Christine Sawyer on tue 27 jul 99
What is a =22food safe=22 level of the total oxides in a glaze=21 Is there =
a rule of
thumb or is it depended on each type of oxide being used.
Gavin Stairs on thu 29 jul 99
At 05:46 PM 27/07/99 -0400, you wrote:
>----------------------------Original message----------------------------
>What is a "food safe" level of the total oxides in a glaze! Is there a
>rule of
>thumb or is it depended on each type of oxide being used.
An interesting concept, since glazes are, by assumption at least, 100%
oxides of one cation or another. I guess you meant oxides other than, oh,
lets see: silicon, aluminum, boron?, phosphorus?, what about iron? I don't
think we get anywhere without specifying the rest of the glaze as well. In
other words, it's not so much the level of a particular oxide as it is how
it performs in a glaze. Or did I miss the point entirely?
Gavin
John Hesselberth on fri 30 jul 99
Christine Sawyer wrote:
>----------------------------Original message----------------------------
>What is a "food safe" level of the total oxides in a glaze! Is there a
>rule of
>thumb or is it depended on each type of oxide being used.
Hi Christine,
Unfortunately, no one knows the answer to your question. Different
colorant oxides behave differently in glazes and each glaze behaves
differently from the next. For example, copper is particularly difficult
to keep in a glaze while iron is easier. The only way to know is to have
your glazes tested by a professional testing lab. It is not very
expensive--$10 for the extraction and an additional $10 for each metal
you want tested at the Alfred Analytical Laboratory.
There are complete instuctions for doing this on my website at
http://www.frogpondpottery.com/glazetest.html
Maybe in another year or two we will be able to give a better answer to
your question with all the testing that I and others are doing. In the
meantime, if you do test your glazes, please share the results with the
list so we can all learn together.
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"
Ron Roy on sun 1 aug 99
The Hamer dictionary has some information on this subject - look up copper
oxide for instance.
Yes it does depend on the amount of oxide - also the type of glaze and the
durability of the glaze. Some oxides are safer than others.
RR
>----------------------------Original message----------------------------
>What is a "food safe" level of the total oxides in a glaze! Is there a rule of
>thumb or is it depended on each type of oxide being used.
Ron Roy
93 Pegasus Trail
Scarborough, Ontario
Canada M1G 3N8
Tel: 416-439-2621
Fax: 416-438-7849
Web page: http://digitalfire.com/education/people/ronroy.htm
Michael Banks on tue 3 aug 99
Regarding the solubility of metals in silicate melts and glasses, there
hasn't been as much cross-over in knowledge from the historical studies of
Geochemistry to Ceramics, as there might have been. It's a bit of a shame,
because some simple concepts were thrashed out by geochemists earlier this
century which can give an insight into this subject for ceramists and
potters. They were worked out by the pioneering geochemist V. M.
Goldschmidt in the 1930's and were added to by the discovery of the
importance of crystal field stabilisation energies in the 1960's.
Goldschmidt grouped the chemical elements into four divisions relating to
their actual abundance in the four physical environments that make up planet
earth (and a few others planets as well). The Earth's crust and mantle are
composed of silicates, the core of iron alloy (plus a sulphide fraction) and
the atmosphere, gases. Goldschmidt grouped the elements to reflect their
preference to concentrate in either silicates (lithophile), sulphide
(chalcophile), iron alloy (siderophile) or gas (atmophile). These groups
correlate fairly well with the measured electrode potentials (Eo, a measure
of chemical activity) for each element, lithophile elements have the highest
Eo, siderophile low Eo and chalcophile intermediate Eo.
The relevance to potters though, is that the Goldschmidt groupings reflect
fairly well the stability of metals in glazes and clay bodies. Lithophile
elements (K,Na,Li,Mg,Ca,Sr,Ba,B,Al,Si,Ti,Zr,Cr,W,Mn,V) form strong bonds to
silica and could be expected to stay put in a glaze and this is generally
observed to be true. There are anomalies though, for example Ba, which in
both nature and glazes, is somewhat unpredictable. Note the inclusion of Cr
and Mn in the stable lithophile group. This would predict that these two
toxic ions are much more stable and immobile in glazes than, say Cu, Pb and
Cd, which belong in the chalcophile group.
The chalcophile elements are only weakly attached in silicate materials and
include quite a few nasties. Of relevance to ceramists, they include
Cu,Cd,Sb,Ag,Bi,Pb,Zn, which suggests that these elements will be only weakly
attached to the Si and O atoms in glazes. The anomaly in this group is Zn,
which does not always behave like a typical chalcophile although it
partitions strongly into sulphides in meteorites and smelter concentrates.
Zinc seems to be dual in nature, being also able to form stable silicates.
This anomaly is known to occur because of zinc's high electrode potential,
which is more typical of the lithophile group. The prediced behaviour of
this group in glazes (excepting zinc) is born out by experience, as this
group are easily induced to exsolve from the glass by subtle changes in
composition, atmosphere, etc. I think it would be wise to exclude
chalcophile group elements (excepting zinc) from glazes to be used in
contact with food and drink.
The siderophile group (Fe,Sn,Au,Pt,Mo,Co,Ni,) is not a very good predictor
of metal-silicate stablity, particularly for the latter two elements, both
of which have strong chalcophile affinities, along with iron itself.
However, comparing electrode potentials (highest to lowest), Fe has the
highest (and is equal to some of the lithophiles), Co next and Ni lower
down. I would tend to exclude Ni and Mo from foodware glazes, given their
low electrode potentials and their predominant occurrence in nature as
sulphides.
The atmophile elements (H,N,and C(in oxidizing conditions) are of course,
fugitive in glazes.
I've ignored the crystal field energy relations of the transistion metals
for the purposes of this discussion, because (although these over-ride
Goldschmidts system), they pertain to crystal-liquid interactions and will
not affect transparent gloss glazes. However, elements with high crystal
field energy, dominate partitioning into mineral octahedral sites where
crystals are precipitating, as in matt glazes. The toxic transistion metals
copper and nickel may become more stable in matt glazes where spinel
minerals occur, but I wouldn't bet on it!
Michael Banks,
Nelson,
New Zealand
> ----------------------------Original message----------------------------
> Christine Sawyer wrote:
>
> >What is a "food safe" level of the total oxides in a glaze! Is there a
> >rule of
> >thumb or is it depended on each type of oxide being used.
& John Hesselberth Wrote:
> Hi Christine,
>
> Unfortunately, no one knows the answer to your question. Different
> colorant oxides behave differently in glazes and each glaze behaves
> differently from the next. For example, copper is particularly difficult
> to keep in a glaze while iron is easier. The only way to know is to have
> your glazes tested by a professional testing lab. It is not very
> expensive--$10 for the extraction and an additional $10 for each metal
> you want tested at the Alfred Analytical Laboratory. (snip)...
Ray Aldridge on thu 5 aug 99
At 02:17 PM 8/3/99 EDT, you wrote:
(big snip of very interesting information)
>
>The chalcophile elements are only weakly attached in silicate materials and
>include quite a few nasties. Of relevance to ceramists, they include
>Cu,Cd,Sb,Ag,Bi,Pb,Zn, which suggests that these elements will be only weakly
>attached to the Si and O atoms in glazes. The anomaly in this group is Zn,
>which does not always behave like a typical chalcophile although it
>partitions strongly into sulphides in meteorites and smelter concentrates.
>Zinc seems to be dual in nature, being also able to form stable silicates.
>This anomaly is known to occur because of zinc's high electrode potential,
>which is more typical of the lithophile group. The prediced behaviour of
>this group in glazes (excepting zinc) is born out by experience, as this
>group are easily induced to exsolve from the glass by subtle changes in
>composition, atmosphere, etc. I think it would be wise to exclude
>chalcophile group elements (excepting zinc) from glazes to be used in
>contact with food and drink.
>
I don't know anything about this, which is why I'm asking, but is the
toxicity of copper really a problem in pottery? I've never been able to
understand why it's okay to have copper in the water pipes, but not in
glazes. Evidently lead pipes are unhealthy and have been associated with
lead poisoning, but are there any epidemiological data supporting the idea
that copper pipes are dangerous? In some locales, the water is acidic,
which would make lead pipes an even worse idea-- but if there were some
morbidity associated with copper pipe, wouldn't these locales pop up in any
analysis of copper-related illnesses, if such exist? After all, most of us
drink water that's passed through copper pipe.
Also, in the spirit of your excellently eclectic post, I wonder if there
are any archaeological data available that might show the effect of copper
in the high alkaline Middle Eastern copper blue wares. I understand that
analysis of Roman remains shows a high lead level in many cases, apparently
due to use of lead serving vessels. The copper blue glazes are extremely
unstable, and must certainly leach a large amount of copper.
Ray
Edouard Bastarache on sat 7 aug 99
------------------
Hello Ray,
copper may enhance the leaching of lead from glazes.
NIOSHTIC RECORD NUMBER : 99867
TITLE :
Some Facts About Lead Glazes For Workshop And Studio Potters
NIOSHTIC CONTROL NUMBER : NIOSH-00137245
AUTHOR(S) :
Anonymous
SOURCE :
National Health and Medical Research Council, Commonwealth of Australia,
Canberra, Australia, 3 pages
PUBLICATION DATE : 1975-00-00
ABSTRACT :
Formulation, application, and firing procedures used in glazing of lead
(7439921) are presented. Ceramic glazes, which are thin glossy coatings
fused onto clay are formulated from basic compounds such as alumina
(1344281) and silica (7631869). A flux is an additive that allows the
glaze
components to fuse at a lower temperature and form a homogenous mass. A
frit
is a preformulated glaze in which lead is bound with the other
constituents
to prevent the leaching of lead from the finished product. Many glazes
are
designed for product decoration only and with normal use are not a
health
hazard. The practice of reducing the melting point of a glaze by
addition of
greater amounts of flux or the use of some formulation mixtures should
be
avoided. They pose a serious health hazard to consumers and potters
because
they increase the leaching of lead from the clay mixture. The spraying
of
glaze should be done only in well ventilated booths exhausted to the
exterior. Kilns used during firing should be placed away from
residential
areas. Coloring agents should be used with care because certain oxides
and
carbonates such as those of copper (7440508), cobalt (7440484), and
nickel
(7440020) tend to release lead from the matrix. Glazes with green and
blue
finishes should not be used on utensils intended for serving food and
beverages. Kiln temperature, firing time, and kiln atmosphere are
important
factors during glaze maturation. Kiln temperature should be maintained
above
1100 degrees-C to provide for adequate reaction of the glaze
constituents
with the clay body during maturation. Good personal and occupational
hygienic practices are essential to avoid transfer of lead from hands to
mouth and exposure to dust and fumes. Incidents of leaching of lead
glaze
have prompted the National Health and Medical Research Council of
Australia
to recommend enactment of legislation to prohibit sale of pottery that
may
result in release of lead.
DESCRIPTOR(S) :
Pottery glaze / Pottery workers lung / Lead poisoning / Work practices /
Occupational exposure / Occupational hazards / Worker health /
Industrial
hygiene / Safety practices
=2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =
=2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A
=2A N I O S H T I C(R)
=2A
=2A
=2A
=2A Produced by : US National Institute for Occupational Safety and Health
=2A
=2A Provided by : Canadian Centre for Occupational Health and Safety
=2A
=2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =2A =
=2A =2A =2A =2A Issue : 99-1 (February, 1999)
=2A
NIOSHTIC RECORD NUMBER : 35751
TITLE :
Lead in Ceramic Glazes
NIOSHTIC CONTROL NUMBER : NIOSH-00037257
AUTHOR(S) :
Meranger, J. C.
SOURCE :
Canadian Journal of Public Health, Vol. 64, pages 472-476, 17 references
PUBLICATION DATE : 1973-10-00
ABSTRACT :
A wide range of domestic and imported earthenware, stoneware and china
was
leached with acetic acid and leachable lead values were determined. Of
the
262 samples analyzed 10=25 showed levels of extractable lead in excess =
of
100
ppm and 36=25 had levels above 7 ppm. The highest levels found for
earthenware, stoneware and china were 8,000 ppm, 3,910 ppm and 55 ppm
respectively. Repeated leachings of lead containing glazes resulted in a
sharp decrease in extractable lead values which increased significantly
when
the surface was abraded. The effect of copper in increasing the
solubility
of glazes is also shown. As a result of this study a Canadian limit of 7
ppm
leachable lead was established for ceramics intended for use with foods.
DESCRIPTOR(S) :
Analytical methods / Ceramics industry / Pottery industry / Pottery
glaze /
Standards / Vitreous enameling / Heavy metals / Chemical analysis /
Chemical
extraction / Containers / 7439921 / 7440508 / 64197
=2A=2A=2A END OF RECORD =2A=2A=2A
The main point would be to have an idea of the severity of your exposure to
copper
compounds.Are you using copper sulfate, copper oxide or copper carbonate?
Do you mix glazes very often, what amount of copper compounds are used over
a given period of time?
Do you use personal protection?
Do you fire often,indoors or outdoors?
Do you use ventilation in firing in your studio?
Etc.
Copper is an essential metal and the daily requirement has been estimated
at 30 micrograms/kg of body weight for an adult.
The normal concentration of copper in blood plasma is 1 mg/liter.
The gastrointestinal absorption of soluble copper is regulated by the need
and is usually around 50=25=3B uptake may also take place after inhalation.
Elimination of copper is slow and takes place via bile.
Wilson's disease is caused by an inborn error of the copper metabolism
characterized by a diminished capacity to eliminate copper via bile.This
leads to copper accumulation and secondary lesions in the liver and the
brain.
Copper sulfate has clinically (medicine) been used as an emetic and it is
well established that soluble copper salts at high concentrations cause
gastrointestinal irritation with vomiting, epigastric burns, and sometimes
diarrhea.
High copper content in drinking water and food may contribute to the
development of severe liver damage in infants.
Inhalation of copper dust and fume results in irritation of the respiratory
tract, ulceration and perforation of nasal septum, metallic or sweet taste,
and in some instances, discoloration of the skin and hair.
Vineyard sprayers using a solution containing aqeuous copper sulfate
developed granulomatous and fibrotic lung lesions.
Liver granulomatas containing copper have also been found in vineyard
sprayers
Experimental studies in mice support that copper is an etiologic(cause) for
fibrosis.
Copper fumes may also cause an influenza-like(benign) illness called =
=22metal
fume fever=22.
Reference:Occupational Medicine,Carl Zenz =26 al.
Copper exposure can result in granulomatous liver disease with hepatic
granulomas near or within the portal tracts.
Clinical liver disease is usually not significant, but granulomas
occasionally result in hepatomegaly, necrosis and fibrosis.
Copper sulfate is used in the whitewashing and leather industry.Toxicity is
primarily due to accidental and suicidal attempts, and results in
intravascular hemolysis, methemogloninemia, renal failure and often death.
Reference: Occupational =25 Environmental Medicine, Joseph Ladoue =26 al.
Copper is a metal used as an alloy in brass and bronze, as a component in
some solders, in pigments such as emerald green, and as a salt in the
lithographic process.The salts are irritants to the skin, eyes,and mucous
membranes.The inhalation of metal fumes produced at high temperature, such
as welding, may cause metal fume fever, nasal ulcers, and respiratory
irritation.Ingestion of copper salts may cause vomiting, diarrhea,
hemorragic diarrhea, and excessive salivation.The chronic accumulation of
copper in the body is rare and occurs in the progressive and sometimes fatal
autosomal recessive condition called Wilson's disease(hepatolenticular
degeneration)
Reference:Hazardous Materials Used in Arts and Crafts, chapter 60 of
Hazardous Materials Toxicology, Sullivan =26 Krieger
I dont think the way potters use copper compounds pose a major threat.
The same applies to pottery users if lead-glazed pots are not used.
Later,
Edouard Bastarache
edouardb=40sorel-tracy.qc.ca
http://www.sorel-tracy.qc.ca/=7Eedouardb/
-----Message d'origine-----
De : Ray Aldridge =3Cpbwriter=40fwb.gulf.net=3E
=C0 : CLAYART=40LSV.UKY.EDU =3CCLAYART=40LSV.UKY.EDU=3E
Date : 5 ao=FBt, 1999 11:24
Objet : Re: Let me rephrase =22oxides=22
----------------------------Original message----------------------------
At 02:17 PM 8/3/99 EDT, you wrote:
(big snip of very interesting information)
=3E
=3EThe chalcophile elements are only weakly attached in silicate materials =
and
=3Einclude quite a few nasties. Of relevance to ceramists, they include
=3ECu,Cd,Sb,Ag,Bi,Pb,Zn, which suggests that these elements will be only
weakly
=3Eattached to the Si and O atoms in glazes. The anomaly in this group is =
Zn,
=3Ewhich does not always behave like a typical chalcophile although it
=3Epartitions strongly into sulphides in meteorites and smelter =
concentrates.
=3EZinc seems to be dual in nature, being also able to form stable =
silicates.
=3EThis anomaly is known to occur because of zinc's high electrode =
potential,
=3Ewhich is more typical of the lithophile group. The prediced behaviour of
=3Ethis group in glazes (excepting zinc) is born out by experience, as this
=3Egroup are easily induced to exsolve from the glass by subtle changes in
=3Ecomposition, atmosphere, etc. I think it would be wise to exclude
=3Echalcophile group elements (excepting zinc) from glazes to be used in
=3Econtact with food and drink.
=3E
I don't know anything about this, which is why I'm asking, but is the
toxicity of copper really a problem in pottery? I've never been able to
understand why it's okay to have copper in the water pipes, but not in
glazes. Evidently lead pipes are unhealthy and have been associated with
lead poisoning, but are there any epidemiological data supporting the idea
that copper pipes are dangerous? In some locales, the water is acidic,
which would make lead pipes an even worse idea-- but if there were some
morbidity associated with copper pipe, wouldn't these locales pop up in any
analysis of copper-related illnesses, if such exist? After all, most of us
drink water that's passed through copper pipe.
Also, in the spirit of your excellently eclectic post, I wonder if there
are any archaeological data available that might show the effect of copper
in the high alkaline Middle Eastern copper blue wares. I understand that
analysis of Roman remains shows a high lead level in many cases, apparently
due to use of lead serving vessels. The copper blue glazes are extremely
unstable, and must certainly leach a large amount of copper.
Ray
Ray Aldridge on sat 7 aug 99
At 02:30 PM 8/5/99 -0400, Tom Buck wrote:
>John Hesselberth has been through this at least twice on Clayart
>and Monona Rossol has repeated the figures on copper allowance. Check the
>archives. The body needs 2-3 mg of Cu++ ion daily but any more and the
>body has real problems.
>
Tom, I did indeed check the archives, but they were not informative in
respect to my curiosity. To restate it: why is copper piping considered
proper building practice, but copper-bearing glazes considered potentially
unsafe? John alluded to the fact that there are no standards for leaching
copper, and suggested we attempt to approach the water standards, though I
cannot quite grasp the logical foundation of this advice, since leaching
tests are conducted with acids much more reactive than even the most acidic
tap water. David Hendley mentioned that the state of Texas tests for
copper concentration, but again, this does not answer my question, since
the state of Texas has not seen fit to outlaw copper pipe.
I did, however, find a post by you in which you say:
"Since copper is an essential element for some of the body's enzyme
systems, and since many texts recommend one consume 1000-3000 micrograms
of copper daily to make up for losses in urine, then one can say that a
smooth-surfaced glaze will not likely release that level of copper into
acidic foods, especially a glaze that has appropriate amounts of alumina
and silica for the firing temperature. And further, since copper oxide is
an excellent flux, any amount up to 6% will probably be well incorporated
into the glaze's glass matrix."
This seemed to conflict with John's assertion that (as I understood him)
copper was very difficult to keep in a glaze. He said:
"Copper is a relatively low toxicity material, but it also has problems.
At least here, there are some reference points. First, it is very
difficult to keep in a glaze compared to other colorants so I believe it
is important to check it for leaching."
Confusing.
Ray
Michael Banks on sat 7 aug 99
Dunno Ray, but there has been a lot of informed comment in the past on this
forum about copper. If it leaches, it can't be good for you. Personally I
don't like using any copper glaze on foodware, tested to be low-leach or
not.
Purely anecdotally, I've heard that the higher than ideal levels of copper
in our diet, is implicated in causing the declining sperm counts in western
men (sorry, can't remember where I read it). So if you want a chemical
vasectomy, go for it! But remember, all that cytotoxic metal is also
carousing around inside your other organs too.
Michael Banks,
Nelson,
New Zealand
----- Original Message ----- >
> I don't know anything about this, which is why I'm asking, but is the
> toxicity of copper really a problem in pottery? I've never been able to
> understand why it's okay to have copper in the water pipes, but not in
> glazes. Evidently lead pipes are unhealthy and have been associated with
> lead poisoning, but are there any epidemiological data supporting the idea
> that copper pipes are dangerous? In some locales, the water is acidic,
> which would make lead pipes an even worse idea-- but if there were some
> morbidity associated with copper pipe, wouldn't these locales pop up in
any
> analysis of copper-related illnesses, if such exist? After all, most of
us
> drink water that's passed through copper pipe.
>
> Also, in the spirit of your excellently eclectic post, I wonder if there
> are any archaeological data available that might show the effect of copper
> in the high alkaline Middle Eastern copper blue wares. I understand that
> analysis of Roman remains shows a high lead level in many cases,
apparently
> due to use of lead serving vessels. The copper blue glazes are extremely
> unstable, and must certainly leach a large amount of copper.
>
> Ray
>
Ray Aldridge on sun 8 aug 99
At 02:37 PM 8/7/99 EDT, you wrote:
>----------------------------Original message----------------------------
>------------------
>Hello Ray,
>
>copper may enhance the leaching of lead from glazes.
(big snip of very helpful information)
>I dont think the way potters use copper compounds pose a major threat.
>The same applies to pottery users if lead-glazed pots are not used.
>
Thanks, Edouard. Just what I was hoping to learn. I did a little online
research into the copper toxicity problem, out of curiosity, and one figure
that kept jumping out was that the lethal dose was 23 grams (which as you
know is an absurdly unscientific statement) Anyway, if this is indeed the
case, then my little sister Amy is a miraculous case of death-defiance,
because she liked to swallow pennies when she was little.
Yet another reason not to use lead.
Ray
Edouard Bastarache on mon 9 aug 99
------------------
Hello Ray,
copper compounds, used as fongicides in the USA, have the following =
=22Lethal
Dose50=22=3D 200-10,000mg/kg. It depends on the compound, these =
are=22killers=22
for fungi.
Red copper(I) oxide has the following LD50 by ingestion in the =
rat=3D470mg/kg.
I dont have one for black copper(II) oxide for the moment.
The TDLo (lowest published toxic dose) in the rat is=3D152mg/kg. for copper
metal by ingestion.
Later,
Edouard Bastarache
edouardb=40sorel-tracy.qc.ca
http://www.sorel-tracy.qc.ca/=7Eedouardb/
-----Message d'origine-----
De : Ray Aldridge =3Cpbwriter=40fwb.gulf.net=3E
=C0 : CLAYART=40LSV.UKY.EDU =3CCLAYART=40LSV.UKY.EDU=3E
Date : 8 ao=FBt, 1999 19:05
Objet : Re: Let me rephrase =22oxides=22
----------------------------Original message----------------------------
At 02:37 PM 8/7/99 EDT, you wrote:
=3E----------------------------Original message----------------------------
=3E------------------
=3EHello Ray,
=3E
=3Ecopper may enhance the leaching of lead from glazes.
(big snip of very helpful information)
=3EI dont think the way potters use copper compounds pose a major threat.
=3EThe same applies to pottery users if lead-glazed pots are not used.
=3E
Thanks, Edouard. Just what I was hoping to learn. I did a little online
research into the copper toxicity problem, out of curiosity, and one figure
that kept jumping out was that the lethal dose was 23 grams (which as you
know is an absurdly unscientific statement) Anyway, if this is indeed the
case, then my little sister Amy is a miraculous case of death-defiance,
because she liked to swallow pennies when she was little.
Yet another reason not to use lead.
Ray
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