vince pitelka on fri 22 dec 00
> I also apply it to leather-hard clay and wait to
> burnish. I do apply multiple coats, though. With
> me, it flakes too much from shrinkage when applied
> to dry clay. How do you do it, Vince?
Snail -
I might over-answer your question below, but better that than too little
information.
How are you mixing your sig? You might want to check out my recipe and
directions for terra sig on Tony Hansen's Digitalfire website at
http://digitalfire.com/education/glaze/terasig.htm
If you are preparing your sig properly to the right density (1.15 times that
of water), and applying repeated flowing coats, not allowing each one to dry
too much before applying the next (just wait until all shiny surface
moisture has soaked in and you can touch the surface), then you normally
should have no problems with flaking. However, this does depend on the
surface preparation. If the surface of the clay is too smooth the sig can
chip or peel no matter how well you do the rest of the process. Some
gritless claybodies can have an almost burnished surface where they are
trimmed, or where you have rubbed the surface for any reason. That is why I
like to sand the surface. Needless to say, if you are going to sand the
surface you need to take all the necessary precautions concerning dust. Do
your sanding outdoors or in a spraybooth with proper exhaust, and wear a
good dustmask.
Be sure to wipe the surface with a soft cloth to remove all dust before
applying the sig. This is doubly important. It gives better sig coating,
but it also prevents serious contamination of the sig when the brush carries
the dust back to the sig container.
One other thing that can aggrivate chipping - if you use a red-clay sig, and
then fire to low-fire or low-midrange temperatures in reduction, the iron
in the sig will flux, causing the sig coating to shrink, and it can chip and
peel, depending on the integrity of the interface. And of course if you
fire a red clay sig higher than that, it just becomes a glaze and really
isn't sig any more.
And one last circumstance where I have seen chipping - when sig is applied
to bisque-fired wares. This is a tenuous process at best, and again depends
on the clay-sig interface. If the clay was sanded before bisque-firing,
with all dust removed from the surface, then it is still possible to get a
good interface. But on bisque-fired wares don't expect to build up layers
the way you can on bone-dry wares.
Best wishes -
- Vince
Vince Pitelka
Home - vpitelka@dekalb.net
615/597-5376
Work - wpitelka@tntech.edu
615/597-6801 ext. 111, fax 615/597-6803
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
http://www.craftcenter.tntech.edu/
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