of the bones are not collected, the black is obtained by breaking up the bones into small fragments and placing these in clay crucibles, fitted with a cover, which is not fastened down; this arrangement allows the volatile matter to escape, but excludes the air, the access of which would cause the carbon to burn away. Any kind of furnace for heating crucibles will answer. One form of such furnace is shown in Fig. 27. The furnace has a flat hearth, measuring 7 yards long by 5 yards wide, and in the

G

Fig. 27.-Bone-black furnaces.

centre is placed the fireplace, which is fed from the outside. A low firebrick arch extends over the hearth, from the bottom of which a number of flues pass round to the top of the arch, where they are in connection with the single large flue that carries away the products of combustion of the fuel used and the small proportion of volatile matter from the bones which is not burnt up in the furnace. Doors are provided in the sides of the furnace and the arch for the purpose of introducing the crucibles. The crucibles are made of fireclay, and are usually eighteen in number; each is provided with a lid; in some places this is simply placed loose on the crucible, in others it is luted on with clay, a few holes being left for the purpose of permitting the escape of the volatile matter from the bones. When all the crucibles are placed in position in the furnace the doors are bricked up and

the fire lighted. The temperature is slowly raised to a red heat, at which it is maintained for from six to eight hours. The temperature should be kept as uniform as possible during the whole of this period; at the end the fire is withdrawn and the furnace allowed to cool down. Comparatively little fuel is required as the bones give off a good deal of combustible matter, which assists in heating the furnace.

When the furnace is sufficiently cool, the doors are opened and the crucibles withdrawn; it is not usual to allow the furnace to cool down completely, but only sufficiently so as will allow the workmen to enter and work comfortably; to let the furnace get quite cold means a loss of time, and, therefore, fewer charges worked in a given time, as also a greater consumption of fuel to heat up the furnace to the required temperature. It is important, however, that the crucibles should not be opened until they and their contents are quite cold; if opened while they are hot, the black contained in them would take fire and burn away, thus leading to a loss of material. When the furnace is emptied of one charge it is ready for filling with a second charge. Two charges can be easily worked in a day in such a furnace, and, by working hard, three charges may be got through, each charge being about half a ton, and yielding about 60 per cent. of its weight of black.

After the black is taken from the crucibles it is ground in a mill, care being taken to exclude grit and other materials liable to generate sparks by friction and thereby to set fire to the black. If the black is to be used as a pigment, it is ground into a fine powder; if it is to be used for decolorising purposes, it is ground into a coarse granular material.

2nd Process.-When it is desired to collect the volatile bodies given off during the process of charring the bones, the operation is usually carried out in earthenware retorts, such as is used in the distillation of coal or wood; a red heat being used. The volatile portions are passed through a series of condensers formed of iron tubes exposing a large surface to the cooling action of the air, by which operation they are separated into three portions, viz. :-(1) an aqueous liquor containing a fair proportion of ammonia, which can be recovered by the usual methods; (2) oily or fatty matter having a very peculiar and unpleasant odour, which may be used for preparing lamp-black, and is used, to a small extent, for currying leather; and (3) an uncondensable gas, which can either be discharged into the atmosphere or used, if necessary or thought desirable, as fuel for heating the furnaces.

1

This method of preparing bone-black is rarely used. It is important that after the operation is ended that the fires be withdrawn, and the retorts be allowed to cool down completely before they are emptied; if opened while hot the black will take fire.

PROPERTIES AND COMPOSITION OF BONEBLACKS.-Bone-black, as sent out for use as a pigment, is a fine powder, of a greyish-black hue, varying a great deal in various samples. It has not the brilliant hue of lamp-black, nor its depth of colour. As a pigment it is quite permanent, and works well in both oil and water, mixing easily with both these vehicles. It is a slow drier when used as an oil-paint. The chief use to which bone-black is put is in the preparation of blacking, where the large quantity of calcium phosphate and carbonate it contains gives to it properties which are as important, perhaps even more important, than its colouring powers.

While bone-black owes its colour to carbon, yet it consists principally of calcium phosphate and carbonate derived from the mineral constituents of the bones. The following analyses, made by the author, of bone-blacks will give some idea of their mineral composition:

The first sample contained an abnormal quantity of water for a black pigment, and also contained some lamp-black.

The average quantity of ash contained in bone-blacks is 75 per cent., of which 60 per cent. is phosphate of lime, the rest being carbonate of lime, with traces of silica, iron, and alumina. The carbon ranges from 15 to 20 per cent. As a rule, the ash is white, or of a pale greyish tint; sometimes it has a red tint showing that it contains iron. The author has some suspicion that blacks leaving a reddish ash are not pure boneblacks.

ASSAY AND ANALYSIS OF BONE-BLACKS.-Boneblacks can be assayed for tint or colour, colouring power, &c., by the usual methods. An analysis of it may be made in the same way as described under lamp-black. The mineral matter may be further tested if there are suspicions that the sample under analysis has been made by mixing lamp-black with some mineral matter. The ash which is left behind on heating the black in a platinum crucible should almost entirely dissolve in strong hydrochloric acid without much effervescence; on adding ammonia a

copious white precipitate should be obtained; on drying and weighing, this should amount to about 60 per cent. of the weight of the black. To the filtrate from this precipitate ammonium oxalate should be added and the resulting precipitate filtered off, dried, and weighed. The amount of it should be about 15 per cent. of the weight of the black. The precipitate with ammonia should be powdery, not flocculent, in appearance; the latter would indicate the presence of alumina, of which traces only are present in the ash of pure bone-blacks.

IVORY-BLACK.

Ivory-black is, or should be, made from the waste cuttings of ivory by the same process as bone-black is made from bones. Much bone-black is sold as ivory-black. In composition and properties ivory-black resembles bone-blacks. The following are some analyses, made by the author, of ivory-blacks, but the genuineness of the samples is not guaranteed :

Ivory-black has usually a finer and more brilliant hue than bone-black. It is used for making printing ink, blacking, &c.

ANIMAL BLACK.

Under the names of animal black and animal charcoal a number of black pigments, made from animal matters of all kinds and by various processes, are sold for use as pigments and for decolorising purposes. Animal black is made, much in the same manner as bone-black, from all kinds of animal products, waste pieces of leather, skins, hoofs, horns, hair, &c. In the process of making yellow prussiate of potash there is produced a large quantity of black, which was at one time thrown away, but it is now largely used for decolorising sugar, paraffin wax, &c.

The composition of animal black is very variable; the following are analyses of several samples of these blacks sent to the author for analysis:

No. 6 had a most peculiar fetid odour and contained a quantity of unburnt oil; from its appearance and composition it is evident that its only claim to the name of animal black was that it was made by the lamp-black process from bone oil.

No. 7 was a true bone-black. With these two exceptions the above analyses show that this black is made from a variety of materials.

Animal black can be used for all purposes to which black pigments are applied.

FRANKFORT OR DROP-BLACK.

This black is named "Frankfort black," because it was first prepared in the old German town; "drop-black," on account of the shape into which it is made up for sale.

Drop-black is made from a great variety of materials of an organic character, such as vine twigs, refuse of wine-making, peach stones, hop bine, bone shavings, ivory cuttings, &c. These are calcined in a closed vessel until they are thoroughly charred. The black so obtained is then ground up as fine as possible with a little water; then the mass is lixiviated to free it from soluble matters, and dried. Then it is mixed with a little glue water and made up into pear-shaped drops for sale, for which purpose they are ready when dry.

Drop-black is a black of fine texture, varying in hue from a bluish-black to a somewhat reddish-black, which is due to the different materials of which it is made; vegetable matters yield a black of a bluish hue, while animal matters give a black of a greyish hue.

Drop-black owes its colour to carbon, the amount of which varies in different samples; it also contains some mineral matter which will vary in amount and kind according to the character of the material from which the black was prepared. following is an analysis by the author of a sample of drop-black:

The

The mineral matter contained phosphate of lime, which showed that bones had been used in making this sample.

Drop-black is used for all purposes for which black pigments are required.

German black is a synonym for drop-black.