for use in potteries are obtained by a treatment which differs in many
respects from that described above for the commoner kinds. In the
first place, the direct contact of fuel or even flame is avoided,
since this reduces some of the sulphate to sulphide of calcium, the
presence of which is in many respects objectionable. Secondly, it is
necessary that there should be a better control over the temperature,
since, as has been seen, if the heating be carried too far the
plaster, if not partially dead burnt, will set too quickly for the
particular purpose to which it is to be put.

The arrangement employed in France is known as the _four a boulanger_,
or baker's furnace. The temperature attained in the furnace itself
never exceeds low redness. The material preferred is the softer kind
of the granular variety of gypsum. This is put in in pieces of about
21/2 inches in thickness. After the baking several lumps are broken up
and examined to see that there are no shining crystalline particles,
which would indicate that some of the gypsum had remained unchanged.
Before use the plaster is ground very fine. This point is of
considerable practical importance. The consistency attained should be
such that the material may be rubbed between the finger and thumb
without any feeling of grittiness. Should there be particles of a size
to be characterized as "grit," these will after use appear at the
surface of the mould, with the result that the mould will have to be
abandoned long before it is really worn out, i.e., before the details
have lost their sharpness.

It is manifestly of considerable practical importance to understand
the conditions which determine the time of the setting up of plaster.
According to Payen, the rapidity of setting, provided the plaster has
dehydrated at a temperature sufficiently low, depends entirely on the