English: Calcining — The lead obtained by the various processes above described, generally contains a significant amount of silver to render its extraction of much importance; but, in addition to this, it is not unfrequently associated with antimony, tin, copper, and various other impurities, which require to be removed before the separation of the silver can be effected.

This operation consists in fusing tlie hard lead in a reverberatory furnace of peculiar construction, and allowing it to remain, when in a melted state, exposed to the oxidizing influences of the gases passing through the apparatus. By this treatment the antimony, copper, and other impurities become oxidized, and on rising to the surface of the metallic bath are skimmed off, and removed with an iron rake. The hearth of the furnace in which this operation is conducted consists of a large cast-iron pan, which may be 10 feet in length, 5 feet t; inclics in width, and 10 inches in depth. The fire-place, which is 1 foot 8 inches in width, has a length equal to the width of the pan, and is separated from it by a fire-bridge 2 feet in width. The height of the arch at the bridge end is 1 foot 4 inches above the edge of the pan, whilst at the outer extremity it is only about 8 inches.

The lead to be introduced into the pan is first fused in a large iron pot fixed in brick-work at the side of the furnace, and subsequently laded into it through an iron gutter adapted for that purpose. The length of time necessary for the purification of hard lead obviously depends on the nature and amount of the impurities which it contains; and, consequently, some varieties will be sufficiently improved at the expiration of twelve hours, whilst in other instances it is necessary to continue the operation during three or four weeks. The charge of hard lead varies from eight to eleven tons.

When the metal is thought to be in a fit state for tapping, a small portion taken out with a ladle, and poured into a mould used for this purpose is found on cooling to assume at the surface a peculiar crystalline appearance, which when once seen is readily again recognized. As soon as this appearance presents itself, an iron plug is withdrawn from the bottom of the pan, and the lead run oif into an iron pan, from which it is subsequently laded into moulds.

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The construction of a furnace of this description requires 5,000 common bricks, 3,500 fire-bricks, and i tons of fire-clay.

Figs. 380 and 381 represent an elevation and vertical section of the calcining furnace.

A is the fire-place ; b, ash-pit ; c, fire-bridge; d, cast-iron pan ; e, flue ; F F F, channels for allowing the escape of moisture ; g, one of the working doors ; H, spout for running off calcined metal.