The hardest material is
frequently obtained from the quick-setting plasters, but for certain
purposes this rapidity in setting is of great practical inconvenience.
Thus the moulder in pottery work must have leisure to fill in every
detail of a design often complicated and intricate before the material
with which he is working becomes intractable. Thus for many of the
more refined purposes to which plaster is applied, extreme hardness in
the set plaster is of less vital importance than a convenient period
of setting. On the other hand, plasters which set very slowly give as
a rule too soft a material, as well as being inconvenient in use.
Plasters which hit off the happy medium are alone suitable for the
work of the potter. The finer varieties of plaster prepared especially
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.
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