Charcoal for chemical purposes may be extemporaneously prepared by calcining pieces of wood covered with sand in a crucible, till no more volatile matter exhales.
The charcoal of some woods contains silica, and is therefore useful for polishing metals. Being a bad conductor of heat, charcoal is employed sometimes in powder to encase small furnaces and steam-pipes. It is not affected by water; and hence, the extremities of stakes driven into moist ground are not liable to decomposition. In like manner casks when charred inside preserve water much better than common casks, because they furnish no soluble matter for fermentation or for food to animalcules.
Lowitz discovered that wood charcoal removes offensive smells from animal and vegetable substances, and counteracts their putrefaction. He found the odour of succinic and benzoic acids, of bugs, of empyreumatic oils, of infusions of valerian, essence of wormwood, spirits distilled from bad grain, and sulphureous substances were all absorbable by freshly calcined charcoal properly applied. A very ingenious filter has been constructed for purifying water, by passing it through strata of charcoal of different fineness.
When charcoal is burned, one third of the heat is discharged by radiation, and two thirds by conduction.
The following table of the quantity of charcoal yielded by different woods was published by Mr. Mushet, as the result of experiments carefully made upon the small scale. He says, the woods before being charred were thoroughly dried, and pieces of each kind were selected as nearly alike in every respect as possible. One hundred parts of each sort were taken, and they produced as under:—
| Lignum Vitæ | afforded | 26·0 of charcoal of a greyish colour, resembling coke. |
| Mahogany | 25·4 tinged with brown, spongy and porous. | |
| Laburnam | 24·5 velvet black, compact, very hard. | |
| Chesnut | 23·2 glossy black, compact, firm. | |
| Oak | 22·6 black, close, very firm. | |
| Walnut | 20·6 dull black, close, firm. | |
| Holly | 19·9 dull black, loose and bulky. | |
| Beech | 19·9 dull black, spongy, firm. | |
| Sycamore | 19·7 fine black, bulky, moderately firm. | |
| Elm | 19·5 fine black, moderately firm. | |
| Norway Pine | 19·2 shining black, bulky, very soft. | |
| Sallow | 18·4 velvet black, bulky, loose and soft. | |
| Ash | 17·9 shining black, spongy, firm. | |
| Birch | 17·4 velvet black, bulky, firm. | |
| Scottish Pine | 16·4 tinged with brown, moderately firm. |
Messrs. Allen and Pepys, from 100 parts of the following woods, obtained the quantities of charcoal as under:—
| Beech | 15·00 |
| Mahogany | 15·75 |
| Lignum Vitæ | 17·25 |
| Oak | 17·40 |
| Fir | 18·17 |
| Box | 20·25 |
It is observable that the quantities obtained by Messrs. Allen and Pepys are in general less than those given by Mr. Mushet, which may be owing to Mr. Mushet not having applied sufficient heat, or operated long enough, to dissipate the aqueous matter of the gaseous products.
To those persons who buy charcoal by weight, it is important to purchase it as soon after it is made as possible, as it quickly absorbs a considerable portion of water from the atmosphere. Different woods, however, differ in this respect. Messrs. Allen and Pepys found that by a week’s exposure to the air, the charcoal of