Sunday, August 6, 2017

How the wood fired kiln works!

Back in the mists of time, when I last posted I did promise to upload some photos and information about the kiln and how it works. The kiln is rather unusual, but is showing signs of working very well, and I do hope that some of the design ideas might help someone else to make a kiln.

I think that the ladder type grate shows a lot of promise, and may well make a very welcome alternative to someone who has battled with a more conventional design. The other refreshing thing for me was to find that the single firebox with entry flue at the back left hand corner made it perfectly possible to have an even heat distribution front to back in the kiln, and only a 2 cone variation from top to bottom. Having the shelf over the firebox may help with this, and it is certainly a useful "bonus" place to put an extra pot or two in the kiln.

Chamber with front shelf removed to show flueway to chimney under shelves. Inlet flue is visible at the left rear of the chamber.

Loading kiln. Back shelf stack full, front stack incomplete.

Loading kiln. Front and back stacks of shelves full.

Chamber door bricked up 6 inches thick with 2 layers of insulating fire brick on edge.

Fire box door in place. Small fire lit under ladder grate at beginning of firing.

Chimney damper on left used to control the draught through the kiln. Right side of chimney no longer in use since alterations to the chamber.

Kiln in daylight with metal drum of kindling rather blocking the view!

Kiln firing at night. The first firebox door was an old ceramic fibre lined dust bin lid!

In February of 2017 I demolished the external firebox and stripped out the interior of the kiln. The following are photos that I took of the rebuild.

Kiln stripped out leaving arch and back up insulation bricks.

Floor of chamber was made on a foundation of concrete blocks that are laid on their sides.

The chamber floor consisted of one layer of standard fire bricks over a layer of insulating fire bricks. On the left the firebox is under construction.

The floor of the ashpit is two fire bricks thick to help protect concrete foundation slab from excess heat.

The actual floor of the firebox has an air gap under it. I used some large second hand fire brick slabs to make this floor.

Fire box nearing completion. Large fire bricks make a ladder grate.

Firebox from the front. "Ladder" just visible.

Heavy large fire brick slabs form the top of the firebox.

Relining the chamber side walls and back wall is complete and the permanent left hand front wall is under construction.

The left side of kiln almost complete. Concrete blocks that are under the chamber will be left "open" so that any build up of heat inside them is allowed to escape.

A kiln shelf covers most of the firebox inside the chamber. A gap is left in the rear corner to form an inlet flue. I initially thought I would use a single stack of 18 x 24 inch shelves in the chamber, but decided that 2 stacks of smaller shelves would make stacking the kiln considerably easier, the shelves less likely to bend or break, and offer more versatility.

Hope the photos have been of help to someone. Apologies if they have been rather slow loading due to their quantity, but I thought it useful to have them all in the one place.

I am happy to try to answer any questions regarding this kiln, but do regard what I have made as a starting point! The shape and size of the kiln was dictated to some extent by the kiln shelves and other materials that were available to me. You may well have other materials on hand, so could build quite differently.

Wednesday, May 24, 2017

Blessings from the Kiln!

Detail of a wood fired copper red bowl from the recent firing of the kiln.

 I am often a little depressed before the opening of a kiln, which may seem odd or be odd... I couldn't possibly say. There are just so many unknowns and variables when glazing and firing pots, especially in a wood fired kiln, that I can only do my best with everything I do, the making, glazing, packing and preparing of the kiln, and the firing, and then wait for 2 or 3 days for the kiln to cool, and the jury to reach its verdict!! The waiting is not an easy time, but it is a time when studio floors can be cleaned, shelves in the store room organised, and some glazing materials put away.

Most of the glazes used in this firing were copper reds, Shinos or celadon. All require an atmosphere in the kiln where oxygen is in limited supply for transforming magic to happen to them, and it is like magic! Green copper carbonate can be transformed to cherry red or pink, a rather nondescript white Shino glaze can become rust coloured or golden, and a honey coloured iron bearing glaze becomes green or grey green, or even blue-green celadon.

Copper red, pink and celadon glazes.

 The firing has been a careful one, abundant oxygen until about 800 Celsius (1472 F) was achieved, and then a deliberate choking and slight over stoking of the kiln until temperatures had climbed to their peak of about 1300 Celsius (2372 F). Firing was about temperature, and time taken to get there, but also about atmosphere, and atmosphere was monitored by eye. Was the chimney smoking, did flames come out of spy holes under pressure, or was air sucked in? Was the flame in the chamber of the kiln hazy, or was visibility in the chamber clear and bright? These are the sort of things that had to be constantly observed and considered throughout the firing.

So there is the firing, and then the waiting for the kiln to cool. Days pass. It is winter and snow falls. There is sleety rain and wind.

Laura having a peep into the kiln.
On the third day after the firing the kiln door is unbricked carefully, one brick at a time. The opening breathes warmth. Hands are put into the darkness of the kiln chamber to check pots for warmth. It is the first contact with the pots. The surface is smooth with a slight dust of wood ash. The pots have a lingering warmth that is almost blood heat. More bricks are removed, and weak sunlight enters the kiln and there is colour and form.

As to the work in the kiln... most things turned out well, or very well, and there was a copper red bowl and a Shino pot that went above and beyond anything I could have anticipated and I will hold onto them for a while and enjoy them!

Bowl approximately 30 cm diameter (12 inches). Copper red and Shino glazes. Stoneware.

The thick copper red glaze was extraordinary.

Stoneware pot with poured Shino glaze. 29 cm high (11.5 inches).

 Those two are keepers. For now!

Shino and celadon platter. 30 cm (12 inches). Stoneware.

Shino platter with sprinkled wood ash decoration. 30 cm (12 inches). Stoneware.

Copper red vase. 26.5 cm (10.5 inches). Porcelain.

Detail of copper red showing pale purple flecks in the glaze.

Copper red and celadon with splash of copper/rutile blue. 18 cm (7 inches). Porcelain.

Carbon trap Shino bowl with copper/rutile blue splash. Dia. 11.5 cm (4.5 inches). Stoneware.

Carbon trap Shino glaze over porcelain, the result is much quieter.  24 cm (9.5 inches).
Inside the pale Shino vase, the same glaze is much warmer in colour.

Vase with poured black Shino glaze. 22 cm (8.75 inches). Stoneware.

Sculptural vase. Copper red glaze on porcelain. 22.5 cm (9 inches).

I was delighted to be contacted by Jane, a potter living in Vancouver Island B. C. when I was glazing pots for this firing. She asked for some thoughts about a tricky copper/rutile blue glaze that she was using, sometimes with great success, but not all the time! She sent the glaze recipe with the email, so I was able to try it and see if it worked for me.

The results were most interesting and rather beautiful, and I have greatly enjoyed corresponding with her by email about the glaze. From time to time the internet can be a wonderful thing, and it is very nice to feel part of a much wider community of potters and other interested people from around the world. It has been a joy to have met some of the readers of this blog too when they have ventured to the East Coast of the South Island of New Zealand.

I recently watched a documentary about an 83 year old Korean potter and his sons who were making celadon glazed pots. The documentary was made a few decades ago and the colour is somewhat faded, but I found it quite a moving glimpse of the life of potters working in a traditional manner, where making and firing pots was really a form of prayer. The heart and life and work of the potter was conducted in a spiritual manner, in a way that goes far deeper than a particular religion or dogma.

*Technical Notes about the firing (for those who might be interested in such things!!).

All areas of the kiln had good reduction, and the temperature variation from top to bottom of the kiln was about 2 cones, from an estimated cone 11 at the top to cone 9 at the bottom, and most of the kiln being an even cone 10. In every day terms, that is about a 30 degree Celsius (86 F) temperature variation. This was a marked improvement from the firing before that had a 3 cone variation from top to bottom, or something in the order of 45 degrees Celsius (113 F). The improvement had been achieved by spacing the lower shelves wider apart than in the first firing, so as to let a greater flow of flame through the setting there.