John Baymore on fri 16 apr 99
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Dorothy,
Couple thoughts on the oribe glaze situation:
(clip)
We have recently started using an oribe glaze, that produces some very
interesting results. ........... If used on a bowl it produces a
translucent green glaze, turn metallic where thin and where thick will have
red/cranberry areas, on lidded vessel the interior will turn cranberry red.
(snip)
The metallic black you get with this glaze occurs due to the saturation of
copper in the glaze. During cooling the glaze can't hold it all in the
glass matrix and it separates out. The black color is from black copper
oxide. Because this copper is on the surface and is in crystaline form, it
is not bound very well in the glaze and so leaches pretty easily.
Copper is not the most outrageously toxic stuff to have leaching out....
but it is still not desireable. This is not a glaze for food contact
surfaces.
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............ we did a leach test with the vinegar in both. ............
(clip)
I think it is important to differentiate between the =22home=22 test with
vinegar and a real leach test. If the =22home=22 test shows change of color=
in
either the vinegar or the glaze or both........ you can bet that a true 24
hour acetic acid leach test a lab performs would show leaching of something
out of the glaze. However... it is important to note that if the =
=22home=22
test does NOT show either or both of these situations that it DOES NOT
prove that the glaze is not leaching anything. This is a truly crude test
that only shows leaching at a high magnitude.
(snip)
Interesting it was still clear when poured into a glass.
(clip)
It was still clear because you can't SEE the copper in the solution. But
it is there, I'd bet.
(clip)
The cup/bowl did change to a lighter shade (left a line) after 3 days.
.................. Then the lidded vessel after one week never altered the
interior. What's happening? Can anyone tell me about these
chemicals/changes?
(snip)
Got an idea here. Never tried this but I am guessing that it'll work.
Maybe not. I am trying to remember back to my high school chemistry
classes. Something called a flame test. It is crude too.... not a
replacement for a real leaching test at a lab.
Take some of the used vinegar liquid from a piece that has soaked a long
while and saturate a small piece of cotton with it. Let it dry fully.
Take a blowtorch and ignite the cotton ball in the hottest part of the
flame. I bet you get a greenish tinge to the flame produced for an
instant. That's the copper causing it. (If it doesn't work it is probably
because the copper concentration is too low to color the flame.....still
get a real leach test.)
The red portions of the glaze have been more heavily reduced and there is
more glassy matrix to hold the copper in solution. The copper is in the
reduced phase and more involved in the melt, and has not precipitated out.
My guess is that more silica has been =22stolen=22 from the body due to the
fluxing action of reduced iron at the body glaze interface, so there is
just a tad more silica glass to disolve the copper in.
It is still leaching copper I bet, but you can't see the difference from
the vinegar.
(clip)
custer feldspar 80
whiting 20
bentonite 2
copper carbonate 5
(snip)
The recipe calculates out a bit low in silica by about .2 mols and is
predominantly CaO fluxed....so I would sumise it is stealing a bit of SiO2
from the body interface, (along with some body iron). This makes it very
dependant on application thickness. This low silica figure makes the glaze
a bit soft and probably prone to leach..... particularly with the secondary
fluxes being all akalies and toward the high end of the limit. (But you
need the alkalies for the nice color response out of the copper =3Cwg=3E)
With the saturation amount of copper added to the base, this is a glaze
that probably should not be used on the insides of food contact surfaces.
To tell for sure, spend =2420 and send off a sample to have tested for the
leaching of copper. The base glaze by itself would still be soft.... but
without the copper there is notheing much to be concerned about leaching
out.
I am sure you will get a lot more info from others on the list. Hope
something here is of help.
Best,
........................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
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