Ceramic Design Group on thu 13 jun 02
To jump in on this thread.......
Many manufacturers tout some of their clay bodies as having a range of cone
6-10. I have seen some advertised as cone 4-10. While this may work for
some, I would offer the following.
1. The dilatometry for cone 4 is going to be different than at cone 10. In
fact, this will be the answer to the question if the clay body would be
fully vitrified at lower temperatures ranges.
2. In fact,clay bodies have entirely different characteristics at cone 4,
5, 6, and 10. Absorption, shrinkage, vitrification etc.
3. I doubt very seriously whether these multi-range formulas would exhibit
the characteristics one would need at cone 4-6 and would lead to glaze fit
issues,. and the atmosphere doesn't really matter.
So it seems to me that the real crux here is to design a clay body that
works at cone 4-6 and gives you the shrinkage, absorption, vitrification you
need and can be well supported by the necessary dilatometry.
We work alot at this range in terra cotta clays bodies, whiteware-porcelain
clay bodies, and some stoneware type bodies. What works is understanding the
relationship between plastics and non-plastics in the body and the necessary
fluxes at that range.
Once these are understood, and it is not rocket science, you can produce a
clay body at this firing range that is even stronger and more versitile than
that in the upper ranges. You save on gas/electricity, and learn something,
by the way. And you can design clay bodies that have no garbage like
fireclays, so often used in high temperature bodies. As the availability of
"decent" fireclays such as GreenStripe, Lincoln, and Sutter have all but
diminished, its important to have flexibility when designing clay bodies, at
any temperature range.
Ceramic Design Group
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Ron Roy on thu 20 jun 02
I have no problem with fire clays in bodies - but it is necessary to
understand what they are and how to use em.
I work for two clay companies and combined - they mix and sell millions of
pounds of bodies each year with Hawthorn fire clay in them.
Tuckers alone has 9 cone 6 bodies with fire clay in them. They are big
sellers and we get very few complaints - and when we do the advice that
works is usually - clean fire your bisque and soak a little at the end of
Fire clays are useful - not if you are doing whiteware or porcelain - but
they help drying and - in reduction give the black iron spots so many
If you are mixing your own clay make sure at least the fireclay - whichever
you use - is screened through 30 mesh. That eliminates lime popping which
is the main problem with fire clays - any piece of lime bigger than the
head of a pin will probably lead to pops.
Other clays can be dirty as well - carbon and sulphur can come in ball
clays, many earthenware clays and even in stoneware clays.
I am fussy about which fire clays I recommend - they need to be stable - no
significant changes in absorbency, colour and shrinkage. Even so I don't
recommend using more than 30% in a body.
I'll say that again - in a different way - don't use more than 30% of any
clay in your body - if you know the history of the clays you use minimize
the ones that are not stable and maximize the ones that are.
You cannot take just any ones advice about which clays are the best -
unless they can give you the test data from which they base their opinions.
Better still do some clay testing yourself - if you are a production potter
the little amount of time it takes will save you a lot of money in the end.
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