JOHN NEELY on thu 25 apr 96
Hodaka Hasebe wrote:
> I am interested to try "Tanka" (reduction cooling ?) with a gas kiln.
> All I know is to fire to cone 9-10, and reduce down to 1650-1750F
> with fire overflowing from a burner port. (Or, In Japan, they put a
> raw wood into a kiln through a special entrance and closed up after
> cone 9-10.) This process makes clay body reduce heavily and traps
> many carbon to glaze. Does anyone know the process in details ?
> Should I close damper completely ? (I assume not) Should I use a
> pilot burner or a burner ? How heavy should I reduce? How much smoke
> should I expect in the kiln room ? etc.
Though the terms are often used interchangeably, there is an important
difference between "tanka shousei" (literally carbonizing firing, but perhaps
more accurately carbon impregnation) and "reikyaku kangen" (cooling
reduction). If what you want is to trap carbon in the body and glaze, it is
much easier to do this by creating a smoky atmosphere during the firing, as
vitrification takes place, rather than trying to make carbon penetrate during
cooling. One common method of carbon impregnation for unglazed ware is to pack
it into saggars filled with charcoal. American style "raku," where hot pots
are put into combustible organic material is another example of carbon
impregnation that is often ( and usually inaccurately) referred to as
"reduction."
Reduction cooling, on the other hand, relies on carbon monoxide (not carbon!)
produced by a small reducing flame in the kiln as it cools. If the clay and
glaze have been reduced on the way up, reduction cooling prevents reoxidation.
If the firing has been in oxidation, the reducing flame will affect only the
surface. Either can produce interesting results with some clays and glazes; I
usually use the former procedure. Incidentally, most clays will still be
reactive at 1750; to insure that there is no reoxidation, I fire down to 1450.
How much you open the damper, what burner you use, etc. depends entirely on
your kiln - how tight it is, whether you have sealed burners, etc. Smoke is
certainly not necessary - carbon monoxide is a colorless gas.
FWIW, we are offering an intermediate/advanced level workshop June 1-12 titled
"Reduction Cooling and Other Firing Processes." Cost is $328 for non-credit,
and while I'm not positive, as I read the catalog it looks like it is four
dollars more to get four quarter credits, they may stick you for a $35
application fee though. Registration number is 1-800-538-2663.
* John Neely neelyjc@cc.usu.edu
* Department of Art, Utah State University, Logan, Utah 84322-4000
BTW, for a good laugh, if you have netscape, take a look at :
http://www.coara.or.jp/~silicon/textJ/enbuJ.html
A portable mini anagama six feet long, five cubic feet of stacking space, you
can put it on casters and wheel it around. The text reads something like " the
crackle of firewood, the flame dances pouring new energy into the clay - the
woodkiln to fire feeling the romance of our ancestors. The kiln that fans of
ceramic art have been waiting for." Better yet, they sell firewood, too!
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