bruec@anv.net on fri 16 nov 01
I would have to say the same thing that I tell the students at the art center. "Once made, they have no control over wha=
t someone else will use their creations for." And sure, she may not be making things that would ever be used "functional=
ly" for food. Fine.
Another question might be, Is she NEVER going to sell the kiln to someone else?
Original Message:
-----------------
From: Heidi Haugen redhat@digisys.net
Date: Fri, 16 Nov 2001 19:23:03 -0700
To: CLAYART@LSV.CERAMICS.ORG
Subject: lead contamination
quick question about the volatilization of lead.
i have a friend who may use lead-containing glazes on non-functional
ware in her electric kiln. if she only uses earthenware for now and
always, should she be concerned with lead contamination if she never
fires to cone 6 (volatilization temp for lead)?
i hated to ruin her day today with a lecture about lead glazes and their
lasting, detrimental effect on electric kilns. was hoping to make her
feel better by suggesting that if she never fires above earthenware
temps she can also put domestic ware in her kiln.
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Heidi Haugen on fri 16 nov 01
quick question about the volatilization of lead.
i have a friend who may use lead-containing glazes on non-functional
ware in her electric kiln. if she only uses earthenware for now and
always, should she be concerned with lead contamination if she never
fires to cone 6 (volatilization temp for lead)?
i hated to ruin her day today with a lecture about lead glazes and their
lasting, detrimental effect on electric kilns. was hoping to make her
feel better by suggesting that if she never fires above earthenware
temps she can also put domestic ware in her kiln.
interested to hear the answers.
heidi haugen in montana.
John Hesselberth on sat 17 nov 01
on 11/16/01 9:23 PM, Heidi Haugen at redhat@DIGISYS.NET wrote:
> quick question about the volatilization of lead.
>
> I have a friend who may use lead-containing glazes on non-functional
> ware in her electric kiln. if she only uses earthenware for now and
> always, should she be concerned with lead contamination if she never
> fires to cone 6 (volatilization temp for lead)?
>
> i hated to ruin her day today with a lecture about lead glazes and their
> lasting, detrimental effect on electric kilns. was hoping to make her
> feel better by suggesting that if she never fires above earthenware
> temps she can also put domestic ware in her kiln.
>
> interested to hear the answers.
> heidi haugen in montana.
Hi Heidi,
I'm not sure I know a definitive answer to you question, but I know how I
would check it. I would buy one of those kits sold in hardware stores for
about $10 that let you check for lead. They are intended for both painted
and ceramic surfaces. Then I would test the interior surface of the kiln for
the presence of lead. I would probably find it.
If I then wanted to use the kiln for functional work I would fire several
times with the kiln empty to a significantly higher temperature with lots of
ventilation. I would check for lead after each firing. When/if I no longer
found it I would fire my functional work.
I think a lot of people don't realize that even fritted lead-containing
glazes can contaminate a kiln and future work, but I suspect the kiln can be
cleaned up with repeated lead-free firings.
Regards,
John
Web site: http://www.frogpondpottery.com Email: john@frogpondpottery.com
"The life so short, the craft so long to learn." Chaucer's translation of
Hippocrates, 5th cent. B.C.
Gwyn Ace on sun 18 nov 01
Heidi .. from my firings in the distant past I think the answer would be =
Yes.
Firing in a small electric kiln to Seger cone 1 using a Lead Bisilicate =
Glaze and a Red/Brown body which was fairly vitreous at this temp ...any =
unglazed exposed part of the pot was 'flashed' This indicated Lead =
Vapour to me. I also found that I could not refire any pot with this =
glaze where the glaze was fairly thin without it becoming 'dry' .Again =
this looked to me as though I was losing some of the Lead flux through =
vapourisation.
I don't know of any other explanation.as this behaviour was consistent.
GWYN in N.Z.
Gwyn Ace on sun 18 nov 01
Heidi... Try a search on the net for Lead Glazes.. I found at some =
time..a method recommended for removing Lead from an electric kiln...I =
cannot remember any details but will post later if I find them.
Good Hunting GWYN in N.Z.
John Baymore on mon 19 nov 01
At a pressure of 101.325 kN/sq M (roughly atmospheric pressure) the =3D
vaporisation temperature is 1477 deg Celsius. At 5 kN/sq M the =3D
vaporisation temperature is 1212 deg Celsius. So the pressure inside a =3D=
kiln would have to be very low for an appreciable amount of lead oxide =3D=
to evaporate at Cone 6.
Ivor,
Question.........
Once you have a liquid, doesn't there occur some volitilization from the=
surface of that liquid even below the true "vaporization" point? This
certainly occurs with water below 212 F / 100C until the partial pressure=
of the water vapor in the surrounding gas (air) has reached a level at
which it is holding as much water vapor as it can hold.
Yes, be cautious. But the only way to find things out is to have some =3D=
analyses done on surface chips of kiln refractory. Look at it this way. =3D=
If Lead in one form or another is fuming from the glazes your friend is =3D=
using then eventually they will attack and corrode the refractories =3D
which line the kiln. Now have you, or has anyone else, ever read or =3D
heard of a kiln being decommissioned because of this occurring? =
Yes,.... I have done lead tests on numerous electric kiln refractories an=
d
yes they have turned up positive a number of times. As to "decomissionin=
g"
........ I simply suggest that someone not buy them unless they
themselves are firing lead containing glazes. (Not that I reccommend THAT=
.)
Best,
..............................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752 (s)
800-900-1110 (s)
JohnBaymore.com
JBaymore@compuserve.com
John.Baymore@GSD-CO.com
"Earth, Water, and Fire Noborigama Woodfiring Workshop 2002 Dates TBA"=
Michael Banks on tue 20 nov 01
Quite so John,
In fact many oxides (even refractory ones) are significantly volatile well
below their boiling points. The most striking one in ceramics is chromium
sesquioxide (chrome green), which by virtue of it's strong colour is
obviously volatile, fuming across ware as low as 1150 degrees C.
Lead too shares this high vapour pressure characteristic - well below it's
boiling point and is notoriously weakly bonded in molten glaze. It is easily
displaced by copper, which is itself only weakly attached to the molten
silicate solution.
The makers of the most widely used commercial glaze here (a Ferro Australia
product), specify a maximum firing temperature of 1150 degrees C for this
gloss borosilicate clear. This low limit is a prudent one for users to
adhere to, (even though glaze has only ~1% lead and remains fairly
unblemished to 1260 degrees C or higher), but like all lead-bearing glazes,
lead vaporisation becomes progressively more problematic above 1180.
Michael Banks
Nelson
New Zealand
----- Original Message -----
John Baymore wrote:
Question.........
Once you have a liquid, doesn't there occur some volitilization from the
surface of that liquid even below the true "vaporization" point? This
certainly occurs with water below 212 F / 100C until the partial pressure
of the water vapor in the surrounding gas (air) has reached a level at
which it is holding as much water vapor as it can hold.
iandol on tue 20 nov 01
Dear Michael Banks,
You say <volatile well below their boiling points. The most striking one in =
ceramics is chromium sesquioxide (chrome green), which by virtue of it's =
strong colour is obviously volatile, fuming across ware as low as 1150 =
degrees C.>>
Things can happen to refractory oxides even before they approach their =
melting points. Silica, which is a constituent in many refractories used =
by ceramicists can be reduced to silicon monoxide, which is volatile, at =
very high stoneware temperatures (Kingery et al).
Please note that I did not say Lead compounds were not volatile. I said =
the volatility would vary with the pressure of the atmosphere. I did =
suggest that if lead volatility was heavy or persistent that one would =
expect the refractories to glaze over a period of time. The fact that =
blushing can occur on free clay adjacent to glazes indicates that there =
is a lot going on in a kiln which we know little about. And as you say, =
the movement of Chromium sesquioxide is easily traced by the strong =
colour it induces on contaminated surfaces. But are you sure this is due =
to evaporation and not some other process, especially under conditions =
of high concentrations of Carbon Monoxide.
In saying that lead compounds are notoriously volatile due to their high =
vapour pressures, do you have values for the Lead silicates, or rates of =
loss in micro/milligrams per K per Min. over a range of temperatures?
Best regards,
Ivor Lewis. Redhill, South Australia
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