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more on terra sig problems

updated thu 23 jan 03


iandol on tue 21 jan 03

Dear Antoinette,

I think that just making a slight upward temperature adjustment is the =
best first step. It may just encourage the materials which are in your =
clay to begin a process called Fluid Phase Sintering. The one thing you =
do not need to do is to get your Kaolin up to a temperature where it =
changes into Mullite and Silica. Nor would I be tempted to substitute =
alternative fluxes until you have tested the increased temperature =
option. In the article "Terra Sigillata", p 50, Technical Ceramics No =
12, I suggest that the highest temperature to which you may take the TS =
is 1040=B0 C.

One thing which I think should be thought about when preparing your =
materials is that although your TS is made from your clay body, the =
objective is to retain the very smallest particles and discard the =
coarse material. Because of the differences in mesh size and density of =
your original ingredients it is most probable that your two materials =
are entirely different even though they have identical origins.

I wish you every success in correcting what I see to be a minor problem.

Best regards,

Ivor Lewis.

Vince Pitelka on wed 22 jan 03

It should be pointed out that ANYTHING done to the sig to increase the
degree of flux activity, and thus the connection to the claybody surface,
will diminish the shine. There is no way you can have both. Flux activity
causes the sig to move around, and disrupts the platelet alignment on the
surface. It is the platelet alignment that creates the shine, as a result
of polishing or burnishing.

I think the problem is just that the sig was applied to bisqueware. It is
impossible to get a good bond applying sig to bisqueware, because if you
want a true terra sig effect you cannot add the fluxes you would add if you
were making an underglaze for bisque application. Flaking is a very
frequent problem with sig applied to bisqueware. Antoinette is lucky not to
have experienced more severe flaking problems. But this indicates how
precarious the surfaces are, and it is to be expected that customers could
experience flaking and chipping on their pots when the bump them against
something or scrape them slightly.

Antoinette, you say that you cannot apply the sig to greenware because your
pots are paper thin. After so many years teaching clay I still like making
pinch pots with my intro class, and I keep challenging myself to make them
thinner. The ones I make these days are usually about 1/16" thick. When
bone dry I sand them, and then apply multiple coats of sig. With the
thinnest ones I have to wait and let the sig dry out before adding more, but
I do manage to get enough sig on there to polish them to a high sheen.
These are usually blackware bonfired, and the sig has never flaked off. I
think you need to apply the sig at the bone dry stage.
Best wishes -
- Vince

Vince Pitelka
Appalachian Center for Crafts
Tennessee Technological University
1560 Craft Center Drive, Smithville TN 37166
Home -
Work -
615/597-6801 ext. 111, fax 615/597-6803