An older caleiner, known as Brunton's, consists of a horizontal revolving table, about 12 ft. in diameter, enclosed in a shallow reverberatory furnace; the table is slightly conical in shape, its surface sloping downwards from the centre to the circumference. The ore delivered on the centre of the table, through a hopper in the roof of the furnace, is exposed to the flame passing through the furnace, and is continuously stirred by a set of scrapers fixed in the roof, whilst the table rotates very slowly below them, making only about six revolutions per hour. The scrapers being set obliquely, shift the stuff gradually from the centre to the circumference of the table, where it falls off, and is collected in a chamber beneath.
Fig. 276 represents the system of flues and condensing chambers employed at a large arsenic works in Cornwall. The flues are built of bricks, and are from three to four feet in width ; they are built underground, and covered with iron plates, which 'nay bo removed in order to gain Wccess to the interior ; in some cases they arc lined inside with slate.
Tho chambers, which consist of a series of brick haffle-walls, are wider and higher than the ordinary flues. At stated periods, called " cleating up days," the ealciners are laid off, the iron plates removed, and the flues entered by workmen, who sweep the deposited arsenic into heaps, and shovel it out. . In some eases, there aro two sets of flues from the calciners to the chimney, in order that while one set is being cleared out, the vapours from the calciner may be turned into the other, so as to avoid a stoppage of work. The flowers of arsenic, or rough white arsenic, of the smelters (the giftinchl, or poison-flour, of the Germans), obtained in this way, is next purified by re-sublimation in suitable iron pots, or other iron vessels, before it is fit for the market. It then forms a semi-transparent, vitreous cake, which gra dually becomes opaque, and of snowy whiteness, by exposure to the air, and at length acquires a more or less pulverulent state on the surface.
The cbief seats of the arsenic manufactmo on the Continent are Altenberg, in Saxony, and Reichenatein, in Silesia. In these places the modus operandi is somewhat different from that pursued in England. After being crushed moderately small, the arsenical ores are roasted in a muffle furnace, in charges of about 10 owt., spread carefully over the bed of the furnace in an eveu layer two or three inches deep. A fire is lighted beneath and the charge slowly raised to a full red beat ; the heat should afterwards he slightly lowered, care being taken to maintain it at the same level throughout the operation. The charge must be constantly agitated, and air freely admitted during the whole process by allowing the furnace door to remain open. Twelve hours
is sufficient to volatilize the whole of the arsenic, when the old charge is withdrawn and a new one introduced. The muffle furnace in common use is about 10 ft. long by 6 ft. wide, and has a Ere under its whole length ; its bed should he slightly inclined. The raw ore is poured into the furnace by means of a funnel, and the vapours of arsenic and sulphur are conducted through a channel into a condensing arrangement placed above, and called the " poison tower." This arrange ment consists of a system of chambers placed one above the other, and numbering usually six in all. In their course through these chambers, the vapours are condensed and fall in a light powder on the floor ; that found in the first two is the purest, the rest being contaminated with more or less sulphur. After the withdrawal of each charge, the arsenic deposited in the upper chamber is raked into the lower ones, which are emptied about once every six weeks. The rake is inserted through small doors in the sides of the tower, and communication is effected between the upper and lower chambers by means of trap-doors. Owing to the poisonous nature of the arsenic fumes the greatest caution is necessary in effecting the removal of the sublimed acid. The workmen should have their mouths and nostrils protected by moist linen, and should he covered from head to foot with a leathern dress and helmet, the latter being furnished with glass eyes.
The further purification of the crude arsenic obtained in the above manner is carried on in an arrangement consisting of a series of iron pots, upon which are fitted cylinders of sheet iron, each terminating in an iron pipe connected with a condensing chamber. The pots, being open at the top, are charged with 3 cwt. of the crude acid, and the cylinders are fitted on by means of handles, the connections being carefully luted together. A fire is lighted beneath each pot, and heat is applied, at first moderately, but, after the lapse of half an hour, more strongly. The volati lized arsenic is carried upwards, and condenses in the sides of the pipe, and on the floor of the chamber, from which it is afterwards detached in a thick, transparent crust, which becomes opaque on exposure. Great care is necessary in regulating the fire ; if the heat be either too strong or too feeble, the quality of the product is much impaired. It is customary to regulate the fire by the warmth of the upper part of the cylinder, as felt by the hand. The end of the operation is ascer tained by inserting a metal rod into the cylinder ; if the process be complete, no arsenic will be sublimed upon it when withdrawn. The sulphur always contained in the crude arsenic is converted by the heat into sulphurous acid, and is conducted away by means of a chimney.