Vogl's* Furnace. This furnace, which is represented by Figs. 29 and 30, is so constructed as to give the greatest possible yield of metal. It possesses,

1. A great depth, so that the slags travel some distance before flowing out, in order to permit the separation of the contained lead and matt.

2. A great breadth on the back wall.

3. The furnace is worked at a low pressure, and with a light blast, whilst the tuyere has an inclination of 25°. The more refractory the ores are, and the greater the necessity for economising fuel, the more blast and pressure are required.

4. A great height, in order to form a sufficient covering. 5. A furnace shaft as narrow as possible, and standing in great contrast to the wide lower part of the furnace. The shaft must be contracted abruptly on its commencement, so as to concentrate the heat in the lower part of the furnace, and to keep cool the upper part, which serves to condense the lead fume, which, when once heated, and beginning to escape, cannot be restrained either by the great height of the furnace or by a low temperature. The small amount of blast employed causes the fuel to be less completely consumed, and also more fuel is required by the swelling of the furnace in the smelting zone; but all these disadvantages are more than balanced by a greater yield of lead. Whereas in the old furnaces 100 cwts. of mixture were smelted with 600 cubic feet of coal, causing a loss of at least 14 per cent of lead, in the new furnace the loss only amounts to 2 per cent at the outside, and consumes 800 cubic feet of coal, which is certainly a saving.t

The neglect of any of these five conditions is at once shown by a flame on the top of the furnace, causing a great loss of lead. The gases emitted from the furnace must be so cool that the hands may be held on the top of the furnace and not burnt, provided the charges have sunk to two-thirds of its height. The same laws regulate the construction both of the iron blast furnaces and the lead cupola furnaces,

* B. u. h., Ztg., 1855, p. 1, 243. + Oesterr. Zeitschr., 1862, p. 34.

modified only in the manipulation, or by local circumstances, as, for instance, when the mixture facilitates the formation of deposits in the sump, the furnace must not have a depth of 4 feet 8 inches, and its upper part must also be shallower.

Markus states that the loss of lead is considerably lessened by a high furnace, and that each pressure requires a certain height, and that the height of a furnace is determined by the highest pressure intended for it. The following comparison of different furnaces will be found interesting:

Joachimsthal. Przibram. Freiberg, Schemnitz. Hartz. Silesia.

a, depth; b. breadth; c, section at the tuyere; d, number of tuyeres; e, height from the tuyere to the funnel head; f, difference of depth in the upper and lower parts; g, pressure of blast; h, quantity of blast; i, loss of lead.

Lead furnaces have either a regular prismatic form, e. g., the furnaces in Przibram, Schemnitz, &c., or, which is preferable, an irregular form (Freiberg, Joachimsthal, Hartz): round or elliptical furnaces do not answer at all.

The following may be stated with reference to the requirements for the proper construction of lead furnaces mentioned in the preceding page :

1. The depth of the hearth of the furnace depends chiefly on the fusibility of the ores, on the fluxes at disposal, and on the pyrometric effect of the fuel. An increased depth of the hearth causes a greater consumption of fuel. When it is not deep enough, the smelting masses will not separate properly according to their specific gravity; the blast impinges against the front wall, and causes a strong draught of the gases in the shaft, and a more rapid waste of the hearth takes place. In most cases, a medium depth of 3 feet 6 inches, as in the furnaces of the Upper Hartz, is found most suitable; the limits are 2 feet 6 inches, and 4 feet 6 inches.

2. A great breadth of the back wall is better than the smaller breadth used in most lead furnaces, but the corners projecting into the furnace are so near to the point of the noses as to be rapidly melted. This is almost remedied in Vogl's new furnace.* (See Fig. 31.)

3. Little pressure and a small amount of blast are certainly advantageous, as preventing the volatilisation of the

* Oesterr. Zeitschr., 1862, No. 5.

lead, but this may be more than counterbalanced by the subsequent smaller production; besides which, the degree of pressure depends on the density of the fuel, and the quality of the ore.

4. A greater height of the furnace acts favourably with regard to the loss of lead by volatilisation and the consumption of fuel; Vogl's new furnaces have a height of 28 feet, and pounded sulphuretted ores require a higher furnace than similar ores in lumps. Oxidised ores and products, when either porous or in the form of powder, are best worked in lower furnaces, as they have been already reduced in the upper parts of high furnaces. To work these ores advantageously in higher furnaces, they should be moulded into the form of bricks, or roasted in a reverberatory furnace till they cake together or become completely scorified.

5. The sudden contraction of the furnace in its shaft has a favourable influence in keeping back the ascending gases; the upper part of the shaft of Vogl's newest furnace is contracted to 18 inches; if the ores contain zinc, this part of the furnace must be at least 21 inches wide. When using a mixture containing much oxide of iron, and employing a dense fuel, it is advisable to widen the shaft of the furnace, and to use a lower pressure,* in order to prevent the formation of much deposit of iron in the hearth; Vogl's construction, with slight modifications, is adopted in many lead works.

Furnaces with two tuyeres, and either with or without a partition wall, have, in some places, yielded a greater production than furnaces with one tuyere, a saving of fuel taking place at the same time. In other smelting works (Upper Hartz, Joachimsthal) this disadvantage is not found, and as it is also more difficult to keep two noses than one, the furnaces are worked by one tuyere.

These division walls did not answer at Joachimsthal in furnaces with two tuyeres, on account of the great quantity of zinc contained in the ore, which occasioned too much metallic fume in the furnace.

* v. AMON, in Oesterr. Ztschr., 1857, p. 332, 334.

G

The hearth, or bottom of the furnace, is formed of a mixture of loam and charcoal dust, firmly rammed down. The basin outside the tymp contains the lead, which is tapped off by opening a tap-hole communicating with the bottom, and the slags are conducted by a slope to another basin, where they accumulate for re-smelting.

The length of the nose must be so regulated that the front wall is kept free from deposit; and by lengthening or shortening the nose, the hearth can be made wider or narrower according to requirement.

B. DECOMPOSITION OF RAW ORES BY MEANS OF IRON. PRECIPITATION PROCESS.

This process is best adapted for ores rich in lead and poor in silver, associated with siliceous and aluminous gangue, and not containing foreign sulphides to any great amount, as these compounds are more or less decomposed by iron; their base either deteriorating the lead (copper, antimony, arsenic), or causing a loss of metal by facilitating the volatilisation. of lead, or the formation of metallic fume (zinc, antimony, arsenic), or by rendering the smelting mass difficult to fuse (zinc blende); the sulphides also cause the formation of products such as speiss, and the matt becomes richer in valuable metals. This process will be found more advantageous the higher the price of lead is, in comparison with iron; as the price of iron is now comparatively high, the only ores profitable to work by this process are those rich in lead; but ores poor in lead may be worked by it if they contain sufficient silver, though the extraction of silver by this method is imperfect.

The advantages of the precipitation process are :-That it saves the roasting of the ore, and allows a more perfect composition of the mixture, thus insuring a safer result than would be possible with roasted ores, as their composition varies according to the degree of roasting. The silica contained in rich ore makes it easier to scorify the lead in oxidised than in sulphuretted ones; the produc