The Blaize kiln possesses a most interesting feature in the novel method of manipulating the revivifying-pipes with regard to the escape of the gases and watery vapour dining the beating. The black, after washing and vaporizing, still retains some internal humidity, which can only be driven off by calcination. If the kiln is charged with too-wet black, this forma a plug at the top of the tube, preventing the escape of the vapour, which is then forced throughout the column of red-hot black ; the latter is thereby decomposed, the carbon is calcined, and the combustible gases escape at the first opening which presents itself, usually between the joints of the pipes, dislodging them, to the deterioration of the kiln, the formation of white char, and the general interruption of the procesa. Ordinarily these evils are avoided by drying the black as strongly as possible before putting it into the tubes ; the moisture then remaining can force a passage between the grains. In the Blaize kiln, the liberation of the vapours is facilitated in the following manner. The heads of the tubes are furnished with a transverse iron bar, composed of two sections, united by a simple covering of sheet iron, and supporting in the axis of the tube another pipe of smaller diameter, made of wrought iron, pierced with slots throughout its whole length, and which, pene trating the mass of black to its hottest part, favours the ready escape of the vapours, and conducts them to the chimney. The black reaches the tubes in a dry atate, as it previously passes through the drier,—a chamber traversed by a large number of metallic tubes, through which travel all the combustion-gases, and which can be cleaned by opening the end; there is a trap for discharging the drier. The black has to undergo many changes of position before reaching the floor, thus ensuring its complete desic,cation.
The second important feature in the Blaize kilo, is the construction of the tubes, which are of enamelled fireware. Cast-iron tubes wear out rapidly, and unenamelled fireviare tubes produce white char, by reason of their great porosity, which allows air to pass. The enamelled tubes do not suffer from the beat of the kiln, as the enamel is put on at a white-red heat, such as is never attained in the black-kiln. Broken tubes can be readily mended by a special composition, and thus rendered as good as new. Moreover earthenware tubes afford a much superior black to iron ones. The construction of the kiln is very simple, and obviates the use of arches, which never withstand fire well. The upper bed and the second floor are formed of square blocks of fireware, through the centre of which pass the tubes. The tubes aupport the blocks, so that the expansion is uniform, and does not damage the kiln. Broken tubes or blocks can be removed and replaced without pulling the kiln about. The second floor rests upon the cooling-tubes, which are of cast iron, and furnished at bottom with traps and drawers, facilitating the discharge of a set every 20 minutes.
Other forms of revivifying-kiln are describe(' under Refining.
Concentration of the Syrup.—The next operation is the concentration of the " thin " juice, the
removal from it of the excess of water, so that the liquid may become sufficiently dense, or saturated with sugar, to enable the latter to crystallize out.
Principles.—While the primary object of concentration is to get rid of useless water and form a solid material, the purification of that material by mere crystallization must not be overlooked. By this act, the particles leave in solution those bodies which are present in too small proportion to admit of their crystallizing out, as well as those incapable of crystallizing. The crystals, freed from their mother-liquor, are considerably purer than the original solution from which they have formed. Crystallization is the property which many bodies (including true sugar) possess of assuming a definite solid form out of a saturated solution when cooled : it is based upon the power of water to hold these bodies in solution in a degree varying with the temperature, this power (in most instances) increasing with the temperature. Thus if a gallon of hot water is made to dissolve as much sugar as it is capable of holding in solution at the temperature exhibited, and this " saturated solution " is cooled, the decreasing solvent power of the water compels the sugar to separate from it in crystals. These crystals are a combination of sugar and water : but the water is chemically combined, and cannot be driven off without decomposing the sugar ; consequently this " water of crystallization " is regarded as an integral part of the substance, and the crystals are looked upon as pure bodies. Their size depends partly upon the conditions under which they are formed, these conditions being chiefly the duration of the operation, the bulk of water present, and the agitation or quiescence of the liquor.
Ia concentrating sugar-liquor to a saturated solution, it is necessary to bear in mind the changes which sugar suffers when subjected to heat. First it melts ; then, if the heating be continued slowly and regularly, it parts with successive molecules of water, becoming converted into a number of nnorystallizable non-saccharine bodies, and ultimately int,o " caramel," a dark-brown substance used for colouring porter and other liquids (see pp. 598-9). This conversion takes place concentrated solutions, as woll as in the dry state. As evaporation proceeds, the mass thickens, and the difficulty of equalizing its temperature increases, with consequent liability of certain portions becoming transformed into caramel. Another change which is constantly proceeding in tbe liquor is the inversion of orystallizable sugar into uncrystallizable. This is caused by pre existing uncrystallizable sugar, acids, and mineral salts, and is favoured by exposure to air and heat. The consequence of these changes is "molasses,"—an artificial product, composed of uncrystallizable sugars, and coloured by caramel. The value of molasses being far below that of sugar, the prevention of its formation is one of the chief aims of modern improvements in sugar making plant.