firing "the BISCUIT"
term is used to indicate an object modelled with clay and
baked the first time at a temperature oscillating between
980°C and 1000°C. Before the clay objects are inserted
into the oven for the first baking, they must be completely
dry so as to avoid breaking during baking. The pieces are
placed on shelves of refractory material, which are supported
by small columns. During the first baking the objects in the
oven may even touch each other, but this must be absolutely
avoided when they are enamelled, because the melting of the
enamel may cause the objects to stick together.
The features of a high-quality biscuit are: rosy, lightness
and sonority. If the colour is dark and the sound is dull
it means that the biscuit is not well baked; if the colour
is yellow-green or greenish it indicates an excessive baking
and the objects will probably present a deformed aspect, because
the clay is melting.
In the second case, the biscuit would have little absorbance
and it would not absorb the necessary quantities of enamel
during the enamelling phase.
The baking temperature is checked through suitable pyrometers,
which precisely indicate the degrees inside the baking chamber;
in ancient times it was customary to use biscuit specimens
covered with enamel which were extracted while checking the
melting, or else the temperature was controlled according
to the colour inside the oven. This control was carried out
through specially provided lights.
The temperature check is accomplished also through cones which
are composed of a mixture of clays and melting materials called
pyroscopic cones, i.e. a pyramidal element which is about
5 cm high, that softens and bends when the oven reaches the
melting temperature, thus warning us that we are approaching
the desired temperature.