Sarah Bailey on tue 27 jan 04
We recently had several low-fire glazes (cone 01) tested by Alfred
Analytical Lab for copper leaching and several came back with levels of
leaching between 4 mg/L and 30 mg/L. I've been reading the archives and the
Mastering Cone 6 Glazes book but haven't found much information on stability
or copper leaching in low fire glazes. I'm wondering if our base glaze is
unstable to begin with (based on information I've read about stability in
cone 6 glazes) and if it's possible that by adjusting the base glaze to
increase the alumina and silica levels, we might have less leaching. If
anyone has had experience with copper leaching in low-fire glazes or has
suggestions on improving stability, I'd be very grateful for your advice. We
fire to cone 01 at a rate of 150 degrees F/hour with a 30 min. soak at cone
01. Our base glaze recipe from the VC handbook:
1 - AC Line Base Glaze
=================================
FRIT 3124........... 52.00 50.49%
F-4 FELDSPAR........ 15.00 14.56%
Gerstley Borate..... 17.00 16.50%
WHITING............. 3.00 2.91%
EPK KAOLIN.......... 2.00 1.94%
FLINT............... 11.00 10.68%
BONE ASH............ 3.00 2.91%
========
103.00
CaO 0.71* 15.66%w 17.77%m
MgO 0.04* 0.63 0.99
K2O 0.03* 1.18 0.80
Na2O 0.22* 5.29 5.43
TiO2 0.00 0.01 0.01
Al2O3 0.23 9.39 5.86
B2O3 0.46 12.56 11.49
P2O5 0.03 1.44 0.64
SiO2 2.28 53.79 56.99
Fe2O3 0.00 0.05 0.02
Cost/kg 12.83
Si:Al 9.72
SiB:Al 11.68
Expan 7.64
Copper leaching with 2% copper ox, and 4% chromium ox: 4.41 mg/L (chromium
leaching was .056 mg/L)
Copper leaching with 3% copper ox., 1.5% rutile, and 2% red iron oxide:
9.52 mg/L
Copper leaching with 6% copper ox.: 30.4 mg/L
Thank you for any suggestions!
Sarah
John Hesselberth on wed 28 jan 04
On Tuesday, January 27, 2004, at 03:25 PM, Sarah Bailey wrote:
> . I'm wondering if our base glaze is
> unstable to begin with (based on information I've read about stability
> in
> cone 6 glazes) and if it's possible that by adjusting the base glaze to
> increase the alumina and silica levels, we might have less leaching.
Hi Sarah,
Yes your base glaze is low in both silica and alumina. If you want to
get it to hold 4% copper (or even 3%) you will have to get your silica
level up to about 3.0 and your alumina level up to about 0.3. Keep the
ratio of the Si/Al about the same as it is in your current glaze. You
can do this by adding clay and silica using your calculation software
to guide you on how much.
The question then will be whether or not it will still melt. I am just
guessing that it will at cone 01; however if you are up to a good glaze
reformulation exercise you might also try a second test raising the
boron level to about 0.6 while keeping everything else the same (except
for the silica and alumina as noted above).
It's great that you are checking this. While I personally don't worry
too much about the leaching level you got at 2% copper, the levels at 3
and 4% are definitely something I would not put on functional work.
Even if they don't cause anyone health problems, that amount of copper
can make food taste bitter. In additional, the 4% sample, and maybe the
3% one, probably won't pass the vinegar test and will show fading in
use.
Regards,
John
http://www.frogpondpottery.com
http://www.masteringglazes.com
Ron Roy on sat 31 jan 04
Hi Sarah,
At first I did not know what to do - how to increase the alumina and silica
and keep the melting - the Boron was almost at 12% and going over that
increases expansion - what I did know was that if I increased the frit
(premelted material) at the expense of the materials that don't melt worth
a damn at 04 (spar, whiting, bone ash) - I could add more silica and
alumina.
Anyway - it dawned on me - reduce the GB and increase the 3124 (GB has
about 25 to 28% B2O3 and frit 3124 only has 13.7. This approach keeps the
Boron at about 12% and so keeps the expansion low to avoid crazing. This
also means you need to test it on your clay body for fit.
So this is what I came up with - have it tested with 2, 3 and 4% copper and
let us know the results please.
Keep in mind - 5% rutile made a big difference in stability at cone 6 - why
not so a set with some added Rutil or TiO2 - 5 or 6% would be a good
amount.
Protect your shelves till you know if this runs too much - if it does we
can make it even more stable.
Frit 3124 - 77.0
Gers Borate - 6.0
EPK - 4.0
Silica - 13.0
Bentonite - 2.0
Total - 102.0
I don't keep copies of recipes anymore so include this if you have questions.
RR
>We recently had several low-fire glazes (cone 01) tested by Alfred
>Analytical Lab for copper leaching and several came back with levels of
>leaching between 4 mg/L and 30 mg/L. I've been reading the archives and the
>Mastering Cone 6 Glazes book but haven't found much information on stability
>or copper leaching in low fire glazes. I'm wondering if our base glaze is
>unstable to begin with (based on information I've read about stability in
>cone 6 glazes) and if it's possible that by adjusting the base glaze to
>increase the alumina and silica levels, we might have less leaching. If
>anyone has had experience with copper leaching in low-fire glazes or has
>suggestions on improving stability, I'd be very grateful for your advice. We
>fire to cone 01 at a rate of 150 degrees F/hour with a 30 min. soak at cone
>01. Our base glaze recipe from the VC handbook:
>
>1 - AC Line Base Glaze
>=================================
> FRIT 3124........... 52.00 50.49%
> F-4 FELDSPAR........ 15.00 14.56%
> Gerstley Borate..... 17.00 16.50%
> WHITING............. 3.00 2.91%
> EPK KAOLIN.......... 2.00 1.94%
> FLINT............... 11.00 10.68%
> BONE ASH............ 3.00 2.91%
> ========
> 103.00
>
> CaO 0.71* 15.66%w 17.77%m
> MgO 0.04* 0.63 0.99
> K2O 0.03* 1.18 0.80
> Na2O 0.22* 5.29 5.43
> TiO2 0.00 0.01 0.01
> Al2O3 0.23 9.39 5.86
> B2O3 0.46 12.56 11.49
> P2O5 0.03 1.44 0.64
> SiO2 2.28 53.79 56.99
> Fe2O3 0.00 0.05 0.02
>
> Cost/kg 12.83
> Si:Al 9.72
> SiB:Al 11.68
> Expan 7.64
>
>Copper leaching with 2% copper ox, and 4% chromium ox: 4.41 mg/L (chromium
>leaching was .056 mg/L)
>Copper leaching with 3% copper ox., 1.5% rutile, and 2% red iron oxide:
>9.52 mg/L
>Copper leaching with 6% copper ox.: 30.4 mg/L
>
>Thank you for any suggestions!
>
>Sarah
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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