BLACKS. Several manufactured carbonaceous substances are known in commerce under the generic name of "Blacks." The most important of these are animal-black, bone-black, Frankfort-black, ivory-black, and lamp-black. They are usually obtained by carbonizing organic matter, particularly bones, in closed vessels or crucibles, or by collecting the soot formed by the combustion of oily, resinous, and bituminous substances. Other blacks than those enumerated are manufactured, but only on so small a scale as to be of no commercial importance.
Animal-black. (FR., Noir animal ; GER., Knochensolazarz.) This substance is almost identical with bone-black, but is generally in a more finely divided state. Any animal refuse matter may be used in its preparation, such as albumen, gelatine, horn shavings, &c. These are subjected to dry distillation in an earthenware retort. An inflammable gas is given off, together with much oily matter, aramonia, and water, while a black carbonaceous mass is left behind. This is washed with water and powdered in a mill, the product being animal black. It is largely used in the manufacture of printing ink (see Ink), and of blacking (see Blacking).
(Fa., Noir d'os ; GER., Knochenschwarz.) When bones are heated in a retort or crucible, the organic constituents are decomposed and carbonized. A mixture of combustible gases is giveu off, which do not condense on cooling; and others, which condense in the form of a heavy oil, called bone-oil, and also much water oontaining tarry water and ammoniacaI salts in solution. The residue in the retort or crucible consists of finely divided carbon iu intimate mixture with the inorganic constituents of the bones : this mixture eonatitutes ordinary bone-black, or animal chareos,l, as it is sometimes called. The inorganic portion may, if required, be removed by washing the residue in dilute hydrochloric) acid.
The process, as worked on the large scale, is carried on in different ways, according as it is desired to collect the volatile condensable portion of the distillate, or to allow it to escape. In the latter case, when it is required to obtain only bone-black, the apparatus employed is of a very simple nature, and the amount of fuel needed is comparatively small. The carbonization is effected in fire clay crucibles, 16 in. high and 12 in. in diameter. These are to be preferred to crucibles made of iron, which were much used at one time, since they do not lose their round form when subjected to a high temperature ; in consequence of this, they fit more closely together in the furnace, less air can penetrate, and therefore leas of the charcoal is consumed by oxidation. The furnace is an ordinary flat hearth, having a superficial area of about 40 square yards, and is covered in with a flat arch, all of briokwork. The fireplace is situate in the middle of the hearth ; the crucibles are
introduced through doors in the front, which are bricked up when the furnace is filled ; each furnace holds eighteen crucibles. Tbe cruciblea, filled with the coarsely broken bones, are covered with a lid luted on with clay. To economize fuel, the furnaces should be in a row, and placed back to back.
Tbe arrangement of the furnace and pots is shown in Figs. 312 and 313. A is the fire place ; B, the cricibles, eighteen in number, spread over the floor of the furnace in a single layer ; c, d, e, and f are the flues for conducting away the heated gases arising from the calcination of the bones, as well as the waste heat itself ; the last portion of the flue is fitted with a damper g.
The furnaces are intended to be built in fours, back to back, the waste heat serving in a great measure to conduct the operation of the revivifying apparatus placed in the centre and marked C.
When the furnace is filled and the doors are bricked up, the heat is slowly raised to redness, at which point it is kept for six or eight hours. The combustible gases are evolved and consumed in the furnace as soon as the bones begin to decompose, and by this means so much heat is produced that only a small quantity of fuel is needed to maintain the required temperature. When the car bonization is complete, the doors are taken down and the crucibles removed to cool, their place being immediately filled with fresh ones. The heat must be kept as uniform as possible through out the process : if it be not sufficiently high, the bone-black will contain a portion of undecom posed organic matter, which renders it quite unfit for use ; if, on the other hand, the temperature be raised too high, the bone-black will become dense and compact, whereby its efficacy as a de colorizer is much reduced. When the charcoal in the crucible has become perfectly cool, it is removed and crushed. When required for decolorizing or deodorizing purposes, it is only roughly broken up into small lumps, in which form it is most readily applicable. The crushing is effected by means of two grooved cylinders, consisting of toothed discs, alternately 10 and 12 in. in diameter. These are so placed that the 10-in. discs of one cylinder are opposite the I2-in. discs of the other, and thus, in revolving, the carbonized bones are crushed to fragments between them, but are not reduced to powder. They are passed sue,cessively through six of these mills, the cylinders of each couple being nearer to each other than the last. Finally, the crushed bones are carefully sieved ; the powder is placed apart from the lumps, again passed through finer sieves, and sorted out into different sizes.