The clays should be as pure as possible, and they are selected and mixed in such proportions as will give a paste easily worked and behaving well under drying and firing. They are then subjected to. a preparatory treatment, which is more or less thorough according to the quality and degree of finish required: crushing, cleaning, drying, blending, etc. The machines used are the same as those previously described.
The paste is divided into fragments called balls \ sometimes it is roughly shaped into slabs called crusts. Balls or crusts are placed in the mould and pressed down with the hand until the clay takes its shape. Care must be taken to compress it evenly, for fear of ruptures in drying and firing.
When the pieces to be manufactured are of complicated form, they are moulded in parts, and the parts are afterwards united. Demoulding is facilitated by the use of moulds in several parts which are taken off separately. Compact pieces are always hollowed inside to lighten them and ensure a uniform drying. In pieces which are moulded in parts, we must take care that the join does not pass through delicate portions, and for this reason we must study the most suitable form to be given to the moulds.
Pieces which are symmetrical about an axis, such as balustrades (Figs. 653, 654), are made with the potter's wheel. The hollow inside is produced by means of a wooden axis surrounded by a string on which the paste is laid, and the outer surface is formed by means of a calibre cut according to the design of the balustrade.
Drying is carried out in storeyed drying - sheds, which are heated when necessary. Firing is done in various kinds of kilns, which are sometimes specially constructed when the pieces are of exceptional dimensions, such as the pyramids of the grand entrance of the Palais des Arts Liberaux at the Exhibition of 1889. These fine products, designed by the talented sculptor Michel, were executed by the ceramist Muller. One of them, Labor, is represented in Plate II.