All the metals and alloys, with the exception of iron and the very fusible metals, are melted in crucibles, of which there are several different kinds. The principal ones in use are the Hessian pots, the English brown or clay pots, the Cornish and the Wedgwood crucibles - all extensively used for melting alloys of brass, bell-metal, gun-metal, etc.; but they are very brittle, and seldom stand more than one heat, yet are generally sold cheap, and some founders prefer to use a crucible only once, for crucibles often crack or burn through on the second heat. The best crucibles for all kinds of alloys are made of graphite (miscalled plumbago and blacklead). These are sold higher than any of the clay crucibles, but they are more refractory, and may be used for 3 or more successive heats without any danger of cracking or burning through. They are not so open and porous as the clay crucibles, and do not absorb so much of the metal, and for this reason they are to be preferred for melting valuable metals. When about to use a crucible, it should be heated gradually by putting it in the furnace when the fire is started, or by setting it on the top of the tyle or covering of the furnace, with the mouth down; it should be heated in this way until it is almost too hot to hold in the hands.

Some founders stand a fire-brick on end in the bottom of the furnace to set the crucible on. This prevents the crucible from settling with the fuel as it is burnt away. This way of supporting the crucible is a good idea when the furnace has a poor draught and the metal is melted slowly and it is necessary to replenish the fuel before the metal can be melted; but in furnaces where the metal is melted quickly, and it is not necessary to replenish the fuel in the middle of the heat, the crucible should be allowed to settle with the fuel, as the heat will then be more concentrated upon it. After the metal has been poured from the crucible into the mould or ingot, the crucible should always be returned to the furnace, and allowed to cool off with the furnace to prevent it from cracking. In forming alloys of brass, etc, a lid for the crucible is seldom used, but a covering of charcoal or some kind of flux is generally laid on the metal. The metal to be melted in the crucible is generally packed in before the crucible is put into the furnace; and when it is desirable to add to the metal after some has been fused, it is put in with the tongs, if in large pieces; but when the metal to be added is in small pieces, it is put into the crucible through a long funnel-shaped pipe.

The small end of this pipe is used for putting metals into the crucible, and the large end is used for covering the crucible to prevent the small pieces of fuel from falling in.

Crucibles 5