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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, which also resists severe temperature differences. 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.