iandol on wed 21 may 03
Dear Friends,
I would like to follow up on what Edouard Bastarache has told us. I =
think the suggestion he gives, to fire the kiln on three separate =
occasions to a temperature above the Volatilisation Temperature of Lead, =
is a little misleading, but he is recalling the suggestion, long past, =
of another person so I accept this comment in good faith.
What we need are some solid facts.
Take Metallic Lead. It melts at 327.46 deg C. The liquid metal Boils at =
1749 deg C The molten metal tends to oxidise when heated in air as do =
many other common metals.
Neither Kay and Laby, "Tables of Physical and Chemical Constants" nor =
the CRC "Handbook of Chemistry and Physics" give information about the =
temperatures at which compounds of Lead volatilise. Many compounds of =
Lead decompose when heated though there is one oxide PbO with a melting =
point of about 800 deg C which at atmospheric pressures boils at 1750 =
deg Kelvin (K&L) To achieve rapid volatilisation at Cone 6 the pressure =
inside the kiln would need to be reduced to around 5 kilo Newtons / =
square metre. One atmosphere is about 101kN/Sq.Mt .
The questions relating to a potters kiln depends on the properties of =
lead frits and glaze compositions under the conditions which exist =
during a firing. Unless these have been measured we just do not know. =
Analysis of brick or refractory fragments would give a true measure of =
the degree of contamination. This would have to be done twice, prior to =
the treatment and after treatment, before the method suggested by =
Edouard could be considered a valid options.
In the meantime, be suspicious and take precautions.
Best regards,
Ivor Lewis.
Edouard Bastarache on thu 22 may 03
Hello Ivor,
my definition of volatilization of lead comes from
"Edouard's own Chemistry 101".
What I meant was to heat the kiln above the temperature
at which it is not usefull anymore in a glaze because it has
left the glaze whatever the leaded compound used.
"Another limitation of lead oxide is that above about 1200
it becomes volatile. For this reason lead glazes are seldom
used above about cone 6." (Daniel Rhodes in Clay and Glazes
for the Potter, revised edition).
That is what I meant by volitization of lead.
Later,
"Ils sont fous ces Quebecois"
Edouard Bastarache
Irreductible Quebecois
Indomitable Quebeker
Sorel-Tracy
Quebec
edouardb@sorel-tracy.qc.ca
http://sorel-tracy.qc.ca/~edouardb/
http://perso.wanadoo.fr/smart2000/index.htm
iandol on fri 23 may 03
Dear Edouard,
Much as I respect the opinions of the late Daniel Rhodes, and I have =
that book to hand, I doubt that he did any chemical investigations which =
would validate the information you give or quote.=20
If you peruse his "Books on Pottery and Ceramics" given on p 325 you =
will note that he draws on Cullen Parmelee and W. G. Lawrence. I do not =
have a copy of Parmellee but Lawrence and West is with me as I write =
this.
W. G. Lawrence and R. R. West make the following observation relating to =
a suspect example of a lead bearing glaze.. "The offending sample was =
found to be perfectly safe if fired to a higher temperature".. They also =
give another quotation from a private communication.... "The most acid =
resistant of all ceramic glazes are lead-fluxed glazes, properly =
formulated, properly applied and properly fired"...=20
In the text they quote R. A. Eppler regarding the elemental oxides which =
inhibit or enhance the instability of a lead bearing glaze. Boron oxide =
is implicated in intensifying the rate of lead release. I find it =
enlightening that oxides from Group 1 and Group 2 of the Periodic Table =
as well as Zinc oxide are .... "More or less effective in increasing =
lead release from a glaze"... They also point out that when lead glazes =
are fired in an electric kiln there should be vents provided to ensure a =
good circulation of air.
By now you will have read my posting in which I give a value for the =
Boiling Point of Lead Monoxide at atmospheric pressure.
Thank you for your response.
Best regards,
Ivor Lewis.
=20
Ron Roy on tue 27 may 03
It is not hard to see that lead is volitile at cone 6 - for what ever
reason - just look at the inside of such kilns and the kiln furniture -
it's all glazed with lead to some extent.
If reduced the probelm becomes worse. I would not be surprised to find lead
in the atmosphere of a kiln at lower temperatures as well.
No wonder that lead was thought to be of no use over cone 6.
We should also understand that the oxides we use - when in close proximity
to each other become more reactive - and at a lower temperature than we
would think possible - otherwise CaO could not be helping the melt at cone
6 - but it does.
Keep in mind - one of the reasons we had lead in gasoline for so long - the
measuring instuments were not sensitive enough to give the warnings.
RR
>I would like to follow up on what Edouard Bastarache has told us. I think
>the suggestion he gives, to fire the kiln on three separate occasions to a
>temperature above the Volatilisation Temperature of Lead, is a little
>misleading, but he is recalling the suggestion, long past, of another
>person so I accept this comment in good faith.
>What we need are some solid facts.
>Take Metallic Lead. It melts at 327.46 deg C. The liquid metal Boils at
>1749 deg C The molten metal tends to oxidise when heated in air as do many
>other common metals.
>Neither Kay and Laby, "Tables of Physical and Chemical Constants" nor the
>CRC "Handbook of Chemistry and Physics" give information about the
>temperatures at which compounds of Lead volatilise. Many compounds of Lead
>decompose when heated though there is one oxide PbO with a melting point
>of about 800 deg C which at atmospheric pressures boils at 1750 deg Kelvin
>(K&L) To achieve rapid volatilisation at Cone 6 the pressure inside the
>kiln would need to be reduced to around 5 kilo Newtons / square metre.
>One atmosphere is about 101kN/Sq.Mt .
>The questions relating to a potters kiln depends on the properties of lead
>frits and glaze compositions under the conditions which exist during a
>firing. Unless these have been measured we just do not know. Analysis of
>brick or refractory fragments would give a true measure of the degree of
>contamination. This would have to be done twice, prior to the treatment
>and after treatment, before the method suggested by Edouard could be
>considered a valid options.
>In the meantime, be suspicious and take precautions.
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
iandol on wed 28 may 03
Dear Ron Roy,=20
<reason - just look at the inside of such kilns and the kiln furniture - =
it's all glazed with lead to some extent.>>
This may or may not be a true statement. But the only way to =
discriminate between fact and assumption or conjecture is to carry out =
an analytical determination on fragments of the refractories which =
exhibit a glassy surface from kilns where lead bearing glazes are fired. =
<proximity to each other become more reactive - and at a lower =
temperature than we would think possible - otherwise CaO could not be =
helping the melt at cone 6 - but it does.>>
This can be shown as untrue by making a glaze to match the formulation =
of the 1170 deg C Eutectic (CaO~23%-Al2O3~14%-SiO2~63%) from Alumina, =
Silica and Whiting, quite easy for someone with a glaze calc program. =
Common sense tells us that it will be well fused at Cone 6. Observation =
may show this is not the case. Do the test and tell us of your result.
Reactivity is determined by Chemical bonding and the measure of internal =
and applied energy within the system, in other words,Thermodynamics. The =
route of the reactions is such that, in the systems we deal with, the =
material or substance with the lowest fusion point acts, when it becomes =
a liquid, as a solvent for the other materials.
<- the measuring instruments were not sensitive enough to give the =
warnings.>>
I would like to see the evidence for this assertion. What were the =
levels of discrimination for the determination of atmospheric =
pollutants, decade by decade through the twentieth century?
Best regards,
Ivor Lewis
Ron Roy on fri 30 may 03
Hi Ivor,
Here a quote from Taylor and Bull's Ceramic Glaze Technology - Page 37 -
first class book by the way.
"Glazes containing lead are characterized by their low surface tension, low
viscosity, wide firing range, high index of refraction and resistance to
chipping"
"Glazes containing lead are used principally below 1150C (RR - cone 6 is
about 1200C) to minimize volatilization which occurs significantly at
higher temperatures, with lead compounds condensing on the cooler zones of
the kiln as the temperature falls. The volatilization losses of PbO are
dependent on time and temperature and upon the level of reducing gases in
the furnace."
I did do the test CaO/Al2O3/SiO2 - many years ago - took it to cone 6 - got
some melting - used wolastonite, kaolin and silica. If I had soaked it long
enough it would have melted. We must keep in mind - if you want something
to melt fast you don't just use a mid temperature flux like CaO and try to
melt at a temperature that is close to it's starting effective temperature
in a firing that terminates as soon as the temperature is reached.
If you really want to find out about this particular mixture - vary it and
include some other fluxers like KNaO. The value to working with a eutectic
mixture is indeed to melt quicker - but at the cost of a shorter firing
range - I'm not sure that is the best thing unless your other glazes fit
into the same firing schedule.
I don't understand the question about lead in gas - I don't remember where
I read that but there has to be some reason it took governments so long to
discover there was a problem - there must have been some reason that
industry was able to say there was no problem with lead in gas in the first
place.
RR
><>reason - just look at the inside of such kilns and the kiln furniture -
>it's all glazed with lead to some extent.>>
>
>This may or may not be a true statement. But the only way to discriminate
>between fact and assumption or conjecture is to carry out an analytical
>determination on fragments of the refractories which exhibit a glassy
>surface from kilns where lead bearing glazes are fired.
>
><>proximity to each other become more reactive - and at a lower temperature
>than we would think possible - otherwise CaO could not be helping the melt
>at cone 6 - but it does.>>
>
>This can be shown as untrue by making a glaze to match the formulation of
>the 1170 deg C Eutectic (CaO~23%-Al2O3~14%-SiO2~63%) from Alumina, Silica
>and Whiting, quite easy for someone with a glaze calc program. Common
>sense tells us that it will be well fused at Cone 6. Observation may show
>this is not the case. Do the test and tell us of your result.
>
>Reactivity is determined by Chemical bonding and the measure of internal
>and applied energy within the system, in other words,Thermodynamics. The
>route of the reactions is such that, in the systems we deal with, the
>material or substance with the lowest fusion point acts, when it becomes a
>liquid, as a solvent for the other materials.
>
><>the measuring instruments were not sensitive enough to give the
>warnings.>>
>
>I would like to see the evidence for this assertion. What were the levels
>of discrimination for the determination of atmospheric pollutants, decade
>by decade through the twentieth century?
>
>Best regards,
>
>Ivor Lewis
>
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Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
Phone: 613-475-9544
Fax: 613-475-3513
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