CIIHING.-" Curing '' einbi aces the drying and whitening or bleaching of the sugar. The several plans will be discussed in succession.
Simple Drainage.—This is the oldest and -crudest method. To remove a certain amount of the molasses and other impurities, the semi-liquid mass, dug out of the coolers as soon as sufficiently cold, is placed in casks with perforated bottoms.; the holes in the casks are loosely filled with canes, twisted leaves, or rushes (the latter long enough to reach above the contents of tbe casks), in such a manner as to form a rough strainer. The casks stand meantime on rafters over an immense tank. Here the draining process slowly and imperfectly goes on, a portion of the molasses escaping iuto the tank below, but much still remaining in the mass of sugar, imprisoned between the minute crystals. Even after months of standing, the separation of the molasses is so incomplete that very gmat leakage and waste continue while the sugar is on its way to European markets. Sugar cured in this way is termed " muscovado," and is the most impure form of " raw " (" grocery," "moist," or " brown ") sugar. It is nearly obsolete in the English and French colonies, and its manufacture is decreasing rapidly in Louisiana.
Claying.—The first improvement introduced is based upon the fact that the impurities of muscovado sugar are much more soluble in water than the sugar itself : thus washing with water effects considerable purification. The earliest -manner of carrying this out was by placing the sugar in inverted cones with a minute aperture in the apex, stopped up during the filling and for about 12 hours afterwards; upon the mass of sugar in the cone, was placed a batter of clay and water (hence tho term " claying"), the object being to ensure a very gradual percolation of the water through the mass. This water carries with it the un crystallizable sugar and colouring matters im bedded between the crystals. The resulting sugar is much lighter-colmired than musco vado, but the grain is very soft, and the opera tion is most wasteful. In Bengal, a wet rag is sometimes substituted for the clay batter.
The process continues but little in vogue.
Spirit-washing.—The very slight solubility of sugar in alcohol, coupled with the ready solubility in that medium of many of its impurities, suggested the practice called "spirit-washing." This
consists in substituting cold alcohol or alcohol and water for simple water. The results are not perfect, however, and the costliness of the method soon caused its abandonment in this connection.
Vacuum-chest —The vacuum-chest consists of an iron box with a tray of wire-gauze above, and connected MU] air-pnmp suction below. The sugar is spread on the tray, and the downward &lotion produced by working the air-pump creates a tendency in the flui.I portion of the mass to separate itself, Effectual separation, however, can only be attained when the grain or crystal of the sugar dealt with is large, hard, and well formed ; with small or soft grain, the process is utterly inapplicable. This fault has restricted its use.
Centrifugals.—The preceding modes have been generally superse led by centrifugal machines or hydro-extractors. There are many varieties, but all consist essentially of a cylindrical basket revolving on a vertical shaft, its sides being of wire-gauze or perforated metal, for holding the sugar. The basket is surrounded by a casing at a distance of about 4 in., the annular space thus left being for the reception of the molasses, which is expelled by centrifugal force throngh the sides of the basket when the latter revolves at high speed. A spout conducts the molasses to a receiver. An example of a simple centrifugal is shovrn in Fig. 1382 ; more complicated forms are used in refineries (see Refining). The machine comprises a revolving basket a, carried by a cast-iron dome b upon v. central shaft, arranged with driving-pulley, footstep, and neck-bearing, on the central bracket c, the whole being supported by the outer cast-iron casing d, which collects the liquid thrown off from the material in the basket, and conveys it away through a discharge-pipe. The brake e, for stopping the motion of the basket, is applied by the lever-handle f acting upon the angle-iron ring h riveted to the cylinder bottom, The sugar is discharged through two copper doors i covering openings in the cylinder bottom, and passes down the shoot k cast in the outer casing, into a receptacle below. The treatment of the molasses separated from the sugar has been already described (see p. 1895).