greater vacuum and lower temperatures necessary for boiling. With a long series of vessels, as in a quadruple, quintuple, or sextuple effect, the thin juice in the first body may be boiled under atmospheric pressure or even at a few pounds above this; this is necessary in order to get a high enough temperature to carry sufficient heat through to the last evaporator. The sub

FIG. 33.

Vacuum pans in sugar factory. Operator in foreground using "proof stick" to withdraw test portion of contents (American Photo Co., Havana).

ject of multiple evaporation is a science in itself, and exhaustive treatises have been written upon this one single phase of sugar manufacture.

After the clarified juice has been evaporated to a sirup we come to the fourth stage of the process of modern sugar making, the graining or crystallizing of the sugar. This is accomplished in a vacuum pan (Fig. 33) which is operated in much the same way as one of the vessels of an effect; in the case of the vacuum

pan, and the same is true with many other effects, the process is assisted by connecting the outlet with a vertical condensing column 34 feet or more high (often extending above the roof, as may be seen in Figs. 35 and 36 beyond). A stream of cold water flows through the column, and this serves both by rapid condensation of the steam and by the barometric pull of its column of liquid to maintain a high degree of vacuum.

A charge of sirup is first drawn into the vacuum pan; this sirup as it leaves the evaporators has a specific gravity of about 1.25 (or about 50 per cent solids) and is boiled down in the vacuum pan to a specific gravity of 1.50 or about 90 per cent solids. The ebullition in the vacuum pan is violent and unless the sugar boiler is careful some of the sirup may be carried over with the vapor into the condenser; this is called entrainment and is a source of frequent losses in sugar manufacture. In all modern sugar factories the chemist makes constant examination of the condensation water, so that any loss due to this cause may be promptly detected and stopped.

The handling of the vacuum pan requires more skill than any other operation of the sugar house; care must be taken to avoid entrainment and care must be taken to build up crystals of uniform grain or size. The usual practice is to boil down the first charge of sirup to what is called "string proof," i.e. to the point when a few drops of sirup withdrawn from the pan will draw out between the fingers in fine strings or threads. When this point is reached, a large charge of fresh cold sirup is drawn into the pan, the sudden cooling of the supersaturated contents starting the formation of innumerable fine crystals. These first crystals constitute the foundation so to speak of all the sugars obtained in a given boiling or strike of the pan. The boiler aims to build up these crystals without forming new ones; he aims from now on to avoid supersaturation and to avoid sudden chilling through drawing in too much sirup at one time. He controls his process by drawing out samples every few minutes

and examining these upon glass against a light; if he sees fine new crystals appearing among the old ones, he reduces the vacuum a little, thus raising the temperature and dissolving this false grain as the fine crystals are called. By skillful manipulation, which only comes with long practice and experience, the sugar boiler is able to build up his crystals to any desired size. The

usual practice is a crystal about the size of ordinary granulated sugar; in certain localities, however, a large crystal is favored, as, for example, in Peru, where the sugar is boiled slowly and for a long time, thus building up a very large grain. The attachment for withdrawing samples of sirup from the vacuum pan is called the "proof stick."

When the vacuum pan is filled with a thick magma of sugar

crystals, of about the consistency of mortar, the steam is shut off, air is admitted, the bottom of the pan opened, and the entire contents dumped into a mixer, which keeps the mass in slow movement by means of revolving arms. This mixer is situated over a row of centrifugal machines; the mass of crystals (sometimes called masse cuite from the French, or Füllmass from the German) is drawn off gradually in successive charges into the centrifugals. The inner walls of the latter are lined with fine brass meshing and as the drums are rotated the masse cuite is whirled against the meshing, which retains the sugar but allows the molasses to pass through. After spinning for a few minutes until as much of the molasses is removed as possible, the revolving mass of sugar may be sprayed with a fine spray of water or a jet of steam in order to remove more of the film of molasses which remains adhering to the crystals; the amount of spraying depends upon the whiteness of sugar desired. In Louisiana a very pure, white sugar is made by spraying with several sprinklings of water; such sugar is over 99 per cent pure sucrose, the remainder being mostly moisture. In Cuba and Porto Rico they aim to make a 96 per cent sugar. In Hawaii and Java a sugar testing about 97 per cent is desired. Spraying will, of course, dissolve some of the sugar, so that the process is one which must be carefully controlled.

When the molasses has been removed as completely as possible, the centrifugals are stopped and the sugar emptied through the bottom of the drum into a conveyor, by which it is carried to the bagging department, where it is prepared for shipment. The raw sugar from the centrifugal contains considerable moisture, and in some countries the sugar is dried in revolving drums before being bagged. This drying is advantageous for two reasons: first, the excess moisture is removed, thus saving the cost of transporting water; and, second, the sugar is sterilized and protected against the attacks of ferments and bacteria. The drying of raw sugar is not practiced in Cuba, Porto Rico,