Tom Buck on sun 31 oct 99
David:
Results are what count, not the probable mechanism. In a reducing
environment, there COULD occur this solid phase - gas phase reaction:
ZnO + CO (or H2) = Zn (metal) + CO2 (or H2O).
The CO2 and H2O would be gases at C10 so they go up the flue immediately.
Zinc metal boils at 907 oC so as soon as any atoms of Zn are created at
C10 they go up the flue too IF they do not immediately find a "home" in
the alumino-silicate matrix (glass).
1) Gas/solid reactions are slow and difficult; some Zn/ZnO will go
into the glaze no matter what. Do enough make it to warrant the cost and
the improvement in the glaze? You decide.
2) Not all reduction kilns work the same, or are operated in a
similar manner. Some gas-fired kilns teeter on the edge of reduction, with
a few locales getting some reduction, others none at all. Hence, a glaze
with Zinc Oxide in it may well exhibit effects common to the Bristol type
of a lower cone.
3) One glaze type (Copper Red) needs no ZnO as D. Henley has
shown. There are however some variants of Cu red that won't reduce well
(too viscous) in some kilns unless one adds ZnO to flux the base glaze and
make it more fluid and thus allow the red colour to form. If the ZnO
disapppears after it has played this role, the loss (and cost) is part of
the process.
In summary: there are no absolutes in making good pots, just some
ways that work better than others.
Tom Buck ) tel:
905-389-2339 (westend Lake Ontario, province of Ontario, Canada).
mailing address: 373 East 43rd Street,
Hamilton ON L8T 3E1 Canada
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