Lynne Berman on wed 7 oct 98
Vince,
Here's a progress report from Philadelphia. My ball mill birthday present was
immediately put to use. I made terra sig of ball clay, got a specific gravity
of 1.15. After ball milling, and only a little water on top, no bottom heavy
layer, the specific gravity is 1.10. Now I am waiting for water to evaporate
to get the specific gravity up to 1.13. Here's the question: why start with a
specific gravity so low that the resulting terra sig will have too low a s.g.
and you have to wait for it to get higher? Or, why not start with a higher
s.g. so that the resulting terra sig is at 1.13 or so, no more waiting?
Also, with all the information I have from you, there is no discussion of fit.
You recommended against using grolleg because of the larger particle size. I
had originally chosen it because I was throwing with porcelain. Does ball clay
terra sig work best on all clays?
That's all my questions for now but can't promise not to conjure up more in
the future.
Thanks for all your help,
Lynne Berman
Vince Pitelka on thu 8 oct 98
>Here's a progress report from Philadelphia. My ball mill birthday present was
>immediately put to use. I made terra sig of ball clay, got a specific gravity
>of 1.15. After ball milling, and only a little water on top, no bottom heavy
>layer, the specific gravity is 1.10. Now I am waiting for water to evaporate
>to get the specific gravity up to 1.13. Here's the question: why start with a
>specific gravity so low that the resulting terra sig will have too low a s.g.
>and you have to wait for it to get higher? Or, why not start with a higher
>s.g. so that the resulting terra sig is at 1.13 or so, no more waiting?
Lynne -
Since you are using an approach completely different from mine, I'm not sure
I can help much. I do not use a ball mill, because we do not have one. I
wish we did, because it opens up lots of possibilities for adding colorants
to the sig and ball-milling the mixtures. Most people who ball mill terra
sig do it after settling, siphoning, and evaporating the sig. They normally
ball mill a fairly heavy concentrate - perhaps 1.4 to 1.6 g/ml density (same
as specific gravity), and then thin it to the appropriate density just
before using it.
It sounds like you are ball-milling the initial mix without settling,
siphoning, and evaporating. I guess I am too much of a purist to do that,
because no amount of ball milling is going to insure that you have
eliminated all of the larger particles, whereas the traditional method of
settling and siphoning does insure that. Of course, the terra sig that you
siphon off is very thin, and therefore must be concentrated by evaporating
(which can be accelerated by heating).
>Also, with all the information I have from you, there is no discussion of fit.
Fit is not an issue with sig the way it is with engobes or glazes. We can
certainly eliminate a few clays which are simply too coarse, but otherwise
we normally select the clay which gives the color and surface we want in our
sig, and then we figure out if there is a way to apply it so that it stays
on the pot through firing and subsequent use. This may require a great many
very thing coats, or you may be able to get away with a few slightly thicker
coats. Thickness is relative, because ANY true terra sig coating will be
VERY thin in comparison to conventional slips and glazes.
>You recommended against using grolleg because of the larger particle size. I
>had originally chosen it because I was throwing with porcelain. Does ball clay
>terra sig work best on all clays?
Again, I recommend that you select the clay according to the color and
surface you want, and then figure out a way to make it work. It does happen
that ball clay terra sig works well on all claybodies, but personally I
don't much like pure white sig, and I like to get the color from the clay
itself whenever possible. Actually, I like the surface quality I get from
redart sig much better than ball clay sig, but obviously the color
possibilities are pretty limited with redart sig.
Hope this all helps -
- Vince
Vince Pitelka - vpitelka@DeKalb.net
Home 615/597-5376, work 615/597-6801, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
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