John Sankey on sun 24 feb 08
John Hesselberth:
"> H&R's finding that RO2 should be 3.5 and R2O3 0.4 (again, using
> their definition of Seger) is clearly not adhered to by
> clayarters. We prefer our glazes much more fluid and/or colorful.
John, where did we say this? Our 4 "rules" for stable glazes include
silica being greater than 2.5, preferably greater than 3 and alumina
greater than 0.25 and not greater than 0.45 at cone 6."
Re silica: page 52 shows a graph with a minimum at 3.5
The text says, "begins to level off at a silica level of about 3.0"
I can't find any reference in the book to 2.5
Re alumina: your graph page 53 shows a sharp minimum at just below 0.4
It's true the text says 0.25-0.45
My words "more fluid" were too hastily chosen.
However, the huge and consistent preference of Clayarters for
silica well below 3 for gloss cone 6 glazes deserves
investigation. What factors would be the most useful to get
numbers on from the database? There must be something other
than silica and alumina that can result in a glaze being stable
enough to be valuable.
John Sankey
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John Hesselberth on sun 24 feb 08
On Feb 24, 2008, at 8:58 AM, John Sankey wrote:
> There must be something other
> than silica and alumina that can result in a glaze being stable
> enough to be valuable.
Certainly. Low levels of colorant results in a glaze leaching less.
But remember only a tiny fraction of those so-called "stable" glazes
in your database have been lab tested for stability. And probably
none other than those from MC6Gs have been challenged with 5% copper
carbonate. Some have been vinegar tested and others, probably,
nothing more than looking at it and deciding it looked like "good
glass" as was done in the technical literature for several decades
(if it looks good it must be stable).
There are thousands of glazes in wide use by potters at all cones
that are not stable by the standards Ron and I have set for the
glazes in our book. Those thousands of glazes are sometimes/often OK
when they have small or zero amounts of easy-to-hold colorants, but
they do not make good base glazes for use with a wide variety of
colorants.
On the other hand, if you can improve on our 4 "rules" for making
stable glazes, Ron and I would welcome it. But please make sure you
do it with hard data.
Regards,
John
John Hesselberth
www.frogpondpottery.com
"Man is a tool-using animal....without tools he is nothing, with
tools he is all" .... Thomas Carlyle
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