A heavily reducing fire ot this stage would desosit carbon into the ware and there might
be some problem getting rid of it laver.

The main work of reduction hos to take place before vitrification closes the glaze and body
to the circulating gases. The stort of vitrification of various bodies and glazes differs so to be sure
that you are getting every advantage reduction should start early, 1400 degrees, and continue until
the maturing cone is down,

The ideal reduction firing would have your kiln full of carbon-monoxide (CO), no free oxygen
and no free carbon; but this is not practically possible, As long as there is not too much free oxygen
it will combine with the CO which is there for thet purpose. In being sure thot one has a kiln full
of CO a little free carbon is certain to be formed, it is seen as smoke when o peep-hole is opened.
A little can be taken care of later. The accomplishment of such o balance is really not at all dif-
ficult if you have a workable kiln. The following should help but it doesn’t relieve the burner of
the necessity to think,

(1) Keep the domper adjusted so that the top 34 of the kiln is constantly under pressure,
flames should just escape through a peep-hole one-third of the way up from the floor of the kiln. This
assures that no cir can leak into the kiln through the walls and that the entire kiln is filled to
that level with the gases of combustion,

(2) To judge the gas-air mixture one must realize that CO is entirely transparent and connot
be seen in the kiln but burns with o blue flame that can be seen at the damper after the kiln has
reached about 2000", Also that free carbon is opague and obscures the view in the kiln, from a
slight hazyness ta a dense, incondescent fog, ond burns with a bright yellow flame like a candle.
So, if we have an atmosphere that is just right, CO with just a bit of free carbon, the view within
the kiln should be nearly clear with a hazy yellow cast, Ifo little air is leaking in here and there
you will see G movement of transparent yellow flame and details will be herder to distinguish,
There should never be so much free carbon that one cannot make out the cones but brief periods
of over-doing it will do no real harm.

When this kiln atmosphere passes the damper into the flue it is exposed to unlimited air so
it burns. If this were a pure CO flame it would be transparent blue. If it is too high in free carbon it
will be opaque and yellow. But if it is neither, it is just right so it will be a semi-transparent mauve-
orange. This should be the fire to maturity.

Again: Smoke escaping two-thirds the way down the kiln, view within the kiln only a bit ob-
scured and the flame passing the damper mauve-orange.

Now to take care of that little bit of free carbon.

(3) To finish off the reduction firing open the air intakes to a normal oxidizing setting, turn
down the gas so that the temperature will drop 25 to 50 degrees during the next half hour and
open the damper a bit so thot the flame now escapes two-thirds up from the floor of the kiln.
Hold this “clearing” fire for 40 minutes, it will burn off the carbon that has deposited into the sur-
face of the ware and re-oxidize the exposed iron of the body and glaze.

There is ever so much more thet should be said but | hove run out of space.

LUKE O. LINDOE, Plainsman Clays Limited, August, 1967.