May Luk on fri 2 nov 07
Hello all;
It is easy to assess the maturity of glossy glaze. It is not as easy to assess matte glaze as the surface is not 'flawless'.
How does one determine if a matte glaze is thoroughly melted? Without a standard specimen to compare against, what are the ways to determine if the crystals are from devitrification, but not non-soluble primary crystals? Without the set up and backing like John Sankey, how can one freely spot the crystals?
I am evaluating 3 sets of high magnesia Currie Grids and I am stumped at some tiles. Of course I can just arbiturarily decide, like I always do. It would be good to really get to the bottom of it and learn something from the experts.
Best Regards
May
Kings County
>>>>>>>>>>>>>>>>
[...]Published limit formulas are virtually useless for a functional
potter who wants her pots to hold up in use. They include glazes that
are very unstable (usually low silica mattes) and exclude glazes that
are very beautiful and durable mattes (high alkaline earth glazes
with plenty of alumina and silica).
I believe a better set of "limits" are found in the 4 rules that Ron
and I developed for Mastering Cone 6 Glazes". In brief they are 1)
enough silica 2) enough alumina 3) thoroughly melt and 4) don't
overload with colorants, particularly copper.
>>>>>>>>>>>>>>>>>>>>>>
John Hesselberth
John Hesselberth on fri 2 nov 07
On Nov 2, 2007, at 3:23 PM, May Luk wrote:
> How does one determine if a matte glaze is thoroughly melted?
Hi May,
Excellent question! I would offer this suggestion. A matte glaze
when thoroughly melted will be glossy if rapidly cooled. If it is
matte when rapidly cooled it was probably not thoroughly melted. Why
is this? When thoroughly melted it will be a homogenous solution.
Crystals take time to form from that kind of melt and if it is
rapidly cooled they will not form and, hence, the glaze will be
glossy instead of matte. On the other hand if it is matte when
rapidly cooled, it means some of the material was never melted in the
first place. For an example of the difference cooling rate makes go to
http://www.masteringglazes.com/Pages/faqframe.html
and click on 'click here' in the 2nd question. The glaze on the left
was thoroughly melted but cooled rapidly. It is as glossy as a glaze
gets. As you progress to the right crystals have begun to form
matting the glaze as the cooling rate is slowed. The difference is
dramatic. When I show these samples in my glaze workshop, the potters
are in total shock at the difference slow cooling can make. But the
fact is many matte glazes have been formulated as unmelted mattes by
potters who didn't understand the chemistry and the physics of
glazes. And, unfortunately, some of these potters are well-known
names who have done excellent work in other aspects of our craft.
The other point to be made is that you must control cooling rate if
you want reproducible results from well formulated matte glazes.
Regards,
John
John Hesselberth
http://www.masteringglazes.com
http://www.frogpondpottery.com
Ron Roy on sun 4 nov 07
Hi May,
You could add 5% copper carb and 6 % rutile and then have them lab tested.
You could then compare the leaching results with the results of our glazes.
I'm sure there would be a lot of interest in the results. Perhaps a
magazine article?
We need to know these things!
RR
>Hello all;
>
>It is easy to assess the maturity of glossy glaze. It is not as easy to
>assess matte glaze as the surface is not 'flawless'.
>
>How does one determine if a matte glaze is thoroughly melted? Without a
>standard specimen to compare against, what are the ways to determine if
>the crystals are from devitrification, but not non-soluble primary
>crystals? Without the set up and backing like John Sankey, how can one
>freely spot the crystals?
>
>I am evaluating 3 sets of high magnesia Currie Grids and I am stumped at
>some tiles. Of course I can just arbiturarily decide, like I always do. It
>would be good to really get to the bottom of it and learn something from
>the experts.
>
>Best Regards
>May
>Kings County
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
| |
|
