MATURING TEMPERATURE is the temperature or cone at which a
body has acquired the hardness or density necessary to its intended use.
For pottery this means 3% absorption or less while for sculpture it
might mean 8% or more. Many potters working in the low-fire range
or with fireclays never bring their bodies to maturity. Deformation
and sticking are always a danger with bodies requiring low absorption,
more about that at a later date.

FIRING RANGE is the temperature spread over which a reasonable
working maturity may be had. For low-temperature clays the firing
range is usually only about 2 cones while for high-temperature stone-
ware it can be as much as eight or 10 cones.

LOW FIRE generally refers to a red-burning clay that matures some-
where between cone 08 and cone 4. The low maturing temperatures
are the result of the combination of iron and sodium or potassium in
the clay crystal; such a clay is obviously not made up of kaolin as is
china clay or ball clay but rather of illite or chlorite. To achieve
such a low maturing temperature in a white body requires the addition
of so much non-clay fluxing material to kaolinitic clays that the body
loses much of its clay quality.

Because of the chemistries involved, low-temperature clays have
relatively short firing ranges, the lower the maturing temperature the
shorter the firing range. Natural clays with maturing temperatures
below cone 04 are not common and potters working in that range
usually have to be satisfied with porous bodies and crazing glazes.
The firing range shows very marked improvement in bodies maturing
at cone 02 and higher,

If it becomes necessary to lower the maturing temperature of a low
temperature clay a body frit or nepheline syenite may be added. 5% or
10% of body frit may be enough to do the job and workability should not
be seriously affected, but the firing range will be further reduced.
Nepheline syenite is effective in lowering maturing temperatures down
to about cone 04 but anywhere up to 30% may be required, seriously
reducing plasticity. Any attempt to improve the plasticity by the

addition of ball clay will elevate the maturing temperature. The most

reliable low-fire reds mature above cone 04 and are made of 7
natural clays and sands. °