The first point to which it is well to direct the manufacturer’s attention is the preparation of the plaster moulds.  When it concerns an object of large dimensions, of a vase a yard in height, for example, the moulder is obliged to cut the form or core horizontally into three parts, each of which is moulded separately.  To this effect, it is placed upon a core frame and surrounded with a cylinder of sheet zinc.  The workman pours the plaster into the space between the latter and the core, and, while doing so, must stir the mass very rapidly with a stick, so that at the moment the plaster sets, it shall be as homogeneous as possible.  In spite of such precautions, it is impossible to prevent the densest parts of the plaster from depositing first, through the action of gravity.  These will naturally precipitate upon the table or upon the slanting sides of the core, and the mould will therefore present great inequalities as regards porosity.  Since this defect exists in each of the pieces that have been prepared in succession, it will be seen that when they come to be superposed for the moulding of the piece, the mould as a whole will be formed of zones of different porosities, which will absorb water from the paste unequally.  Farther along we shall see the inconveniences that result from this, and the manner of avoiding them.

[Illustration:  FIG. 1]

The mould, when finished, is dried in a stove.  Under such circumstances it often happens that there forms upon the surface of the plaster a hard crust which, although it is of no importance as regards the outside of the mould, is prejudicial to the interior because it considerably diminishes its absorbing power.  This trouble may be avoided by coating the surfaces that it is necessary to preserve with clear liquid paste; but Mr. Renard advises that the mould be closed hermetically, so that the interior shall be kept from contact with warm air.  In this way it is possible to prevent the plaster from hardening, as a result of too quick a desiccation.  I now come to the operation of moulding.  In the very first place, it is necessary to examine whether it is well to adopt the arrangement by pressure of air or by vacuum.  The form of the objects will determine the choice.  A very open piece, like a bowl, must be moulded by vacuum, on account of the difficulty of holding the closing disk in place if it be of very large dimensions.  The same is the case with large vases of wood form.  On the contrary, an elongated piece tapering from above is more easily moulded by pressure of the air, as are also ovoid vessels 16 to 20 inches in height.  In any case it must not be forgotten that the operation by vacuum should be preferred every time the form of the objects is adapted to it, because this process permits of following and directing the drying, while with pressure it is impossible to see anything when once the apparatus is closed.

[Illustration:  FIG. 2.]