39. The door frames are two feet long by eight inches high in the clear, are set two inches above the hearth, and are built into the wall as solidly as possible. An exception is made as to the back door on the first hearth, when it is to be used for discharging the roasted ore. It must then be set flush with the hearth, and is two inches higher in the clear, to allow of using a large hoe for discharging, but when the discharge is effected through a trap in the hearth, this door frame is the same as the others. The masonry is arched over the frame, and as the wall at this point is liable to be rather thin, it is a good plan to arch the skewback a little, in a horizontal plane inwards, by which the thrust of the main arch against this weak part is lessened, and to some extent transferred to the points at each side, where it is received by the backstays and tie-rods; notwithstanding which, it is also well to place above each door frame, outside of the wall, a flat bar of iron two inches wide and half an inch thick, the ends of which are tucked between the adjoining backstays and the wall.

40. Doors are sometimes made of cast iron, and pivoted to the frames, but sheet iron doors detached from the frames answer very well, and are furnished with long handles, made of iron rods riveted to them at right angles; when in place they rest against the rabbet formed by the junction of the frames with the masonry, the projecting handles being supported by the roller in front. Doors sliding vertically on the outside of the wall are very convenient, being counterpoised by a weight attached to one end of a rope which passes over a pulley, the other end being attached to the door by a short chain or a link. A slot in the lower part of the door allows it to be closed while the hoe rests on the roller, the handle of the hoe being supported horizontally by a hook depending from a beam above.

41. When the walls are high enough above the hearth, which is 17 inches at the points opposite the highest part of the arch, and not less than eight inches at the lowest points, the skewback is set up with bricks on end, the lower end being cut to the proper angle, and then the walls are continued up, level with the top of the arch. All openings in the walls are then temporarily stopped, and the enclosed space is filled with moist sand, up to the skewback along the sides and ends, and higher in the middle, and this is carefully shaped to the required form of the furnace top. On the sand, as a support, the arched roof is laid dry, the bricks on end, one course thick, working from the skewback all along on both sides, and keying in the centre line. All openings in the arch, such as flue or feed holes, are circular, formed with a course of "rollers." When the arch has been well keyed, and the openings filled with sand, it is wetted by pouring water on it, and then grouted with a mixture of clay and sand, thinned with water so as to run into and fill all the interstices between the bricks. The best way to lay the bricks in the arch is the style called "herring bone," but it requires well porportioned bricks, of which the width is just twice the thickness, otherwise straight courses are to be preferred.

As soon as this is done, the backstays and the upper tie rods are placed in position, and all the nuts tightly screwed to support the walls, yet not so much so as to move them. The doors, which were temporarily stopped with loose bricks, or pieces of board to retain the sand, are opened, and the sand, on which the arch was laid, is drawn out, to allow the arch to settle as it dries. In a single hearth furnace, the arch has very little spring transversely, because it abuts the other way against solid end walls, but in a long furnace a transverse spring is more necessary, especially near the junction of the two hearths, where the roof descends and rises again, so as in fac to be an inverted arch, as to the longitudinal section, as may be seen in Plate 2, but, as this is almost the narrowest part of the furnace, there is no danger of its falling, if it has a good spring crosswise.

42. In building the fireplace many masons, very improperly, let the bearing bars for the grates rest on an offset in the brickwork, so that though space be left for the lengthening of the grates by expansion when heated, yet that space is soon filled with ashes and other debris, so that the grates are forced to bend or "buckle.." When they become cool again they shorten but do not straighten, the spaces at the end, left by their shrinkage, are refilled, and, when again expanded by heat, the grates buckle still more and are soon ruined.

The end walls of the ash-pit should be perpendicular, without offsets; the bearing bars should be well clear of them, and the ends of the grates should also clear the walls by at least half an inch. The space thus left for expansion is then bottomless, and remains always open, and the grates remain straight.

43. The flue-holes are connected by flues in any convenient way, with a stack which is two feet square inside, and 20 to 30 feet high. In some part of the flue or stack is a damper, similar to that of a stovepipe, operated by the roaster through the agency of a cord or wire.

44. It is desirable to utilize as much as possible the waste heat from the furnace, and it is a good plan to carry the flue in front of the stamp battery, if dry crushing is done, and there enlarge it to form a dust chamber. The top of the chamber is of sheet or boiler iron, which forms a drier on which to dry the ore for crushing. Even where only concentrated sulphides are treated, so that no crushing is required, a drier is convenient, and may very properly be the top of the dust chamber. The walls are built two bricks high above the iron plate, and topped with two-inch plank, held down by anchor bolts built into the walls. If, however, there is a battery, the side of the drier next to it is without a wall above the plate, which, if thin, is secured by an anchor bolted iron strap along the edge.

45. If silver ore is treated, a drier is necessary for the precipitate. It may be made as described above, to be heated by the waste heat from the roasting ore, by steam, or by a special fire. A small roasting furnace is also requisite for roasting the dried precipitate. It is built similarly to the large furnace, except that it has but a single hearth and one working door. A hearth containing 36 to 40 square feet of surface, will suffice for the roasting of from one to two thousand ounces of silver, in the form of precipitate, in each twenty-four hours.

46. It will be observed, by those accustomed to furnaces, that there are two small innovations in the plan given. Firstly, the ash-pit is open entirely across the furnace. This gives the operator the choice of leaving it so, or of closing either end, which is sometimes an advantage on account of the draft. It is generally preferable, though contrary to custom, to have the opening on the rear side of the furnace, that is the side opposite to the fire door, because the cold air, entering under the hotter end of the fireplace, tends to equalize the heat. Especially is this the case when the workman pushes the half burned wood back, when introducing fresh fuel, instead of drawing it forward as he should do. Secondly, the doors on the first hearth are not placed in the middle of its length, as is usual, but a foot nearer to the fireplace. Here again the object is to equalize the heat, by causing the cold air which enters by the door, to pass over the hotter portion of the ore near the fire wall. Another advantage is that the hearth is made wider near the fire wall, and narrower at the other end, thus concentrating the heat toward the part which is farthest from the fire.

47. When a long furnace, which may have any required number of hearths, is built on a hillside, it is a good plan to make each successive hearth two feet, or even more, higher than the preceding one, the one next to the fire-place being the lowest. By this plan the cost of grading is lessened, and the dropping of the ore from one hearth to the other assists greatly in the oxidation. This is called a "step furnace."

48. Furnaces are often built with an arched chamber under the first hearth, as shown by the dotted line in Plate 2. This chamber is closed on the working side of the furnace and open on the other, and the ore is discharged into it through an opening about a foot in diameter, in the hearth near the working door. The opening is closed by an iron plate, which rests on a rabbet a couple of inches below the level of the hearth, and the depression thus formed is filled with roasted ore. A small flue from the chamber, leading through the wall to the interior of the furnace, removes the fumes rising from the hot ore, which is not drawn out of the chamber to the cooling floor for some time.