Raw sugar is refined by redissolving it in water, adding to the solution albumen, under the form of serum of blood or white of egg, and, sometimes, a little lime-water, and heating the whole to the boiling-point; the impurities are then removed by careful skimming, and the syrup is decoloured by filtration through recently burnt animal charcoal; the clear decolorised syrup is next evaporated to the crystallising-point in vacuo, and at once transferred into conical earthern moulds, where it solidifies, after some time, to a confusedly crystallised mass; this, when drained, washed with a little clean syrup, and dried in a stove, constitutes ordinary loaf, lump, or refined sugar. Sometimes in washing the crystallised mass for the purpose of removing the coloured syrup which is mingled with it, the process known as ‘claying’ is followed.
In this case, instead of white syrup being used, a layer of thin mud or a paste of thin pipe-clay is poured into the mould on to the base of the inverted sugar cone, through which the water escaping from the mud or pipe-clay permeates, and carries with it the coloured syrup. Neither the mud nor the pipe-clay mix with the sugar, but remaining on the top soon become hard, when they are removed. As the syrup running from the moulds still contains a large quantity of crystalline cane sugar, this is recovered as follows:
The syrup, after being sufficiently concentrated by boiling in the vacuum pan, is removed and allowed to cool, when it assumes the appearance of a crystalline magma known as ‘crushed sugar.’
Crushed sugar is a mixture of a large quantity of sugar crystals with uncrystallisable syrup. To get rid of this latter from the crystals, the mass is placed in quantities of 3 or 4 cwts. at a time in a ‘centrifugal machine.’ This, of which an engraving is given below, consists, as will be seen, of a drum fixed on a vertical axis. The walls of the drum are made of perforated metal, or are formed of meshed wire work, and the drum itself enclosed in an outer metal cylinder, which is fixed, and, of course, unperforated. When the drum is made to revolve on its axis at the rate of 1000 or 1200 revolutions in a minute, the syrup flying off by centrifugal
action, and escaping through the perforation at the sides of the drum, is received into the outer cylinder, whence it escapes by a trough into a proper receptacle, leaving behind the crystals in the interior of the drum.
a is an open drum of fine meshed wirework, caused to revolve in the cast-iron vessel (b b), by means of the bevel wheels (c d), gearing with a motive power. The motion of the drum can be stopped by means of the brake (e), and regulated by the weights placed at o.
When the crystallisation of sugar is allowed to take place quietly and slowly, the product is sugar candy. The evaporation at a low temperature in vacuum pans has the effect of diminishing the yield of treacle.
Prop. Sugar requires for its solution only 1⁄3rd of its weight of cold and still less of boiling water; it is slowly dissolved by cold rectified spirit; it dissolves in 4 parts of boiling rectified spirit and in 80 parts of boiling absolute alcohol; it melts by heat, and cools to a glassy amorphous mass (barley sugar); at about 400° Fahr. it suffers rapid decomposition,