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Vegetable- or Wood-Charcoal (i.e. turn-coal, from O.E. char, to turn; therefore wood turned to coal). - Charcoal-burning is an ancient process, and in countries where wood is plentiful it is still carried out as originally. Logs of wood are piled nearly vertically in a conical mound round a central shaft. The mound is covered with turf or moistened ashes; ventilating holes are left at the bottom. The pile is ignited by brands thrown down the shaft, and thick white smoke, consisting of the volatile products of combustion, escapes up the shaft during the process of burning, the latter being checked if necessary by stopping up the ventilating holes. When the combustion is finished the pile is carefully covered and left to cool for some days. After this the charcoal is removed, and further cooled, if necessary, by water thrown upon it. Sometimes, as in Austria and Sweden, fir logs are piled horizontally for charcoal-burning.

This process is evidently very wasteful, since all the volatile products are lost, except so far as their combustion within the pile helps to maintain its temperature. A process of destructive distillation in closed vessels was known to the alchemists, and is employed at the present day. Wood in small pieces or sawdust is carbonised in cast- iron retorts, and the volatile products are collected. These consist of wood spirit or methyl alcohol, acetone, pyroligneous acid, which is impure acetic acid, and wood-tar or creosote.

The production of charcoal from wood may be compared with the formation of coal from carboniferous vegetation,. the pressure of the superincumbent strata in coal-formation having a similar effect to high temperature. And just as there are different grades of coal, reaching a maximum carbon-content in anthracite, so there are different grades of charcoal, the percentage of carbon in which depends upon the temperature of carbonisation.

Wood begins to turn brown at about 220° C., at 280° C. it becomes deep brown, and at 310° C. black, soft, and friable; when prepared at high temperature charcoal is, however, brittle.

Stein has traced the process of carbonisation by heating wood with water in sealed tubes between 245° C. and 290° C., with the following results:

Temp. ° C.245°250°255°265°275°280°290°
Per cent. Carbon64.369.270.372.874.077.681.3
Hours heating9665555

The amount of carbon in the product depends upon pressure as well as temperature; it never exceeded 78 per cent, under atmospheric pressure, even at red heat.

The following table shows the composition of charcoal produced at different temperatures:

Temp. ° C.270°350°432°1023°1100°1250°1300°1500°Over 1500°
Oxygen with some nitrogen24.0618.6115.2414.1313.799.256.463.030.93

Thus by very strong ignition nearly, but not quite, all the hydrogen can be driven out of charcoal. Wood-charcoal ordinarily contains, however, from 85 to 90 per cent, of carbon and from 2 to 3 per cent, of hydrogen.

During carbonisation wood shrinks in volume as well as losing in weight. Thus 100 parts of wood yield about 65 parts of charcoal by measure and 25 parts by weight.

Density of Charcoal

Since wood shrinks during carbonisation the density of charcoal is greater than that of the wood from which it has been produced. It varies according to the nature of the wood and the temperature of formation of the charcoal, but lies between 1.45 and 2.0. This value, however, applies to charcoal from the pores of which air has been removed. The density of charcoal containing air in its pores varies from 0.1 to 0.2. The effect of removing air from the pores of charcoal may be shown by floating some fragments of charcoal on cold water and then boiling the water for some time. The air is thus driven out of the pores, and the charcoal eventually sinks in the water. The same effect may be produced by placing the charcoal and water in a flask from which the air is continuously pumped.

Absorption of Gases by Charcoal

If a piece of freshly ignited wood-charcoal is introduced into a tube of ammonia gas standing over mercury, the gas is slowly taken up by the charcoal, so that the mercury rises in and may eventually fill the tube. A similar effect may be observed with hydrogen-sulphide gas. This phenomenon, which might simply be called absorption, is termed adsorption because it is essentially a surface phenomenon. It is a general property of solid bodies to condense gases, vapours, and liquids upon their surfaces; this property renders difficult the washing of finely divided precipitates in gravimetric analysis.

The greater the specific surface of a solid the greater its power of adsorption. Now wood charcoal presents an exceedingly great surface by reason of its cellular structure, for the walls of every microscopic cell throughout its mass contribute to the superficial area; therefore, after the adsorbed air has been removed by ignition the charcoal is able to adsorb many times its own volume of another gas.

This phenomenon, which is connected with surface energy, was first observed in 1777 by Scheele and Fontana, but first investigated quantitatively by Saussure, who found that the quantity of gas adsorbed by a solid depended on its properties and its pressure, but that Henry's law of gas-absorption does not apply.

The following results were obtained by Saussure. One volume of charcoal adsorbs the following volumes of various gases measured at 12° C. and 724 mm.:

Ammonia – 90; Nitrous oxide – 40; Oxygen – 9.25; Hydrogen chloride – 85; Carbon dioxide – 35; Nitrogen – 6.50; Sulphur dioxide – 65; Ethylene – 35; Hydrogen – 1.25; Hydrogen sulphide – 55; Carbon monoxide – 9.42;

It will be observed not only that there is a great difference between the volumes of different gases adsorbed, but also that extent of adsorption is connected with condensibility.

Similar experiments were carried out by Hunter.

Subsequent work of Chappuis, Joulin, and Kayser enabled Ostwald to deduce a law of adsorption which is expressed by the equation

c = kam

where c = concentration or pressure of the gas, a the amount adsorbed, while k and m are constants which depend upon the nature and temperature of the gas.

Dewar has shown that the volume of gas adsorbed increases greatly with lowering of temperature, as the following figures indicate:

Vol. at 0° C.Vol. at -185° C.

This property is now utilised for separating the rare gases of the air; for charcoal cooled in liquid air will adsorb oxygen, argon, and nitrogen readily, but has much less influence on hydrogen, helium, neon, krypton, and xenon. On the same account charcoal is employed for producing high vacua, since it adsorbs the last traces of gas from an enclosed space. Condensation takes place during adsorption, and therefore heat must be evolved in the process.

Favre obtained the following results for the volumes of gases adsorbed by 1 c.c. of charcoal at ordinary temperature, and the heats of adsorption per gram of gas:

Gas.Volume adsorbed. c.c.Heat of adsorption. cals.
Hydrogen chloride166274
Hydrogen bromide-191
Hydrogen iodide-173
Sulphur dioxide165168
Nitrous oxide99169
Carbon dioxide97158

Wood-charcoal also possesses the power of adsorbing colouring and other matters from solution, but in this property it is surpassed by animal charcoal, under which these phenomena will be described.

Catalytic Influence of Charcoal

Not only does wood-charcoal adsorb various gases, but it also possesses the power of promoting chemical changes between them.

If, for example, some freshly ignited charcoal is immersed in hydrogen- sulphide gas, and subsequently in oxygen, combustion of the hydrogen sulphide takes place within the pores of the charcoal and sufficient heat is developed to ignite the latter, which then burns brilliantly in the oxygen. It was, moreover, observed by Melsen that when charcoal saturated with chlorine is brought into dry hydrogen formation of hydrogen chloride takes place even in the dark. Further, it was shown by Stenhouse that charcoal not only absorbs obnoxious gases - as, for instance, from sewers - but also causes their oxidation by the atmosphere. On this account charcoal finds practical application as a deodorant.

Thus wood-charcoal has the power of hastening chemical action between adsorbed gases; that is to say, it is a catalyst.
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