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terra sigillata as glass???

updated tue 27 feb 01


vince pitelka on sun 25 feb 01

> This is much akin to the quiet entries I have made on ClayArt
> with regards to Terra Sigillata - which is really Attic Ware
> related . A fine slip being oxidised , reduced and then
> oxidised again.
> Terra Sigillata cannot be scratched by a razor blade and is a
> glass , fluxed by black iron oxide and fine particle size.
> There is no mention of having to Burnish to get a gloss .

Khaimraj -
This is a bit misleading. Terra sigillata is not a glass. A shiny glass or
glaze of course achieves its shine by melting, and terra sig, by definition,
does not. And much terra sigillata can certainly be scratched with a razor
blade. Through the tradition of terra sig, the surface rarely approached
vitrification. It all depends on the particular sig and the firing
temperature. The black slip used on Attic ware that you refer to was a
special-case circumstance which does not fit the classic Roman terra sig.
As you have indicated, the Attic black slip was fluxed just enough so that
it locked in the reduced black iron oxide, preventing reoxidation to red
iron oxide.

I have never seen a terra sig, no matter how finely prepared, that would
give a gloss without at least a good polishing. The shine on terra sig
depends on particle orientation and compression, not on formation of a
glass. The best terra sig is of course refined to the very finest clay
platelets, and when applied to a surface the platelets naturally tend to fit
together and lay flat on the surface, creating a reflective sheen. Any
pressure at all will cause them to lay down flatter and denser, creating a
higher sheen. Burnishing with a shiny hard object will create a sheen like
glass, but one can get a very good shine by polishing with a soft brush, a
soft cloth, or a piece of plastic film.

In my opinion, if a material coating fuses in the firing to creates a gloss
that was not there before the firing, then it is a glaze, not a slip or a
terra sig.
Best wishes -
- Vince

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