John Baymore on mon 2 nov 98
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(snip)
When using a glaze with lead in it, doesn't it leave a residue in the kiln
that in effect could cause problems later on down the road as far as
contamination goes?
(clip)
Lori,
Yes, lead volitalizes out of glazes. Many other things also do.........
check the green tinged flame of a high fire kiln in reduction with a lot of
copper in the glazes. The problem with lead is it's (well known) toxicity
in very, very low amounts and the low temperature that it boils off. Not
to mention the federal (and some state) regulations that apply to lead
=3Cwg=3E.
There may be similar problems with other things too....... but they haven't
been studied as much or at all (manganese fume comes to mind as a likely
candidate for study in this regard).
If a kiln is used to fire lead glazed wares, the refractories (and the
furniture) keep picking up a little lead oxide in each firing. How much is
is a function of the number of loads fired, the density of the loads, the
formulation of the glazes, the temperature it is fired to, if the kiln is
vented in some fashion, and so on.
As you would expect... the odds of the kiln being objectionably
contaminated after a single firing with one lonely pot having lead based
china paint to a low temperature like cone 017 are pretty much
non-existant.... while a kiln that has been firing load after load of
heavily lead bearing glazes to a relatively high temperature like maybe
cone 02-01 could probably be well contaminated.
One of the things that I recommend to clients looking at purchasing a used
electric kiln is to apply a lead test swab (from the hardware store kits)
to the kiln BEFORE purchasing it. Test a number of locations. While ANY
kiln certainly could have been used to fire lead glazed wares.... it is/was
a more common practice in electric kilns. Be particularly careful of kilns
that are older and/or have been used by people in the =22hobby ceramic=22
field.
If the kiln tests positive.... you shouldn't buy it. It would be almost
impossible to =22clear=22 a kiln of lead contamination by repeated firings.
The lead in the kiln will continue to volatilize in every firing and as it
cools will be redeposited on the interior surfaces of the kiln and
everything in it. With a small partial pressure of lead in the atmosphere
of the kiln while the glazes are molten, a small amount of lead will also
tend to get into the outer layer of the molten glaze too. So even if the
ware you are firing has no lead in the formulation...... it is possible
that the outer surface of the glazes (and the body) are affected by this
redeopisition of the lead from the kiln itself.
If the kiln tests positive for lead the only way to know for sure how much
is getting on the ware is to test a piece that has not been =22cleaned =
off=22
before testing. This test should be done by a lab... the hardware store
kits are not sensitive enough for this. It is also of concern that the
lead fume is getting out of the kiln into the kiln room, and/or is being
vented somplace by the vent system that you might not want lead to
accumulate.
There is much information available in the ceramics technical press on the
lead issue. It is one of the few things that HAS been studied a lot.
Check out this information if you are interested in =22things lead=22.
BEst,
......................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752
JBaymore=40Compuserve.com
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