Jeff Tsai on sun 7 apr 02
Someone wrote:
<bisque?>>
and the reply given was:
<>
This is true, no doubt, but I also remember hearing once that terra sigs
don't shrink very much if at all. Because the sig is already the finist
particle size that you can obtain, the amount of shrinkage is almost nil. It
is as if you were to apply water to the surface of a bisque piece almost
(almost).
So far, as long as the terra sig was well made and not tampered with too much
afterwards, my sigs have never flaked off, applied bisque or green.
-jeff
vince pitelka on mon 8 apr 02
> Low shrinkage might account for the shivering we sometimes experience with
> Goldart-based terra sig applied to greenware. I'll do some green- vs
bisque
> terra sig experiments this week and report to the list.
> Vince, any comments?
Roger -
I don't think you can call it shivering unless it is a glaze. With terra
sig it is just peeling or flaking. And in my experience this only happens
when the sig-to-claybody interface is weak (as with excessively thick
application of sig on bisqueware), or when the claybody starts to shrink and
the sig doesn't. It is the characteristic of a very fine-grain clay that it
shows high drying shrinkage and low firing shrinkage. More water layers in
the wet sig causes greater drying shrinkage. Less air spaces in the dry sig
causes lower firing shrinkage. So if the claybody starts to shrink and
there is a considerable buildup of sig, the sig simply separates from the
claybody.
Best wishes -
- 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/
vince pitelka on mon 8 apr 02
> This is true, no doubt, but I also remember hearing once that terra sigs
> don't shrink very much if at all. Because the sig is already the finist
> particle size that you can obtain, the amount of shrinkage is almost nil.
It
> is as if you were to apply water to the surface of a bisque piece almost
> (almost).
Jeff -
Just the opposite is true. "Because the sig is already the finest particle
size that you can obtain," it will shrink MUCH MORE than other clay or slip
mixtures. The smaller the particles, the more water layers, and therefore
the more drying shrinkage. It is only because we apply it in such thin
layers that it works at all, and that is double true when it is applied to
bisqueware.
Best wishes -
- 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/
Roger Korn on mon 8 apr 02
Hi Jeff,
Low shrinkage might account for the shivering we sometimes experience with
Goldart-based terra sig applied to greenware. I'll do some green- vs bisque terra
sig experiments this week and report to the list.
Vince, any comments?
Roger
Jeff Tsai wrote:
> Someone wrote:
> <> bisque?>>
>
> and the reply given was:
> <>
>
> This is true, no doubt, but I also remember hearing once that terra sigs
> don't shrink very much if at all. Because the sig is already the finist
> particle size that you can obtain, the amount of shrinkage is almost nil. It
> is as if you were to apply water to the surface of a bisque piece almost
> (almost).
>
> So far, as long as the terra sig was well made and not tampered with too much
> afterwards, my sigs have never flaked off, applied bisque or green.
>
> -jeff
>
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--
Roger Korn
McKay Creek Ceramics
In AZ: PO Box 463
4215 Culpepper Ranch Rd
Rimrock, AZ 86335
928-567-5699 <-
In OR: PO Box 436
31330 NW Pacific Ave.
North Plains, OR 97133
503-647-5464
Michael McDowell on tue 9 apr 02
I am hardly an expert on this subject, but I do have a little anecdotal
experience to share on the subject. I have a number of small refrigerator
magnets that we dipped, as bisqueware, in terra sig from Vince's "Ancient
Clay" workshop here one year. Then we fired them in the bonfire along with all
the workshop pieces. What I have noticed is that while the terra sig layer has
stayed intact on the outward facing surface, the surface that I apply the
magnet to has not held up to the strain. When the magnet finally separates
from the little stamped tile, it takes the terra sig layer with it. Actually,
it is possible to re-glue them at this point, and the problem is solved.
Luckily, I hadn't sold too many before I discovered the problem. I pulled the
remaining ones off the market. I'm using them myself so I can re-glue them as
they come apart.
Vince recommends multiple coats of very thin terra sig mixture (sp. g. of
1.15). I would suggest not cheating on that stipulation. Even if pieces seem
to come out of the fire OK, it is still possible for the terra sig layer to
flake off at a later date. As with any other interface between surfaces,
differential shrinkage must be taken into consideration. For good ware, you
must get it right. Being only close to right allows for these kind of delayed
problems. I know that it sometimes happens to us all, but when it does,
delayed failure of a piece that has already sold can work to the detriment of
every clay worker. If our customers get the feeling that our work may have
hidden flaws, they'll be hesitant to purchase, not only our work, but anything
they see as "of that type".
Michael McDowell
Whatcom County, WA USA
michael@mcdowellpottery.com
http://www.mcdowellpottery.com
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