Marcie McConville on mon 2 sep 96
Has anyone had an electric kiln go into reduction without the aid of a
reducing material?
Although it is hard to believe I have! Smoke was pouring out of every
available crevice above the top bung hole at 1000 degrees C. Ten minutes
after this discovery the kiln was turned off. It was a tight pack with a
high percentage of fire clay. There was no carbon deposit on any of the
work but there was evidence of iron spots on some of the pieces. Believe
it or not this actually happened and no I wasn't drinking!! This defies
everything I have ever learnt or experienced.
Marcie McConville
Ron Roy on tue 3 sep 96
>----------------------------Original message----------------------------
>Has anyone had an electric kiln go into reduction without the aid of a
>reducing material?
Hi Marcie,
I don't find it unusual to see an electric kiln reducing although it is
usually in a bisque firing. The carbon can build up in the fire bricks
though. Many clays come with more or less amounts of lignite (coal.) You
can see it sometimes as small black bits. This is why slow firing a bisque
between 700 and 900C is recommended - to make sure there is enough oxygen
in the kiln to burn off the carbon. This can be even more of a problem in
an electric kiln because there is no air circulating - a plus for those
using a vent system.
Some one, correctly, attributed this problem to causing glaze blistering in
a recent post - but recommended (incorrectly) firing bisque higher to get
rid of it. If there is not enough oxygen to burn off the carbon the iron
will be reduced and become a strong flux leading to many faults - higher
bisque firing cannot undo this. Slower firing and some ventilation is the
answer.
Complaining to your clay supplier may do some good but developing a safer
(read longer) firing cycle will be best in the long run.
What is a good rate to fire - as usual it depends - if the kiln is tight
and tightly packed I would say 50C per hour. If there is a vent I would say
100C per hour. There is a way to find out if there is oxygen in the kiln -
aside from a probe - If there is no oxygen combustibles will not burn so a
piece of cardboard or paper in the spy should tell you - you don't have to
be close to see this - no need to burn any eyebrows - if there is oxygen it
will burn inside. Actually the smoke coming out of the kiln is visible - if
you are there simply slow the rate of increase down and open a spy hole or
two till it stops and then proceed slower.
Ron Roy
Toronto, Canada
Evenings, call 416 439 2621
Fax, 416 438 7849
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