Eleanora Eden on wed 15 mar 06
Hi everybody,
There must be such a thing as limit formulas for clay bodies.
I am specifically looking for information on area of ^1-3. But
I think I would learn more if I could see a spectrum of analyses
like the ones we see for glazes.
Anybody?
Eleanora
Taylor Hendrix on wed 15 mar 06
On 3/15/06, Eleanora Eden wrote:
> Hi everybody,
>
> There must be such a thing as limit formulas for clay bodies.
> I am specifically looking for information on area of ^1-3. But
> I think I would learn more if I could see a spectrum of analyses
> like the ones we see for glazes.
>
> Anybody?
Eleanora,
I know Ron and John are going to want to comment on this one.
Here are some things I have learned through my studies: Technicians
developed 'limit formulas' because they wanted rules of thumb to help
them make good glazes. That meant when they looked at a fired pot the
glaze looked like "good glass", it was flawless. Over time they
realized "Hey, if we keep X ox somewhere in this range, we don't get
the pinholing we used to get" and so on. It was an oxide thing.
The criteria for a "good glass (glaze)" and the criteria for a good
clay body are not the same. John is a great person to give the low
down on those concepts. Some interesting history of the early
technicians and their quest for good glazes.
There ARE rules of thumb for making clay bodies, but most people don't
refer to them as limit formulas. Most people don't seem to think of
clay bodies in 'oxide' terms like they do glazes perhaps because they
are not as "melted" as glazes are.
I too am interested in rules of thumb for clay body formulation, so I
hope others put in their two cents as well.
Taylor, in Rockport TX
Michael Wendt on wed 15 mar 06
Eleanora,
I think it is harder to construct an accurate limit
formula
for a clay body for several reasons.
Most notably is that it is not normally fired until it
fully
melts like a glaze is. As a result, characteristics
like the
grind size of the clay can affect the fired character.
I ran into this a few years ago with Plainsman Clays
in Canada. They used my clay body recipe with Helmer
and got a body that was very porous and seemed to
be underfired.
When I make the same recipe and fire it to cone 10,
it has less than 1% absorption. We both used the same
recipe, the same Helmer Kaolin and fired to the same
cone. I fluid slipped my clay and they dry mixed
theirs.
As a result, they had large granules of Helmer
surrounded by a glassy matrix. Those granules are cone
34 clay and
so are very porous. More of my Helmer Kaolin
particles were taken into the melt because they were
fluid slipped to break them up. The only other way I
see to
really get consistent and meaningful results in this
is to hold the particle size distributions for the
ingredients
constant, something that is very hard for us as potters
without a lab to fall back on to do.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, Idaho 83501
USA
wendtpot@lewiston.com
www.wendtpottery.com
Eleanora wrote:
Hi everybody,
There must be such a thing as limit formulas for clay
bodies.
I am specifically looking for information on area of
^1-3. But
I think I would learn more if I could see a spectrum
of analyses
like the ones we see for glazes.
Anybody?
Eleanora
skiasonaranthropos@FSMAIL.NET on wed 15 mar 06
Hello Taylor,
You=92re spot on with noting that oxides alone are insufficient for bodies a=
s
unlike glazes they do not form an entire melt. The oxide approach to glazes
primarily goes back to Zachariasen s work on glasses; of course whilst
glasses are amorphous bodies most certainly are not
Even for glazes whilst the oxide approach is very useful it is not the
entire story, and it is flawed for bodies
Thinking about raw materials for either glaze and body use as oxides is
only part of the story as they are minerals and not, as sometimes
described, a collection or warehouses of oxides
Regards,
Antony
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