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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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  • 1 year later...
  • 1 year later...
1 hour ago, Steven Goldate said:

You wrote: "It's not expansion of air that causes explosions.  It's expansion of water."

OK, thanks. But what about the saggar? Is it fully sealed? How was it sealed?

Thanks again, Steven

If you look at the first photo, the saggar top fits inside the base, forming a decent seal. There's no way to 100% seal a saggar, nor is it necessary.

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@Steven Goldate, the person you are asking the questions to, @why_not hasn't been on the forum since 2018 so it's unlikely they have seen your post. You could try sending them a message, they'll receive an email that they have a message from you via the forum. To message them click on their screen name or avatar, this takes you to their homepage, from there click on the envelope icon near the top of the page and follow the prompts. (to see when a member was last logged in on the forum just hover your mouse over their screen name)

Saggars were traditionally used in fuel burning kilns to stack pots within thus avoiding the need for kiln shelves. They also prevented unwanted deposits of ash etc from being deposited on the pots. For using saggars for localized reduction in an electric kiln your main concern in regards to the kiln is keeping the reducing atmosphere away from the kiln elements. Running a kiln vent will help with this as will using as little in the way of combustibles as possible to achieve the results you are after.

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  • 6 months later...

Hi @Steven Goldate I expect you could properly seal the saggar with sand. Make a base dish with 2 rims an inch apart and with plenty of depth. Make another vessel (the saggar) where the rim fits between the 2 rims of the dish. Put sand between the 2 rims and then nestle the saggar into the sand. Theoretically, this should create a very nice seal, while simultaneously allowing excess gas to be vented through the sand.

I'll be trying this technique later this year and will report back if I find anything interesting.

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Which reminded me of an early paper on local reduction. 

One of the most interesting ideas was the use of a SiC containing engobe/slip  which then influenced the colour of a SiC-free glaze applied over the bisqued piece.
- They give combinations for copper reds at cones 012 and cone 9.
- They suggested that silicon carbide achieves reduction effects over this remarkably wide range by only reacting when in contacted with liquid glaze.

I haven't seen this idea mentioned elsewhere (and would be very interested in pointers).

More detailed summary of  the 1932 paper in an earlier post

PS If the slip idea does work I wonder if an SiC/CMC "underglaze" would also work.

PPS A full copy of the paper is now online via
https://ur.booksc.eu/book/9078584/3dc696

PPPS I get the english version of this paper using this button

image.png.bffee6d4efc2e05422a87f109058eccc.png

... your mileage may vary. If so see the later posts from those who had difficulties..

Edited by PeterH
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Thanks Martin S. I'll be interested to hear about your results. I have had some limited success using black iron oxide with charcoal in a saggar as described above. I'll be looking at combining the technique with a silicon carbide glaze, maybe a SiC underglaze slip (a very intriguing idea!) as most recently posted by Peter H. Peter - I couldn't download a copy of that paper- it seemed to be in Farsi or Arabic, but I have requested a copy from researchnet.

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  • 1 month later...

I've been using a saggar in an electric kiln for a few years now (even wrote a book on it). I'm now also experimenting using charcoal to create a reduction . I had good results in a small saggar (fired to cone 05). I'm going to try higher temperatures aswell. If I succeed it will be in my next book :-) 

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  • 7 months later...
On 3/23/2022 at 3:03 PM, JolandavdG said:

I've been using a saggar in an electric kiln for a few years now (even wrote a book on it). I'm now also experimenting using charcoal to create a reduction . I had good results in a small saggar (fired to cone 05). I'm going to try higher temperatures aswell. If I succeed it will be in my next book :-) 

Hello, what is the book called? 

Thanks 

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