Question:

How do I do a reduction firing?

How to

The usual method of creating a reducing atmosphere is to close off the flue or chimney. This can be done by laying a piece of kiln shelf or kiln brick across the top of the flue/chimney, or if it is a larger chimney, by pushing in the damper (a piece of tile which can be pushed into a slot in the chimney, which is at an accessible height, reducing the air flow). Reduction firings are generally not recommended for electric kilns, although it is possible to achieve reducing effects by introducing combustible materials (e.g. twigs) into the kiln chamber through the spy hole. But the carbon monoxide tends to attack the kiln elements (electrical coils), which then deteriorate quickly.

Description

In a reduction firing, the oxygen supply to the kiln chamber is restricted, resulting in a saturation of free carbons in the kiln atmosphere, mostly in the form of carbon dioxide and carbon monoxide. In a strong reduction firing, flames will lick out of the flue and through other openings or cracks in the kiln walls. This is because the flame needs oxygen to burn, so it will try to get it from wherever it can. The reduction cycle generally begins between 1600o-1900o F (900o - 1000o C). Typical reduction glazes are 'copper reds' and 'shinos'.

Mixing oxidation and reduction

Sometimes a firing cycle for a particular glaze (e.g. a 'copper red') will include various stages of oxidation and reduction. Re-oxidation brightens up a glaze, although the effect will usually still be different from a pure oxidation firing. Re-oxidation is often done towards the end of a firing cycle, in order to eliminate excessive carbon trapping, which could result in dullness.

History

Pit Firing
Early cultures found clay in the ground and must have discovered its plastic and fired qualities by accident, probably discovering some burnt clay in a camp fire. This very basic firing then evolved into the pit-firing. Pit-firing involves placing unfired or bisque fired pottery in a pit in the ground, then covering the pottery with suitable burning materials, e.g. dried grasses and branches. Depending on the amount of work to be fired, a pit of the appropriate size is dug.

A bed of dry leaves and twigs and possibly coal, which will burn slowly, is placed at the bottom of the pit and the pottery placed on top of this. The work is then covered with more leaves and twigs and dung, if available, building up a mound over the pieces. Once the stacking process is finished, the pile can be lit around the edges and left to smolder for several hours, if not until the next day. Towards the end of the burning process, it is possible to bury the pit in earth or sand, which will cut off the oxygen supply and create a strong reducing atmosphere inside the mound. Not all clays are suitable for such a firing, especially the more refined types available from suppliers. Additions of grog 'open up' the clay and make it more resistant to heat shock. Clays dug directly from the earth may be suitable 'as is', or might profit from additions of grog or volcanic ash, if using a commercial clay, get a clay suitable for raku firings. The best color results can be achieved with iron bearing, or red clays.

What is a kiln

High-temperature furnace used commercially for drying timber, roasting metal ores, or for making cement, bricks and pottery. Oil- or gas-fired kilns are used to bake ceramics at up to 1,760ºC/3,200ºF; electric kilns do not generally reach such high temperatures.