Karl P. Platt on tue 8 apr 97
Let's start this by saying that of fundamental importance here is that
those among us who are in tableware want to have the hippest glaze at no
expense to long term durability. I will submit that essentially all
potters in the business understand this at this point and am not going
to proceed on the premesis that they (we) are idiots. Good glaze does
not come casually and I will call for a discussion aimed at getting
beyond recipes.
Now,.......
>> 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....
This scenario is largely implausible. Let's go on to the net item.
>>What are you talking about: glaze or crystal?<<
In the sense of crystal glass or crystalline materials?
>>Why would it be BaSiO3?<<
Because carefully taken mesurements of the differences in thermodynamic
properties of these reactions have been made and show this to be the
case. See A. Paul., Chemistry of Glasses, Chapman and Hall.
>>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.
Glasses which have the same composition as some crystal, let's use
BaSiO3 as an example, show chemical properties that are quite similar to
the crystalline cousin. Generally, the Gibbs energy will be a little
higher for the glass ( reflecting it's weaker structure) and the entropy
will be larger reflecting the same thing.
In the case of a pure BaO-SiO2 melt, which is difficult to produce in a
homogeneous form owing to immiscibility, BaO does, indeed serve to
disrupt Si-O-Si bonds. However, in the presence of Alkali it's behavior
is fundamentally different. Ba (Mg,Ca,Sr,etc) have the effect of
_stabilizing_ the melt rendering it more stable than it would have been
without the addition.
>>pH is _NOT_ the only criteria. Citrates can complex Al+++ and make it
soluble so why should Ba be different.<<
Who said it was? But it is and the substance of that discussion my be
more appropriate elsewhere. The use of the concept of pH has been very
cavalier here. I simply stated a series of facts for a clearly defined
set of circumstances. Besides, in the real world, even if these
reactions do proceed, their rates are extremely slow againt the well
made glaze. We're not discussion the pace of reactions one might see
when they light a match -- or a stick of dynamite.
>>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.<<
I'm not sure what this is getting at. We're talking about a cup, not a
tile floor. Assumng evaporation doesn't take place it remains that a
given volume of solvent can only dissolve "x" amount of a substance in
any event. If it is spread thinly over a wide area the net amount of
corrosion (leaching) which might occur in a given area is significantly
smaller than in the case where it is confined in a cup.
>>Who says they wash the cup between each drink?<<
Who cares. In this case there's probably more risk from bacteria than
the glaze.
>>Yes and increase surface area for attack.. and _possibly_ produce two
>>phase glaze with one phase much more soluble than the other..
The odds of having two phases in a ceramic where one is more soluble
than the other in a given medium are very high -- even if these are,
say, ZrO2 and Al2O3. It is the relative solubility that we are concerned
with and this should be elaborated.
>>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.
This doesn't mention that the enamel frit likely contained up to 8 or 10
mol% B2O3 in combination with the BaO. Let's be clear that enamels are,
by definition, substantially weaker in structure than most all
commercial glasses and ceramic glazes and it is unreasonable to draw
comparisons between the two.
Anyway, this is all to show that there's a great deal occuring within
ceramic glazes that are worth considering in more detail here. And,
again, I am urging this. However, I will remain firm in the conviction
that the grand majority of tableware glazes which contain BaO are
perfectly safe to use -- remember, there are _no_ leaching standards for
Ba reflecting either that there are no problems and/or that the no-one
has yet managed to create one. Show me one potter or a customer of
theirs in the recent history of potting laid low by Ba and I will
shut-up and go away. I will also cite myself (again) as a living
example of someone who has been working with these "deadly"substances in
quantity, eaten off of Ba blue plates, used Pb glazed plates my whole
life (so have you), enjoyed my Pb glass stemware and so on since the
time when I had no gray hairs ( but I ain't old) and have suffered no
maladies as are gravely predicted here....and there. Guys, if anyone is
the guinea pig for all of this dire spooge it's me and I am not a
moon-suit sort of guy. Tidy, yes. Paranoid - why bother. I'd rather
enjoy the sunrise over Lagoa da Conceicao instead and reckon I'll be
doing that for another 40 years or so. I'd suggest it to you, too
Regards,
KPP
Gavin Stairs on wed 9 apr 97
At 07:05 AM 08/04/97 EDT, you wrote:
....
>Anyway, this is all to show that there's a great deal occuring within
>ceramic glazes that are worth considering in more detail here. And,
>again, I am urging this. However, I will remain firm in the conviction
>that the grand majority of tableware glazes which contain BaO are
>perfectly safe to use -- remember, there are _no_ leaching standards for
>Ba reflecting either that there are no problems and/or that the no-one
>has yet managed to create one. ...
I have three comments:
First, we need to be clear that not all glazes in use by studio potters end
up as mature, tough, glass coats. The discussion of glasses and
thermodynamically equilibrated structures is part of the question, but it
is by no means the end of the discussion. It is extremely hard to get ANY
species out of the matrix of a mature, homogeneous glass melt or crystal.
That is what I was referring to in my earlier comments concerning the
initial vs. the long term rates. The reported resistance of zirconia
encapsulates is attributable to this, and to its insolubility in the
siloxide matrix, which prevents disruption of the crystal trap. I cannot
credit any large release of Ba, or any other species, from a structure such
as these mature glasses. But from an immature glaze melt... that's an
entirely different question. It is not proper to dismiss thermodynamically
disadvantaged species in these circumstances.
Second, the existence or non-existence of standards is no evidence of
anything.
Third, I also suspect that the great majority of Ba glazes are safe under
ordinary use. However, I am not so cavalier as to dismiss the rest with a
wave of the hand. Virtually all of the safety problems we are discussing
here are low percentage risks. As I have stated previously, even lead
glazes, which are universally acknowledged to be "a bad thing", have a long
and glorious history of largely innocuous human use. It is a matter of
good practice on the part of potters to ensure that THEIR products are safe
for general use. If we do not KNOW if a thing is safe, it may be the
better part to avoid its use.
One last remark. Studio practice is not the same as the tableware
industry. First, there are few lab resources available to studio potters.
Next, they have been educated as artists, for the most part, not
technologists. And they strive for artistic effect by unusual means, which
would not be contemplated by an industrial concern. The cases are
different, and require different approaches. We need to learn from all
sources, but translate for our own circumstances.
Gavin
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Gavin Stairs
http://isis.physics.utoronto.ca/
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