The materials mixed with it are fire-clay (burnt and ground to an impalpable powder), emery, rotten-stone, hammerscale, etc, etc, the only object being to obtain a very fine and fire-resisting powder. Old crucibles powdered up are highly recommended by no less an authority than Cellini. Some material has also to be used that will bind this powder together. The celebrated founder, Jean Balthazar Keller, used white of egg and horse-dung. Cellini preferred rotten rags and cow-dung. The result sought for and attained in either case was the same. The body of the mould was composed of a very fine fire-resisting powder, held together by some other less refractory body, the result of which was that the mould, when tolerably dry, was firm and solid enough, but when exposed to a great heat it became very friable and porous. These moulds took a long time to make, but the operation Was not one that required great skill on the part of the workmen employed.

This is the method of construction: - A sufficient quantity of the material having been prepared of the requisite degree of purity and firmness, a small quantity of it is placed in a mortar, and ground up until it becomes of the smoothness and consistency of oil paint. Armed with a soft bristle brush, and with a pot of this paint, the assistant proceeds to give the work one complete coat - an even, thin coat, just as if he wished to colour it. This is allowed time to dry,, when, if it has been laid on thinly and evenly, it will be found to have dried without a crack. A second coat is now laid on in the same way, and with the same care, as thinly as possible. This is again allowed to dry, and so on for about 30 coats. After the first 5 or 6 coats, however, a portion of cow-hair may be worked into the paint, and it may be spread a little thicker. After about 30 coats have been given, there will be a thickness of about } in., and the mould may now be completed by building up in thicker layers until a sufficient strength is obtained. The mould must be hooped about in all directions with iron bands to strengthen it. After this, it being apparently pretty dry, a temporary kiln is built round it, and a gentle fire is kept up until the wax flows out through the drains before mentioned.

A considerable quantity of wax will be lost, but some of it may be saved by placing tubes at the drains to lead it outside the kiln. This operation is usually, but not invariably, conducted in the pit beneath where the metal is to be melted.

When the wax has ceased To run, the fire must be let out, and the kiln taken down, in order that the drains may be stopped. These must be plugged with great care with the same material used for the mould; the kiln is then built up again, and the firing is carried on as before, until the wax has been completely dissipated. When there seems to be no trace of wax left, the fire must be still kept up, and indeed urged on.

until the mould and core are both of a line cherry-red, after which the fire may be let out, and the whole allowed to cool slowly down. If the mould is at the right heat, it can be seen on looking down the openings of the vents and jets, or on putting a piece of tarred rope down, when it will take fire if the heat is right.

The kiln being cool enough, it must be pulled down, and the pit must be filled up with fine dry earth, pressed firmly and evenly all round the mould, so that the top alone, showing the openings of the jets and vents, is visible above the ground. Be the work small or large, this process of firing is exactly the same, only that a mould for a very small figure can be fired in about 8 hours, and a statue may take 5 weeks.

tor small works it is not advisable to use a luto mould. Compo is quite good enough, any mixture of which plaster and brickdust are the chief components. Many founders use for the moulds exactly the same composition that they use for the core, and it saves trouble to do so. It if better practice, however, to use a somewhat stronger compo for the mould. Some make an inner skin to their moulds, of a closer and harder composition than they use for the backing up, and this is in accordance with theory and good practice. But unless the heat is carefully regulated, these layers sometimes separate in the firing. We will suppose, however, that the work has been successfully fired, and is in the pit earthed up and ready to cast.

The next consideration is the furnace. This may be made in many different forms.

First in power and capacity is the reverberatory furnace, with its saucer-shaped hearth, and its flat-domed roof. It may be made of every varying size, but it is inconvenient when made for small charges. These furnaces are a very ancient invention, but until the end of last century they were made without any chimney, the draught being supplied through two or three long trenches, with the aid of wind-sails. Modern practice has dispensed with these last, and added a tall chimney, besides flattening the dome of the roof very considerably.

The furnace most suited for small works in the artists' studio is, however, the common air-furnace for crucibles. This furnace is cheap and easy to build, and takes up but little room. It is usual to build a row of them in the form of a low wall along one end of the workshop. The usual size takes a crucible with a charge of about 40-60 lb. of metal; this is easily lifted out by one man. If more metal is required, several crucibles are poured at once. These furnaces are, however, sometimes made of much greater size, and crucibles are used which will take 1000 lb. of metal; but these, of course, are not likely to be used by artists very frequently. Very convenient is a portable furnace, exactly suited for artists' occasional use; it will take a charge of 32 lb., quite enough for a small bust, or a sketch, or statuette. Fig. 200 shows an ideal foundry tn section: A, pit; B, entrance to pit; c, shed or store over stairway; d, crucible furnace; E, ashpit with gratings; F, firebox of reverberator; G, hearth of reverberator; H, chimneys; I, flues; K, wall of foundry; L, cross-beam; M, foundry crane.