Doug Trott on tue 21 oct 08
When reading about the dangers of certain chemicals in glazes=2C it's not a=
lways clear if the danger is only in handling the material while making the=
glaze=2C or if the glaze is not food-safe=2C too.
A couple weeks ago I mixed my first glazes (whoopee!) from John Britt's boo=
k. One was the Ranch Butter=2C which includes 5.2% zinc oxide. There was =
some discussion of toxicity=2C but it sounds like it burns off completely i=
n c10 reduction. There's a photo of a pitcher with that glaze=2C inside an=
d out. Can I assume it's food-safe?
I also mixed a Magnesium Matte Purple that includes 1% cobalt carbonate. T=
hat seems like more of an issue than the zinc. But is it automatically not=
food safe? Would it be not safe as a liner glaze holding liquids=2C but s=
afe on a plate or bowl holding solid food? Or should it be on decorative w=
ork only?
Or do I need to test the glaze for leaching every time? Are there times wh=
en leaching is not a worry=2C due to low concentrations?
Thanks for your advice!
Doug
The Fuzzy Chef on wed 22 oct 08
Doug,
> When reading about the dangers of certain chemicals in glazes, it's not always clear if the danger is only in handling the material while making the glaze, or if the glaze is not food-safe, too.
Depends on the glaze.
> A couple weeks ago I mixed my first glazes (whoopee!) from John Britt's book. One was the Ranch Butter, which includes 5.2% zinc oxide. There was some discussion of toxicity, but it sounds like it burns off completely in c10 reduction. There's a photo of a pitcher with that glaze, inside and out. Can I assume it's food-safe?
You can't assume *anything* unless the author says that they tested the
glaze for leaching, and gives test results. The authors of MC6G do
this, but no other glaze book author I've seen does. Without that,
*you* need to test the glaze yourself. MC6G has a whole chapter about
glaze toxicity testing; it's worth the $25 price of the book.
> I also mixed a Magnesium Matte Purple that includes 1% cobalt carbonate. That seems like more of an issue than the zinc. But is it automatically not food safe? Would it be not safe as a liner glaze holding liquids, but safe on a plate or bowl holding solid food? Or should it be on decorative work only?
It really depends on the base glaze. Cobalt isn't espeically toxic
AFAIK (compared to Manganese or Lithium) but it's really a question of
how able it is to escape from the glaze.
> Or do I need to test the glaze for leaching every time? Are there times when leaching is not a worry, due to low concentrations?
Presumably if you made a glaze which contained no toxic materials, based
on only feldspar, clay, silica, and iron you could ignore testing. As
far as I know, all of the colorant metallic oxides are toxic if you end
up drinking them. It's not a question of how much oxide is in the
glaze, it's a question of how much leaches *out* of the glaze.
You only need to test each new *glaze* though, not every piece. So
that's a modest burden. Also, I tend to believe that if you're a
hobbyist doing the "home" testing and using formulas for safe glazes is
adequate; doing extensive lab testing is mostly for people making a
living off their pottery.
--Josh
Ron Roy on mon 27 oct 08
Hi Doug,
We cover the subject in our book - Mastering Cone 6 glazes - the info on
glaze stability applies to cone 10 glazes as well.
There are ways to tell from a glaze recipe - if a glaze is likely to be
stable - or unstable. That is a separate issue from the toxicity of any
materials in a glaze. If there are no toxins in a glaze then the glazes
stability is not the same kind of issue. If however a blue glaze loses it's
colour in use - over time or immediately under some circumstances - that is
another issue.
The worst situation is to find a glaze with toxic materials in it which
shows changes in use.
By the way - you can test to see if the zinc does go away - just mix the
glaze up without the zinc and see if it melts the same. Pretty silly to
have an expensive material in a glaze that does no good.
Let me know if you have questions - RR
>When reading about the dangers of certain chemicals in glazes, it's not
>always clear if the danger is only in handling the material while making
>the glaze, or if the glaze is not food-safe, too.
>
>A couple weeks ago I mixed my first glazes (whoopee!) from John Britt's
>book. One was the Ranch Butter, which includes 5.2% zinc oxide. There
>was some discussion of toxicity, but it sounds like it burns off
>completely in c10 reduction. There's a photo of a pitcher with that
>glaze, inside and out. Can I assume it's food-safe?
>
>I also mixed a Magnesium Matte Purple that includes 1% cobalt carbonate.
>That seems like more of an issue than the zinc. But is it automatically
>not food safe? Would it be not safe as a liner glaze holding liquids, but
>safe on a plate or bowl holding solid food? Or should it be on decorative
>work only?
>
>Or do I need to test the glaze for leaching every time? Are there times
>when leaching is not a worry, due to low concentrations?
>
>Thanks for your advice!
>
>Doug
Ron Roy
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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