Paulaclay@aol.com on sat 8 jun 96
My friend was offered an electric kiln that fired white lead glazes to cone 6
for about 25 years. If she now fires leadfree cone 06 glazes, is there
residual from the previous lead? Can the white lead get into the low fire
leadless glazes and how can this be prevented? Can the kiln be purged?
Thanks from Paula Sibrack, sweating during construction of my attic studio in
Sherman, CT, with a seasonal view of Candlewood Lake.
Teri and Bill Seeley on sun 9 jun 96
Paulaclay@aol.com wrote:
>
> ----------------------------Original message----------------------------
> My friend was offered an electric kiln that fired white lead glazes to cone 6
> for about 25 years. If she now fires leadfree cone 06 glazes, is there
> residual from the previous lead? Can the white lead get into the low fire
> leadless glazes and how can this be prevented? Can the kiln be purged?
According to the "Handbook of Chemistry and Physics (43rd Edition)",
white lead (PbSO4) melts at 1000 degrees C, the approximate
temperature of cone 06. No boiling point is listed. Lead itself
vaporizes at 1620 degrees C (approximately cone 26). I'd use the
latter as a working figure for "purging the kiln". Somehow I doubt
that a 25 year old kiln it is likely to be able to reach cone 26.
I'd look this gift horse in the mouth!
Bill
--
Theresa and William Seeley 410 486-3171 (voice)
Villa Nova Pottery 410 484-6273 (fax)
4015 Buckingham Rd. Baltimore, MD 21207
"186,000 miles/second is not just a good idea - its the law!"
Bob Kavanagh on sun 9 jun 96
I fire my kiln to cone 9-10 about every 25-30 firings to clean it out.
I put shelves in it, no pots at all and I leave the spy holes open
and the lid very slightly ajar.
rdowns on sun 9 jun 96
At 05:03 PM 6/8/96 EDT, you wrote:
>----------------------------Original message----------------------------
>My friend was offered an electric kiln that fired white lead glazes to cone 6
>for about 25 years. If she now fires leadfree cone 06 glazes, is there
>residual from the previous lead? Can the white lead get into the low fire
>leadless glazes and how can this be prevented? Can the kiln be purged?
>Thanks from Paula Sibrack, sweating during construction of my attic studio in
>Sherman, CT, with a seasonal view of Candlewood Lake.
>
Paula,
Is there residual lead in the kiln? YES. Is the lead fuming into the
kiln atmosphere durring firing? YES. I don't think that you can prevent
the lead from fuming. You might try a coating like ITC to contain the lead,
however I would not count on it working as the lead will continually be
working into the coating and looking to release into fumes again. To
determine the amount of contamination, try a lead test kit on ware that has
been fired in the kiln.
Good Luck,
Rose
Dianna Rose Downs
in north central Texas
214-734-1329
http://www.why.net/home/gdowns/terra.html
http://www.dfw.net/mckinney/rdowns/rdowns.html
Bill Aycock on sun 9 jun 96
At 05:03 PM 6/8/96 EDT, Paulaclay@aol.com wrote: **** (in part) ****
>----------------------------Original message----------------------------
>My friend was offered an electric kiln that fired white lead glazes to cone 6
>for about 25 years. If she now fires leadfree cone 06 glazes, is there
>residual from the previous lead? Can the white lead get into the low fire
>leadless glazes and how can this be prevented? Can the kiln be purged?
>Thanks from Paula Sibrack, sweating during construction of my attic studio in
>Sherman, CT, with a seasonal view of Candlewood Lake.
>
Paula- 25 years ago I had an L&L that had been used for ^6 firing with
balanced lead glazes.
When the BIG scares hit about lead glazes , (thank goodness), I made a
simple test (almost all that was available, at the time), using a pot that
had no lead in the glaze ingredients. The results were positive, and scary.
I have just run some Thermochemistry calculations on the problem, and the
point where lead oxide just begins to vaporize significantly is about ^06.
At ^6 it really boils.
I have seen a suggestion that firing to ^9-10 with an empty kiln is a way of
purging. I would not trust it. I scrapped my old L&L, and saved only the
lid, to put on the floor under a small test kiln.
I also saw a suggestion to test a piece fired in the kiln. I suggest that a
simpler test, without wasting the ware that is fired in it, is to test the
kiln itself. If you have a swab type of test- use it on the brick walls. A
good additional place to test is the upper edge, where the lid sits. This is
where the lead vapors will tend to condense out during firing.
Bill- sitting out a peaceful Sunday on Persimmon Hill.
Bill Aycock on sun 9 jun 96
At 03:47 PM 6/9/96 EDT, Teri and Bill Seeley wrote: **** (in part) ****
>----------------------------Original message----------------------------
>According to the "Handbook of Chemistry and Physics (43rd Edition)",
>white lead (PbSO4) melts at 1000 degrees C, the approximate
>temperature of cone 06. No boiling point is listed. Lead itself
>vaporizes at 1620 degrees C (approximately cone 26).
Bill- I dont have the 43rd Ed., but most are the same- and PbSO4 IS NOT
white lead, and it doesnt melt at 1000 c, it decomposes at that temp. Lead
sulfate would REALLY be a threat; not only would it leave its PbO to leach
out, it would give off SO2, which (in the presence of lead, as a catalyst),
reacts with atmospheric water and oxygen to make sulfuric acid. Without the
lead, it makes sulfurous acid, and the difference, as far as your health
goes, is not worth reporting.
White lead is also called basic lead carbonate, has the formula
2(PbCO3)-Pb(OH)2, which
decomposes at 400C. The product of decomposition that we are concerned with
is the Oxide, PbO, which melts (according to my edition of the handbook) at
888 C. Like any other ceramic ingredient, however, there are interactions
that make this only an approximation, when a glaze is the vehicle.
As far as the vaporization of lead is concerned, there is an appreciable
vapor pressure of lead at any stoneware temperature, especially if the
atmosphere is not kept nicely into the oxidizing range. In a glaze, local
reduction can occur even in an electric kiln, if some contaminant such as
sulfur is present.
I ran a series of calculations this afternoon, using some of my old Rocket
motor combustion software, with burning Hydrogen and Oxygen to get the
temperature, and Nitrogen as a diluent, with PbO as a contaminant. I was
surprised at the care I had to use to be sure I was not reducing, in order
to keep lead vapor from resulting. I could have used propane and air, but
the mix to get a stoichiometric balance with H2 and O2 was easier.
Bill- having fun with his retired library on Persimmon Hill
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