Fig. 21. Fire-clay Crucible for Lemel.
The sal-enixum prevents the rising of the mixture in the crucible—which should be of the skittle shape ([Fig. 21])—and keeps it from overflowing; it also possesses a refining capacity the same as saltpetre, and is much cheaper. The burning of the lemel has a great tendency to destroy all organic matter that would be likely to cause the mixture to overflow during the period of fusion; but if such a thing should be at all likely to take place, the addition of a little dried common salt would remedy the evil, a small quantity of which ought always to be provided for the purpose. The common carbonate of soda is also a cheap and useful flux to the silversmith. Five-sixths of the above flux should be well mixed with the stated proportion of lemel, then placed in the pot, and the one-sixth remaining placed upon the top of the mixture, when it may at once be transferred to the furnace. Great heat is required in this operation, and it also requires careful supervision to prevent, if possible, waste of material. When the mixture has become perfectly liquid, the heat of the furnace should not be allowed to decrease, but continued for half an hour longer, and if the use of it be not further required, the fire may then be allowed gradually to die out. The mixture will require repeated stirrings during the period of fusion, in order to dissolve such portions as might otherwise not come immediately under the action of the flux. When the operation of fusion has been completed, the crucible is withdrawn and allowed to cool, the solidification of the metal is then perfect, and it may be recovered by breaking the pot at the base, when it will fall out in a lump corresponding with the shape of the crucible. The lump of metal should then be carefully weighed, the loss ascertained—which always varies in proportion to the amount of organic matter contained therein; it may then be sold to the refiner, or exchanged for new metal.
In this process it will be observed that the crucible is broken every time a fusion takes place, consequently some little expense is incurred in providing crucibles for the purpose. To obviate which the following plan may be economically and successfully employed; and especially when the metal is sold to the refiner by assay, the method about to be described will be found most advantageous, for it should be borne in mind that the lump of metal from the previous fusion has to be again run down in another crucible and poured into an ingot mould before the refiner will consent to take his assay from it. In this latter process the whole work is performed in one fusion, and the expense of a new crucible thereby saved. The flux employed in the reduction of the metal is also considerably reduced. The plan is performed after the following manner:—
Fig. 22. Plumbago Crucible for Lemel.
Take a plumbago crucible of the shape shown in [Fig. 22], and capable of holding the required mixture; put the lemel into it, and then place on the top one ounce of finely powdered carbonate of soda; this is all the flux the mixture requires, and it is then quite ready for the furnace. When the lemel has become properly fused, for facilitating which it is repeatedly stirred with a thin iron rod, it is withdrawn and poured into an ingot mould prepared for it as previously described. The flux and other organic matter, which always accumulates upon mixtures of this kind, is held back by the timely application of a thin piece of flat wood to the mouth of the crucible. After the withdrawal of the bar of metal from the ingot mould, it is cooled and weighed, and then it is quite ready for the operations of the refiner.