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Copper Reds…How Do I Know If My Kiln Is In Reduction?

Copper Reds…How Do I Know If My Kiln Is In Reduction?

It’s important to know how to tell if you gas kiln is in reduction!

I’m reposting this blog post and have added some info as I feel that it is pertinent to all of us potters that are trying to do reduction gas firings, whether they are cone 6, 7, 8, 9, or 10!

For some reason, I thought achieving reduction in a gas kiln was almost unattainable.  It seemed to me the “holy grail” of pottery and firing.  So when the kiln went into reduction without great effort or fanfare, I was totally euphoric!!

Why did I covet the “reductionist” of glazes?  Well, it’s pretty obvious when you look at pots that have been fired with the same glaze, to the same cone/temperature and compare the results.

I know that even the glaze gurus, like Steven Hill, are converting back to electric and cone 6 glazes.
Here is the link where a comparison is made:

I’m sure they have their reasons.  But for me, I’ve always wanted to high fire in a gas reduction kiln.  I DO think it looks magical – better colours, better flow, better tactileness, better everything!

Reduction is achieved in a kiln when the fire burns up all of the oxygen in the kiln and the fire must then seek oxygen in the glazes and the clay body in order to keep burning.  The resulting effect is, in my opinion, very different from the electric glazes.

The benefit of gas reduction firing is well-put by Scott Cooper at St. Earth Pottery, “Unlike electric kiln firing, fuel gives you real flame, atmospheric currents and draft, the ability to fire in a reduction atmosphere, and a nice component of drama.”

So, how did we get the kiln into reduction?  Simply by cutting off the oxygen supply by:
1.  Closing the kiln flue by slightly covering the opening with fire bricks.
2.  Closing down the oxygen wheel on the gas torches going into the kiln.

Note:  I now only reduce from the flue and leave the oxygen at the burners alone.  I start reduction anywhere from 750 degrees C to 900 degrees C and get copper reds with no worries!

To determine if the kiln is in reduction you can remove one of the peep hole bungs and if a 3 to 4 inch flame comes out, then you are in reduction.


In addition, the flames will be burning out of the flue.



Yahoo!  Reduction!!! Euphoria!!!  Glorious Reduction!!!



Written by Marian Williams


  1. helen sparrow · April 29, 2015

    Are firing with natural gas or LPG. With natural gas you can have a flame coming out of the spy hole and not be in reduction.

    • Marian Williams · April 30, 2015

      Hi, Helen, I fire in LPG, and have never fired natural gas so I don’t know about the difference. I have learned that one of my kilns doesn’t reduce by just closing off the flu- I have to close off the burners AND close off the flu-(about halfway on both) to get that one in redux – it has the flame out the bung hole as you mentioned, but there wasn’t enough reduction, so I’ve had to really crank down on the oxygen that it can access. Good luck! Thanks for reading and commenting!
      All the best,

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