Vince Pitelka on wed 4 aug 04
Anomalous or contradictory? Perhaps I can clarify. The point here is that
the connection of a terra sig coating to the claybody beneath is tenuous at
best. As Ivor points out, a flux might help the sig to stick. We usually
do not add fluxes to sig because they are so coarse and can interfer with
the shine. The iron contained in the clay is an exception, and a little
iron turning to flux could increase the shrinkage of the terra sig coating,
possibly causing it to flake off the pot. This has never happened to me
with multiple thin (SG 1.15) coats of sig, but it can happen with a thicker
accumulation of the material.
I guess in the long run it is not worth it to worry about reduction, because
the sig coating SHOULD survive a reduction atmosphere just fine. Instead, I
would focus on the integrity of coating resulting from the application
proceedure.
Best wishes -
- Vince
Vince Pitelka
Appalachian Center for Craft, Tennessee Technological University
Smithville TN 37166, 615/597-6801 x111
vpitelka@dtccom.net, wpitelka@tntech.edu
http://iweb.tntech.edu/wpitelka/
http://www.tntech.edu/craftcenter/
Ivor and Olive Lewis on thu 5 aug 04
Dear Vince Pitelka,
Thank you for that clarification. Had you said why you considered the
Red Iron Oxide to be a the root of the problem I would have found it
immediately acceptable. The idea of a fluxing agent causing separation
clouded my vision of the process. But a change in unit cell dimension
and form when an oxygen atom is discharged from the Fe3O4 is a whole
new ball game.
Best regards,
Ivor Lewis.
Redhill,
S. Australia.
Wally on thu 5 aug 04
Hello Vince and Ivor,
I have been following this discussion at the tip of my chair, as it
has allways been a little mysterious to me why terra-sig sometimes
produces these kind of "allergic reactions".
I do agree with Vince about his theory on the influence of the
fluxing materials.
This phenomenum is very likely to reduce the bond between the
terrasig layer and the clay underneath.
During my hands-on "naked raku" workshops many students do bring
along some terra-sigged objects to be fired.
Sometimes they turn out marvellous with great cracklings, sometimes
it is a big disaster, the terrasig just chipping off at random
spots...
One thing one has to bear in mind in this respect is the aggressive
nature of the firing process, where the 3 totally different mediums
(terrasigged bisque, slip layer, glaze layer) are under varying
kinds of severe pressure during the complete cycle of firing and
reducing.
Of course there is a difference between how ts-mixtures are made,
and their varying kinds of viscosity.
Up to now, I was quite convinced that the 2 main reasons for the
chip-offs were a low viscosity of the terra-sig, and the method of
applying TS to a specific clay.
Thick mixtures do seem to create problems, certainly if applied by
brushwork in multiple layers.
Best results seem to originate from applying a medium viscosity by
spraying in consecutive layers with an airgun.
I also think that there is an influence from the way that the TS
syrface is "buffed" after applying.... The highher the gloss, the
more the pores will close down, creating a surface less likely to
accept any other extra layer.
The composition of the ingredients of the TS do seem to be a very
valuable "fourth reason", when the negative results of the fluxing
starts to influence the bond.
Of course, all 4 above will probably have (what I call)
an "amplifying effect" on the final result....
If all 4 are at high level, chanches of good results are shivering
at the horizon...
If all 4 have bad kharma, failure is hiding at the doorstep.
Just my 4 Neurocents worth.
Wally, Belgium.
www.wallyasselberghs.be
--- In clayart@yahoogroups.com, Vince Pitelka wrote:
> Anomalous or contradictory? Perhaps I can clarify. The point
here is that
> the connection of a terra sig coating to the claybody beneath is
tenuous at
> best. As Ivor points out, a flux might help the sig to stick. We
usually
> do not add fluxes to sig because they are so coarse and can
interfer with
> the shine. The iron contained in the clay is an exception, and a
little
> iron turning to flux could increase the shrinkage of the terra sig
coating,
> possibly causing it to flake off the pot. This has never happened
to me
> with multiple thin (SG 1.15) coats of sig, but it can happen with
a thicker
> accumulation of the material.
>
> I guess in the long run it is not worth it to worry about
reduction, because
> the sig coating SHOULD survive a reduction atmosphere just fine.
Instead, I
> would focus on the integrity of coating resulting from the
application
> proceedure.
> Best wishes -
> - Vince
>
> Vince Pitelka
> Appalachian Center for Craft, Tennessee Technological University
> Smithville TN 37166, 615/597-6801 x111
> vpitelka@d..., wpitelka@t...
> http://iweb.tntech.edu/wpitelka/
> http://www.tntech.edu/craftcenter/
>
>
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