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reduction coooling

updated mon 30 sep 96


JOHN NEELY on thu 5 sep 96

"Darrol F. Shillingburg" <74353.2316@CompuServe.COM> wrote:

> The question is , "What cone temperatures do I need to fire to, how
> much body reduction and at what temperatures?" ( I am able to do
> reduction firing) I reduced the first kiln load with a yellow flame
> burning during "cooling" until the wares "turned dark". OK, so it's
> not scientific, but I am a 'converted painter'


Good to see you online - clearly Elephant Butte is not so remote as I had
believed. Oldtimers on Clayart will recognize this as a topic that comes up
periodically. You'll forgive me if I quote earlier posts...

> The CM reference to reduction cooling might well be an article I
> wrote back in 1988 called "Nice Cooling," which dealt with, *among
> other things,* that subject. Variations in cooling speed and
> atmosphere will affect finished results was the upshot of the
> article. One of the variations that I discussed was maintaining a
> reduction atmosphere during cooling - in fact the pots illustrating
> the article were all done in this fashion. It is entirely different
> from soaking - which is maintaining a constant temperature.
> Without going into all of the variables, a basic firing schedule to
> start with would with would be the one that I use most - reduction
> sufficient to stall the kiln at cone 08-04 range for about 45
> minutes, light reduction up to top temperature of cone ten, and heavy
> reduction down to about 800 degrees centigrade. Then the burners are
> turned off and cooling to room temperature is with no manipulation of
> atmosphere. Need I add that it takes very little gas to keep the kiln
> reducing while it cools to 800?
> Please understand that I'm not advocating this as THE way to fire -
> this is just one way that will yield significantly different results
> than some other firing/cooling schedules.

FWIW, all my initial tests on this were cone with a pyrometer that read in
centigrade. 800 C is actually about 1472 F but I often use the figure of
1450 F as a benchmark.

For example, in another post I wrote:

> Reduction cooling....... 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.

A pyrometer really helps here. The problem with using cones to gauge the
cooling is that is very difficult to get them to stand back up as the
temperature decreases. (g)

Alas, I read Clayart in digest form (which means it is a day old at least
before I see it) so I'm afraid this won't reach you in time to help with your
Thursday firing. How'd it go?

John Neely in Logan, Utah (in the middle of one of these firings as he types)

Darrol F. Shillingburg on fri 6 sep 96


Thanks for your response to my inquiry about reduction cooling. The information
was new to me, regardless of how "old".

I don't have a pyrometer and couldn't afford those fancy 'resurrection cones'
(g) so had to eyeball the end point of reduction.

Got great results from yesterdays firing. Fired three different high iron clay
bodies, a^5 porcelain(?) some mixed with mason stains as well as some terra
sigillatas. Resultant colors ranged from matt black (looks true black) to brown
black, red black and a metalic black with violet overtones. Since I was refiring
some pieces I brought the temperature up to ^04 in six hours in oxidation, then
up to ^5 in a couple of hours in very light reduction, then closed up the kiln
with a dirty yellow flame. However, I did have some air comming in with the
flame this time, thus the red blacks. All the clays I am using are ^5. In my
kiln it takes about two to three hours of "cooling".

Will an earthenware body be reactive enough at ^04 to yield a color shift? Seems
a narrow temperature range to work in. But then I am pretty new at this kind of

Thanks again.

Darrol in Elephant Butte, NM

David B. Buck on sat 7 sep 96

>----------------------------Original message----------------------------
>Thanks for your response to my inquiry about reduction cooling. The information
>was new to me, regardless of how "old".
>I don't have a pyrometer and couldn't afford those fancy 'resurrection cones'
>(g) so had to eyeball the end point of reduction.
>Darrol in Elephant Butte, NM
> Instead of using a pyrometer a voltmeter or digital
multimeter can be used to gauge temperture. A voltage to temperture
conversion table is needed so this method is a pain, but cheep. Also the
same thermocouple probe a pyrometer uses is needed. If you have already got
the voltmeter you can save about 3/4 of the cost of a pyrometer.

David B. Buck
Nepean, Ont.

JOHN NEELY on sat 7 sep 96

"Darrol F. Shillingburg" <74353.2316@CompuServe.COM> wrote:

> Will an earthenware body be reactive enough at ^04 to yield a color
> shift? Seems a narrow temperature range to
> work in. But then I am pretty new at this kind of firing.


I'm not sure what you are asking here. Do you mean, "can one fire an
earthenware body that matures at cone 04 using this reduction cooling
If so, then the answer is yes. Yes, but... The intensity of the color will be
dependent on how vitreous the body is. Many earthenware bodies are not
vitreous at all. This has to do with what I call"the Jello theory of
ceramics." You know that strawberry Jello in the box is pale pink, but when
you add hot water the color turns to intense red. Same thing with clay. In a
vitreous body everything is "wetted" with glass - and the colors are darker,
or intensified. Another thing to remember is that black iron oxide is a flux,
where red iron is pretty refractory up to about cone 8, when it begins to
spontaneously reduce (which is why you can have oxidation tenmoku at 8 and
above, but that is another story.) I don't know about cone 04 bodies, but with
some cone 6 red bodies, they will mature a cone or two lower, i.e., cone 4-5
if they are fired in reduction.

Then again maybe this isn't what you asked at all. Sounds like you are getting
the hang of it anyway.


Terrance Lazaroff on sun 8 sep 96

Just wanted to relate a reduction cooling method I used a few years back. I
worked with an red earthenware clay, When the clay reached the leather hard
stage I burnished the pieces. When bone dry I applied terrasig and polished
with a soft cotton cloth. When I loaded the kiln I placed a pot just under a
peep hole just inside the kiln. This pot was situated so that it would catch
my reduction material once inserted into the kiln.

I fired the kiln to cone 04 and shut it down. When the kiln lost the red
heat I rolled news paper into a ball small enough to pass through the peep
hole. I continued to add the news paper balls until the kiln was full of
smoke. If the damper is closed the reduction is fairly solid. If the damper
is open the reduction will follow the flow of the heat current and thus give
a stronger reduction on one side with a flame lick reduction towards the side
opposite. This is an eyeball method. So be ready for any result.