John Christie on fri 15 mar 02
Chris Clarke writes:
"Why can I only get really good carbon trapping when it's on the verge of
raining?"
My suggestion:
Carbon trapping occurs when unburned carbon is deposited in the pores of the
clay or glaze in the early part of the firing and sealed by subsequent
vitrification.
When water vapour passes through (or over) a bed of red-hot coals, it forms
"water gas" - a mixture of carbon monoxide and hydrogen. One volume of
water vapour produces two volumes of "water gas". Carbon monoxide and
hydrogen are both powerful reducing agents and "use up" some of the
surplus oxygen passing through the kiln
.
The presence of "water gas" lessens the probability of condensed carbon
burning off (usually between 900 and 1000 degrees C.) It can also assist
later in the firing in
the reduction of metallic oxides in glazes. Oxidised saturated iron glazes
may also work better in humid weather because they are usually better when
they have been reduced and then re-oxidised.
Some people value the effects of carbon trapping in body or glaze but it can
also be responsible for serious defects - bloating, pinholing and crawling.
John Christie.
www.blackhills.co.uk
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