Two principal modifications of furnace may be described according as the jackets are of cast- or of wrought-iron, the dimensions and other details of construction not necessarily differing in the two cases. Figs. 61, 62 and 63* show a furnace

with wrought jackets erected at Great Falls, Montana, in 1891. Fig. 64 shows the lower part of the most recently erected furnace at the Globe Smelting Works, Denver, 1895; while Figs. 65 and 66 show the new furnaces of the Broken Hill Proprietary Block 14 Company at Port Adelaide, South Australia (1897).

* Borrowed from Hofman, Metallurgy of Lead, 1892.

Dimensions. The sectional area of modern oblong furnaces has been increased during the past fifteen years by increasing the length, it being, as already pointed out, impracticable to widen these furnaces without serious disadvantage. Fifteen years ago or so, there were few furnaces in existence over 6 feet long, whereas nowadays 120 inches to 140 inches is the usual size, and some have been built even 160 inches or over 13 feet long. The width at tuyere level varies between 2 feet 6 inches and 3 feet 6 inches; a few years ago the tendency was to reduce the width to 30 or 33 inches in order to use a lower blast pressure and prevent the heat from creeping up, but of late most of the new furnaces erected are 40 or 42 inches across tuyeres, corresponding with the increase in height and with the increased rapidity of driving.

As already mentioned, the height of water-jacket furnaces has been increased of late years to cope with the more refractory ores now smelted and the more infusible slags produced, the most recent furnaces except those working, on zinciferous ores with very ferruginous slags being generally 17 feet to 20 feet from tuyere level to stock line.

The

1. Foundation.-A good foundation is essential, and solid rock is always best, unless more than 10 feet from surface, in which case a frame work of heavy planking placed crosswise and embedded in concrete may serve instead. The excavation should be at least 3 feet larger each way than the furnace bed plate, and is filled either with good rubble masonry, brick work, or a block of concrete into which are rammed as many large stones as possible, cement being in any case mixed with the mortar. best of all possible foundations is formed by filling the excavation with molten slag, forming, when cold, a single block; but this is only attainable for additions to plant in an already existing works where a sufficiently quick succession of slag pots can be obtained. In any case, the topmost course must be perfectly smooth and level which, in a new works, is secured by making it of brick, and when molten slag is available can be easily obtained by forming shallow dams with pieces of bar iron (the tops of which are carefully levelled) and then filling them successively with molten slag. When a perfectly even and level foundation has been obtained, a layer of clay mortar is spread over it, and then the wrought-iron or steel "base plate," inch or inch thick is laid in position. This plate has generally a rim of 13-inch angle-iron rivetted on to assist in holding the hearth castings or "caisson plates" in position, and its function is to prevent leakage of lead from the well or crucible.

2. Crucible with Automatic Tap.-The crucible of this furnace, built up on the bed plate, of firebrick laid as closely as possible with thin fireclay mortar, is surrounded by thick "crucible castings" or caisson plates, which have to be very strong in order to resist the pressure of the lead in the crucible, frequently 20 to 25 tons dead weight of lead alone, without counting the downward pressure of the column of charge which floats upon it. These castings are commonly strengthened with projecting ribs, and bolted together. Their great weight, however, renders them very awkward for use in localities remote from railroad facilities, and at many modern works in America, as well as in the various Australian plants, they have been done away with, being replaced by bent rails enclosing a boiler plate outer casing, as shown in Figs. 61 and 62. Hofman indeed suggests that the outer casing of the crucible should be oval in form, when it could be enclosed in a rivetted boiler plate shell strengthened by means of continuous bands of flat bar iron, but, though a very practical suggestion, the author is not aware that it has ever been put into practice.

Inside the plates the well is built up, a space about 2 inches wide being left between them and the firebrick, which is afterwards tamped with sand or brasque, so as to allow for expansion. The Arents automatic tap is simply an inclined channel half

* Op. cit., p. 186.

a-brick wide, which is left from the bottom of the cruciblegenerally at the centre of one side-to the outside, where a basin is formed called the "lead well." Formerly this was formed by a semi-circular casting bolted to the crucible castings, but it is now wholly enclosed by the latter, and either a spout or a taphole is provided for the lead to flow into a cooling pot, which may be either fixed or, more generally, run on wheels. In some old furnaces a lead well on each side was provided, but, except in the case of large furnaces running on very rich lead ores, this is simply a waste of heat, and in most modern furnaces only one is seen. Two slag spouts are, however, frequently found, one at each end of the furnace, as with the help of a spout at the back which can be used occasionally or on alternate shifts, when the side instead of the end of the furnace is presented to the dump, crusts can frequently be prevented from forming.

3. Shaft.-At each corner of the foundation stands a column (usually of hollow cast-iron) about 8 inches diameter and 1 inch thickness of metal with capital and base, each 16 to 20 inches square and 2 inches thick. Upon these columns is set the "deck_frame," which supports the whole of the furnace walls with their lining. In old furnaces this was composed simply of cast-iron plates 1 to 2 inches thick, with strengthening flanges 3 to 9 inches deep, part of the direct pressure being taken off by arches in the brickwork. These light plates, however, were always giving trouble by bending and breaking, owing to unequal expansion, and in all modern furnaces the deck frame is composed of three beams supporting a plain flangeless castiron deck plate, as shown in the Figs. 61 and 63. The beams are bolted firmly to each other and to the capitals of the columns, the cast-iron plates simply resting on the former, so that there is no lateral thrust on them caused by unequal expansion. The lateral thrust of the brick arches was formerly taken up by the tie-rods only, but now strong flanges at the corners of the deck frames assist in taking it, as shown in Fig. 64. In some of the most modern furnaces arch beams are introduced to support the heavy furnace walls, as shown in Fig. 64.

The furnace walls resting on the deck plates are usually of common brick with an inner lining of firebrick, an expansion space being left between the two layers. In places, however, where firebricks are obtainable at not more than double or two and a-half times the cost of common brick, it is true economy to build the stack of firebricks throughout, their greater strength giving practical immunity from repairs. The walls of all modern furnaces are made much thicker at the bottom than the top, to diminish the loss of heat by radiation.

The whole shaft is well braced with tie-rods and corner-irons, which are usually cast-iron "back stays" with heavy lugs to receive the tie-rods, as shown in Fig. 65.