This keeps both bridge and furnace bottom in their places, and prevents any fluxed material from escaping from the bed of the furnace. It is usual to give the bridge plate sides about 2 in. high, and a lining of thin firebricks to strengthen it. The bed of the furnace is divided into two parts : tbe " working bed," that nearest the fire, is 6 in. or so lower than the " shelf" or charging hed ; the hopper in which the charge of sulphate, coal, and chalk is contained, is built into the arch over tbe centre of the " shelf." Each bed is provided with a working door, closed by ca,st-iron covers lined with half thicks. Concerning the pan placed at the end of the furnace more will be said presently. A ball furnace requires very careful and substantial building to stand the heat (about 1200°), the wear and tear, and contraction and expansion. The walls are about 14 in. thick, the arch 9 in., the sole 9 in., formed of 9-in. firebricks, of best possible quality, set on end and " grouted in " with a thin mixture of finely ground fireclay and water. Below this bed a foundation of, first, Concrete, and then brickwork is laid, vvith as thin jointings as possible. Bevelled portions of brick work, as shown in Fig. 245, allow the workman to reach every corner of the beds with his paddle and rake. The arch goes in a nearly horizontal line over the first bed and then dips down towards the pan, so as to carry the beat well into the material. The connection between furnace and pan is foimed of a bridge and air-course somewhat similar to the fire-bridge and its air-course, a large flat gnarl, which projects some 4 in. over the edge of the pan, preventing the flames from coming into contact with the iron. The whole erection of furnace and pan is firmly bound up by strong iron binders running over upright bars set into the ground or foundation. The outside of the furnace is usually cased with plates of cast iron, as shown in Fig. 243, but in Lancashire it is customary simply to pass strips of metal behind the binders, as shown in Fig. 220, when describing a close or blind roaster. The bricks used in building a ball furnace, and especially in the beds, must be as free as possible from silica—to prevent the formation of silicate of soda, and hard burnt. The fireclay must be as well ground as possible so that all joints may be kept fine. The charging by hopper is a great improvement over the old custom of throwing the materials down in front of the charging door and shovelling them in by hand. Not only is a considerable saving of labour effected, but less cool air is admitted into the furnace. The best plan is to arrange the furnaces en that one tramway, as direct as possible, may run overhead above the back beds and deliver its hourly charge into each hopper. By tho old method, besides a "ball furnacernau," or man to work and draw the furnace, a " mixer " is required, whose business it is to fetch the charge of sulphate, chalk, and coal from the various depots, throw the materials upon the shelf, and spread them to the flames. Adopting the hopper and tramway system, and arranging the depots in contiguity to one another, half a dozen furnaces can be served by one boy. A fresh charge is always kept in the hopper to late it, the simple withdrawing of a slide causing the materials to fell down upon the shelf of the furnace.
It has been said that the wear and tear of a ball furnace is very great. The working bed requires renewal about every three months, the arch immediately over the fire lasts about the same time, while the whole furnace, except plates and foundation, requires recon,truction about once iu every three years. With inferior workmanship in the construction, or inferior materials, the life of a furnace is even shorter than three years, and the renewals from time to time of the several portions mentioned, very frequent indeed.
The placing of a pan at the end of the ball furnace, as set forth in the drawings, is simply a matter of convenience and economy, to utilize the waste heat of the furnace in concentrating the black ash liquors. The pan is usually of the description shown io Figs. 243, 244, and 245, a large rectangular vessel built of sheet-iron plates, three-eighths of an inch thick and thoroughly riveted together. The size varies from 18 to 24 ft. in length, from 2 ft. to 2 ft. 6 in. iu depth, and is of the same width as the furnace. It is best to have it a little deeper at the fire end than at the damper. Round the top runs a strong angle iron which carries a 41 in. (occasionally 9 in.) arch thrown across and joined to the sloping arch of the hall furnace. This arch also slopes down towards the far end of the pan to bring the heat well upon the liquors. Some manufacturers take the arch along almost level, and close down upon the angle iron, a eustom that tends to yield burned salts. Tho arch and pan are bound together by square 2 in. iron binders, placed along
the sides at intervals, and a series of cross rods, above and beneath, gripping the upright binders. The products of combustion, after passing over the surface of the liquor, finally pass to the chimney down the flue shown at the end of the pan, the draught being regulated by a hanging damper. In the front are constructed two or three doorways, depending upon the length of the pan, with projecting necks, closed by strong cast-iron slabs, which screw up against an angle iron rim running 'round the jambs of the doorways, and are further made tight by a bedding of clay. Yhrotigh these doors the "black salts " are raked ont into the " drainer," so planed iu front of the pan, and at a loWer level, that the projecting necks overlap by a few inches. The drainer is built of sheet iron in a manner similar to the pan, and is about 18 to 20 ft. long, 3 ft. wide, 2 ft. 3 in. deep at the end nearest the ball furnace, and 3 ft. at the other end. This sloping bottom allows the drainings from the salts to collect at the lower end, where a well, shown in Figs. 243 and 245, receives them. In this well, a pump is fixed whereby they are returned from time to time into the boiling-down pan. To assist the draining operation, a false bottom, perforated with a great number of holes, lies upon an angle iron running along the sides and ends of the drainer, about 6 iu. from the bottom at the shallowest end, and level throughout. The pan is built upon pillars, as shown in the engravings, that all leakage may be readily apparent. The precise working will be detailed when treating of the lixiviation of the balls. After traversing the pan the waste heat from the furnace may be still further utilized by placing over the flue iron tanks of any suitable description, in which the liquor from the vats may be kept at the requisite temperature while settling.
The balling operations are as follows :—The required quantities of chalk, sulphate, and small ooal are weighed off and introduced into the furnace—upon the back bed—by some such means as has been described. The workman with his "slice " then spreads the charge over the bed so as to thoroughly expose every portion to the action of the flames, and shuts down the door. After a short time the charge—already called a " ball"—is raked up, half of it transferred to the bed nearest the fire, and the other half again "spread." This splitting of the ball is not a universal method of working, but is upon the whole preferable. Again the doors are closed and the split ball exposed to the fluxing heat for about ten minutes. The second half is now transferred to the working bed, and the really hard labour of the ball furuaceman begins, hardly ceasing until his ball is drawn. As the materials begin to soften and flux—the sulphate first—every portion must be continually turned over so as to get an even fusion, and prevent any portion being burned. This is done with the paddle, and requires great experience, strength, and judgment on the part of the workman, as his materials are constantly varying, and, technically speaking, will " stand more fire " and need more fining down at one time than another. As soon as the fused mass begins to get stiffer, and the jets of flame, or " candles " begin to die down, the ball furnaceman takes his rake— the heavy cast-iron head about 10 in. by 7 in.—and after thoroughly mixing up every portion of the ball, draws it out as rapidly as possible into a wrought-iron barrow, or "bogie," placed under the furnace door, and just overlapped by the door-plate. All this finishing and drawing must be timed u 2 to a nicety, and calls into practice all the skill of the workman. If the ball be drawn a shade too soon, it is "green," and certain to contain undecomposed sulphate; if left for a moment too long exposed to the heat it is burned, and solidifies into a close hard mass, difficult to break up and lixiviate. A badly judged mixture is at once apparent at the finishing of the ball. If too little coal has been used, the whole mass remains 'oft ; if too little chalk, it becomes thoroughly stiff and is difficult to draw. It takes about forty minutes to dry, work, and draw a ball. A fresh charge is introduced upon the shelf a few minutes after transferring the second half of the previous ball to the working bed, and, after drawing, this part of the furnace is left empty for ten minutes or so, to get up a thorough heat again, almost a white heat being required to flux rapidly. After the ball has been raked out into the bogie, it is left for a short time to cool and solidify, the " candles " or "pipes" rapidly dying out, and the mass assuming a creamy brown appearance ; it is then wheeled away and tipped up in convenient contiguity to the lixiviating tanks.