0
>Whiting 19.0
>Kentucky Ball Clay 9.0
> Calcined Kaolin 8.0
> Zinc Oxide 3.0
>
> Cobalt Oxide .5
> Copper Oxide 5.0
> Rutile 1.0
>
> I would like to use it on a baking or serving dish, but I do not yet
>understand enough about glaze chemistry to know if it is food safe.
>
> Can any of you who are knowledgeable about glaze chemistry tell me:
>
> Is it food safe?
> Could it be used for baking at around 400 degrees F?
>
> If the answer to these questions is No, could anyone recommend
>substitutions that would make it functional?
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
John Hesselberth on thu 2 mar 06
On Mar 1, 2006, at 12:44 PM, Janet Haugen wrote:
> I have a recipe for a beautiful green-black satin matt cone 9 =20
> reduction glaze:
>
> Green Black Satin Matt, Cone 9 Reduction
>
> Kona F4 Feldspar 41.0
> Cornwall Stone 20.0
> Whiting 19.0
> Kentucky Ball Clay 9.0
> Calcined Kaolin 8.0
> Zinc Oxide 3.0
>
> Cobalt Oxide .5
> Copper Oxide 5.0
> Rutile 1.0
>
> I would like to use it on a baking or serving dish, but I do not =20
> yet understand enough about glaze chemistry to know if it is food =20
> safe.
>
> Can any of you who are knowledgeable about glaze chemistry tell me:
>
> Is it food safe?
>
Hi Janet,
The answer, in my opinion, is probably not. It does have plenty of =20
alumina and silica (although the silica is at the lower end of what I =20=
have found to be necessary for a stable glaze). But it also has a lot =20=
of copper which is very difficult to keep in a glaze. And of course =20
using it for a baking dish--particularly when someone bakes something =20=
that is fairly acidic like tomato-based dishes--is tough duty. Also =20
it calls for copper oxide (red or black?) instead of copper =20
carbonate. I have found that about 5% copper carbonate is about as =20
much copper as any glaze can hold and, depending on whether you use =20
red or black copper oxide, this could have as much as 8-12% on a =20
copper carbonate equivalent basis--I'm doing this in my head from =20
memory of molecular weights so don't hold me to the exact number--the =20=
point is it is a LOT of copper.
Another potential problem is crazing. This glaze has a very high =20
coefficient of expansion. Sometimes, though, I have seen a high level =20=
of copper mitigate the tendency to craze--I don't know if it would =20
happen here or not.
I have pasted the Seger unity formula below (I didn't know which =20
Kentucky ball clay you used--there are several but using a different =20
one won't make enough difference to change my opinion). As to what to =20=
change to make if more stable--take 1/2 or more of the copper out. Of =20=
course that will probably change the color substantially.
Sorry I can't be more positive about this one, but I would not use it =20=
on functional work.
Regards,
John
Recipe Name: Green Black Satin Matte
Cone: 9 Color:
Firing: Reduction Surface: Satin
Amount Ingredient
41 Feldspar--Kona F4
20 Cornwall Stone
19 Whiting
9 Ball Clay--Kentucky #6
8 Kaolin--Calcined
3 Zinc Oxide
100 Total
Additives
.5 Cobalt Oxide
5 Copper Oxide--Black
1 Rutile
Unity Oxide
.174 Na2O
.088 K2O
.008 MgO
.619 CaO
.11 ZnO
1.000 Total
.496 Al2O3
.003 Fe2O3
2.592 SiO2
.008 TiO2
.002 P2O5
5.2 Ratio
80.2 Exp
Comments:
-----------------------------------
Calculations by GlazeMaster=99
www.masteringglazes.com
------------------------------------
Susanne Notenboom on mon 6 mar 06
Hai,
You can try a simple test (it is not garanteed)but it helps.
Take a sample off your glaze and put paper sticky tape on a part off this
sample and put it in vinegar for three (3)days. if there is a difference
when you remove the sticky tape than don't put it on functional ware.
The acid in the vinegar will change the look and feel of the sample.
more information look at the website of ian.currie.com
Greetings,
Susanne Notenboom from Rotterdam the Netherlands
| |
|
