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 produces. 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 he withdraws from it by means of a suitable apparatus. The sugar-boiler holds these drops of thick fluid on his finger and thumb, between his eye and a strong light, and is thus able to de tect 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 consistency 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 It is allowed to cool and complete its crystallization before the plugs, which close the bot tom of the pots, are withdrawn. When this is done, from one fourth to one-third of the contents of the form, which has re mained in a fluid state, runs off into gutters leading to large tanks, from which it is again pumped up into the vacuum-pan, vid re boiled, yielding a second quantity 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 Muscovada 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 pans 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 clayed 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 end 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, 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 bean 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 difficulty in getting rid of the smell of alcohol in the sugar.
Sugar Iltfining is described as follows in an article in the New England Grocer, and by Mr. Dresser, of the Mass. Institute of Technology : "At the refinery the melting process is the first in order, and is usually carried on in a building separate from the main refinery. The coarse, raw sugar, is emptied into a melting pan to which is added the sweetened water obtained from clearing the empty hogs heads by steam. The melting pans contain 4 to 6 hogsheads of sugar, and the crude mass is kept in motion and the lumps pulver ized by stirrer knives. The liquid is drawn off through a sieve to rid it of chips, nails, etc. It is then raised to the clarifying house where it is placed in clarifiers or "blowups," large shallow pans, having at the bottom copper pipes through which steam circulates. The liquid is heated up to a temperature of 210 degrees Fahr. The process is a long one and requires careful watching, as to the moist sugars which contain more or less acidity, lime is added, while dry sugars—Manilas in particular—contain too much lime and have to be treated with an acid or sulphurous gas. Alum is sometimes used. Before filtering, the liquid must contain an ex cess of lime to prevent fermentation. Bullocks' blood, fresh or dried, is used with the lower grades. The albumen of the blood gradually rises to the top as the heat increases, taking with it all the light impurities and leaving the heavier ones at the bottom. Between the top and bottom a clear liquid is left which is drawn off and passes through bag filters several feet long, which are dou ble consisting of an outer and an inner bag. Where formerly but 100 bags were put into a single filter, 400 to 800 are now used. The filtration is usually very rapid.
From the filters the liquid passes to the receiving tanks below. It is of a dark color, but very clear, being freed from great impu rities, but still retaining gum, lime, salts and various mineral sub stances which can only be removed by charcoal or boneblack fil ters. This has the power of removing these organic impurities and assists the granulation of crystals. Chartmal filters are gen• erally 10 ft. in diameter, 20 ft. deep and hold 90,000 lbs. of char coal. This charcoal is made from beef bones which are burnt and ground for the purpose. So important an agent is it that refiners term it the "soul of the sugar refinery." Some of our refineries have 4.000,000 lbs of it in constant use. It is used over and over again, being made as good as new by a process called revivi fication. This filter was first introduced in the French beet sugar factories by M. Dumont. One pound of bone black is used to every pound of sugar manufactured.