Another modification of the steam process is to subject the rough fat to the action of about its bulk of water, containing 2-3 per cent, of sulphuric acid, boiling the whole by steam at atmospheric pressure, i. e. in an open, or loosely closed, lead-lined tank. There ex ists, however, a prejudice against tallow or lard in whose preparation any chemicals have been used.
In all cases, it is highly desir able to render the fat as soon as possible after its removal from the animal's body, since the animal tissue rapidly decomposes, and sets up fermentation, producing rancidity in what would otherwise be a neutral fat, and also injuring the colour, &c., of the ultimate product.
The different kinds of fat should also be sorted, and each description melted separately. In large esta blishments, such as the Union stock yards of Chicago, the stockyards at St. Louis, and the packing-houses of Cincinnati, where thousands of beasts are killed every day, there is a row of these digesters, each one allotted to its own kind of fat, which is placed therein a few minutes after the death of the animal. In this way, is secured -• great uniformity in the various grades of tallow and lard produced.
The waste liquor, containing large quantities of nitrogenous matter, is sold for manure.
Many soap-makers "render" rough fat in a soap-copper (see Soap) by the simple action of wet steam upon it, while the cover of the copper is secured by bolts or weights. When all the tallow has been skimmed off, a weak impure solution of caustic soda, technically known as "half-spent legs," is run in, and the whole is boiled. The object is to saponify the last remaining portions of tallow, and the discoloured imperfect soap is used in lower grades of the pure article. The process, how ever, is a wasteful one, since large quantities of the soda are used up in forming ammonia, by its action upon the nitrogenous animal tissue, and this ammonia, being volatile, escapes with the noxious vapours, and thus a valuable fertilizing agent is lost. As far as concerns the avoidance of noxious odours, rendering by steam has undoubted advantages, and it is altogether a more rapid and satisfactory process.
The foregoing descriptions of the various processes for the "rendering" of beef-, mutton-, and pig- fat, apply equally, mutatis mutandis, to that of kitchen-stuff, ship's-grease, &c. In these cases, however, the liability to rancidity, and production of noxious fumes, is much greater than with fresh fat.
A large quantity of fat, known as " bone-fat," " bone-grease," or " bone-tallow " is now extracted from bones, and is chiefly used by the soap-maker. The common method of extract ing the fixed fat frum bones by boiling in open vessels has been described under Manures (p. 1256). A great improvement consists in the application of steam in closed vessels, as adopted by most large firms. The form used by Morris and Griffin, at Wolverhampton, is shown in Fig. 1024 It is composed of alb in. iron cylinder a, 6 ft. Iong by 3 ft. 6 in. in greatest diameter,
furnished with hinged doors b c at top and bottom, which are tightly closed during operations by 11-in. screw-threaded bolts and nuts d, and further pro vided with 2-in. tap-holea at e and f. The bones are intro duced at the top door b, which is then secured ; steam at about 141° (286° F.) is introduced for 40 minutes ; the steam is shut off, and that remaining in the cylinder a is let out into a con denser ; hour later, the fat is drawn off by the tap f at the bottom of the cylinder ; the bottom door c is then opened, and the bones are allowed to fall upon the floor below. The bones are rendered much more brittle than by ordinary boiling, and are much drier and more easily ground, requiring no previous storing (see Manures, p. 1256). The cylinder shown will steam at a charge 4ei cwt. of bones, yielding 87-88 lb. of fat per ton.
One of the best and most inoffensive arrangements of the bone-boilers fur a large establishment is that adopted at Adams' knackery, in Birmingham, erected under the direction of Dr. Alfred Hill, and shown in Figs.1025,1026. A set of six pans are set and heated in the usual way. They are enclosed above in a sort of closet, formed by a wooden partition, reaching up to the roof of the building and down to the front of the top of the pans. Opposite each pan, is a shutter in the partition, which can be slid up whenever it is requisite to obtain access to the pan. Each pan is closely covered with a wooden lid, and from the upper and back part of each pan, beneath the cover, starts a pipe leading to a 10-in. main running the whole length above the pans and within the hopper, receiving contributions of vapour from each pan. This pipe finally has exit outside the building ; it here communicates with an oblong condenser, made of sheet iron, filled with coke, measuring 14 ft. long, 3 ft. in vertical section, and 14 in. in horizontal section, inclined a little in its long diameter. At the lower end, it receives the pipe from the boilers ; and from its lower border, another small pipe conveys away the water produced by the condensation of the vapour, the cooling agent being the outer air, to which the condenser is exposed on all sides. The upper end of the condenser communicates by two air-pipes with the flues from the fire-placea. When the pan-lids are raised, the escaping vapour rises to the roof, and is conveyed from the interior of the closet into the chimney-shaft, by escape-steam pipes provided for the purpose. The reference-letters indicate : a, fire-holes ; b, boiling-pans; c, steam-pipes ; d, main steam-pipe ; e, escape-steam pipes ; f, air pipes ; g, chimney-shafts ; condensed-steam pipes ; water-trough ; k, wrought-iron condenser ; 1, doors for cleaning smoke-flues ; m, smoke-flues ; n, closet with sliding door.