Elements of the Theory and Practice of Chymistry, 5th ed.

by Pierre Joseph Macquer

Moreover, this event infallibly follows whenever a bit of coal happens to fall into the crucible; and this is one reason why it is necessary to cover it.

You may be certain that the operation hath succeeded if the scoria be smooth when cold, and have not in part escaped through the lute; if the Lead be not dispersed in globules through the whole mass of the matter contained in the crucible, but is, on the contrary, collected at the bottom, in the form of a solid Regulus, not very shining, but of a blueish cast, and ductile. Moreover, the scoria ought, in the present case, to be hard and black, and should not appear full of holes like a sieve, except only in that part which was contiguous to the Salt.

Here it is proper to observe, that the Sea-salt doth not mix with the scoria, but floats upon it. After the operation it is black; which colour it gets, no doubt, from the charred parts of the flux. The absence of these signs shews the operation to have miscarried.

When the ore to be smelted is pyritose and refractory, it may be roasted at first with a much stronger degree of fire than is used for ores that are fusible; because the martial earth, and the unmetallic earth, which are always mixed in pyritose matters, hinder it from growing readily soft in the fire. Besides, such an ore requires a greater quantity of the black flux and of borax to be mixed with it, and a higher degree of fire to fuse it.

It is generally needless to mix iron filings with this sort of ore; because the martial earth, with which pyritose matters are always accompanied, is reduced during the operation by the help of the black flux, which for that purpose is mixed with it in a large proportion, and furnishes a quantity of iron sufficient to absorb the heterogeneous minerals mixed with the Lead.

Yet, if it should be observed that the pyrites which accompany the Lead ore are arsenical, then, as such pyrites contain but a small quantity of ferruginous earth, iron filings must be added; which are, on this occasion, so much the more necessary for absorbing the Arsenic, as this mineral remains in part confounded with the ore, is reduced to a Regulus during the operation, unites with the Lead, and destroys a great deal of it by procuring its vitrification.

The Lead obtained from such pyritose ores is commonly not very pure; it is blackish and scarce ductile; qualities communicated to it by a small mixture of Copper in the pyrites, which always contain more or less thereof. We shall presently shew the method of separating Lead from Copper.

Lead ore may also be reduced by melting it amidst coals. For that purpose first kindle a fire in the furnace in which you intend to melt your ore; then put a layer of your ore immediately upon the lighted coals, and cover it with another layer of coals.

Though the melting furnace used for this operation be capable of giving a considerable heat, yet it is necessary further to increase the force of the fire by the means of a good pair of perpetual bellows, which will produce an effect like that of a forge. The ore melts, the earth of the Lead unites with the phlogiston of the coals, and so is reduced to metal, which runs through the coals, and falls into an earthen vessel placed at the bottom of the furnace to receive it. Care must be taken to keep this vessel well filled with charcoal-dust, to the end that the Lead may be in no danger of calcination while it continues there; the charcoal-dust constantly furnishing it with phlogiston to preserve its metalline form.

The earthy and stony matters that accompany the ore are scorified by this fusion, just as they are by the other which is performed in a close vessel. With regard to the Sulphur and Arsenic, they are supposed to have been first accurately separated from the ore by roasting. This is the method commonly employed for smelting Lead ore at the works.