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Cone 6 Reduction Electric Kiln Firing in Saggars

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I have recently carried some successful trials using saggars to produce cone 6 copper red reduction glazes in an electric kiln.  I generated a reduction atmosphere by adding a small amount of black iron oxide mixed with powdered charcoal to the bottom of a saggar with a test piece painted with Selsor red copper reduction glaze (see formula in "The Complete Guide to Mid-range Glazes " by J. Britt pg 100).  I started with a copper red cone 6 glaze because it is easy to see if there is reduction; red=reduction / green=no reduction.  The iron oxide/ charcoal mixture reacted to produce a reducing carbon monoxide atmosphere within the saggar, as the electric kiln was fired to cone 6, which changed the glaze oxblood red (see attached photo).  The atmosphere in the kiln remained oxidizing so that test patchs of the same glaze painted on the outside of saggar remained green.

I hope that by sharing these results I can generate feedback on this this technique.

 

Saggar_trial.jpg

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3 hours ago, Callie Beller Diesel said:

Im curious why the iron oxide?

Callie,
I'll take a stab on why this procedure works to produce copper reds in an oxidation kiln:
The iron oxides and powdered charcoal provides a oxygen fugacity buffer where the dominant carbon gas form is carbon monoxide (CO).  The CO reacts with the copper ions in the glaze to reduce the copper to the copper specie that produces the red colour.   I haven't done the calcs, so consider this as an SWAG.   Your question would make a good exercise for a Geo-PChem exam.
LT
 

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I'm with Callie- why the iron? There should be plenty of CO produced by the charcoal. Iron is sometimes used in copper red glazes (although tin is much more common) to provide a nucleus for the growth of the red crystals. I'm not sure how it would affect the glaze by being in the saggar, though. Plus it'll goober up the bottom of your saggar,  which should otherwise last for many firings. I would run the same test without the iron and see if there's a noticeable difference.

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This is interesting. Did you seal the saggar or is it just a loose dome over the bottom?Just curious because all together its interesting to think about saggars in electric. I know in the old days with wooden kilns people used to saggar fire porcelain to get perfect tea bowls etc,  but never thought about it in electric to do the opposite.

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I got ^10 copper reds in Oxidation using silicon carbide for local reduction back in 1973. hadyn

red was the base. It was a common approach back then.

there are many solutions to challenges in glaze chemistry. I might add that the Selsor in Selsor red copper reduction glaze is me.- and I used this glaze for many years as did my students.

 

Marcia Selsor

 

 

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The saggar has a tight seal to prevent carbon monoxide from escaping which both ensures strong reduction and protects the kiln elements but it prevents oxygen , from the atmosphere , from entering so a source of oxygen  must be provided.  The iron oxide is added to provide a source of oxygen to react with the carbon .   Black iron oxide was chosen because, at temperatures around cone 6, it reacts easily with carbon to produce carbon monoxide.   Black copper oxide could also be used as an oxygen source but it's more expensive.  Black iron oxide is cheap, readily available and comes as a very fine powder.

To protect the saggar,  the iron oxide and charcoal are placed on a tile which sits in the base.  Residue from the reaction can be easily scraped from the tile so it can be reused and it can be replaced if damaged.  The aim was to make the saggar last as long as possible.

Thank you for your responses.  Also my complements to Marcia Selsor for your excellent cone 6 copper red glaze!

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