Bruce Girrell on thu 9 nov 00
1) When modifying a clay body to improve its ability to tolerate thermal
shock, kyanite is often added. I understand why kyanite, but why is the
kyanite always coarse, like 35 mesh? These boulders make trimming a real
pain. Why not use 200 mesh (if you could find it)?
2) Next to kyanite, what other additives are most effective? I'm trying to
create a porcelain body that can handle the shock. Adding 200 mesh molochite
seems to make the clay rather short, though certainly it could stand 5% or
so. Would adding in some ball clay return some plasticity without negating
the shock resistance of the molochite? What about wollastonite? Doesn't it
have acicular crystals?
I understand that I'll have to do the requisite experiments, but I'm trying
to limit the field a little before mixing up batch after batch of
experimental clays. Is there a systematic approach to this as there is for
glazes? Mixing up test batches of glazes is one thing, but mixing hundreds
of pounds of clays is quite another.
Thank you for your suggestions,
Bruce "lost in space" Girrell
Jonathan Kaplan on thu 9 nov 00
>1) When modifying a clay body to improve its ability to tolerate thermal
>shock, kyanite is often added. I understand why kyanite, but why is the
>kyanite always coarse, like 35 mesh? These boulders make trimming a real
>pain. Why not use 200 mesh (if you could find it)?
Works fine. I also don't know why one would put 35 mesh kyanite in a clay body.
>2) Next to kyanite, what other additives are most effective? I'm trying to
>create a porcelain body that can handle the shock.
Pyrophyllite is the material most widely used. I have used it extensively
in both stoneware and porcelain bodies in both plastic and casting bodies.
Adding 200 mesh molochite
seems to make the clay rather short, though certainly it could stand 5% or
>so. Would adding in some ball clay return some plasticity without negating
>the shock resistance of the molochite? What about wollastonite? Doesn't it
>have acicular crystals?
Molochite is basically a known formula "white" grog type material that is
often used
in porcelain bodies. It to will pose some trimming issues as very fine
lines in the ware.
Too much ball clay in a porcelain body will, as I am sure you know,
compormise the whiteness.
Wollastonite is another good material to add, It is hydroscopic though and
needs to be stored in a very dry place.
>
>I understand that I'll have to do the requisite experiments, but I'm trying
>to limit the field a little before mixing up batch after batch of
>experimental clays. Is there a systematic approach to this as there is for
>glazes? Mixing up test batches of glazes is one thing, but mixing hundreds
>of pounds of clays is quite another.
Sure there is a systematic approach. If you have a body that works for you
and totals 100 with clays and fluxes and fillers (Grolleg, Tile 6, ball
clay, flint. spar) start a procedure by adding material in a percentage
basis on top of the 100 parts of the body.
Make 10 pound mixes. This will give you enough to have some to throw, and
some to run shrink and absorption tests, test tiles for glaze fit, and what
ever else you may want to do with it.
Good Luck
Jonathan
Jonathan Kaplan
Ceramic Design GroupLTD/Production Services
PO Box 775112
Steamboat Springs, CO 80477
(970) 879-9139 voice and fax
http://www.sni.net/ceramicdesign
UPS: 1280 13th St. Unit13
Steamboat Springs, CO 80487
Michael Banks on sat 11 nov 00
Bruce,
When formulating shock-resistant bodies, two parameters are of interest.
Firstly using a low expansion mineral and second (if coarse sand-size
textures are acceptable); utilising the grog effect.
Coarse grog works better than fine because the larger grains block more
cracks per unit volume. Blocking the propagation of cracks is sort of a
second-tier defence to body failure due to thermal shock. Even high
expansion materials like silica sand can be effective in this role, but the
best are the low expansion acicular minerals.
But using a low expansion acicular mineral such as kyanite as fine as 200
mesh only utilises it's low expansion character and little grog effect is
evident. And kyanite this fine detrimentally affects plasticity. To be
effective in crack-blocking, grog needs to be at least 80 mesh and up to 10
mesh. 30 or 35 mesh is often used because it is a quite effective grog size,
but not too unpleasant to throw.
Next to kyanite, I've found sillimanite, pyrophyllite, talc, molochite
(high-temp calcined china clay), wollastonite, mica and mullite to be
useful, even in fine mesh sizes. In fact sillimanite usually has a higher
particle aspect ratio (longer needles) than kyanite (but identical
chemistry) and therefore superior. Wollastonite is excellent too, but check
with the supplier the aspect ratio of each product. There are many
wollastonites out there and some have low aspect ratio crystals which are
not effective. Similar comments apply to talc, very many talc products of
different grain morphology are marketed for different purposes.
One system you could adopt is to make up identical porcelain plates with
identical body recipes, except for the non-plastic shock resistant
ingredient (e.g. kyanite, sillimanite, mica, pyrophyllite), -each plate with
only one of these ingredients added. These are all fairly inert alumino
silicates. (Avoid talc or wollastonite initially as these add extra fluxing
effects which affect relative vitrification). Heat up the plates to
identical temps, dunk test 'em in water or abuse in other systematic ways.
The surviving ones will provide data regarding the relative effectiveness of
the individual additive. The next stage is to try mixtures of the key
ingredients, and evaluate colour, translucency etc. Some additives are not
very white-firing for porcelain. Then attack them with more diabolical
tests.
Michael Banks,
in Nelson,
New Zealand
(waiting to see if the Tua Man can clobber Lennox Lewis)
----- Original Message -----
From: Bruce Girrell wrote:
> 1) When modifying a clay body to improve its ability to tolerate thermal
> shock, kyanite is often added. I understand why kyanite, but why is the
> kyanite always coarse, like 35 mesh? These boulders make trimming a real
> pain. Why not use 200 mesh (if you could find it)?
>
> 2) Next to kyanite, what other additives are most effective? I'm trying to
> create a porcelain body that can handle the shock. Adding 200 mesh
molochite
> seems to make the clay rather short, though certainly it could stand 5% or
> so. Would adding in some ball clay return some plasticity without negating
> the shock resistance of the molochite? What about wollastonite? Doesn't it
> have acicular crystals?
>
> I understand that I'll have to do the requisite experiments, but I'm
trying
> to limit the field a little before mixing up batch after batch of
> experimental clays. Is there a systematic approach to this as there is for
> glazes? Mixing up test batches of glazes is one thing, but mixing hundreds
> of pounds of clays is quite another.
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