As the concentration of the liquid in the vacuum-pan proceeds, crystals of sugar begin to form, and the skill of the sugar-boiler is shown by the uniformity of the crystals he pro duces. The boiling is commenced by filling in only about a third or fourth of the quantity the vacuum-pan will hold, and gradually adding more liquid as the crystals increase in size. • The sugar-boiler is able to watch the changes going on in the vacuum-pan by means of small samples lie withdraws from it by means of a suitable apparatus. The sugar boiler holds those drops of thick fluid on his finger and thumb, between his eye and a strong light, and is thus able to detect those minute changes in its condition which show that it is time to add an additional quantity. By the time the vacuum pan is full, the contents have thickened, by the formation of crystals of sugar, into a mass of the con sistency of thick gruel; it is then allowed to descend into a vessel called the heater, where it is simply kept warm until it can be run out into the "forms," which, in the sugar-growing colonies, are generally conical earthen pots, holding from one to two cwts. of sugar. It is allowed to cool and complete its crystalization before the plugs, which close the bottom of the pots, are withdrawn. When this is done, from one-fourth to one-third of the contents of the forms, which has remained in a fluid state, runs off into gutters leading to large tanks, from which it is again pumped up into the vacuum pan, and reboiled, yielding a second quality of sugar. This reboiling of the drainings is repeated, with a continually decreasing result, both as to quantity and quality of the solid sugar obtained, and it is rarely carried beyond the fourth boiling. If the planter wishes to obtain Muscovado, or unclayed sugar, the process is now complete, and the sugar is turned out of the forms, and packed for shipment, In some cases, the sugar is run direct from the vacuum-pan into casks or hogsheads, which replace the forms, holes being bored in the bottoms of the casks, to admit of the uncrystallized portion of the sugar draining out.
If elayed sugar is to be made, the forms are allowed to stand for a few days until all the treacle has drained out; and a quantity of thin mud, about the consistency of good thick cream, is then poured over the sugar to the depth of one or two inches. The water contained in this thin mud slowly steals down through the sugar, and mixing with the coatings of treacle still adhering to the outsides of the crystals of sugar, renders them less viscid, and facilitates their descent to the bottom of the form. The mud re mains, at the mid of a few days, in the form of a dry hard cake on the top of the sugar, and none mixes with the sugar.
The process of claying sugar is simply washing off a coating of black or yellow treacle from a crystal of sugar, which is always white. This operation is possible without dissolving the crystal of sugar, simply because the treacle has a greater affinity for water than the crystallized sugar has. Anything that would yield a very slow and steady supply of water to the sugar, would do as well as mud or clay. There is always some loss of crystallized sugar in the process of claying, and attempts have been made to use strong alcohol for washing off the coatings of treacle from the crystals; but although alcohol dissolves treacle very freely, and scarcely acts on the crystals at all, still it has not been found to answer commercially. Besides the cost of the process, there is a
difficulty in getting rid of the smell of alcohol in the sugar.
The centrifugal machine of Messrs. Manlove, Alliott & Co. has been very ex tensively used for getting rid of the treacle. Its action depends on precisely the same principle as that called into play when a sailor twirls a mop to expel the water from it. The centrifugal machine is simply a drum of 3 or 4 feet diameter, and 12 to 18 'inches high, revolving at a great velocity on a vertical axis. The sugar, either direct from the vacuum-pan or after it has been allowed to cool, is put, still mixed with the treacle, into the machine. As soon as the drum acquires a high velocity its contents are forced by the centrifugal action against the drum, the cylindrical portion of which is made like a sieve, and admits of the escape of the treacle, but retains the crystals of sugar. Some idea of the efficiency of those machines may be formed when it is stated, that in a ma chine of 3 ft: diameter, revolving at the usual speed of 1000 revolutions per minute, the tendency of the treacle to escape will be 514 times its own weight; that is to say, the treacle will have 514 times more force to fly off than it has to drop off the crystal by the mere force of gravity.
Sugar-refining was unknown to the ancients, and even the refining previously referred to as having been established in Germany in the Ifith c. consisted merely in clarifying the sirup, and producing a sort of sugar-candy; but one improvement followed another, until the process may now be considered as almost perfect. The chief difficulties attend ing the operation arise from the circumstance that the material to be operated upon is ever varying in quality. Not only is there a difference between the produce of two different plantations, Lut even the manufacture of the same plantation shows differences of quality; these differences arising chiefly from the presence of foreign substances, which seriously interfere with the operations of the refiner. The attempts made to test the exact quality of solutions of raw sugar by means of polarized light (see above) have hitherto been attended with little success in practice. Sugar-refining, as practiced in Britain, has three distinct objects—(1) the production of loaves of thoroughly refined sugar; (2) crushed sugar; and (3) white sugar in separate crystals. The last is of com paratively recent introduction. In some existing sugar-refineries, old fashions still pre vail; but our description must be confined to the most recent methods.
Sugar-refining is carried on in this country on a great scale; London, Bristol, and Greenock being the principal seats of the trade. There is comparatively little raw sugar used in Great Britain. Nearly all the yellow and dark-colored sugar sold in the shops has passed through the hands of the refiners, and is simply inferior sugar, made out of the sirup which drains from the white loaf-sugar.