Warren Smith on tue 8 jun 99
While attending a recent upscale arts festival in my area I was amazed
that some of the potters I encountered were using a clear glaze base on
their functional work which contained lead bisillicate.I was told by one
that this stopped the crazing problem which I knew but that it was also
perfectly safe for functional ware.
I have tried to be on the safe side when using ceramic materials so I
may be an alarmist but this has me puzzled.I was under the assumption
that lead bisillicte was "safe" when fired under industry controlled
standards and even at this standard there could be questionable
results.What say the more knowledgable potters in this forum.Is it
safe?The implications for problems being both legal and medical seem
daunting!
Warren Smith
John Hesselberth on wed 9 jun 99
Warren Smith wrote:
>----------------------------Original message----------------------------
>While attending a recent upscale arts festival in my area I was amazed
>that some of the potters I encountered were using a clear glaze base on
>their functional work which contained lead bisillicate.I was told by one
>that this stopped the crazing problem which I knew but that it was also
>perfectly safe for functional ware.
>
>I have tried to be on the safe side when using ceramic materials so I
>may be an alarmist but this has me puzzled.I was under the assumption
>that lead bisillicte was "safe" when fired under industry controlled
>standards and even at this standard there could be questionable
>results.What say the more knowledgable potters in this forum.Is it
>safe?The implications for problems being both legal and medical seem
>daunting!
>
>Warren Smith
Hi Warren,
In my view, responsible studio potters do not use lead-containing glazes
on functional work. Period! We have neither the control of our kilns
nor the required equipment in our studio to handle and fire lead safely.
If we do choose to use it, there are Federal and/or state laws and
regulations in the U.S. and most other parts of the world, which must be
followed, regarding carrying out a regular testing program of lead that
leaches from the pots and the lead that accumulates in the bodies of the
potters.
I am also of the view that non-functional potters should only used lead
in a fritted form--if they feel they must use it at all. Handling raw
lead bisillicate is just asking for health problems unless your studio is
extremely well equipped for doing it and you are very well trained in
handling dangerous materials.
I worked for thirty years in the chemical industry handling all kinds of
dangerous materials and, yes, any material can be handled and used safely
if you take proper precautions. That said, I have never had lead in my
studio and never will.
Below is a post I wrote in late April on the subject. Please print it
out and give it to your friend.
Regards, John
--------------
Lead in Glazes Used on Functional Pottery
Most responsible studio potters have long ago given up using lead in
their glazes--at least on those pots that might end up being used for
food or drink by living things (animals, including humans, or plants).
The potential hazards are simply too great and the precautions one must
take to assure safety of both the potter and the end-user are beyond the
capability of most potters. There are a few, however, who continue to
insist on using lead and it is to those few this document is addressed.
There are some common myths (or defense mechanisms??) promulgated by
these people that must be discussed head on!
Myth: Using lead in fritted form makes it safe
Fact: Lead-containing frits are indeed safer for the potter to handle
than the various forms of raw lead. The use of frits, however, has
ABSOLUTELY NOTHING to do with the safety of the final glaze. Once the
glaze is melted in the kiln, the lead no longer knows it was contained in
a frit. The only things of importance from this point on are the
composition of the glaze and the firing conditions.
Myth: If I use a commercial lead-containing glaze which indicates that
it is food safe, it is.
Fact: This is only true if your firing conditions and things like
underglazes are exactly the same as those used by the glaze manufacturer.
In fact, most kilns have significant variation in temperature and your
firing conditions, at least in part of your kiln, are probably
significantly different from those used by the glaze manufacturer. A
copper-containing underglaze may significantly affect lead release. The
only way to assure that a lead-containing glaze is safe for use with
foods is to have representative pots from every firing tested by a
qualified laboratory. In addition FDA regulation in the United States
requires that potters have a qualified laboratory periodically check
their ware. Ware that might be used for food, but does not pass the
leaching standards must have a hole drilled in the bottom or be
permanently labeled that it is not safe for food use. California
requirements are even more specific and stringent.
Myth: I only use lead-containing glazes on things like flower pots,
planters and bird baths so I don't need to worry.
Fact: Lead is toxic to animals just as it is to humans. Plants raised
in a lead glazed pot will absorb lead. If that pot is used for growing
something like herbs the lead has just worked its way into the food chain.
Myth: I only use lead-containing glazes on my sculptural work. I don't
need to worry about my functional work.
Fact: If your sculptural work and your functional work are fired in the
same kiln--even at different times--you need to worry. It is well known
that lead can migrate to the walls of the kiln and then contaminate
future firings. Of course, if functional and non-functional pottery is
fired at the same time, contamination is even more certain.
Myth: I use a lead-containing blue glaze which I have had tested and
know is safe. In fact it was way below the allowable limit. I wanted to
get a blue-green color so I added some copper. Because it was so far
below the allowable limit in my first test, I don't need to retest it.
Fact: Addition of copper to a lead-containing glaze is widely reported
in the literature to significantly increase the release of lead. COPPER
SHOULD NEVER BE USED IN A LEAD-CONTAINING GLAZE.
In Summary:
Although lead-containing glazes have been a favorite of potters for many
years and have many desirable properties, IT IS PAST TIME FOR POTTERS TO
STOP USING LEAD-CONTAINING GLAZES ON FUNCTIONAL WORK. Val Cushing, a
long time user of lead glazes, speaks for many potters when, after
reviewing all the data on lead glazes he wrote in Cushing's Handbook:
"It has been hard for me to accept the fact that, generally speaking, the
world is a safer place if we potters eliminate lead glazes from our
repertoire. Good-by lead glazes -- we shall miss you..."
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"
Tom Buck on wed 9 jun 99
Warren:
Here is my take on this quesation of raw lead/partially frited
(ie, Pb bisilcate)/fully fritted lead mix.
Raw leads are dangerous to the POTTER as well as to the user of
lead glazed foodware. The Pb bisilicate and the Lead frits are less
dangerous when formulating glazes because the frits are very insoluble at
room temperature; the main way they would pose a problem would be by
inhaling them (the frits themselves); but if by chance we ingest some,
the lead compounds would be made soluble by stomach acid(s).
Any lead-based glaze, once it is fired, will behave identically no
matter what the lead ingredient was in the slurry stage. The lead compound
at C06 + becomes PbO (the oxide) and silicate or other glass-forming
oxide. So if the PbO is taken into the alumina-silica network polymer (ie,
glass) it will be well bound and not likely to be released in food
service.
Industrial plate makers (etc.) have rigorous standards and
operational control, so they can guarantee government that their pots will
meet the current standard for lead-release. And of course they do routine
daily checks on lead release.
We potters, most of us, are not able to meet the same quality
standards and controls. So we take our chances, make lowfire pots using
lead-based glazes and their lovely colours, and sell them to users who
believe there is no danger from lead-release, a condition very very few
can say with assurance. We trade on our ignorance and hope the pots don't
come back to haunt us through the courts. And we shrug off any guilt we
might feel.
If you make foodware, you should abandon lead-based glazes on your
pots; only by doing so can you have a clear conscience.
Tom Buck ) tel: 905-389-2339
(westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
On Tue, 8 Jun 1999, Warren Smith wrote:
> ----------------------------Original message----------------------------
> While attending a recent upscale arts festival in my area I was amazed
> that some of the potters I encountered were using a clear glaze base on
> their functional work which contained lead bisillicate.I was told by one
> that this stopped the crazing problem which I knew but that it was also
> perfectly safe for functional ware.
>
> I have tried to be on the safe side when using ceramic materials so I
> may be an alarmist but this has me puzzled.I was under the assumption
> that lead bisillicte was "safe" when fired under industry controlled
> standards and even at this standard there could be questionable
> results.What say the more knowledgable potters in this forum.Is it
> safe?The implications for problems being both legal and medical seem
> daunting!
>
> Warren Smith
>
Ray Aldridge on thu 10 jun 99
At 03:13 PM 6/9/99 EDT, you wrote:
>
>I am also of the view that non-functional potters should only used lead
>in a fritted form--if they feel they must use it at all. Handling raw
>lead bisillicate is just asking for health problems unless your studio is
>extremely well equipped for doing it and you are very well trained in
>handling dangerous materials.
>
John, perhaps I misunderstand you, but lead bisilicate *is* a frit.
Otherwise, I agree with you. I have no lead in my studio.
Ray
John Hesselberth on fri 11 jun 99
Ray Aldridge wrote:
>----------------------------Original message----------------------------
>At 03:13 PM 6/9/99 EDT, you wrote:
>>
>>I am also of the view that non-functional potters should only used lead
>>in a fritted form--if they feel they must use it at all. Handling raw
>>lead bisillicate is just asking for health problems unless your studio is
>>extremely well equipped for doing it and you are very well trained in
>>handling dangerous materials.
>>
>
>John, perhaps I misunderstand you, but lead bisilicate *is* a frit.
>
>Otherwise, I agree with you. I have no lead in my studio.
>
>Ray
Ray,
You are, of course, right. My fingers got ahead of my brain. I should
have said "Handling raw lead and even lead in fritted form like lead
bisillicate is just asking for health problems unless ......"
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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