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 structure of gypsum employed. Thus, according to him, the fibrous kinds gives a plaster setting almost instantaneously. The water, he says, penetrates the material freely, setting takes places almost simultaneously throughout the mass. The hydration of each particle is accompanied by an expansion, and under the conditions specified, this expansion being unresisted takes place to the maximum extent, with the result of leaving cavities between the crystals, and producing a set plaster of less coherence and density. On the other hand, where granular crystalline gypsum has been used, setting begins at the surface of each group of crystals before the water has penetrated to the interior; the hydration is in consequence more gradual, and resistance being offered to the expansion of the inner parts, a harder and denser material is obtained. That this expansion contains an element of truth is indicated by the practice of employing the granular crystalline variety for the preparation of moulding plaster.

The explanation appears, however, to be inadequate in several respects, especially in view of the fact that plasters for moulding are reduced to a fine state of division before use. It seems as if this treatment must, in great part at any rate, break up the crystalline aggregates.

In order to discover a more satisfactory explanation, let us examine the results of the chemical analysis of plasters used in commerce. One is struck by the large percentage of water they usually contain. Thus, four samples of ordinary plaster analyzed by Landrin have an average of 90.17 per cent. of CaSO4 and 7.5 per cent. of water, while two samples of best plaster contained 89.8 per cent. of CaSO4 and 7.93 per cent. of water. These numbers do not add up to 100, the difference being due to silica and other impurities of the original gypsum, amounting altogether to about 3 per cent.

It might be suggested that the reason why these plasters set more slowly than completely dehydrated plaster is owing simply to the fact that they contain, apparently, some unaltered gypsum, which serves to dilute the action. Were this so, a similar result, as far as time of setting is concerned, should be obtained with a plaster containing a corresponding quantity of dead-burnt material. This, however, is not found to be the case. The time of setting appears, then, to be connected in some special and peculiar manner with the retention of water by the burnt plaster.

The following explanation of this connection is offered, an explanation only tentative at present, owing to want of experimental data.

The following substances are known:

Gypsum, and set plaster, CaSO4 + 2 H O, containing 20.93 per cent. of water.

Plaster completely burned at moderate temperature, CaSO4, probably amorphous.

Anhydrite and dead-burned plaster, CaSO4, crystalline.

Selenitic deposit from boilers, 2 CaSO4 + H O, or CaSO4 + 1/2 H O, containing 6.2 per cent. of water.

The circumstance that the hot calcium sulphate can crystallize with 1/4 its normal amount of water indicates that for this proportion of water it has a greater attraction than for the other 3/4. Having a similar bearing is the fact that when burned at lower temperatures, gypsum only loses the last portions of water with extreme slowness.

Now, if it be the case that anhydrous calcium sulphate has a greater attraction for the first half molecule of water, then the operation of hydration will proceed very rapidly at first, more slowly afterward. Many such cases are known, e.g., that of copper sulphate. Conversely, if only 3/4 of the water of hydration be expelled during the baking of gypsum, the material obtained should hydrate itself more slowly. For our present purpose it will be convenient to recalculate the numbers given by Landrin (vide supra) so as to make the calcium sulphate and water add up to 100. This treatment of the numbers gives a mean result for the six analyses of 7.68 per cent. of water, the amounts not varying by more than 1 per cent.

It will be seen that the dehydration has never passed the composition corresponding to 2 CaSO4 + HO; indeed, the material approximates more nearly to the composition 3 CaSO4 + HO. It appears probable, therefore, that in the successful preparation of plaster the whole, or nearly the whole, of the gypsum is changed, but that this change does not result in the production of CaSO4, or of a mixture of CaSO4 and CaSO4 + 2 H O, but of a lower hydrate of calcium sulphate.

In the case of the analyses, given by Landrin, of fine plaster for potteries, the percentages of water (8.14 and 8.08) correspond closely to that of a hydrate, 3 CaSO4 + 2 HO, which would contain 8.1 per cent. of water.

Some surprise may have been excited by the fact that the well known method of revivifying hydrated calcium sulphate has recently formed the subject of a patent (Eng. pat., No. 15,406).

The method described in the specification consists in reducing the materials (waste moulds, etc.) to small lumps, and baking between the temperatures of 95° and 300°. It is mentioned that the whole of the water must not be expelled. This is no doubt correct, but it must be effected by regulating the time of baking, since by prolonging the operation all the water of crystallization can be expelled far below 300°. To secure even baking the mass is kept stirred by mechanical stirrers, a necessary precaution, since the operation is to be carried out in an ordinary kiln. The process is stopped when a portion of the plaster is found to set in the required time, a method of regulation which will probably be found to work well in practice. - Chem. Trade Jour.