Saturday, May 30, 2009

Firing the wood fired kiln 03

Well, it is 10.45 am and we are at 970 C (1778 F). Things are going well so far, and the weather is nice at the moment, with blue sky and a strengthening breeze from the south.

The kiln firing in oxidation, there is little visible smoke.

Firing in Reduction results in some smoke,

and a hazy oxygen starved atmosphere in the kiln.

At around 800 C (1472 F) I went from firing the kiln in oxidation (enough air to burn the fuel efficiently with a bright flame and almost no visible smoke) to firing in reduction. Reduction is where the kiln receives more fuel than it can efficiently burn, and the atmosphere in the kiln becomes hazy and the flame in the firebox looks orange and lazy. Smoke comes out the chimney, and also through any gaps in the kiln.

Starving the atmosphere in the kiln of oxygen has a profound effect on the colour of the glazes and of the clay that the pot is made of, because the hungry flame will take oxygen from where ever it can, including the glaze and the clay. A glaze with some copper in it, would normally fire a green colour in oxidation. In reduction the same glaze is likely to turn red. A glaze with 1 to 3 percent of iron oxide in it, would probably give you a honey colour in oxidation. In a reduction firing the same glaze could turn celadon green. I have some glazes in this firing that I would like to be copper red, I have some celadons too, and I also have some carbon trap shinos, which, if I am lucky, should have gray carbon sealed into the glaze.

Firing in reduction is fairly easy in a wood fired kiln, just add more fuel to the fire box than will burn cleanly. The trick is to ensure that the temperature will still keep on rising, and that you don't waste an unnecessary amount of timber. I control the temperature rise by adjusting the chimney dampers. Just half an inch in or out with the damper can make the difference between the kiln just sticking at a temperature, or rising.

We fired from 840 C (1544 F) to 900 C (1652 F) in an hour under quite strong reduction. That is about the point where some of the glazes are turning from a powder to a glaze. If you fire in reduction at that stage, the flame can drag oxygen from deep in the glaze. If you leave reduction too late, than only the surface of the glaze is reduced.

We are going faster now, and burning the wood more efficiently.


Pat - Arkansas said...

Firing... 01,02 and 03 are fascinating!!! Text and photos!! When I looked at the photo in Firing... 01, it brought to mind Edgar Allen Poe's "A Cask of Amontillado," and the first movie version of "The Canterville Ghost" (1940's - Margaret O'Brien/Charles Laughton) in which bricking folks up behind walls seemed to be the punishment of choice. At least they didn't have horrendous heat applied after the bricking-in! :)

I enjoyed reading your explanation of changes to the various glazes under oxidation/reduction firing. I hope I am learning to have a better appreciation of how beautiful pottery is made; thank you, Peter.

Linda Starr said...

Hi Peter, I am really enjoying your explanations. This is so much fun, it's almost as if I am right there, especially with the hazy shot of the inside of the kiln. I didn't realize oxygen was taken from the clay too during firing. I hope you get good celadons, reds and shinos.

Peter said...

Thanks Linda, it is actually quite fun attempting to do a commentary. I did think that I wouldn't be able to manage it and firing the kiln, but Laura is developing some good stoking skills, and was able to take over whilst I was at the computer. It was also very restorative being able to have a short break as the glare from the firebox, smoke and heat, can certainly drain the energy.

Any iron oxide in the clay can release its oxygen when the kiln is in reduction, the result is black iron, rather than red iron, thus stoneware clay that is fired in reduction is often cool and gray in colour (except where it gets toasted by the flame). In oxidation the same clay would be warmer in colour. The other nice thing that can happen to stoneware clay in reduction if it contains some iron oxide, is iron spotting.

Peter said...

Hello Pat, What an interesting injection of literature into the smoke and grime of a firing! Well done! Fortunately only pots bricked up in my kiln, though I did check first for cats, mice and hedgehogs!

ronald said...

How do you treat the cooling cycle?

Peter said...

Hi Ronald,
Welcome to my site. Regarding the cooling cycle, I usually let the fire burn out after getting to around my cone 11 target, then allow the temperature to fall quite rapidly until around 1100 degrees Centigrade. At that point I begin to block off the fire mouth and adjust the chimney dampers to about half way. After that I find that my kiln takes between a day and a half to 2 days to cool to 200 degrees Centigrade. At that point I open the chimney dampers fully, and once around 150 C, I begin to remove bricks from the doorway and ash pit. I have experimented with clamming up the kiln as soon as the fire starts to die down after cone 11 is reached. This produced some spectacular blue violet crystals in the bottom of some small copper red glazed bowls that were close to the arch brickwork, but it did make one sort of liner glaze I used in some pots quite rough with excessive crystal formation. I have also crash cooled the kiln to just above 1000 degrees C, but worry about doing this as I think it is putting some of the lower kiln shelves at risk due to cool air rushing in.