Charcoal, the solid residuum of the destructive distillation of wood. Exclusive of hygroscopic water and ash, it is composed of carbon, hydrogen, oxygen, and nitrogen, in proportions varying with the kind of wood, with its hygroscopic condition, with the temperature of the distillation, and with the duration of the process. It is insipid and inodorous; is a bad conductor of heat and a good conductor of electricity; is insoluble in water; is attacked by nitric acid with difficulty, and is but little affected by the other acids or by the alkalies. Its carbon constituent is exceedingly refractory to heat, and if secluded in a retort will neither fuse nor volatilize under the highest temperature that can be produced. It retains the organic structure of the wood from which it was produced, except when prepared at very high temperature, when it becomes a black, shining, porous mass, resembling fossil coal, with a considerable increase in density, and without a trace of organic structure. Charcoal always contains a certain percentage of ash derived from the wood of which it is made, but that percentage is much less than what is due to the respective weights of the original wood and the resulting charcoal, showing that a portion of the mineral matter in the wood has been carried off by the fluid distillates.
Charcoal is excessively hygroscopic. A fresh preparation of it strongly attracts moisture from the moment it is exposed to the atmosphere, and the quantity thus absorbed at saturation varies with the temperature of distillation, and with the hygro. metric and barometric conditions of the atmosphere. The lower the temperature at which the charcoal is produced, the greater the humidity of the air, and the higher the barometer, the greater is the percentage of moisture absorbed at saturation by the charcoal. Charcoal is hard and brittle; it rings when struck, and resists a strong pressure gradually applied, but is easily broken by a sudden blow, when it shingles like porcelain and shows a glossy, intensely black and glistening fracture, that scarcely soils the hand until after some expo. ure to the air. The quantity of charcoal obtained from the same weight of wood in the same condition, and the ratio of its carbon constituent to its hydrogen, oxygen, and nitrogen, vary with the temperature and time of distillation; the lower the temperature and the shorter the time, the greater is the percentage of charcoal obtained, and this charcoal contains a greater percentage of hydrogen, oxygen, and nitrogen, and less of carbon; but the highest temperature and the longest application fail to wholly expel the volatile constituents of charcoal.
The ordinary charcoal of commerce, prepared in heaps at the temperature of about 700° F., has the density of 0.42 including its pores, and of 1.50 excluding them. In the merchantable state it weighs 15 1/2 lbs. per cubic foot, and requires 144 1/2 cubic feet of space to contain one ton. The following is its chemical constitution by weight, both including and excluding its ash and hygroscopic moisture:
Inclusive of ash and Hygroscopic moisture.
Exclusive of ash and Hygroscopic moisture.
When the distillation is done under a temperature of about 1,500° F., charcoal has the density, exclusive of pores, of about 1.75, and the following chemical constitution by weight, both including and excluding its ash and hygroscopic moisture:
Inclusive of ash and Hygroscopic moisture.
Exclusive of ash and Hygroscopic moisture.
The ash of charcoal consists chiefly of carbonate of potash, silica, lime, and oxide of iron. When wood is distilled to the uttermost, at the temperature of 2500° F., the resulting dry charcoal, exclusive of its pores, has a density of about 2, and contains about 3 per cent, of volatile matter. The specific heat of good charcoal, free from hygroscopic moisture, increases with the temperature, and in the following ratio: At 212° F. the specific heat is 0.2415; at 392°, 0.2441; at 572°, 0.2407; at 752°, 0.2493; at 932°, 0.2519; at 1112°, 0.2545; at 1292°, 0.2571; at 1472°, 0.2597; at 1052°, 0.2023; at 1832°, 0.2649; at 2012°, 0.2675; at 2192°, 0.2701; at 2372°, 0.2727; and at 2552°, 0.2753. - Charcoal absorbs the fixed gases as well as aqueous vapor with great avidity, the volumes at standard temperature and pressure absorbed by the same charcoal varying with the temperature and pres. ure of the gas; the lower the former and the greater the latter, the greater the absorption. In function of pressure alone, the temperature being constant, the same charcoal absorbs volumes of gas nearly, but a little less than, in the direct ratio of the pressure.
The amount of absorption differs greatly for the charcoal of different woods, other things being equal; and for the temperature at which it was formed from the same wood, the lower that temperature the greater the absorption. If charcoal saturated with one gas be exposed to another gas, it will discharge a portion of the former and absorb a portion of the latter, so that it will contain both gases, but not in so great quantities of each as if it were exposed to each alone. Recently prepared charcoal exposed to the atmosphere absorbs the oxygen of the air to a greater extent than the nitrogen. Damp charcoal does not absorb gases so readily nor to the same quantity as that which is dry. Charcoal loses its property of absorption by use, but regains it completely by simple reheating. The absorption of gases is always accompanied by a rise of temperature in the mass. If dry charcoal saturated with sulphuretted hydrogen be introduced into oxygen, the gases combine with explosion and the production of aqueous vapor and sulphurous acid gas; just previous to the explosion a yellow vapor, that of the separated sulphur, exudes from the pores of the charcoal.
If nitrogen be mixed with the oxygen, the above effect is produced more slowly, and the hydrogen alone of the sulphuretted hydrogen is combined, its sulphur being precipitated as a solid. The absorption by charcoal of gases and vapors attains its maximum in about 36 hours; and damp charcoal does not absorb them so readily as that which is dry. This absorbent property of charcoal is ascribed to catalysis, and resembles the action of spongy platinum on certain mixtures of gases; but there is this difference between the two substances, that platinum has the greater combining power, while charcoal has the greater absorbing power. Coarsely powdered charcoal boiled in a solution of the chloride of platinum until thoroughly soaked, and then heated to redness in a closed crucible, retains a portion of the platinum, and possesses remarkable powers of absorption and combination. Charcoal has a great deoxidizing power even at low temperatures; it is, in fact, the great reducing agent of metallurgists. Fresh-burned charcoal of boxwood, free of ashes, introduced into nearly neutral and very dilute solutions of gold, platinum, palladium, silver, and copper, precipitates the metals, which are deposited on the charcoal in thin films. The deposit of copper, if allowed to remain in the liquid, soon disappears.
Zinc, iron, lead, and mercury are precipitated in the same manner, but redissolve in acid liquors. At a red heat charcoal deoxidizes many fixed and volatile metallic oxides (arsenious acid), reproducing their metallic bases. At a white heat it deoxidizes even potassa, soda, and phosphoric acid, setting free in vapor potassium, sodium, and phosphorus. Charcoal produces in the cold a violent explosion with perchloric acid; and at a red heat it deoxidizes the chlorates, perchlo-rates, and nitrates, with deflagration, producing carbonates with the respective alkaline bases. At a full red heat it converts most of the sulphates to sulphites, and it decomposes water; but it has no action on the haloid salts, namely, the chlorides, bromides, iodides, and fluorides; neither does it decompose, even at a very high temperature, silica and alumina. When the vapor of water is passed slowly over charcoal heated to full redness in a porcelain tube, the resulting gas, called water gas, is composed by weight, in 100 parts, of hydrogen 56.03, carbonic oxide 29.15, carbonic acid 14.65, and carburetted hydrogen 0.17. The absorbing and deoxidizing powers of charcoal are greatly diminished by saturation with water. Charcoal has also a strong deodorizing power, referable to the same property.
If air containing sulphuretted hydrogen be agitated with powdered charcoal, or if water containing sulphuretted hydrogen be filtered through charcoal, the offensive odor of that gas is speedily removed. The sulphuretted hydrogen is first absorbed, and then, by a catalytic action, its hydrogen unites with the oxygen of the atmosphere, while its sulphur is deposited. Charcoal absorbs coloring matters. Powdered charcoal agitated with solutions of dilute sulphate of indigo, of cochineal, of the blue iodide of starch, and of the red permanganate of potassa, entirely removes these colors. Port wine, by a similar process, is rendered tawny or light-colored; and impure solutions of sugar and nitre lose their color by filtration through a mass of charcoal. The taste of liquids in some cases is almost removed by filtration through charcoal when the taste depends on the presence of certain organic substances. The bitterness of the hop in ale is thus removed, and strychnia, too, is completely absorbed. The antiseptic power of charcoal is due to the action of its absorbed oxygen upon organic matter, under the influence of which the decomposition of such matter is quickened instead of retarded.
Charcoal, by its possession of these properties of absorption, decomposition, and combination, is eminently fitted as a filter for the purification of water, removing from that liquid the color, odor, and taste of its impurities, by oxidizing and recombining them into other and inoflen-sivo substances. The following gases stand in their respective order as regards their volumes at standard temperature and pressure, which are absorbed at saturation by dry charcoal; the first named being absorbed in least volumes, and the volumes increasing for each gas to the last named; the absolute number of volumes in each case varies enormously with the charcoal of different woods and with the temperature at which it was made: 1, hydrogen; 2, nitrogen; 3, carbonic oxide; 4, oxygen; 5, marsh gas; 6, nitrous oxide; 7, carbonic acid; 8, olefiant gas; 9, sulphurous acid; 10, air; 11, sulphuretted hydrogen; 12, muriatic acid; 13, hydrochloric acid; 14, ammonia. - When air-seasoned wood, that is to say, wood containing about 25 per cent, by weight of hygroscopic water, is slowly charred in heaps at a temperature of about 750° F., about one third of it is wholly consumed in producing the heat required for the charring, leaving two thirds for the portion charred.
The dry charcoal produced is 16 per cent, of the total weight of wood; that is, of the sum of the weight consumed and the weight charred, or (16x100)/64 24 per cent, of the latter. When the wood is slowly distilled in ovens or retorts at the above temperature, then, as the same quantity of heat is required for producing the same effect, for every two pounds put into the retort one pound will be consumed beneath it to produce the heat for distillation. Of the two pounds thus distilled, 24 per cent, is obtained as dry charcoal; or, of the total weight of wood used, that is, of the sum of the weights consumed and the weight distilled (2/3 of 24), 16 per cent, is obtained as dry charcoal. Consequently, for equal weights used of the same wood, the process of distillation in retorts and that of charring in heaps are equally economical; in the former case, however, the gaseous and liquid distillates belonging to two thirds of the wood are saved, while in the latter case they are lost. When the charring or distilling is very rapidly done, the resulting weight of dry charcoal is only 14 per cent, of that of the wood charred or distilled; and the quantity of wood consumed in the production of the heat required for the charring or distilling is equal to the weight of wood charred or distilled; consequently, in this case, only 7 per cent, of the total weight of wood used is obtained as dry charcoal.
The proportion of charcoal obtained from wood varies with the kind of wood, and for the same kind of wood it varies with the temperature and with the duration of the charring or distillation; the lower the temperature and the longer the duration of the process, the greater is the proportion of charcoal obtained. The charcoal also varies in chemical composition with the temperature and the duration of the charring or distilling process. The lower the temperature, other things being equal, the greater is the percentage of volatile matter and the smaller that of fixed carbon in the charcoal. The hygroscopic power of such charcoal is greater than of that prepared at higher temperatures. The longer the process of preparation is continued, other things equal, the less is the percentage of volatile matter and the more that of fixed carbon in the charcoal. The hygroscopic power of such charcoal is less than of that prepared with shorter processes. No temperature at the command of man, and no time of distillation, will wholly expel the volatile constituents of charcoal, namely, the hydrogen, oxygen, and nitrogen. - The inflammability of dry charcoal varies with the kind of wood from which it is made; and for the same kind of wood it varies with the temperature and with the duration of the charring or distillation; the lower the temperature and the shorter the duration of the process, the more inflammable is the charcoal.
The relative inflammability is measured by the temperature of ignition; the lower that temperature, the more inflammable is the charcoal. The inflammability depends on the porosity of the charcoal and on the percentage of its volatile constituents, and both these are greater the lower the temperature of the charring or the distillation and the shorter the time. After a brief exposure of dry charcoal to the atmosphere, its large absorption of hygroscopic moisture materially modifies the above relations, and tends to equalize its inflammability, as the lower the temperature and the shorter the process of the charring or distillation, the more hygroscopic is the charcoal, and consequently the higher is its temperature of ignition. The temperatures of ignition of dry charcoal prepared at various temperatures are as follows:
Temperature of the Charring.
Temperature of Ignition.
It is obvious that the dryer the wood, the greater is the percentage of charcoal obtained from it, and the less is that of the wood required to be consumed in producing the heat for the charring or distillation; for the uncom-bined water in the part of the wood charred or distilled is entirely driven off as aqueous vapor during those processes; while in the part of the wood consumed to produce the heat for the charring or distillation, the uncom-bined water is not only driven off, but also ab-sorbs for its evaporation a portion of the heat due to the combustion of the ligneous matter. A further portion of that heat is taken, too, for the evaporation of the water in the part of the wood charred or distilled, so that the greater the quantity of uncombined water in a given weight of wood, the greater is the percentage of the part required to be consumed in producing the heat for the charring or distilling. Thus, of the total weight of wood used, that is, the sum of the weight charred or distilled and consumed in producing the heat for the charring or distillation, the effect of the uncombined water is to diminish the charcoal product more largely than is due to the simple subtraction of that water by the amount of wood consumed for its evaporation.
This is the only result attending the presence of water in the wood when the temperature of the charring or distillation is kept below that of a red heat. If that temperature be exceeded, however, the aqueous vapor, in passing over the red-hot charcoal, reacts upon its carbon constituent and converts a portion of it into carbonic acid, carbonic oxide, and carburetted hydrogen; all of which pass off in the gaseous form, diminishing the charcoal products by the amount of carbon they contain. If, of two equal weights of the same wood containing equal weights of uncombined water, one weight be dried while the water is allowed to remain in the other, the two masses will contain equal weights of ligneous matter; and if they be charred or distilled at less than a red-heat temperature, under the same conditions, the same weight of charcoal will be obtained from the charred or distilled part of each. One pound of charcoal, having the chemical constitution given for it in the early part of this article, develops sufficient heat by combustion in oxygen to raise the temperature of 12,306.414 lbs. of water, at the temperature of 32° F., one degree, or to evaporate under the atmospheric pressure 12.743 lbs. of water from the temperature of 212° F. One pound of what remains of this charcoal, after deducting its ash and hygroscopia water, will develop sufficient heat by combustion in oxygen to raise the temperature of 14,288.837 lbs. of water, at the temperature of 32° F., one degree, or to evaporate under the atmospheric pressure 14.796 lbs. of water from the temperature of 212° F. Equal weights of charcoal from all varieties of wood, prepared under equal conditions, have the same heating power. - In the ordinary mode of manufacturing charcoal in heaps, the sticks of wood are piled up, sometimes in horizontal and sometimes in vertical layers, around a central opening which extends from the bottom to the top of the heap.
The heaps are conical or hemispherical, from 10 to 30 or 40 ft. in diameter, and of a height of about 12 ft.; their outer surface, after being made even with chips and twigs, is covered with small branches, leaves, straw, or moss, upon which sods are laid together with the charcoal dust of previous burnings. This dust is mixed with sufficient earth to give it consistency, and when moistened the mixture makes the best of all coverings. The sticks of wood to be charred are of any convenient size, and are closely packed, the interstices of the larger sticks being filled with smaller ones. The central vertical opening in the heap is left for a chimney, and for the introduction of the fire to ignite the heap; to facilitate the latter, a horizontal opening is left in the bottom of the heap, extending from its periphery to the central opening. Around the bottom of the heap other smaller openings are made for the admission of air and the escape of the volatile products. As the charring process proceeds, these small openings are closed, and new ones made nearer the top of the heap, and in other places, as it is found desirable to check the process in some parts and to hasten it in others.
The heap is fired in the centre at the bottom, and the fire gradually spreads in all directions, but especially toward the openings by which the air is admitted. The first matter that escapes is aqueous vapor, which partly condenses in the cover of the heap, moistening it, and then passes off with a yellowish color. After the vapor disappears, it is followed by a lighter-colored smoke that becomes black and dense, emitting the odor of pyroligneous acid, which grows stronger to the end of the process. The carbonization, first completed at the centre and top of the heap, gradually extends down its sides; and in a heap half charred, the finished portion has the form of an inverted cone, the apex of which is at the bottom of the vertical axis of the heap. As the line between the finished and unfinished portions moves downward, the openings for the admission of air are kept in advance of it, and upper openings are closed as lower ones are made. The completion of the charring of each part of the heap is indicated by the smoke emerging from that part changing from its black and dense appearance to a transparent light bluish color. The tarry matters, which collect mostly toward the close of the operation, run out in channels made for that purpose beneath and around the heap.
When the entire heap is completely charred, all its openings are closed, and it is then left for one or two days, after which it is partially uncovered and the charcoal drawn out and spread around in thin layers. This is best done at night, so that if any of the charcoal remains ignited, it can be quickly seen and quenched. The time required for the process varies with the size of the heap and the state of the weather. Small heaps may be charred in a week, and large ones may require three weeks. A common yield from one cord (128 cubic feet) of wood is about 30 bushels of charcoal. - Animal charcoal has been described under Bone Black.