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oxidation & reduction in elec & gas kilns

updated fri 28 feb 97


Ellen Baker on tue 25 feb 97

Gavin Stairs' 2/24 message about oxidation & reduction in electric
(radiant) and gas (combustion) kilns is correct. His chemistry is
right-on. I think it's important to note that the superior oxidation in an
electric kiln depends on good venting.

This does NOT suggest that an un-vented electric kiln will give you the
same atmosphere as you'd have in a "reducing" gas kiln.

When gas burns, you're combining hydrogen and carbon with oxygen = heat,
carbon dioxide and water (vapor). In "reduction," the hydrocarbon fuel
doesn't get enough oxygen to completely burn. This may leave carbon in
clay bodies and glaze temporarily -- although a long last oxidation cycle
can strip (actually burn) some of that carbon back out.

But theres's another function that "lack of oxygen" in a hot kiln provides.
It serves to strip oxygen from metal oxides and carbonates, modifying the
concentrations of colorant metals in the glaze. In this case, the
hydrocarbon works as an agent to "scrub" or "steal" oxygen. It's easy to
see the action of this function when you successfully reduce a raku copper
red glaze. Oxygen in the glaze is stripped away by smoldering organic
(carbon-based) media, leaving behind a very recognizably metallic glaze.

In my quest for great flambe reds, I do believe that careful slowing,
soaking, and cooling are absolutely essential to "catching" the right
color. It's clear that there are many conditions beyond kiln atmosphere
that contribute to really spectacular effects. (Those who produce great
cystal glazes seem to be able to control cooling very, very well.)

It seems that saggar firing, alone, or introducing carbons to vented
electric kilns simply cannot "reproduce reduction" in the strict sense.
But produce amazing, beautiful effects? Yes! I wouldn't worry too much
about the definition if you get results.