Bone-black, as found in commerce, is very variable in its discolouring power, which arises from its having been exposed either to too great a heat which has glazed its carbon, or to too low a heat which has left its albumen imperfectly decomposed. A steady ignition of due continuance is the proper decomposing temperature. Its composition is generally as follows:—
Phosphate of lime, with carbonate of lime, and a little sulphuret of iron, or oxide of iron, 88 parts; iron in the state of a silicated carburet, 2 parts; charcoal containing about one fifteenth of azote, 10 parts. None of the substances present, except the charcoal, possesses separately any discolouring power.
The quality may be tested by a solution of brown sugar, or molasses, or of indigo in sulphuric acid. The last is generally preferred by the French chemists, who have occupied themselves most with this subject, and it contains usually one thousandth part of its weight of this dye-drug of the best quality. Other animal substances yield a charcoal, possessed of very considerable discolouring properties. The following table by M. Bussy exhibits an interesting comparison of almost every kind of charcoal in this point of view.
Table of the discolouring powers of different charcoals.
| Species of Charcoal. | Weight. | Indigo test consumed. | Molasses test consumed. | Blanching by indigo. | Power by molasses. | ||||
|---|---|---|---|---|---|---|---|---|---|
| Gramme. | Litres. | ||||||||
| Blood calcined with potash | 1 | 1 | ·60 | 0 | ·18 | 50 | 20 | ||
| Ditto with chalk | 1 | 0 | ·57 | 0 | ·10 | 18 | 11 | ||
| Ditto with phosp. lime | 1 | 0 | ·38 | 0 | ·09 | 12 | 10 | ||
| Gelatine ditto with potash | 1 | 1 | ·15 | 0 | ·14 | 36 | 15 | ·5 | |
| Albumen ditto ditto | 1 | 1 | ·08 | 0 | ·14 | 34 | 15 | ·5 | |
| Starch ditto ditto | 1 | 0 | ·34 | 0 | ·08 | 10 | ·6 | 8 | ·8 |
| Charcoal from acet. potash | 1 | 0 | ·18 | 0 | ·04 | 5 | ·6 | 4 | ·4 |
| Ditto from carb. soda by phosphorus | 1 | 0 | ·38 | 0 | ·08 | 12 | 8 | ·8 | |
| Calcined lamp black | 1 | 0 | ·128 | 0 | ·03 | 4 | 3 | ·3 | |
| Ditto ditto potash | 1 | 0 | ·55 | 0 | ·09 | 15 | ·2 | 10 | ·6 |
| Bone black treated with mur. acid and potash | 1 | 1 | ·45 | 0 | ·18 | 45 | 20 | ||
| Bone black ditto with mur. acid | 1 | 0 | ·06 | 0 | ·015 | 1 | ·87 | 1 | ·6 |
| Oil calcined with phosp. of lime | 1 | 0 | ·064 | 0 | ·017 | 2 | 1 | ·9 | |
| Crude bone black | 1 | 0 | ·032 | 0 | ·009 | 1 | 1 | ||
With regard to the mode of operation of bone black on coloured liquids, M. Payen showed in his prize essay, 1. That the decolouring power of charcoal depends in general upon its state of division; 2. That in the various charcoals, the carbonaceous matter acts only upon the colouring matters, combining with and precipitating them; 3. That in the application of charcoal to the refining of sugar, it acts also upon the gluten, for it singularly promotes crystallisation; 4. That according to the above principles, the decolouring action of charcoals may be so modified, as to make the most inert become the most active; 5. That the distinction between animal and vegetable charcoals is improper, and that we may substitute for it that of dull and brilliant charcoals; 6. That of the substances present in charcoal besides carbon, and particularly animal charcoal, those which favour the decolouring action, have an influence relative only to the carbon; they serve as auxiliaries to it, by insulating its particles, and presenting them more freely to the action of the colouring matter; 7. That animal charcoal, besides its decolouring power, has the valuable property of taking lime in solution from water and syrup; 8. That neither vegetable, nor other charcoals, besides the animal, have this power of abstracting lime; 9. That by the aid of the decolorimeter, or graduated tube charged with test solution of indigo or molasses, it is easy to appreciate exactly the decolouring properties of all kinds of charcoal.
Different varieties of lignite (fossilized wood) or even pit coal, when well carbonized in close vessels, afford a decolouring charcoal of considerable value. By reducing 100 parts of clay into a thin paste with water, kneading into it 20 parts of tar, and 500 of finely ground pit coal, drying the mixed mass, and calcining it out of contact of air, a charcoally matter may be obtained not much inferior to bone-black in whitening syrups.
The restoration of animal charcoal from burnt bones, for the purpose of sugar refining, has been long practised in France. Mr. W. Parker has lately made the following process the subject of a patent. The charcoal, when taken from the vessels in which it has been employed for the purposes of clarifying the sugar, is to be thoroughly washed with the purest water that can be obtained, in order to remove all the saccharine matter adhering to it. When the washing process has been completed, the charcoal is laid out to dry, either in the open air or in a suitable stove, and when perfectly free from moisture, it is to be separated into small pieces and sifted through a sieve, the wires or meshes of which are placed at distances of about two and a half in every inch. This sifting will not only divide the charcoal into small pieces, but will cause any bits of wood or other improper matters to be separated from it.
The charcoal, thus prepared, is then to be packed lightly in cylindrical vessels called crucibles, with some small quantity of bones, oil, or other animal matter mixed with it. The crucibles are then to be closed by covers, and luted at the joints, leaving no other opening but one small hole in the centre of the cover, through which any gas, generated within the vessel when placed in the oven or furnace, may be allowed to escape.
The crucibles are now to be ranged round the oven, and placed, one upon another, in vertical positions; and when the oven is properly heated, gas will be generated within each crucible, and issue out from the central hole. The gas thus emitted, being of an inflammable quality, will take fire, and assist in heating the crucibles; and the operation being carried on until the crucibles become of a red heat, the oven is then to be closed, and allowed to cool; after which the crucibles are to be removed, when the charcoal will be found to have become perfectly renovated, and fit for use as before.