Sunday, October 02, 2005

The Raku Kiln

I've been thinking a lot about the raku kiln. It will be a while before we absolutely need one, since we still need to fill the electric kiln with greenware and bisque.

Behind the house, between our porch and a storage shed, is a small patch of gravel. The branches of one of the larger pines, a bit up our sled hill, overhangs the area. This is where I want to place and fire the raku kiln.

I'll use a pad of cinder blocks to keep the kiln off the ground. The air gap in the blocks reduces heat loss into the earth while the kiln is firing. On top of the cinder blocks, will be a square of loose kiln brick; this forms the floor of the kiln. The kiln brick needs to be loose, so I can put them into a box, and shelter them from the elements when the kiln is not in use. Around the outer edge of the kiln floor will be placed a double layer of brick, to form a wall. There will be a gap, half a brick wide, in the wall, to allow flame from the venturi burner to enter. A half piece of brick will be in the center of the kiln, turned so that one corner faces the opening for the burner. This will cause the flames to spread as they enter the kiln. The corners of the kiln wall will be truncated; stepping in to help support a kiln shelf that the wares will sit on while being fired. The there is a gap between the inside edge of the kiln wall and the kiln shelf, to allow heat and flames to rise.

On top of the kiln wall will sit a metal frame supporting a cylindrical ceramic fiber kiln. The kiln will be constucted of galvanized hardware cloth; a wire mesh where the wires have been spot welded together at the crossing points. The ceramic fiber will be held on the inside of the hardware cloth with ceramic buttons on the inside of the kiln, with high temperature wire attached to the buttons, passing through the ceramic fiber, and then passing through and twisting around the hardware cloth. There will be handles wired to the hardware cloth to allow the kiln to be lifted. There will be a thermocouple port, made of high fired clay, attached to the side as well. The top of the kiln will be a disk of fiber with a vent hole cut in, supported either by hardware cloth, or a galvanized metal trash can lid. The top of the kiln can either be affixed to the rest of the kiln, or detachable so that the top can be lifted as a lid.

The burners for the kiln will be a propane fuelled MR750 raku system, from wardburners.com. It has the venturi burner, hoses, regulators, valves, and gauges to hook up to a propane tank, and control the firing of the raku kiln. I've read articles about people using weed burners to fire their kilns, but I want to have more control than a standard weed burner setup will provide.

Pieces will be placed on the kiln shelf, after they have been bisqued and glazed. The thremocople from my gas kiln will be threaded through its port. The fiber upper kiln will be lowered in place, and the burner will be ignited. It will take about forty five minutes to bring the kiln to cone O7, about 1790 degrees F. Through the peeps and vents in the kiln, the work will be orange hot, and some glazes will be shiny.

While the kiln is firing, the reduction chambers are prepared. These are metal containers that have been filled with combustible materials, like shredded newspaper, cardboard, straw, leaves, or pine needles. The chamber itself can be an unused paint can, galvanized bucket or tub, or a gift canister.

When the pieces are ready to be removed, the gas is shut off to the kiln, and the pyrometer is carefully removed and placed on a non-combustible surface. If I am firing with helpers, the entire fiber kiln will be removed, and placed on waiting cinderblocks topped with dry kiln bricks, and the pieces will be removed in a choreographed pounce. If I am working alone, only the top of the kiln is removed, so that the kiln doesn't lose all of its heat as pieces are removed.

If a piece has a metallic raku glaze, like a copper green, the piece is immediately moved into a reduction chamber, using long handled tongs, and the chamber is sealed once the flames start. The combustion inside the reduction chamber uses up the available oxygen, then starts to pull oxygen out of the glazes on the piece. A copper glaze can turn from green to a metallic copper surface, like a new penny. The piece is left in the chamber until it is cool enough to touch.

A glaze like Seth's Luster can turn multiple colors, including copper penny, dark blue, light blue, and magenta. All of the colors have a metallic sheen. The glaze isn't very shiny, due to the tin in it, which causes the surface to be more pebbley. The different colors on the piece are caused by different thicknesses of oxides forming in response to changes in temperature. This temperature difference can be encouraged by 'flashing' the piece in the reduction chamber. This is done by lifting the lid off the chamber to release a billow of greasy green smoke, which bursts into flames. The flames licking against the piece encourage temperature gradients, which induce the colors. The lid is placed back on the piece after the flash occurs, and then the piece is allowed to cool.

A piece with a crackle glaze is allowed to cool a bit before it is placed in a chamber. You can hear the glaze pinging an crazing from thermal shock. After it has been removed from the kiln with tongs, some potters spritz the piece with water, or blow on it to encourage the glaze to crack. The piece is placed in the chamber, flames ensue, and the lid is closed. Carbon is driven into the cracks in the glaze, emphasizing them. Too much reduction can cause the whitening agents in the glaze to go transparent, so it is helpful to either not seal the chamber very tight, or to allow the piece to cool a bit before placing in the chamber.

There are some conventional, low fire glazes that I have used sucessfully in raku firing. I have used a fire engine red glaze on the fins of some of my rockets, and it has matured successfully in the short firing time.

Once the pieces have cooled, the reduction chambers can be opened. If the glaze of a piece has touched the combustable materials, an imprint can be left in the glaze. Anything unglazed on the piece will turn matte black, from where carbon is driven into the piece. This is typically the foot of the piece, but the artist can artistically leave portions unglazed and get the matte black. The pieces need to be scrubbed a bit to remove ash, tars, and soot.

1 Comments:

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