Biscuit - pottery
: First (low) firing (before
glazing).
For stoneware we fire around
900°C. The clay is then still porous, allowing the unfired glaze to
adhere.
Chamotte or grog: crushed fired clay, which can
be then mixed with plastic clay.
Grogged clay : Clay with grog
Flash-glazing : We
fire with pine-wood, making lots of flames that pass through the pots.
There is often much fly-ash in the kiln which, falling on the pots will
leave a "natural" glaze, often of interest - unless it falls on and
obscures a decoration!
Frost
damage : Porous clay bodies will flake
or break after freezing.
Overglaze:
A transparent glaze put over a design, allowing the colours to show
through.
Paper clay : A mix of clay and paper in
different proportions depending on the purpose. Allows easier work with
sculpture (collages ...).
Pyritic clays: contain nodules of iron sulfate.
Temmoku : Iron saturated glaze giving
blacks and red-browns.
Thermal
expansion : Dimensional changes in body and
glaze during firing.
Should thermal expansions be
too large between body and glaze, the tension created could break the
pot.
Raku: A
firing technique from Japan.
The pots are fired and glazed as for
stoneware but the second firing is only at around 900°C. The
pots
are then taken out with tongs and plunged into saw-dust or water to
cool and/or reduce them. One can also use paper or anything that burns.
The contact with flames on different parts of the same pot can give
various colour variations so appreciated in raku
NB: One must choose glazes that are receptive to colour changes in
oxidation/reduction.
Depending on the degree of reduction; copper, for example, can vary
between different blues, greens and reds, even to its own metal colour.
Crazing:
This can
be done using a monochrome glaze that does not change colour under the
smoke treatment, or using a versicolor glaze. One must let the pot cool slowly until the tinkle of
glaze crazing begin. The piece can then be recovered with sawdust or
wood-shaving, the heat still being sufficient to burn the wood and
blacken the crack marks without changing the colour of the glaze.
With such brutal temperature changes we cannot guarantee the
imperviousness of raku.
In fact, all pots fired at low temperatures such as raku or earthenware,
the body stays porous,
and watertightness cannot be assured except with perfectly fitting
glazes, which is unlikely with raku.