Pit firing can be termed a 'primitive' firing technique, although
this is not meant in a derogatory way. The word 'traditional' can
certainly also be used in the case of many cultures, that were amongst
the first to discover this simple firing technique. It is still
widely used on many continents today, but it the sort of thing you
can easily do in your own backyard and it enjoys popularity with
some studio potters.
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. Not all clays are suitable to use in 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.
It is a good idea to bisque fire the work first, as this helps
to prevent shattering and cracking. Pit-fired work is usually not
glazed but rather burnished before the bisque, or decorated with
washes of black or red iron oxide, copper carbonate and mixtures
of these, after bisque. Color effects can also be achieved by spreading
oxides and carbonates around the pieces (particularly copper carbonate),
which volatilize and result in flashes of color appearing
on the fired work. Similar effects can be achieved by wrapping copper
wire around a pot.
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.
This description is not meant to be a complete instruction on
how to pit fire, but rather an inspiration for you to find out more
about the techniques involved in the process. Books on the subject
may help, but a workshop is even better!
Additional Notes on Pitfiring