Themostimportant ore of mercury (quicksilver) is the sulphide called cinnabar, identical in composition with the vermilion of commerce. The most important deposits of this mineral are at Almaden in Spain, Idria, New Almaden in California, Amiata in Tuscany, China, and Borneo.
When cinnabar is heated in a current of air, it is decomposed, the sulphur uniting with the oxygen of the air to form sulphurous acid, and leaving the mercury in a metallic state. But owing to the low temperature at which mercury volatilizes, the heat required to liberate the sulphur distils the mercury in a vapour, and necessitates the use of complicated apparatus to effect its condensation to the liquid state.
The oldest cinnabar-roasting furnace still, or till recently, in use is the Bustamenti furnace. In this apparatus, the ore and fuel are charged together into a short vertical shaft, and air is admitted from below and from the side. When the shaft is incandescent, the air decomposes the cinnabar, and the volatile products, sulphurous acid, mercurial vapour, and carbonic acid, pass off through flues into 2 large square chambers, where the gases are somewhat cooled. At some distance is a second pair of chambers, and the two sets are connected by several tubes. These tubes are formed of short joints, and each joint is an earthen vessel, swelling at the middle, and with one end larger than the other, so that the small end of one aludel, as these vessels are called, may be thrust into the large end of the next. The joint is made with clay. These composite tubes slope from each end towards the centre, and rest on a foundation of masonry. The mercurial vapours passing through these tubes are cooled down, and the metal is deposited in drops, which, on account of the inclined position of the tubes, run down to the central point. A small hole in the under-side of the aludel occupying that position, allows the metal to escape and collect in vessels set to receive it.
So much of the vapour as escapes condensation in the aludels passes into the second set of chambers, where another portion is liquefied, and the remainder passes out into the air and is lost.
Besides the metallic mercury, another product is condensed, called "soot." Its composition varies, but it contains flour of mercury-i.e. mercury in minute globules; undecomposed sulphide of mercury, for cinnabar is itself volatile, and when air is not brought into contact with it, may be distilled unaltered; calomel, when there has been any salt in the ore; and soot proper, formed by the imperfect combustion of the fuel. This product is collected and worked over by hand with lye to cleanse the minute globules of metal from impurities, whereupon a large portion of them unite to drops, and are thus recovered. The remaining matter is added to the ore at the next' charge. The mixing of fuel and ore, as practised in this furnace, has one great disadvantage. If at any time there is more fuel than there should be, too great heat is created, and if there are easily fusible compounds in the ore, such as silicate of iron, a glaze is produced, which in this, as in all roasting, interferes very much with the proper action of the air upon the ore.
The aludels, too, are somewhat difficult of manipulation, and it is very hard to make the joints between them tight; no one would now build a furnace on this plan.
The Bustamenti furnace is shown in Fig. 150. On the fireplace a is kindled a firs of wood introduced through b, the flame of which reaches the cinnabar ticked upon the perforated firebrick arch a. The ore is introduced through the openings fg; In stacking it, large blocks poor in metal are placed first, next shore come smaller pieces of richer ore, and at top are put cakes made by kneading up the "root" before referred to with clay. The sulphurous and mercurial vapours liberated by the heat escape by the flues d, In the side of the furnace, into the condensing arrangement e, consisting ordinarily of about 300 alaffli arranged in 12 separate lines of 25 each, side by side. All but one of each line are placed on an incline as shown; this one is laid horizontally in the centre, and is perforated on the lower side, to enable the condensed mercury to escape into the gutter A, leading to suitable receptacles. The uncondensd escaping vapours pass into the chamber i, where a further deposition of mercury takes place- The chimney j carries off the smoke from the wood fire a, and the chimney k serves for the escape of the sulphurous acid gas. Each roasting occupies about 12 hours, and the furnace has to cool for 3 or 4 days before receiving s fresh charge.
The losses at the numerous joints render it also a very wasteful apparatus.
The New Almaden furnace consists, in the first place, of a large square shaft, two opposite sides of which are broken thruugh with numerous openings. This shaft is filled with lumps of ore so arranged as to leave passages through the mass, but no fuel is mixed with it. The fireplace is placed on the outside of one of the perforated walls. The flame strikes through the perforations into the ore, and the gaseous products of the process pass through the opposite wall into a series of condensers. The con densers are large chambers of brick, wood, iron, or glass, in which are cross- walls, and these cross-walls are broken through alternately at the bottom and top, to admit of the passage of the vapour, but at the same time to compel it to take a circuitous course. This at once lengthens the distance over which it must pass, giving it time to cool, and, by producing gentle eddies in the current, favours the fall of minute particles of solid or liquid matter. The floors of the chambers are curved, and slope towards the outside wall, so that the metal, as it liquefies, may collect at one point in each chamber, whence it may either be allowed to run out continually through a small opening, or may be tapped when it has accumulated.