"Marilyn B. DaleL-Soft list server at University of" on fri 4 dec 98
I have a question about the following ^5-^6, oxidation glaze, please,
thanks in advance to all who answer!
Copper carb......... 4
( Doesn't add up to 100. 5 grams of EPK was added to the original recipe
to help with drips)
Here's the thing....I have just actually just sent this glaze to Alfred
for testing. And, I JUST received an order for 100 mugs, due in 14 days,
and the customer would like this glaze on the mugs! ( Alfred can't
answer me in that time!)
I read on the INSIGHT web page that more than 10% gerstley borate in a
high fire glaze is suspect....Am I in trouble at mid range? I truly am
beginning to learn about balancing chemicals, (i'm scared of poisoning
someone.)Until I order the glaze calculation program, would someone tell
me if this glaze is OK for food surfaces?
John Hesselberth on sat 5 dec 98
Your glaze is very similar to the Broken Blue glaze I posted the other
day which had very low extractables. See that post and compare. Yours
has 4% copper vs. the 3% in my glaze. It is very high in boron and is
also high in calcium, but both alumina and silica are in normal ranges.
My biggest worry is the copper being at 4%. While I don't have enough
data to be sure yet, copper seems to do OK at low levels and then passes
through a limit where a lot of the excess is extractable. Bottom line--I
don't know but I would guess the extractables would be pretty low. This
is not a science, however, and the only way to be sure is to test it. Of
course, even when you get your copper number back, you will still have to
decide whether you will sell that level or not. There are no legal
limits on copper and no one knows how much is safe. The only benchmarks
we have are that 10 ppm (or mg/l) can cause orange juice and similar
liquids to taste bitter and 1.3 ppm is the legal limit for drinking
With lead, the legal limit for pottery varies from 0.1-3.0 ppm depending
on the state you are in and the type of vessel you put it on--that range
is from 7-200 times the amount which is allowable in drinking water
(0.015ppm). Can copper still be safe at 7-200 times the allowable water
limit? No one knows. If you want to be absolutely bullet proof I
suppose you would try for the water limit, but there probably won't be
many glazes brought to market that look very interesting at that level.
My own goal is to be well under 10 ppm on copper for any mugs, pitchers
and the like that are intended to hold liquids. But given that the test
is much more rigorous than a vessel is likely to see in routine use, I
don't feel the need to be at or below the water level. You will
undoubtedly hear disagreement on this point from many clayarters,
however. Some feel is is absurd to even be worrying about copper and
other transition metals while others feel the water limits are what we
ought to shoot for. Sorry, there is no clean answer on this one.
While you are waiting for your Alfred results you can certainly do the 24
hour vinegar test. If you see a color change in that time, I would
recommend you don't sell the glaze. You can also start a dishwasher test
and see if your mugs are durable there.
Good luck and please let us know the results of your test. I am
collecting all such data I can get my hands on so we can gradually get a
better understanding of this subject.
Marilyn B. DaleL-Soft list server at University of wrote:
>I have a question about the following ^5-^6, oxidation glaze, please,
>thanks in advance to all who answer!
>Copper carb......... 4
>( Doesn't add up to 100. 5 grams of EPK was added to the original recipe
>to help with drips)
>Here's the thing....I have just actually just sent this glaze to Alfred
>for testing. And, I JUST received an order for 100 mugs, due in 14 days,
>and the customer would like this glaze on the mugs! ( Alfred can't
>answer me in that time!)
>I read on the INSIGHT web page that more than 10% gerstley borate in a
>high fire glaze is suspect....Am I in trouble at mid range? I truly am
>beginning to learn about balancing chemicals, (i'm scared of poisoning
>someone.)Until I order the glaze calculation program, would someone tell
>me if this glaze is OK for food surfaces?
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: firstname.lastname@example.org web site: http://www.frogpondpottery.com
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