Lili Krakowski on thu 15 sep 11
Megan writes she is using Laguna R-2 Clay, bisquing at 03, glazing at =
=3D
05. She uses Spectrum Low Stone glazes (900's) But they craze right =3D
away..
Is she doing something wrong, and can anyone recommend a better =3D
commercial clay body (good for throwing) that will have a better fit =3D
with the glazes, or glazes that fit the body better.,
Ok.
Let's find some beginning.
I assume you BOUGHT both clay and glazes. So first I would talk to the =3D
dealer and say: " I bought these as I wanted a body and glaze meant for =3D
each other. These appear to be a mismatch...." I found a Spectrum ad =3D
in PMI, so now know that 900s is a series of glazes. Fine.,
Next. What you are doing firing wise is unusual. Commonly used only by =
=3D
industry.
You are biquing HI and glaze firing LOW, which creates a bunch of =3D
problems.
I suspect, Megan, that you have neither looked at a cone chart, nor =3D
cracked a book....(I write this in my severest tones!)
I think you think 03 lower/cooler than 05...but it's the contrary,
When Hermann Seger, of much blessed memory, made his original cones =3D
(called "bowling pins " in the original German) they went 1,2,3, and up. =
=3D
Later, he realized cones also were needed at lower temps. So he made =3D
lower temp cones, that move DOWNWARD. So, just below c.1 comes cone 01, =
=3D
and lower yet c02, 03, and so on.
Ok? Cone 01 is hotter than cone 02. But cone 2 is hotter than cone !.
Hence your bisquing cone is hotter than your glaze fire. Now what does =3D
that mean?
Bisquing is a preparatory firing. In a way, like putting clothes in =3D
the washtub to soak. Bisquing takes care of some of the early work, =3D
early changes in the clay, in order to make the second or glaze firing =3D
smoother. Bisquing is not necessary. Many potters skip it.
Bisquing, however, has the great advantage of hardening the clay by =3D
driving off chemical water--namely that water that is part of the =3D
molecule of clay. Bisquing also burns out organic matter in the =3D
clay..This stuff creates a lot of gas in burning, and when these gases =3D
have to go through a layer of glaze they can, simply put, damage the =3D
surface.Not good.
So a bisqued pot is sturdy, easy to handle, absorbent, so the glaze will =
=3D
stick to it in the raw state, and it already has shrunk from the =3D
original, greenware, size,
When glaze is applied and fired, the ideal is that the pot will shrink =3D
a tidge more than the glaze--so that the glaze is "under compression." =3D
And, again, in much simplified form, if a glaze crazes IT has shrunk =3D
more than the pot, and it is NOT under compression.
It really is important to coordinate shrinkage of pot, and shrinkage of =3D
glaze.
Lettuce take a far fetched example. You buy a cashmere sweater. Your =3D
mother, as mothers will, screams: "How can you buy a cashmere sweater? =
=3D
You cannot afford to have it dry cleaned all the time, and no matter =3D
what, if you wash it, it will shrink!
And you say: "Yes, Mother dear, that is why I bought it. I need to lose =
=3D
20lbs, the shrinking cashmere sweater will give me motivation."
Ok. your sweater shrinks. You plan to shrink in synch.
Pretty similar with clay and glaze, The pot WILL shrink. You want the =3D
glaze to shrink in synch.
But when you fire your bisque high, you have preshrunk it. It cannot =3D
shrink to accommodate a shrinking glaze. If you have fired the clay to =3D
its maturity it cannot, should not, will not shrink more.
Having said all that. Call the dealer ask, what the proper temp for =3D
firing those glazes is. As what a good bisque cone for the clay is. I =3D
would guess 05, even 06 will work for an earthenware body but I do not =3D
know this R body...so ask.
It is NOT the clay, NOR the glaze that are at fault. Sorry. We can =3D
suggest other bodies, other glazes, it will make no difference unless, =3D
until you coordinate the two.
Meanwhile, be a dear, and read a book, such as Daniel Rhodes' "Clay and =3D
Glazes for the Potter".,
Lili Krakowski
Be of good courage
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