TECHNIQUES OF RAKU FIRING

RAKU GLAZE FIRING

Raku is a Japanese technique of glaze firing closely linked to the Tea Ceremony. It involves removing the glazed pot from the kiln when the glaze is molten (around 900 degrees centigrade). The glazed pot is then put into a reduction chamber (usually an oil drum or galvanised bucket) and covered with sawdust. This action gives the typical black crackle caused because the glaze cools quickly and shrinks, the sawdust catches fire and the smoke marks the pot along the line of the crackle. The act of  covering the pot with sawdust also cuts off the oxygen to the pot, but because it is still very hot it takes oxygen from the oxides in the glaze to enable it to keep burning. This chemical action caused the copper in the glaze to come to the surface of the pot, giving a lustrous finish. The amount of crackle/lustre is dependent to a large extent on the ambient air temperature and the amount of time the vessel is left in the reduction chamber.

COPPER FUMING

Copper fuming is achieved by the application of a 90% copper oxide and 10% glaze mixture onto the surface of the pot, which is then fired in the normal Raku way. This gives beautiful vibrant colours and a velvet textured finish (sometimes!). It is notoriously difficult to control.

NAKED RAKU

This term is given to pots which have had the glaze removed after firing. To make this possible the pots are coated with slip (liquid clay) at leather-hard stage and  burnished with a pebble until shiny. The pot is then biscuit-fired in the normal way. A second slip is then applied to the pot and after 24 hours it is glazed. Following the normal Raku firing the pot is removed from the reduction chamber and the glaze will (hopefully) come away easily. Black crackle marks are then revealed. The residue of the slip is removed and a coating of wax is applied to protect the pot.

Because Raku is a low temperature firing technique the body does not fully vitrify and even the glazed pot may be porous. If one is required to hold water then it is suggested a plastic bottle be cut to fit and inserted inside the pot. This works very well. Also strong sunlight may fade the carbonisation – it is (as one very well known Raku potter suggested) best to treat Raku pots as you would a fine painting.