Chris Schafale on fri 13 oct 00
Recently, in discussing glazes, a couple of folks (Tom Buck and
John Hesselberth, I think), alluded to the idea that a glaze might
have too much alumina. This surprised me. My impression had
been that, in general, more alumina was a good thing, as long as
the glaze melts and isn't too stiff. So, help me out -- if a glaze
melts to a glossy surface and doesn't crawl or pinhole, why would
having high alumina be a bad thing? I think John referred to a glaze
having too much alumina to be stable at cone 6, though it might be
at cone 10 -- what does this mean? Thanks for any clarification.
Chris
Light One Candle Pottery
Fuquay-Varina, North Carolina, USA
(south of Raleigh)
candle@intrex.net
http://www.lightonecandle.com
John Hesselberth on fri 13 oct 00
Chris Schafale wrote:
>Recently, in discussing glazes, a couple of folks (Tom Buck and
>John Hesselberth, I think), alluded to the idea that a glaze might
>have too much alumina. This surprised me. My impression had
>been that, in general, more alumina was a good thing, as long as
>the glaze melts and isn't too stiff. So, help me out -- if a glaze
>melts to a glossy surface and doesn't crawl or pinhole, why would
>having high alumina be a bad thing? I think John referred to a glaze
>having too much alumina to be stable at cone 6, though it might be
>at cone 10 -- what does this mean? Thanks for any clarification.
Hi Chris,
I recently ran an alumina test series (varied only the alumina--nothing
else as closely as I could do it). It showed you could have too little
or too much alumina. For the particular test I ran between .35 and .40
was best. I don't understand the mechanism. It could be that I just
didn't get a complete melt as I got to the higher alumina levels
(although this was a glossy glaze and it remained glossy and appeared to
my eye to have been melted). It could also be that the excess alumina
was not incorporated into the glass structure and was sitting there as a
second phase. This is an extremely complex system we are dealing with.
I am certain of only two things so far. If you want to have a stable
glaze you 1) have to have enough silica (>2.5 but >3.0 is better) and 2)
you must get it thoroughly melted. You can't always tell the latter by
just looking.
So right now the only answer is test, test, test. I am continuing to
work at this and will have more to say in the coming months (or years).
John
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is, perhaps, still necessary to say that the very best glazes cannot
conceal badly shaped pots..." David Green, Pottery Glazes
Chris Schafale on sat 14 oct 00
John,
More details please! How did you know that the high alumina tests
were not as good as the mid-range ones? You say that they
looked glossy and melted, and that you can't tell by looking at
them -- are you saying that you sent them all to Alfred for leach
testing, and that the higher ones failed? If so, failed how? Failed
to hold copper? Failed in some other way? Thanks for clarifying.
Chris
> Hi Chris,
>
> I recently ran an alumina test series (varied only the alumina--nothing
> else as closely as I could do it). It showed you could have too little
> or too much alumina. For the particular test I ran between .35 and .40
> was best. I don't understand the mechanism. It could be that I just
> didn't get a complete melt as I got to the higher alumina levels
> (although this was a glossy glaze and it remained glossy and appeared to
> my eye to have been melted). It could also be that the excess alumina
> was not incorporated into the glass structure and was sitting there as a
> second phase. This is an extremely complex system we are dealing with.
>
> I am certain of only two things so far. If you want to have a stable
> glaze you 1) have to have enough silica (>2.5 but >3.0 is better) and 2)
> you must get it thoroughly melted. You can't always tell the latter by
> just looking.
>
> So right now the only answer is test, test, test. I am continuing to
> work at this and will have more to say in the coming months (or years).
> John
>
> John Hesselberth
> Frog Pond Pottery
> P.O. Box 88
> Pocopson, PA 19366 USA
> EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
>
> "It is, perhaps, still necessary to say that the very best glazes cannot
> conceal badly shaped pots..." David Green, Pottery Glazes
>
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Light One Candle Pottery
Fuquay-Varina, North Carolina, USA
(south of Raleigh)
candle@intrex.net
http://www.lightonecandle.com
John Hesselberth on sat 14 oct 00
Chris Schafale wrote:
>
>More details please! How did you know that the high alumina tests
>were not as good as the mid-range ones?
Copper leaching
You say that they
>looked glossy and melted, and that you can't tell by looking at
>them -- are you saying that you sent them all to Alfred for leach
>testing, and that the higher ones failed?
Yes I leach tested them all at BSC, Inc. Alfred Analytical has been
giving pretty poor service recently. Failed is too strong a term since
there are no pass/fail standards. The ones lower than .35 or higher than
0.4 had higher release of copper--significantly higher.
If so, failed how? Failed
>to hold copper? Failed in some other way? Thanks for clarifying.
A general comment Chris. I think you may be trying too hard to find
simple answers in a very complex system. There are none--or at least
very few.
Take your question back to Tom on limit formulas for instance. Remember
where limit formulas came from. They are derived, over the decades, from
visual observation of which glazes make "good glass" and which don't.
There is NO science behind them. None. Just lots of observations of
what seems to work made by some very capable observers. Since each of
those observers worked with a different set of glaze recipes, they came
out with slightly different answers. So they are a guide to be used, not
a set of rules. I think most of us would agree you are more likely to
make stable glazes by staying within limits--and it really doesn't matter
whose-- than by going outside them. But there are stable glazes outside
limits. Also I can make ANY glaze unstable by adding 7-10% copper--in
limits or not.
In my own case I have decided to use ability to retain copper as a test
for stability. If it will leach 5 mg/l or less when it has 5% copper
carbonate it is pretty darn stable. I haven't yet found a glaze that can
pass that test that hasn't held reasonable quantities of other colorants.
But I will keep looking. There are very few absolutes in this glaze
formulation game. Lots of judgement is still required.
By the way, you can derive your own limit formulas if you use HyperGlaze.
Just indicate a bunch of recipes that work for you and HyperGlaze will
figure out what the "limits" are, i.e. what the outer limits of oxides
present in those recipes are.
Have fun, John
John Hesselberth
Frog Pond Pottery
P.O. Box 88
Pocopson, PA 19366 USA
EMail: john@frogpondpottery.com web site: http://www.frogpondpottery.com
"It is, perhaps, still necessary to say that the very best glazes cannot
conceal badly shaped pots..." David Green, Pottery Glazes
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