jklust on mon 3 may 99
I have a blue matt recipe from a pottery school that calls for spodumene. I
understand that spodumene is a lithium containing feldspar so does that =
mean it
is toxic to use in glazes intended for dinnerware? I use the glaze =
underneath a
transparent glossy glaze. I have also seen glazes I would love to try i.e.,
purple icing however it contains lithium carbonate which I understand is =
toxic
and not suitable for dinnerware. Can I substitute the lithium for something
else??? I also have a recipe for emerald that contains 2=25 chrome oxide can=
I
substitute copper carb. or copper oxide so that I can avoid the chrome all
togther???Any help in this is greatly appreciated. Thanks Kim
John Hesselberth on tue 4 may 99
Hi Kim,
I'm glad to see you are thinking about the potential safety issues before
you start to use or formulate glazes. Any of the materials you mentioned
can be used safely in glazes if the glaze is durable to leaching. You
have picked three metals though that deserve some individual discussion.
I should warn you at the start, though, that are no hard answers and you
may get opinions that range over a broad spectrum.
Lithium is a very useful, though sometimes unpredictable, flux. There
won't be much difference in the form of the lithium in the final glaze
whether you get your lithium from spodumenne or lithium carbonate. It is
going to turn into lithium oxide in the kiln. If it leaches from the
glaze into food it will probably be as lithium citrate or lithium
acetate. The main concern I would have is that both lithium citrate and
lithium acetate have biological effects as anitmanics or antidepressants.
I know they are effective at fairly low levels, but I just don't have
enough knowledge or references available to me to be able to tell you
what levels. If you formulate a glaze you like with lithium I would
recommend you have it tested for leaching. If the glaze is stable in the
leaching test, use it.
Chrome is a bit more controversial. A lot of people don't use it in
functional glazes; some do. Again, if you use it, have it tested. I
prefer copper greens to chrome greens anyway, so I don't use it and I
haven't looked into it in any depth. Perhaps other Clayart members have
more knowledge of chrome.
Copper is a relatively low toxicity material, but it also has problems.
At least here, there are some reference points. First, it is very
difficult to keep in a glaze compared to other colorants so I believe it
is important to check it for leaching. At levels of about 10 mg/l it is
known to make food or drink taste bitter--something I don't think any of
us wants to do whether or not that causes a safety problem. The accepted
limit on copper levels in drinking water is 1.3 mg/l. I personally have
decided to try to keep copper as far below 10 mg/l as I can until I know
more. I have seen at least two copper-containing glazes that leach 25-40
mg/l.
Perhaps the most important part of your note is that you use the blue mat
glaze underneath a transparent glaze. I have had great success with one
glaze in dramatically reducing the leaching (by a factor of 10) by doing
this. Why don't you submit a sample for formal leach testing? You just
might learn that you have essentially no leaching and then your concerns
will evaporate. If you do test it, let us all know the results. John
jklust wrote:
>----------------------------Original message----------------------------
>I have a blue matt recipe from a pottery school that calls for spodumene. I
>understand that spodumene is a lithium containing feldspar so does that
>mean it
>is toxic to use in glazes intended for dinnerware? I use the glaze
>underneath a
>transparent glossy glaze. I have also seen glazes I would love to try i.e.,
>purple icing however it contains lithium carbonate which I understand is
>toxic
>and not suitable for dinnerware. Can I substitute the lithium for something
>else??? I also have a recipe for emerald that contains 2% chrome oxide can I
>substitute copper carb. or copper oxide so that I can avoid the chrome all
>togther???Any help in this is greatly appreciated. Thanks Kim
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is time for potters to claim their proper field. Pottery in its pure
form relies neither on sculptural additions nor on pictorial decorations.
but on the counterpoint of form, design, colour, texture and the quality
of the material, all directed to a function." Michael Cardew in "Pioneer
Pottery"
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