M Richens on mon 7 apr 97
In article <334417BE.65DF@glass.com>, "Karl P. Platt"
writes
>
>The kenetic side of things, as roughly described above, are important
>and I sought to address these issues in the original post by noting that
>the Ba ion is the larges of all one might encounted in glazes. As such
>its diffusion rate -- how fast it can make it to the surface where the
>leaching is taking place, is restricted by its size -- at ambient
>temperatures.
True but who is saying the Barium is released by diffusion? If the Al Ca
Mg are attacked and chelated the Ba++ will fall out. This should not
happen with a properly formulated _AND_ fused glaze but....
>
>I was thinking on the way over here today about the EPA Ba Drinking
>Water Standard -- it's 2 parts per million (PPM). This may be visualized
>as 2 grams in 1 million grams -- which is a metric ton. Alternatively,
>this may be viewed as 0.0002% by Wt. Now let's imagine a ceramic glaze
>with, say, 8 wt% BaO -- it'll likely be matte. Further, let's imgine
>this glaze is applied to a drinking cup with 300 cm^2 of surface area
>inside and that 3 times/day there's orange juice in this cup for 6
>minutes. That's 18 minutes/day. Now we want to imagine a case where in
>that 18 minutes the concentration of Ba in the orange juice would rise
>to 2 ppm. Assuming the orange juice has a specific gravity of 1.3 or so
>we know that the weight of the juice is 390 grams. Thus 2 PPM of Ba
>would amount to 0.078 grams. This 0.078 grams would have to be removed
>from the glaze at a rate of 0.004 grams/minute.
>
>Now if Ba resides in the glaze as BaSiO3 (it tyoically does) we can go
>to our handy dandy Hanbook of Chemistry and Physics to discover that the
>Gibbs Free Energy of the reaction BaSiO3 (crystal) + 2H == H2SiO3 + Ba2+
>is -30570 calories or the reaction constant K= -22.4.
What are you talking about: glaze or crystal? Why would it be BaSiO3?
There are more than these three elements in the majority of glasses. The
reactions produce many different solid solutions at various stages of
the melting process.
Glasses are intrinsically different in their behaviour to crystalline
material. The Barium breaks the -Si-O-Si- tetrahedra and weakens the
glass structure as do all the fluxes and modifiers. They make it easier
to soften and work but can reinforce the glassy state. The glass will
take the fluxes in and can make them harder to get at or the glass
former proportion can be so low that the release is worse. It depends on
the formula, thermal history etc...
>If the amount of
>Ba in the OJ is zero, and we assume that the activity of both the H2SiO3
>and the Ba are equal, then we find that for a pH of 3.6 --
pH is _NOT_ the only criteria. Citrates can complex Al+++ and make it
soluble so why should Ba be different.
> in the
>ballpark for OJ -- the amount of Ba+ balanced in the solution is, get
>this, 6.03 x 10^-14 grams. Of course, there's absolutely no way that
>this could ever approach the 0.078 grams that would send the level of Ba
>in the OJ to 2 ppm.
It is not just down to volume of liquid. A small volume spread over a
large surface area can produce more attack than a large volume due to
changes in the state of the solution and attack mechanisms. Who says
they wash the cup between each drink?
If it's for food test it. Don't calculate it.
Some commercial organizations will put a different glaze on the inside
of a cup to that on the outside to avoid problems with liquids.
>
>A barium matte has a couple forms. BaO is, as we know, rather
>refractory. Adding 6-10wt% BaO to any shiny glaze will make it matte.
>The crystalline habit of Ba mattes is, in my experience anyway, miuh
>finer in texture than a Ca matte or an Al2O3 matte. Like the Ca matte a
>Ba matte will become more glassy as the firing time-temperature is
>increased -- all else being equal. This would affect the distribution of
>the Ba between glassy and crystalline material present.
Yes and increase surface area for attack.. and _possibly_ produce two
phase glaze with one phase much more soluble than the other..
>
>
>In an underfired glaze the Ba may wellexist as BaO
How will you know? It may well not.
In 20 years of enamel glass R&D I was able to generally predict
properties of my experimental glass formulae but every so often one
would get up and kick me in the teeth. It is difficult to predict with
accuracy when you deal with 10-15 oxide variables in the formulation and
maybe as many minerals in the batch.
I also found that using > 3% Barium in a glass.. (Remember everything is
fritted together first then milled and applied).. would sometimes give
enamels which were unstable in slip form due to Ba leaching.
If you underfire your glaze, which is not fully fritted, then you are in
an unstable situation wrt binding of materials into the structure.
You have to test. At the simplest level: your reputation is at stake.
Max
--
Max Richens max@richens.demon.co.uk +44 (0) 1925756241
Enamel Consultant - Ceramist - Analyst programmer
Software for Batch Formulation and Millroom control.
Gavin Stairs on tue 8 apr 97
At 12:24 PM 07/04/97 EDT, Max Richens wrote:
....
>You have to test. At the simplest level: your reputation is at stake.
....
Absolutely right. If there is one thing I have learned it is that Mother
Nature has the last word...always. And there is nothing harder in the
world than to nail the truth down. A good experiment, be it ever so
humble, is worth more than the words of the best authority.
Go and try it out: measure, measure, measure. I begin to think we need an
institute for the provision of expensive analyses to indigent potters.
Gavin
=================================
Gavin Stairs
http://isis.physics.utoronto.ca/
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