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Raku firing

Working in the snow

At present, we in are very much engaged in ceramics. One of the firing methods that engages our warm interest is Raku-firing.

We operate a propane gas-fired Raku kiln of quite nice proportions. Creation of a Raku-work requires some special precautions, due to the short heating time, about 1000 °C in about 30 minutes (!). Each Raku firing is a spectacular happening. A very active and exciting way of making art!

Technical information: how to make a work suitable for Raku firing

Two types of clay are suitable for Raku: clay with 30-40% chamotte of at least 0-1 mm grain size and clay with at least 20% silversand. Usually the work is bisque fired in an electric kiln to improve its manageability. It is possible to Raku fire without bisquing the work, at some risk of destroying it.

Maximum size of the work is determined by kiln dimensions and manageability with the long tongs used to place the work in the kiln and to take it out again when hot.

The walls cannot be very thin, as the work will collapse during the quick heating. Too thick walls will explode or peel down due to temperature gradient in the wall, resulting in differences in thermal expansion. About 10 mm will do nicely, as uniform as possible.

The work is glazed with a special lead glaze with a relatively low melting point, dried and then fired. The glaze is molten and covers the clay body. Upon reaching that state, the work is lifted out of the

In the open air, the piece cools quickly. The glaze solidifies and, since it is not free to shrink it crazes. You actually hear cracks in the glaze develop. The so-called craquelé is a special characteristic of Raku work.

kiln with long tongs - vivid red hot and radiating quite a bit of heat.

When the cracking is sufficiently advanced, after perhaps a minute, the work is placed on a heap of combustible material, such as sawdust, shredded newsprint, sheep wool or similar. This stuff immediately catches fire.

Quickly a metal container is placed over the work, completely isolating it from the air. The fire cannot continue, due to lack of oxygen, but it starts to smolder, developing a thick black smoke in the container.

This is the so-called "reduction" stage. The black smoke and soot enter through the cracks in the glaze and colour the clay body intensely black in the typical Raku crack-pattern. Any clay area not covered by glaze will be an indelible uniform black after the reduction.

For the reduction, a wide variety of materials such as sawdust, shredded paper of all kinds, fragments of (natural) textiles, dried leaves, twigs, toilet paper, oil-impregnated tissue paper, hair cuttings, seeweed can be used.

After say half an hour of reduction the piece is taken from the container, cooled down and then cleaned by brushing it with water. This develops the final appearance of the work.

The results of a Raku firing are usually quite unpredictable - which precisely is the fun of this technique. Sometimes very beautiful pieces "just happen".