Karl P. Platt on fri 4 apr 97
Meu Deus! Voces me mandou muitas cartas hoje!
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(leac
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= Kinetic Stability + Thermodynamic Stability.
The kinetic side of things, as roughly described by Mr. Stairs above,
are important
and I sought to address these issues in the original post by noting that
the Ba ion is the largest of all one might encounted in glazes. As such
its diffusion rate -- how readily 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 kinetic 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.
There are also the matters of whether and how often the leaching
solution is replenished and whether it is agitating during the leaching
process.
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. I'm stuck trying
to think of a real-world example to demonstrate how REALLY small this
is..... How about this:
If you drank a 500 ml cup of this water every day you'd ingest 0.001
grams of Ba. That is, you'd have to drink 1000 cups of water or 500
litres (13 or so gallons) to ingest 1 gram of Ba from the liquid
containing 2 ppm.
Now let's consider a 500 ml ceramic cup with a surface area of 50 cm^2
that has been glazed with a glaze containing 5 wt% BaO and that the
glaze film is 1.0g/cm^2 -- lotsa glaze. This means that there's 2.5
grams of BaO (consider this loosely) in the cup. Now, if the glaze
leached so much as to place 2 ppm Ba into the liquid within it after
2,500 cups of water (perhaps a 2 years of using the same cup every day)
you'd have consumed all of the Ba in the glaze and the glaze would look
like a sponge.
There's been talk here about the effects of pH on leaching. To put the
matter of pH in perspective, below are shown equilibrium constants for
various ions in aqueous solution at various levels of pH. These are
based on the general solution reaction:
MSiO3 + 2H == H2SiO3 + M+
Where M is the metal ion in question Ba, Ca, Pb and Na. In calculating
these figures one assumes concentration of the metal in the solution is
constant. Of course, in the real world this is not the case, however, as
more metal gets into the solution the tendency for the solution to
take-up any more is reduced. All of this also assumes room temperature
conditions. Going through the calculations to demonstrate what happens
at higher temperatures is a lot of work, and I'm not into it right now.
I'm also not going to go through all the brincadeira with the details of
these calculations, but would refer anyone interested to any college
level thermodynamics textbook.
In short, if a number if positive, the substance is soluble at that
level of pH and the magnitude of the number reflects how much of the ion
might be present in the solution. Note that Ba is not so dissimilar from
Ca and that Pb isn't terribly soluble and appears to increase the
durability of glass to basic solutions. However, in the case of PbO
glasses with more than 35% or so PbO serious structural changes occur in
the glass which drastically increases its solubility at all levels of
pH. We can also see that Na happily goes into solution regardless of pH.
Obs: This is all VASTLY simplified -- gotta say that, I'm trying to make
this as accessable as possible.
Equilibrium Constants
pH K K K K
Ba Ca Pb Na
1 18.5 16.3 3.206 31.4
2 16.5 14.3 2.206 29.4
3 14.5 12.3 1.206 27.4
4 12.5 10.3 0.206 25.4
5 10.5 8.3 -0.794 23.4
6 8.5 6.3 -1.794 21.4
7 6.5 4.3 -2.794 19.4
8 4.5 2.3 -3.794 17.4
9 2.5 0.3 -4.794 15.4
10 0.5 -1.7 -5.794 13.4
11 -1.5 -3.7 -6.794 11.4
12 -3.5 -5.7 -7.794 9.4
13 -5.5 -7.7 -8.794 7.4
14 -7.5 -9.7 -9.794 5.4
15 -9.5 -11.7 -10.794 3.4
16 -11.5 -13.7 -11.794 1.4
17 -13.5 -15.7 -12.794 -0.6
All of this is intended to establish some of the physical realities
surrounding the matter of leaching.
Here we should also observe that the numbers above are for pure
substances. The situation in a glaze is much more complicated
Mr. Wilt requests:
>>Discussion about (1) glazes with a little barium,(2) barium matts, (3) underfi
glazes.<<
How the Ba ends up being situated in a glaze depends on who else is
around in the glaze. The matter really isn't so simple as noting the
quantity of Ba in a glaze and degree of firing it receives. In general,
lower firing and bright glazes will tend to give-up more of anything
they contain than a less bright and harder fired glaze. This is because
the glassy network holding them together is weak owing to the presence
of larger amounts of the alkalies and/or notable amounts of B2O3 which
makes a glass markedly weaker than the SiO2 based variety -- the B-O
bond is a lot weaker than the Si-O bond.
If the glaze is vitreous and the firing is done at reasonable
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 in the crystalline form, 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 of Ba in the
glaze is small, say 1 to 4 wt%, and the relative proportion of alkali in
the glaze is also small, the amount of Ba that might be leached by
churning battery acid in a week 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, much
finer in texture than a Ca matte or an Al2O3 matte -- some call it
"buttery". 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.
Really high BaO content glazes are another matter entirely. If Ba is the
preponderant element among all of the others it will, of course, be
given preference in any decomposition of the glaze by a solution. I
don't have time to go through phase diagrams and number crunching to
detail the most likely scenario here. Suffice it to say that of all of
the forms of Ba to have in a glaze, the presence of BaO as such is the
worst case as regards leaching potential. This is only likely to be seen
in glazes bearing extremely high amounts of BaO in the composition. To
avoid problems ensure you're working with at least a tri-silicate --
SiO2 is at least 3 in the Seger Formula. Also work with the least amount
of BaO necessary for the intended effect.
Some effects, like blue and purple modifications of Cu colors aren't
possible in other types of glazes except those carrying large amounts of
Bao and/or alkali. These glazes, while beautiful in their effects, are
probably not the best thing to use on the inside of a casserole dish. On
sculpture, who cares.
I also can't find any systematic studies on Ba leaching from glazes.
Perhaps their absence reflects the exceedingly small (absent) nature of
this "problem". If anyone else does discover any peer reviewed studies,
pass them along here. I'm really burned out on reading all of this
flakey anecdote that passes for authority.
This whole discussion about chemical durability raises questions in my
mind about the merit of firing at cone 6. This seems to be an arbitrary
standard in the potting world these days. I would suggest that if we
were to move up to cone 8 a lot of these chemical durability issues go
away by nature of the more "rigid" structures given by the lower alkali
content of higher firing glazes.
This isn't to say that wholly satisfactory and durable glazes can't be
made at cone 6. On the contrary, the tableware industry produced hotel
china (bem resistente) with fritted lead glazes at cone 5 for decades *
absolutely without incident*. I'll stew on some "rules" one might want
to consider when formulating tableware glazes for lower temperature
firings. Art glazes are a bit more complicated in this regard if only
because the many spectacular effects are given by deliberately causing
unstable situations that give color separations, unusual colors,
textures and so on.
OK, I need to go study estatistica for tomorrow's lecture.
Valeu! Tchau!
Karl P.Platt -- no laboratorio
P.S. SrO is typically _not_ a plausible substitute for BaO if the BaO
has been added for color effect. This, however, is anothe post and I'm
outta here
Craig Martell on mon 7 apr 97
At 08:15 AM 4/4/97 EST, you wrote:
>----------------------------Original message----------------------------
>How the Ba ends up being situated in a glaze depends on who else is
>around in the glaze. The matter really isn't so simple as noting the
>quantity of Ba in a glaze and degree of firing it receives. In general,
>lower firing and bright glazes will tend to give-up more of anything
>they contain than a less bright and harder fired glaze. This is because
>the glassy network holding them together is weak owing to the presence
>of larger amounts of the alkalies and/or notable amounts of B2O3 which
>makes a glass markedly weaker than the SiO2 based variety -- the B-O
>bond is a lot weaker than the Si-O bond.
In regard to bright glazes, whether or not they are durable depends on the
ratio of silica and alumina as compared to the alkalies in the seger
formula. There are many bright glazes from cone 6 to cone 10 and above that
contain sufficient silica and alumina to be considered durable.
Craig Martell-Oregon
Karl P. Platt on mon 7 apr 97
>>What is important is not what the maker thinks, its what the buyer
thinks.<<
Agreed. Consider recycling. Anyone who works with it knows full-well
that it conserves nothing or is convieniently ignoring (or enjoying)
their highly subsidized raw-material. The public, on the other hand,
responds nicely to propaganda campaigns to the contrary. Marx, in
considering the role of religon in society couldn't have imagined the
effects of television. Consider that there are now people who have been
persuaded to believe that electric cars somehow have nothing to do with
pollution when if fact the net pollution, regardless of its form of
(highly inefficient) generation involved in producing the electricity
that feeds them is, on a spooge/mile basis, much higher than our best
internal combustion engines - and this says nothing of the metallic
lead/acid batteries they use. Among other things imagine these in a
nasty meeting with a Kenworth. The point is that ignorance needs to be
dealt with honestly, but with the memberd of the Edison Electric
Institute lobbying for from which they are the direct economic
beneficiaries, truth in advertising remains in the murk.
>>It may be safe, you may think its safe,
but if the buyers get the idea that the product
isn't safe, manty of you could be put out of
business.<<
That's why I'd like to beat back the hysterics who want to make money
off of instilling in people unfounded fears of chemical substances. This
isn't to say that there aren't harmful chemical substances, but find me
a credentialed someone who reviles Ba in glaze who can't make money off
making the claim. This presence of this hysteria is of itself the
product of a form of marketing for the involved.
>>If some government or medical research test indicates some
vague safety concerns, people can make all of the
arguments that they want to, but sales are going
be difficult to come by.<<
Hasn't happened and the prospects are small unless un-natural or
ignorant fear prevails -- but then we are considering the Americans who
are known as a manic people -- consider the idea of "Another Ice Age"
that was popular before "Global Warming". Unfortunately, public policy
often has flimsy foundations. When the Ice Age was debunked, and with it
the research $$, attention turned to "Global Warming" (which cannot be
proved to be occuring) or the Ozone Hole -- which has always existed --
notice no-one talks about it much any more. But the chemical companies,
who backed studies of the matter, got their way. Imagine the money to be
made replacing the cooling system in every refrigerated device on the
planet.
>>If you are not correct, its could have an effect on many people who
would probably prefer
that the egotists not make these decisions on their own for the entire
community. <<
Can this be extended to the US EPA, IRS, HUD or even the Congress itself
-- All of whom have succeeded in turning every person in the country
(except HUGE corporations) into criminals of some form or another?
>>I would guess that the people you sell to won't object to barium
potters using themselves
and their own families as test subjects over the next 20 - 30 years and
letting them drink and eat out of more boring less wonderous plates and
cups. <<
This suggests that one has concluded that Ba in glaze is explicitly
harmful. Please substantiate that conclusion. The statement also
concludes that Ba is explicitly necessary to produce spectacular glaze
effects. It is not, however, certain of them have a very unique
personality.
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