High Fire Reduction Question

[Devany]

I have a very large gas (propane, we live on an island with no natural gas) kiln that I bought used. It has four burners. I use ^ 10/11 clay and high fire glazes. I have had the kiln about a year and it has gone through 7 glaze firings. I am having issues with the reduction process. All of my previous experience has been in University and Guilds where we were not allowed to fire, so all I could do was buy books and study about firing. I have no problem getting the kiln to ^ 10/11 and everything comes out nicely, but I know that I am not achieving full reduction. In fact every time I close the damper the flames go out completely. I have to open the damper and then relight after waiting a little while.

 

I am not sure how much I need to starve the atmosphere or for how long, nor am I sure if I am actually achieving reduction if I leave the damper open enough for the flames to continue burning. Can anyone here help me with this problem? I am also not sure at which point in the process the reduction should be taking place or for how long. I am trying to do it at the end of the firing process now.

[TypicalGirl]

Hey Devany...

Not sure how much help I'll be, but here goes.

 

First, you can see my typical reduction firing schedule here:

http://squarepegarts.blogspot.com/2011/05/chun-red-results-and-little-more.html

 

It depends a little on what results you're looking for - carbon trap shinos will need reduction pretty early in the process, I *think* copper reds get it later and heavier. Tenmokus and oribes don't need as heavy (I think).

Having said that, it does sound like you're way over reducing. How far from the opening in the kiln (floor?) are the burner ports? You may be able to help that by giving them more ox as the flame enters the kiln rather than inside the kiln.

 

 

If you look at my schedule, you'll see I soak at 1800 f to ensure I burn out all organic matter, which is important to minimize pinholing, carbon coring and bloating. I usually start a light reduction at that point, and don't usually do a heavier one till about 2150 or so.

But I'm still finding my way with reduction.

 

I think its really important to know what type of results you're hoping for and maybe let us look at your firing schedule?

[Devany]

Thanks for your suggestions. I am doing mostly porcelain work with the same glazes I have been using for several years. No shinos, no tenmokus, but a few copper reds. Most of what I do is in the green/blue/white realm, so maybe I should not be so concerned with reduction.

 

I am not sure what you mean by soaking, but in glass I remember that meant to hold the temperature for a while. Is that what you mean? I have no problems with pin holing or bloating, but some of my bellied work does form cracks at the stress point in the glaze firing, not the bisque firing. This has always been typical for me in the forms that I do and using porcelain only makes it more exaggerated.

 

The top of the burner ports are about 3" from the kiln floor.

 

The only results I am looking for is deeper reds (they are coming out more orange) and more general depth of color in the turquoise/green range. The reds are nice, but not crucial to me for most of my work. Unfortunately I have a very large kiln, which means I cannot do a separate firing just for the few pieces I do with red on them. I do use an over glaze on many pieces using John Toki's Shibu Ice. It is a semi transparent glaze that has a slight iridescence to it. It does come out much better in full reduction.

 

Thanks again for your help.

 

 

 

 

Hey Devany...

Not sure how much help I'll be, but here goes.

 

First, you can see my typical reduction firing schedule here:

http://squarepegarts...ittle-more.html

 

It depends a little on what results you're looking for - carbon trap shinos will need reduction pretty early in the process, I *think* copper reds get it later and heavier. Tenmokus and oribes don't need as heavy (I think).

Having said that, it does sound like you're way over reducing. How far from the opening in the kiln (floor?) are the burner ports? You may be able to help that by giving them more ox as the flame enters the kiln rather than inside the kiln.

 

 

If you look at my schedule, you'll see I soak at 1800 f to ensure I burn out all organic matter, which is important to minimize pinholing, carbon coring and bloating. I usually start a light reduction at that point, and don't usually do a heavier one till about 2150 or so.

But I'm still finding my way with reduction.

 

I think its really important to know what type of results you're hoping for and maybe let us look at your firing schedule?

 

 

 

 

[bciskepottery]

You might want to pick up a copy of Nils Lou's "The Art of Firing". Gives an excellent discussion of firing kilns, updraft/downdraft, reduction/oxidation/neutral atmospheres, etc. Another good reference is Mel Jacobsen's "21st Century Kilns".

 

For diagnosing problems/faults, I recommend Hamer and Hamer's "The Potter's Dictionary of Materials and Techniques" and Harry Fraser's "Ceramic Faults and Their Remedies".