Mark Luton on sat 29 jun 96
Hi from Down Under,
As my first posting, I am hoping someone can clear up a
dilemma I'm concerned about.
There seems to be a variety of opinions as to what
percentage of barium carbonate in a glaze recipe is considered safe for the
user of the vessel; an example being a recently acquired celedon with 16.5%.
A fellow potter felt that the celedon shouldn't be used inside eating or
drinking vessels.
Any advice would be appreciated.
Thanks.
Mark
mluton@ultra.net.au
Cathy Nelson Hartman on sun 30 jun 96
Mark,
The problem is the solubility of the barium in the fired glaze just as
lead. You would need to find a barium test kit similar to the lead tests
on the market. However the main criterion for solubility is the degree of
fusion in the glaze. Matts -being essentially immature glazes -will be
more soluble,whereas transparent glazes-like your celedon- are the least
soluble. The barium would be surrounded by a glass rendering it insoluble.
Your glaze is probably the most safe of barium glazes. How safe that is
is still for you to determine via testing.
walter hartman
Ron Roy on sun 30 jun 96
For Mark,
The amount of Barium in a glaze has little to do with how much will leach
out under specific conditions. What has to do with leaching is the quality
of the glaze or glass.
Example 1 - a cone 6 glaze with 2% barium carb but has a little less silica
than is needed for a durable glaze according to the limit formulas I use
(from Insight) - when tested in a lab leached 0.98 ppm of barium.
Example 2 - a cone 6 glaze with 4% barium and just over the limit formulas
minimum amount of silica - leached .001 ppm.
The real problem, over the long term, is: even if you have the glaze tested
and it proves safe - there is no guarantee that future firings and changing
materials will give the same results.
The safest course for glazes which will be in contact with food is to
substitute strontium Carb .75 to 1.0.
As for glazes oversupplied with Barium - I don't think there is any way to
make them safe.
Just to reiterate in case the thought got lost in the details: The safety
of a glaze - in this case - has to do with the molecular structure of the
glaze - not how much Barium is included - up to a point - after that it's
hopeless.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Richard Burkett on sun 30 jun 96
Walter Hartman says:
>Matts -being essentially immature glazes -will be
>more soluble,whereas transparent glazes-like your celedon- are the least
>soluble
Actually this isn't exactly accurate, Walter. True matte glazes are formed by
crystal growth in the glaze surface during cooling. These glazes actually
reach full fusion during the firing and would be quite glossy if cooled
quickly. Other mattes are indeed immature glazes, but not all.
And merely being a gloss glaze doesn't always ensure that the glaze will be
durable or insoluble. It has more to do with the amount of silica and alumina
in the glaze, along with the formation of a mature glaze, as these have an
effect on hardness and durability. The presence of large amounts of alkaline
fluxes or very high boron glazes without much silica, for instance, may in
some cases be more easily corroded by acids and leach oxides from the glaze.
Sound glaze design and laboratory testing are the best ways to ensure that
barium is not leaching from your glaze. Or don't use barium.
Unfortunately the ClayArt archives are still not accessible on the listserv,
as this is a topic that has been discussed over and over in the past three
years. And still no word on when the archives will again be easily available.
Richard Burkett -
School of Art, Design, & Art History, SDSU, San Diego, CA 92182-4805
E-mail: richard.burkett@sdsu.edu <-> Voice mail: (619) 594-6201
Cathy Nelson Hartman on mon 1 jul 96
I must admit that I misspoke myself on the too inclusive term "matt
glazes " when I was specifically thinking of the high alumina matts as
opposed to the crystalline matts referred to by Richard Burkett.
The comment on transparent glazes and their solubility is more sublime in
the context of celadon glazes but I'm sure he is accurate.
Thanks for clarifying.
walter hartman
Don Jung on tue 2 jul 96
To all the glaze gurus out in clayart, I was wondering if there are any
other ingredients in a glaze that could/should be tested for? Lead and
Barium are the obvious ones, but what about Lithium, Manganese or ...?
Are all the other ingredients rendered inert or have sufficiently low
solubility that it's not a concern?
Ron Roy pointed out that a particular glaze was not suitable for cone 6
and would be more susceptable to break down and release of Barium (in
this particular glaze). Should there be any concern regarding other
ingredients in a similarily 'weak' glaze leaching out?
This is especially a concern when we make a 'mystery' glaze out of the
scrap and leftover glazes.
thanks Don Jung ***Happy Canada Day***
Appreciating all the great info and posts.
Cathy Nelson Hartman wrote:
>
> ----------------------------Original message----------------------------
> Mark,
> The problem is the solubility of the barium in the fired glaze just as
> lead. You would need to find a barium test kit similar to the lead tests
> on the market. However the main criterion for solubility is the degree of
> fusion in the glaze. Matts -being essentially immature glazes -will be
> more soluble,whereas transparent glazes-like your celedon- are the least
> soluble. The barium would be surrounded by a glass rendering it insoluble.
>
> Your glaze is probably the most safe of barium glazes. How safe that is
> is still for you to determine via testing.
>
> walter hartman
Ron Roy on thu 4 jul 96
>----------------------------Original message----------------------------
>To all the glaze gurus out in clayart, I was wondering if there are any
>other ingredients in a glaze that could/should be tested for? Lead and
>Barium are the obvious ones, but what about Lithium, Manganese or ...?
>Are all the other ingredients rendered inert or have sufficiently low
>solubility that it's not a concern?
>Ron Roy pointed out that a particular glaze was not suitable for cone 6
>and would be more susceptable to break down and release of Barium (in
>this particular glaze). Should there be any concern regarding other
>ingredients in a similarily 'weak' glaze leaching out?
>This is especially a concern when we make a 'mystery' glaze out of the
>scrap and leftover glazes.
>> Mark,
>> The problem is the solubility of the barium in the fired glaze just as
>> lead. You would need to find a barium test kit similar to the lead tests
>> on the market. However the main criterion for solubility is the degree of
>> fusion in the glaze. Matts -being essentially immature glazes -will be
>> more soluble,whereas transparent glazes-like your celedon- are the least
>> soluble. The barium would be surrounded by a glass rendering it insoluble.
>>
>> Your glaze is probably the most safe of barium glazes. How safe that is
>> is still for you to determine via testing.
I feel I have to add a bit more here. Shiny glazes can leach as well. It
has to do with the ability of the "glass" to resist attack by acid and
alkaine. The safest policy is to use liner glazes which have no hazards. If
you want to use the doughtful oxides the best policy would be to have the
glazes tested but that is expensive. The third alternative is to evaluate
the integity of the glaze by calculating the molecular formula and
comparing to some standard like limit formulas. The next choice would be to
test for yourself - there must be information somewhere on how to do this -
I don't know how reliable the test kits are and I have only seen them for
lead.
There are many questions here - do the hazardous materials in stains come
out of glazes if the glass is inferior - the answer is yes. Are the new
"encapsulated" stains capable of releasing their contents under the right
(wrong) conditions - they say not but I have not heard of any testing so
far.
I could go on about this being a ripe field for inquiry and publication but
........
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
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