iandol on sat 21 dec 02
Dear Paul,
I had another look at your recent post since it nudged my memory about =
another issue I raised a couple of weeks ago.
I quote <instance)cone 3 as a matte glaze and others use at cone 6 as a glossy =
glaze>>.=20
You, and others, may recall I asked what were the criteria by which we =
could judge or qualify the samples on a test tile series as being =
mature, over fired or underfired. From recollection I was rewarded with =
a resounding silence. Now taking your example I would imagine that the =
difference between cone 3 and cone 6 would mean that a glaze only fired =
to lower cone would still have some of its ingredients in their original =
state, or if they were carbonates which had decomposed, that they were =
still oxides within a vitreous matrix. And if this were true, they would =
be unstable in use.
Any comment you would wish to make?=20
All the best,
Ivor Lewis. Redhill, South Australia
John Hesselberth on sat 21 dec 02
Hello Ivor and Paul,
Underfired "mattes" are all too common among potters. They are a
problem at cone 6; they are very common at earthenware temperatures.
And, yes Ivor, they are less stable. The graph on page 54 of Mastering
Cone 6 Glazes shows this affect clearly. The leaching rate on the glaze
that didn't completely melt until about cone 7 was 4 times higher at
cone 4 than it was at cone 7. Underfiring glazes to get a matte surface
is an outstanding way to make really bad glazes.
Interestingly, the glaze referenced above fired to cone 7 or 8 and
cooled slowly produced just as nice-looking a matte as did the
underfired cone 4 sample--but it was a good, stable glaze.
Regards,
John
On Friday, December 20, 2002, at 11:44 PM, iandol wrote:
> You, and others, may recall I asked what were the criteria by which we
> could judge or qualify the samples on a test tile series as being
> mature, over fired or underfired. From recollection I was rewarded with
> a resounding silence. Now taking your example I would imagine that the
> difference between cone 3 and cone 6 would mean that a glaze only fired
> to lower cone would still have some of its ingredients in their
> original state, or if they were carbonates which had decomposed, that
> they were still oxides within a vitreous matrix. And if this were true,
> they would be unstable in use.
http://www.frogpondpottery.com
http://www.masteringglazes.com
Paul Lewing on sat 21 dec 02
on 12/21/02 12:41 PM, John Hesselberth at john@FROGPONDPOTTERY.COM wrote:
> Hello Ivor and Paul,
>
> Underfired "mattes" are all too common among potters. They are a
> problem at cone 6; they are very common at earthenware temperatures.
> And, yes Ivor, they are less stable. The graph on page 54 of Mastering
> Cone 6 Glazes shows this affect clearly. The leaching rate on the glaze
> that didn't completely melt until about cone 7 was 4 times higher at
> cone 4 than it was at cone 7. Underfiring glazes to get a matte surface
> is an outstanding way to make really bad glazes.
>
> On Friday, December 20, 2002, at 11:44 PM, iandol wrote:
>
>> You, and others, may recall I asked what were the criteria by which we
>> could judge or qualify the samples on a test tile series as being
>> mature, over fired or underfired. From recollection I was rewarded with
>> a resounding silence. Now taking your example I would imagine that the
>> difference between cone 3 and cone 6 would mean that a glaze only fired
>> to lower cone would still have some of its ingredients in their
>> original state, or if they were carbonates which had decomposed, that
>> they were still oxides within a vitreous matrix. And if this were true,
>> they would be unstable in use.
I knew the minute I sent that message off that I was going to hear from
somebody along these lines, and it would probably be John. I should have
included that caveat when I said that some people used the same glaze at
cone 3 as a matte that others used as a gloss at cone 6. It's for sure not
how it should be, but it sure is how it is.
And Ivor, that could very well be the mechanism by which this happens, and
you're probably right that that makes them unstable.
Paul Lewing, Seattle
iandol on sun 22 dec 02
Dear John Hesselberth=20
Thank you for confirming my suspicions. Well, no access here to "The =
Book" though it seems to be rapidly turning into a "Bible". And I think, =
given your remarks that we are inchin' slowly towards accepting the =
concept that there are ways by which we can judge the degree of maturity =
of sample glazes when examining multiple test tiles.
Perhaps people who are interested in the notion of ramping a firing down =
to get Matt or translucent effects from otherwise mature but =
characterless glazes might care to read my article on Transforming =
Glazes in Ceramics Technical 13.
Trusting that you have a most enjoyable time over the Festive season.
Best regards,
Ivor.
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