Patrick Atwood on thu 8 aug 96
Here I am again, with another glaze question. (thanks to all of
you who so generously helped out with my celadon problem) I've got
myself a gorgeous matte purple for cone 10 that I've sort of
conjured up somehow. I was soooooo pleased with myself until I
tried to apply it to a POT (pouring). I dipped my tests, twice,
and it worked out beautifully. But when I try to dip or pour this
glaze onto an actual POT, it cracks and peels away from the pot. I
don't know why, and it makes me sad because I love this glaze so
much, and I was SO darn proud of myself. I am only a semi glaze
guru, not the Real Thing yet, so I know just enough to modify a
glaze to get what I think I want, but not enough to truly
troubleshoot. And I don't know ANYTHING about what chemicals will
give you nice application texture. My recipe is:
Neph. Syenite 20.0
Colemanite 7.1
Wollastonite 6.8
Dolomite 5.4
Kentucky Ball Clay 11.3
Macaloid 6.4
Cobalt Carbonate 3.5
Magnesium Carbonate 7.5
HELP! :)
Thanks in advance.
********************************
* Julie *
* Atwood@gnn.com *
* *
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PJLewing@aol.com on sun 11 aug 96
Patrick,
You have several problems with this glaze. You have not enough alumina or
silica, and the ratio of those two is also quite low- probably wouldn't melt
at all if you didn't have so much boron. The other problem is you might have
too much magnesium. Now, you need a lot of magnesium to get that purple
color, but you are outside the limit formulas for magnesium. However, all
the most interesting glazes seem to be outside the limits one way or another.
You might look in the old Dan Rhodes "Stoneware & Porcelain" book. In the
text, not in the back, there's a recipe for a magnesium base glaze that will
make a purple with cobalt. If you can't find it, let me know, and I'll send
it to you. It has even more magnsium than this.
But your main problem here is not chemical, it's physical. Taking your
recipe out to 100 parts, you have about 13% magnesium carbonate. That's a
HUGE amount of a fluffy material that shrinks like mad when it dries. That
much ball clay just makes it worse.
First thing I'd try would be cutting down the mag carb to the place where it
stops producing that purple color. If that still shrinks too much, I'd get
the magnesium from talc instead of carb. A straight substitution will give
you silica too, but you need more anyway, as well as more in relation to the
alumina.
And it might help to use kaolin instead of ball clay. It doesn't shrink as
much and it will make a cleaner color. To make a stable glaze you need more
of both alumina and silica, but if adding more clay causes too much shrinking
again, you can calcine all or part of the clay. You might have to do that if
you continue to get your magnesium from the carb, but if you calcine all the
clay, the glaze probably won't stay suspended. That's what the Macaloid's in
there for.
Well, I hope that helps.
Testing, testing, and more testing!
Paul Lewing, Seattle
Cobalt1994@aol.com on mon 12 aug 96
Hi Patrick,
I'm sending this message through Clayart since I got a a mail error message
when I tried to email you. My glaze recipe was for cone 10.
Jennifer in Vermont
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email: Cobalt1994@aol.com
Jennifer Boyer
Thistle Hill Pottery
Montpelier, Vt.
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