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cone 6 claybodies...not a rant, but longish

updated mon 28 jan 02


Ceramic Design Group on sun 27 jan 02

Recently there has been some discussion regarding cone 6 clay bodies. Having
been through hell and returned safely using most of what is out there in the
way of prepared bodies, I'll share some generalities with the list regarding
formula that we are now blending.

Most of the prepared bodies, also known as "clay in the box" at that
temperature have a range of cone 5 and 6. Although I have worked with some
that claim to be a cone 6 body and is fully vitreous at cone 4. I'll not
mention any manufacturers and please understand that this is my experience

The prepared white/porcelain bodies bodies that we used were quite poorly
formulated and some were quite badly prepared with impurities from previous
runs in the company's mixing equipment. Some relied on straight kaolins with
some flux. Others had non-plastic kaolins Very few of them had any correct
ball clays and many of them had badly selected fluxes. Many of them had no
silica whatsoever. One even had a few percents of Gerstly Borate. In short,
they were poorly compounded and full of problems.

While I have not seen any dilatometry on these bodies if they even exist or
are current, I would venture they many of them are very seriously flawed. I
know this because I have used them, fired them, thrown them, and Ram pressed
them and jiggered them. Again, uderstand that this is my experience only and
while I know that there are perhaps some white bodies in this range that
have all the necessary characteristics, I have not used them. Regardless,
naming names is not part of my lexicon.

We now have a nice selection of very nice white porcelain/whiteware bodies
that I have formulated, tested, and some into very small samplings until the
dilatometry is run. While I won't share my formulas, I will offer some
generalities that you may want to formulate into questions to ask your

Whiteware/porcelain bodies at cone 5/6 need to have half plastic ingredients
and half non-plastics. Kaolins are quite refractory at at cone 5/6 need more
flux and flint for proper balance.

There are very light to even white burning ball clays that need to be in the
clay body. The weakest link in any clay body are the kaolins. Ball clays
impart the "glue" that keeps the whole thing together, especially for
pressing, and god forbid, in clay bodies that have non-plastic kaolins.
Casting bodies in this temperature range also need ball clays, but more
specifically, ball clays that have the correct particle size distribution
for rapid and effective casting. There are ball clays that are made just for

There are domestic kaolins that are very plastic and wonderful to work with.
WHen blended together, their working characteristics are to me,. superior to
what I have worked with at cone 10 even with plasticizing admixtures.
Imported expensive kaolins need not be used. Please remember, that Grolleg
is a non-plastic kaolin.

Combinations of potash feldspars and soda feldspars can be very successfully
used at that temperature to obtain desireable vitrification also with a
balanced amount of silica and pyrophyllite. There might be some funk
associated with soda spars in terms of the body gelling slightly,(sodium and
deflocculation) but it has not been our experience. Kona F-4, Nephy Sy,
Custer, even some wollastonite can all be used. Pyrophyllite is particularly
effective in this range.

Many manufacturers tout their clay bodies to be useable from cone 4-10 or
cone 6-10. I would submit that at the lower ranges these bodies are poorly
supplied with flint in the melt at that temperature are far from vitreous
and can pose significant problems for functional potters. Please, again,
understand that these are my opinions only and they have stood up to my
experience for years in our facility. Quite frankly, a clay body will have
much different characteristics at cone 4 then it will at cone 10, and the
truth can be seen in the dilatometry.

Ask your clay suppliers the right questions. Take some degree of
responsibility for your ware. Seek out people who are skilled in blending
the correct ingredients for specific temperatures and buy a formula and them
have it made by whom ever you use to buy prepared clay from. These people
will custom blend bodies in a certain minimum and offer confidentiality for
your formula. We are fortunate in that we can blend what we need in our
facility to what ever moisture level we need. We check it with a
penetrometer. Trust me, the money will be very well spent.

Respectfully submitted,


Jonathan Kaplan, president
Ceramic Design Group
PO Box 775112
Steamboat Springs CO 80477
voice and fax 970 879-9139

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