Paulette Carr on mon 17 apr 06
I need some advise from clay gurus,
I have been testing a series of white, low-fire commercial clay
bodies for their suitability as a tile body. I have found two from
the same company that appeal to me based on working properties and
fired result. I tested the clay bodies for shrinkage and absorption
following Ron Roy's procedure at both ^04 bisque and ^01 bisque.
Both shrinkage and absorption were the same at both cones, so I
repeated the firings, and tests and all the samples. Still the
same. The appearance and sound when struck was also identical - at
least to my eyes and ears. I was hoping that something (shrinkage or
absorption) would change at the higher temperature. Does this mean
that the clay is really unchanged even though it has been fired 3
cones higher? If so, does it make any sense to fire to bisque fire
^01? -- or is ^04 really the same/adequate?
I did call the company and talked with the clay formulator and
technical specialist. She indicated that this may be the case
because the body is a talc and ball clays- based body, and might not
begin to mature until I have fired higher - perhaps ^2. She advised
against this, wanting to keep some absorption in the tile, but did
not talk about warping or slumping. I am not sure that I follow her
arguments or agree with her -- just don't know. At ^04 and ^01 the
shrinkage is between 5-6%, and the absorption is 11-12%.
My glazes will all be fired to ^04, and all firings for tiles are
slow going up and down. I will not be using underglazes. This
project also gives me the excuse to develop that low-fire glazes that
I have always been meaning to get to... and all is going well.
Surprisingly, I have used a talc body into which I hand wedged about
5% grog to make tiles and birdhouses that have survived outside over
10 years in an area known for it's high number of freeze-thaw
incidents over the winters. Many have been glazed. The reason that
I am looking at other bodies, is that I want to purchase the clay
already containing grog or aggregate. The behavior of these "new"
bodies is as good or better than the previous body, and the shrinkage
and absorption are nearly identical. I am currently testing the clay
bodies for thermal shock - sans glaze - to see if the clay bodies
hold up as well as the old body.
Incidentally, the tiles that I am planning to make for this large
volunteer project will be installed on interior surfaces. .... but
who knows, I may be tempted to use outdoors as I have before. The
plus of buying something already on the shelf is that I would not be
required to buy and store large amounts that I may not need or use in
this project. I usually work at ^9, so this is a real departure and
challenge for me.
I hope that I have provided enough information. Thanks, in advance,
for any guidance that you can give me.
My best,
Paulette Carr
Paulette Carr Studio
Member/Potters Council
St. Louis, MO
Ron Roy on wed 19 apr 06
Hi Paulette.
This makes sense for a white body with only MgO (talc) as a flux - because
MgO does not work as a flux until around cone 03/04.
High talc bodies are a good idea for low fire because they do not absorb
water like bodies without it - which is a cure for delayed crazing. They
also produce Enstatite - which gives a higher expansion and so helps with
solving crazing problems in low fire bodies.
The reason you should not go higher than cone 02 - when the MgO starts
working as a flux - it happens fast and violently with MgO - so controlling
for over firing becomes very difficult - because the melting is happening
so fast.
RR
>I need some advise from clay gurus,
>
>I have been testing a series of white, low-fire commercial clay
>bodies for their suitability as a tile body. I have found two from
>the same company that appeal to me based on working properties and
>fired result. I tested the clay bodies for shrinkage and absorption
>following Ron Roy's procedure at both ^04 bisque and ^01 bisque.
>Both shrinkage and absorption were the same at both cones, so I
>repeated the firings, and tests and all the samples. Still the
>same. The appearance and sound when struck was also identical - at
>least to my eyes and ears. I was hoping that something (shrinkage or
>absorption) would change at the higher temperature. Does this mean
>that the clay is really unchanged even though it has been fired 3
>cones higher? If so, does it make any sense to fire to bisque fire
>^01? -- or is ^04 really the same/adequate?
Ron Roy
RR#4
15084 Little Lake Road
Brighton, Ontario
Canada
K0K 1H0
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