Mark Mondloch on fri 13 jun 97
For a tile countertop I made I wanted a clay with low enough shrinkage
that the grout lines would make up for all the shrinkage. I used the
following clay body (from CM, sorry I don't remember who submitted it).
It was submitted as a raku body.
1 goldart
1 ap green fire clay
1 kyanite 35m
The tiles were single-fired in a wood kiln to ^10. Shrinkage was
perfect.( about 6% total)and the body seems very strong to me. I've done
the freezing-boiling water thing on it.
Now I'm wondering about using kyanite as an additive to high-fire
throwing clay. Is there any down-sides to it? Should it improve thermal
shock resistence at high temperatures too( I know it's used in raku for
that)? How would it affect glaze fit? How DOES it do what it does?
Sylvia Mondloch, Wi.
Ron Roy on sun 15 jun 97
>Now I'm wondering about using kyanite as an additive to high-fire
>throwing clay. Is there any down-sides to it? Should it improve thermal
>shock resistance at high temperatures too( I know it's used in raku for
>that)? How would it affect glaze fit? How DOES it do what it does?
>
>Sylvia Mondloch, Wi.Hi Sylvia,
Hi Sylvia,
Kyanite can be used in a throwing body.
35 mesh is quite hard on the hands - Kyanite is needle like in shape.
You will probably not like the kind of grooves you get when trimming with
the 35 mesh size.
It provides excellent mechanical strength because of the interlocking
needle like shape of this material.
Kyanite (the raw mined material) grows somewhat in size during firing so it
has some effect on overall shrinkage. Don't expect much of this kind of
effect with only 5% in a throwing body.
Mullite is calcined Kyanite - it has gone through the increase in size
because it is fired.
Kyanite is very refractory so adding it to a clay body will increase the
maturing point of that body. If you decide to add it you could just wedge
it in and then do absorption tests on the original body and new mixture to
see if it needs other adjustments.
Kyanite is a low thermal expander and contractor so there are good reasons
for adding it to cut down heat induced cracking in ovenware. Be aware that
a small amount (up to 5%) will not have much effect in this area. It will
make the ware stronger because of the interlocking.
Adding a refractory to a clay body can increase the amount of cristobalite
in ware fired above 1100C unless it is compensated for.
Any material that influences body expansion/contraction downwards has the
potential to make some glazes craze. Body winds up bigger than before but
glaze still contracts as much as before.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
Studio: 416-752-7862.
Email ronroy@astral.magic.ca
Home page http://digitalfire.com/education/ronroy.htm
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