The view generally adopted with regard to the theory of these processes is that plaster consists of anhydrous calcium sulphate, CaSO4, in a condition probably amorphous, different from that of natural crystallized CaSO4, known to mineralogists under the name of anhydrite.  By the influence of a high temperature it appears probable that a molecular change is gradually induced with production of a crystalline structure, and probably an increase of specific gravity, resulting in the artificial reproduction of the mineral anhydrite.  No determination appears to have been published of the specific gravity of plaster prepared by complete baking at a low temperature.  The theory is, however, confirmed by the results obtained by workers on the subject of mineralogical synthesis, who have shown that the material which has been produced at high temperatures has the specific gravity and other physical properties of the mineral anhydrite.

It was formerly supposed that plaster prepared by baking at a temperature above 300 degrees loses completely its power of setting.  Later observations, however, as those of Landrin, negative this view.  Between 300 degrees and 400 degrees Landrin obtained plasters setting almost instantaneously when mixed with a small amount of water.  When the temperature employed approached 400 degrees, the set plaster was softer, but the setting still took place quickly.  These observations appear to show that the change to anhydrite is a very gradual process at temperatures below a red heat.

Reference has been made to the differences in (1) time of setting of plaster and (2) in hardness of the resulting material.  Both of these properties are affected by the mode of baking.  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.