Snail Scott on fri 14 jun 02
At 10:26 AM 6/14/02 -0500, you wrote:
>I have had trouble using a grogged clay body (cone 10 stoneware mid-iron
>content with 10% grog 10% silica sand)
>and getting a burnished look and feel to the surface.
>I find that after the bisc firing the surface is still rough and after the
>reduction firing it is even more coarse.
You aren't going to be able to get a true
burnished surface at stoneware temps. The
burnishing starts to be altered at temps
as low as ^010 or so, and diminishes as
the temperature rises. It's not totally
true that burnishing has no effect at
stoneware temperatures - I've observed
that there is a visible difference
between a burnished stoneware surface and
and one that's been just smoothed. There
even seems to be an effect on how the clay
responds to reduction. It ain't really a
burnished effect, though, not even close.
This is because as the temperature rises
above low-bisque temperatures, it starts
to melt more thoroughly and shrink, and
the fine clays melt in amongst the coarse
clay particles, disrupting the smooth
burnished surface you put all that work
into.
Unfortunately, you pretty much have to
choose: either burnished surfaces, or
stoneware temperatures, but not both at
once.
-Snail
Steven Lee on fri 14 jun 02
I have had trouble using a grogged clay body (cone 10 stoneware mid-iron
content with 10% grog 10% silica sand)
and getting a burnished look and feel to the surface. I have used two mixes
of slip to smooth out the surface.
I find that after the bisc firing the surface is still rough and after the
reduction firing it is even more coarse.
I am afraid to use a more plastic clay body for the slip because I don't
know if it would be a good fit for
the grogged stoneware.
The slips I have tried include:
Cooper Black Slip
Red Clay 60
Yellow Ocher 30
Red Iron Oxide 10
and
Beige Speckled Slip
Ball Clay 72
Kaolin 25
Red Iron Oxide 3
60 Mesh granular Manganese (after mixing with water and sieving) 12
With and Without 3% Tin Oxide as an opacifier
If anyone could help I would greatly appreciate it.
Sorry that I do not know the recipe for the stoneware clay body.
_____
vince pitelka on fri 14 jun 02
> I have had trouble using a grogged clay body (cone 10 stoneware mid-iron
> content with 10% grog 10% silica sand)
> and getting a burnished look and feel to the surface. I have used two
mixes
> of slip to smooth out the surface.
> I find that after the bisc firing the surface is still rough and after the
> reduction firing it is even more coarse.
Steven -
I'm afraid that you are asking the impossible. A look and feel of a
burnished surface is a low-fire surface. As soon as you high-fire,
especially in reduction, the particles in the claybody will variably shrink,
and that is drastically increased if there is sand or grog present. In
general terms, a true burnished finish starts to diminish above about cone
012. When we refer to a burnished surface, it specifically means a surface
polished with a hard, unyielding object, like a polished stone or a
stainless steel spoon. I am assuming that this is what you are doing.
A true burnished surface is compressed so much that it no longer has any
capacity for shrinkage. That is why we usually burnish with some
re-introduced moisture after the piece is bone dry. That way, the shrinkage
has already taken place, and in a cone 012 firing there will be no further
shrinkage, thus preserving the true quality of the burnished surface.
As soon as you fire higher, even to normal bisque temperatures (cone 08) the
piece starts to shrink, and the burnished surface will crinkle on a
microscopic level, diminishing the quality of the shine. The higher you
fire, the more shrinkage. And as I mentioned above, the shrinkage is
differential among the different clay particles and the non-plastics. Grog
is high fired, and therefore does not shrink at all in the high firing. But
everything else shrinks around it.
I have seen some intriguing surfaces created with burnishing or terra sig
fired to midrange or high-fired temperatures, but they are never anything
like a true low-fired burnished surface. Using a more plastic body for your
slip will not make any difference.
Good luck -
- Vince
Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home - vpitelka@dtccom.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
http://www.craftcenter.tntech.edu/
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