Jennifer Assinck on wed 22 mar 00
I am hoping Clayart members can assist me in determining suitable upper limits
for oxides in food surface glazes.
I belong to a small guild of mostly hobby potters, and, until recently, most
members used the studio facilities and the guild glazes that we have had analyze
for safety.
Some members have started to develop their own glazes at home, using recipes fro
uncertain sources. The guild is not comfortable selling work glazed with unknow
glazes on food surfaces. We do ask them to test their pots with vinegar for
colour loss, and with rapid temperature changes for glaze fit. We are also
considering asking them to confirm that the (food surface) glaze formula is
balanced according to computer analysis.
I should also mention here, that functional pots must be fired to cone 6 or
higher, so there is little risk of toxins leaching through a porous clay body
from an outside glaze.
Although I realize that a balanced glaze can hold on to more oxide than an
unbalanced glaze, I would like to establish upper limits for particular oxides i
food surface glazes. Using a percent limit in the formula would be a clearer
guide for our members than ppm in a leachate test, because that would serve as a
beginning for choosing glazes. Even if a member were to get a mug tested for
leaching (not likely for the hobbyist who is constantly experimenting),
subsequent work may not be subject to the same firing conditions, resulting in a
different degree of leaching.
While I anticipate that Clayart members may respond that one should always test
one's glazes for leachates, please keep in mind that I am discussing hobbyists
who are not likely to do this. This is my attempt to put some control on the
situation, and give us some means of making a decision when we are presented wit
a pot with an unknown glaze on the day before the sale. We will also be spendin
time educating our more adventurous members, and encouraging all members to
consider using white or clear liners more often.
>From my reading of the Clayart archives, I have come up with the following list
of oxides and recommended upper limits. I would appreciate feedback from Clayar
members on these plans in general, and on the following glaze limits.
The Potter's Dictionary (p. 235), by Hamer, gives upper limits for producing ful
strength colour without becoming matt, but these may be more than one would want
in a food-surface glaze.
I chose some of the following limits based on the amounts we have in our balance
guild glazes. For those materials we don't use, I referred to Hamer.
UPPER LIMITS FOR FOOD-SURFACE GLAZES
No lead, barium, cadmium, manganese, iron chromate, nickel or vanadium.
0.1% Chromium oxide (Hamer: max colour at 1%; 0.1% for pinks)
1.6% Cobalt carbonate (Exception: Guild recipe for black glaze containing 5.5%
cobalt carb, used as light decoration on food surface)
1.0% Cobalt oxide (Hamer: max colour at 1%)
5.5% Copper carbonate
3.0% Copper oxide (Hamer: max colour at 5%)
10.0% Iron oxide (Hamer: max colour at 12)
2.0% Lithium carbonate
5.5% Rutile
12.6% Strontium carbonate
7.0% Tin oxide
5.0% Titanium dioxide (Hamer: max opacity at 5%)
8.0% Zinc oxide
If you feel that any of these limits are too high for an interior glaze, or if
you feel the limits could be higher, I would very much appreciate your comments.
Thanks very much,
Jennifer Assinck
Newmarket, Ontario, Canada (just north of Toronto)
Kat Berger on thu 23 mar 00
Be careful about setting strict guidelines. You may lose out on some
wonderful glazes and an opportunity to learn about the whole glaze
mixing and inventing process. Maybe you should sell work with a label
that states it has not been tested for food safety. I have many clients
who never eat out of the bowls they buy from me. They display them.
Do you know that almost all commercial ceramics have lead in the glaze.
The point is it's been tested.
Kat
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