Karl P. Platt on fri 4 apr 97
Meu Deus, quantas cartas!
The first part of this will be dry technical matter, but if you _really_
want to learn something about leaching continue on. If not, ditch this
post.
Let me start with Mr. Stairs who writes in part:
Sure. I'll start out by observing that in all leaching reactions, with
p(leachate) in n:0 imbalance, after the surface layer is depleted,
virtually all reactions are transport limited, and therefore go as some
diffusion rate. This is usually very slow. In other words, unless the
body is dissolving, the leaching will start large and then slow down in
an exponential fashion, with a false asymptote at the diffusion rate.
So the first confusion is about which of these rates one is measuring.
The fresh surface rate is bound to be many times the limit rate.
This is all entirely true, but only addresses one side of the chemical
durability issue.
Chemical Durability= Kenetic Stability + Thermodynamic Stability.
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. As well, the influence of either of these factors depends
greatly on the way the testing is done. At low temperatures the leaching
proceeds largely on the basis of the kenetic side of things. Yet if the
surface area is very large (remember specific surface) or if temperature
is elevated, then the thermodynamic side of things will predominate.
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. 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 -- 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.
Mr. Wilt requests:
>>Discussion about (1) glazes with a little barium,(2) barium matts, (3) underfi
How the Ba ends up being situated depends on who else is around. In
particular, it depends on SiO2. If there's any SiO2 around, Ba would
much prefer to combine with it as BaSiO3. However, this requires
sufficient heating. If the glaze is highly vitreous and the firing is
done at fairly high temperatures (+cone 8), the Ba will likely be
assimilated into the glassy part of the glaze. Here one would expect
it's durability to be somewhat below that of BaSiO3, but not much. The
amount which might leach in any event depends on how much is available
to the leaching solution in the first place. If the amount is small, say
1 or 4 wt%, the amoutn of Ba that might be leached wouldn't be enough to
affect small insects.
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.
In an underfired glaze the Ba may wellexist as BaO
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