Encyclopædia Britannica, Ninth Edition/Tar
TAR is a product of the destructive distillation of organic substances. It is a highly complex material, varying in its composition according to the nature of the body from which it is distilled,—different products, moreover, being obtained according to the temperature at which the process of distillation is carried on. As commercial products there are two principal classes of tar in use—(1) wood tar, the product of the special distillation of several varieties of wood, and (2) coal tar, which is primarily a bye-product of the distillation of coal for the manufacture of illuminating gas. These tars are intimately related to the bitumen, asphalt, mineral pitch, and petroleum obtained in very many localities throughout the world.
Wood Tar.—Wood tar, known also as Stockholm and as Archangel tar, is principally prepared in the great pine forests of central and northern Russia, Finland, and Sweden. The material chiefly employed is the resinous stools and roots of the Scotch fir (Pinus sylvestris) and the Siberian larch (Larix sibirica), with other less common fir-tree roots. A large amount of tar is also prepared from the roots of the swamp pine (P. australis) in North and South Carolina, Georgia, and Alabama, in the United States. In the distillation of wood a series of products, including gas, tar, pyroligneous acid and wood spirit, and charcoal may be obtained, and any of these may be the primary object of the operation. When tar is the substance sought, the ancient and crude method of working is yet largely adopted in the north of Europe. The wood to be treated is closely piled up into a huge conical stack or pile on an elevated platform, the sole of which is covered with clay and tiles. The sole slopes inwards from every side to the centre, where an opening communicates with a vaulted cavity under the elevated platform. The pile of wood is closely covered over with layers of turf and earth or sand to a depth of several inches, but leaving at first near the bottom numerous apertures for the admission of air to promote ignition. The pile is ignited from below, and as the fire spreads through the heap the various apertures are closed up and a slow smouldering combustion goes on for some days till, by the sinking of the pile, the top of the stack falls in, and a bright flame springs up at that point. About ten days after ignition tar first begins to flow, and it is at once collected into barrels. According to the size of the pile, the distillation may continue several weeks, the tar secured amounting to about 17·5 per cent. of the wood operated on. In this method several valuable products—the gas, the crude pyroligneous acid, and much charcoal—are lost or wasted; and a more economical process of treating the wood in closed stills or retorts is now largely used in Russia, the gas evolved serving as fuel under the retorts. The heavier tar products of the distillation collect at the bottom of the retort, whence they are carried off by a pipe to a receiver; the volatile portion passes off at the upper part of the retort, and is separately condensed, the lightest portion passing through a worm condenser. From treatment in close retorts resinous roots yield from 16 to 20 per cent, of tar, with some oil of turpentine and pyroligneous acid.
Wood tar is a semi-fluid substance, of a dark brown or black colour, with a strong pungent odour and a sharp taste. Owing to the presence of acetic (pyroligneous) acid, which is a collateral product, it has an acid reaction; it is soluble in that acid, as well as in alcohol and the fixed and essential oils, &c. Tar consists essentially of a mixture of homologous hydrocarbons, and by redistillation it can be fractionated into a series of bodies having fixed boiling points. Some varieties of tar have a granular appearance, from the presence of minute crystals of pyrocatechin, which dissolve and disappear on heating the substance. Pyrocatechin dissolves freely in water, and to it the tar water (liquor picis) of pharmacy probably owes its value.
Crude tar from retorts, when submitted to redistillation, gives off wood spirit (methyl-alcohol), and then acetic (pyroligneous) acid, and finally, on forcing the heat, pitch oil is driven off. The residuum left in the still hardens into a solid vitreous mass, which forms the black pitch of commerce. Tar and pitch are most largely used as protective coatings for woodwork and other materials much exposed to water and the weather. Thus tar is of great value in connexion with shipbuilding and shipping generally. A considerable quantity is used in manufacturing tarred ropes, and in the "smearing" of highland sheep to afford a protection against the weather. Pitch also is the basis of the Berlin black or Brunswick black used for coating cast-iron goods and for “japanning” preparations.
Coal Tar.—The art of distilling coal for the production of tar was discovered and patented by the earl of Dundonald in 1787, and till the general introduction of coal gas some amount of coal was yearly distilled in Scotland for the production of coal tar. The demand for the substance was limited, it being principally used for coating iron castings and smith work, for making an inferior lamp black, and as a source of a solvent oil. With the extensive use of coal gas the necessity for this separate distillation ceased, and soon tar was produced in the manufacture of gas in quantities that could not be disposed of. It was burned up for heating gas-retorts; it was mixed with coal dust, sawdust, &c., for making patent fuel; and it was distilled for producing a series of hydrocarbon oils, heavy tar, and pitch; but it was only after the discovery and introduction of “tar-colours” that the substance came for some time to be really valuable. Since that time its price has fluctuated greatly; and in the United Kingdom alone there are now distilled annually about 10,000,000 tons of coal for gas-making, producing 120,000,000 gallons of crude tar,—a quantity greatly in excess of the ordinary demand.
If wood be distilled slowly at low temperatures, the gases consist chiefly of carbonic oxide and carbonic acid, mixed with only very little of carburetted hydrogens, and consequently little luminous on combustion; the watery part of the tar includes relatively much of methyl-alcohol, acetone, and acetic acid; the oily part of the tar (tar proper) has a certain proximate composition characteristic of this mode of distillation. Our present knowledge in regard to this last-named point is very incomplete; of definite species the following have been discovered:—
(1) Phenol, (synonym carbolic acid).
(2) Cresol,
(3) Phlorol,
(4) Pyrocatechine, , one of three isomerides.
(5) Guaiacol, methyl-ester of No. 4.
(6) Homo-pyrocatechine,
(7) Creosol, methyl-ester of No. 6.
Genuine creosote consists of (1), (2), (5), and (7). In addition, there are numberless bodies which still await scientific definition.
If the distillation of wood is carried out at a very high temperature,—if, for instance, the wood is placed in a relatively large retort previously brought up to a bright red heat and kept at such temperature, or if the vapours produced at a relatively low temperature are passed through intensely heated pipes before reaching the condenser (Pettenkofer’s method for producing illuminating gas from wood),—the gas produced contains a considerable admixture of luminiferous hydrocarbons, the proportions of methyl-alcohol, acetone, and acetic acid get less, and the tar proper assumes more of the character of coal-gas tar (see below). Similar observations we make in the case of coal. About 1862 Wigan cannel coal used to be distilled industrially at low temperatures to produce "light oils." Schorlemmer examined these and found them to consist chiefly of "paraffins" (see Paraffin) from C5H12 upwards. A similar result is obtained with ordinary coal, although in its case the "benzols" are more largely represented. If we distil any kind of coal at high temperatures—i.e., in the way customary for illuminating-gas making—the distillable part of the tar proper consists chiefly of benzene, C6H6, and benzene-derivatives, i.e., benzols, C6H6+nCH2; phenols, C6H6O, and homologues, (C6H5.nCH2)OH; amido-bodies, C6H5NH2 (aniline), and homologues; condensed benzols, such as naphthalene, C10H8 = 2C6H6 - C2H4; anthracene, C14H10 = 3 C6H6 - C4H8; chrysene, C18H12 = 4C6H6 - C6H12, &c. The paraffins then become an altogether subordinate feature.
A great and meritorious research of Berthelot's has thrown considerable light on the chemical mechanism of dry distillation. As found by him, even the most complex of the substances named are producible by the interaction upon one another of a few bodies of very simple constitution, or even one or other of these by the mere action of a high temperature. To give a few examples. Marsh-gas, CH4, when passed through red-hot tubes, yields olefines, C2H4, C3H6, C4H8, &c., with elimination of hydrogen, H2. The same CH4, if subjected to a spark-current {i.e., local application of intense heat), yields acetylene and hydrogen, 2CH4=C2H2+3H2, and the acetylene produced passes partly into benzene, C6H6=3C2H2. Ethylene, C2H4, when passed through a porcelain tube kept at a moderate red heat, yields benzene, C6H6, styrolene = phenyl-ethylene, C2H3.C6H5, naphthalene, and perhaps also its hydride, C10H10. Acetylene, qua potential benzene, and ethylene yield styrolene and hydrogen, C6H6 + C2H4 = C6H5.C2H3 + H2; and styrolene plus ethylene yields hydrogen and naphthalene, C10H8.
Benzol at a high temperature loses hydrogen, and, so to say, doubles up into di-phenyl, C12H10; and this latter, when heated with ethylene, yields anthracene, C14H10, and hydrogen, C12H10 + C2H4 = C14H10 + 2H2. Conversely, hydrogen may, so to say, turn out its equivalent of a hydrocarbon; thus, for instance, chrysene, C18H12 + 2H2, yields di-phenyl, C12H10, + benzene, C6H6.
Pyrogenic reactions generally are reversible; thus, any of the following three equations is correct, whether we read it from the left to the right or from the right to the left:—
(1) C2H6 (ethane), at a red heat becomes C2H4 + H2.
(2) C12H10 + C6H6 = C18H12 + 2H2.
(3) C14H10 + 2H2 = 2C6H6 + C2H2.
Hence no single pyrogenic reaction goes to the end; if it does not, so to say, check its own progress, other secondary reactions set in and do so, the general result being that ultimately, but in general slowly, a state of dynamic equilibrium is attained in which a set of synthetic reactions on the one hand and a set of analytic reactions on the other compensate one another.
Industrial Working of Coal Tar.[1]—Coal tar, as it comes from the gas-works, is used for a variety of purposes, such as—(1) for fuel, the tar being made into a spray by means of a steam-injector and the spray kindled (2) for the preservation of building materials, porous stones, and bricks, &c.; (3) for making roofing-felt (in 1868, five-sixths of the 9000 tons of tar produced at the Berlin gas-works was thus utilized; the case, however, is different now); (4) for making a low quality of lamp-black. At present, however, most of the tar produced, in centres of industry at least, is worked up by distillation. The tar as it comes from the gas-works is allowed to rest in a "pond" until the tar-water (solution chiefly of ammonia and certain ammonia salts) has gone to the top. The tar proper is then pumped into a large wrought-iron still (of upright-cylinder form preferably) and therein subjected to distillation over a naked fire. A necessary preliminary, however, is the removal of the unavoidable remnant of water, which is best effected by cautiously heating the tar in the still so as to render it more fluid and enable the water to rise to the top and then letting the upper stratum run out by an overflow tap at the side. The distillation is then started. It involves the formation of two sets of volatile products, namely—(1) combustible gases (including sulphuretted hydrogen and bisulphide of carbon vapour), which must be led away to avoid nuisance and danger of fire, and (2) a very complex liquid or semi-liquid distillate. This latter is collected in successive fractions, generally in this manner:—(1) as "first runnings," what comes over at temperatures below 105° to 110°C.; (2) as "light oils," at temperatures between 110° and 210°C.; (3) as "carbolic oil," at temperatures between 210° to 240° C.; (4) as "creosote oil," at temperatures between 240° to 270°C.; (5) as anthracene oil, at temperatures above 270°.
In the earlier part of the "first runnings" and light-oil period the condenser must be kept cold; towards the end it must be kept warm to prevent choking by solidified naphthalene. In practice, the operator does not go entirely by the boiling point, but to a great extent by the specific gravity of the distillate, which, in general, increases as the boiling point rises. As soon as a drop of the last runnings floats in water (exhibits the specific gravity 1), the "light oil" is supposed to be over. That the fractionation is not always and everywhere effected in the same way needs hardly be said. If the manufacture of carbolic acid is aimed at, it is best (according to Lunge) to select the fraction 170° to 230°C. for this purpose. Naphthalene boils as high as 217°, yet a deal goes into this carbolic-acid fraction. As soon as naphthalene begins to crystallize out largely (on cooling down a sample of distillate), the carbolic acid may be presumed to be over. What follows next is put aside as creosote oil, until, after the disappearance of the naphthalene, a new solid product, namely, anthracene, begins to show itself. With any tar that contains a remunerative proportion of anthracene, the anthracene oil is the most valuable of the products, as the raw material for the making of artificial alizarins.
Supposing the anthracene to have been extracted as completely as practicable, the residue in the still consists of "hard pitch," a viscid black fluid which on cooling freezes into a fragile solid. In former times more commonly than now "soft pitch" used to be produced by leaving more or less of the anthracene oil and even creosote oil in the still. At the end of the anthracene stage of the distillation it is as well, if not necessary, to help the very high boiling vapour out of the still by means of superheated steam, and to keep the worm at 100°C. to prevent choking. At a German establishment a vacuum is used with great advantage.
We come now to explain briefly how the several fractions are worked up.
The pitch (which we assume to be "hard pitch") must be run off hot through a tap at the bottom of the still and led into a low-roofed and well closed-in "house," because it would take fire in the open air. After it has cooled down sufficiently in the "house," the pitch is run into pitch-holes in front of the house and allowed to freeze there. The depth of pitch in a hole is about 12 inches. The solid pitch is hacked out with pickaxes and sent into commerce. A superior apparatus for the recovery of the pitch, which precludes all danger of conflagration and many inconveniences of the ordinary system, has been devised for the Paris gas-works by Regnault.[2] Lunge found, from many distillations, that tar from the midland counties yields about 55 per cent, of hard pitch.
Hard pitch is used chiefly for making the following. (1) Asphalt.—The pitch is fused up—perhaps in the still which produced it—with the requisite proportion of creosote and anthracene oil, previously freed from their valuable components. Such asphalt is used for street-paving, i.e., filling up the spaces between the paving-stones, and, in admixture with sand and generally more or less of natural asphalt, for the making of footpaths and floorings generally. In Germany it serves for the making of pipes for conveying acid liquids in works and chemical laboratories, &c. Endless hemp-paper is soaked in liquefied asphalt and wound spirally around an iron core, previously smeared over with soft soap, in about 100 layers. The whole is then exposed to strong pressure while still hot, and is separated from the core after being allowed to cool. Such pipes stand almost any kind of acid, but they must not be used for hot liquids. (2) Varnishes.—The pitch is dissolved in suitable tar oils,—creosote oil for a lower and light oil for a higher quality. (3) Coke.—In former times more frequently than now pitch was made into coke by transferring it to a special flat still and distilling as long as any volatile products came off. The coke which remains is a very pure and consequently valuable fuel. (4) Lamp Black (as a last resource, if no other mode of utilization is practicable).—The pitch is subjected to partial combustion on hot iron plates and the smoke conveyed into chambers to deposit its carbon. The yield is about 40 per cent.
Anthracene Oil.—The oil is allowed to stand cold for a week or so until the anthracene has crystallized out as completely as possible. The mother-liquor is then eliminated, the bulk by means of a filter-press, the rest, at a higher than the ordinary temperature, by hydraulic pressure. The crude product includes far more than half its weight of impurities—phenanthrene, paraffin, naphthalene, &c. To remove these as far as possible, the crude anthracene is ground up and treated with petroleum spirit (boiling at 70° to 100°C.) or coal tar naphtha (120° to 190°), in which real anthracene is relatively insoluble. The insoluble part is separated by filtering arrangements and presses (so constructed as to avoid danger of fire), and at last sublimed, more with the view of bringing it into a customary convenient form than with the object of effecting further purification. Such final anthracene may contain 50 to 65 per cent, of pure substance. The only reliable method for determining its strength, is to convert a known weight into anthraquinone, C14H10O2, by boiling it with a glacial acetic acid solution of chromic acid, separating out the quinone by diluting with water, collecting and weighing the product. One part of quinone corresponds to 0·8658 of anthracene.
Creosote Oil is either used as it is for pickling timber, softening of pitch, &c., or else redistilled to extract from it what there is of anthracene oil and carbolic acid oil, which are worked up with the respective principal quantities.
Carbolic Oil.—Assuming this oil to have been collected (as it should be if intended for the making of carbolic acid) between 170° and 230°, the process of extraction is, briefly, as follows. The oil is mixed with a suitable proportion of caustic-soda ley (ascertained by an assay) in an iron vessel at 40° to 50° C. Charles Lowe recommends ley of 1·34 sp. gr., diluted with water to five times its volume. After settling, the aqueous layer is withdrawn into a lead-lined vessel, and the soda supersaturated by sulphuric acid. Crude carbolic acid rises to the top as an oil, and is withdrawn to be sold as such or purified. See Carbolic Acid.
Naphthalene abounds in the oil left after extraction of the carbolic acid by caustic soda and in the more volatile fractions of the creosote oil. From these it separates out (not completely), on standing, in crystals. These are collected, best in a filter-press, and then subjected to hydraulic pressure to force out the rest of the mother-liquor. The crude naphthalene thus obtained contains an impurity which causes it to become red on standing in the air. To remove it, the crude product is mixed with 5 to 10 per cent. of vitriol of at least 1·7 sp. gr., at a moderate heat (addition of a little binoxide of manganese is an improvement, Lunge); it is then washed, first with water, then with dilute alkali, and lastly again with water, to be ultimately distilled or sublimed. In the latter case it is obtained in the form of thin colourless plates of great beauty. It fuses at 80° C. and boils at 217° C. Naphthalene is used largely in the making of certain tar colours, such as Manchester yellow, C10H6(NO2)2, and the beautiful scarlets and crimsons made y the “farbwerke’’ in Höchst, Germany; these latter are diazo-compounds derived from β-naphthol, C10H6(OH). Coal gas, if impregnated at a suitable temperature with naphthalene vapour immediately before issuing from the burner, gains greatly in luminosity. This is the principle of the “albo-carbon” gas lamps.
First Runnings and Light Oil.—These may be said to include all the industrially valuable "benzols" (taking “benzol” as a generic term for benzol or benzene itself and its higher homologues, C6H6, C7H8, C8H10 &c. As the distiller in most cases does not aim at an actual separation of all the individual benzols from one another, but at the production of certain benzol mixtures demanded by the trade, the mode of working may assume a great variety of forms; yet the first aim in all cases is the same, being the elimination of all the non-benzol from the given oil or oils. For this purpose the light oil is, as a rule, subjected to a preliminary fractionation over a naked fire to split it up into fractions fit to be worked for (crude) benzol (C6H6 and C7H8), for carbolic acid (C6H6O), and to be incorporated with the creosote oil respectively; the carbolic acid is extracted, and the creosote-oil part put aside, and thus one or more mixtures of “benzols” are obtained.
The first runnings contain the bulk of the benzene, (C6H6, and a little of its higher homologues, associated, however, with bisulphide of carbon, low-boiling olefines, traces of carbolic acid, &c. To remove these impurities as far as possible, the oil is thoroughly agitated with concentrated oil of vitriol (which takes up the impurities except the bisulphide of carbon), and the “dirty” acid allowed to settle out. The acid is then withdrawn as neatly as possible, and the residual oil washed, first with water, then with dilute caustic soda, and, lastly again with pure water. The washed oil then is subjected to a preliminary fractionation by distillation over a naked flame in the “crude benzol still.”
The several mixed benzols obtained are subjected finally to a further fractionation in stills worked with steam, to be divided into mixed products known by specific names in commerce. But these we cannot possibly consider here. We will rather give an idea of the way in which the several chemical species (benzene, toluene, &c.) are being isolated in a state of approximate purity to meet the demands of the tar-colour industry. To do so even for one named component by means of ordinary stills would require an endless number of fractional distillations. The work is very materially shortened if, as proposed by Mansfield long ago, we combine the still with an inverted condenser (still-head, dephlegmator), inserted between the still and the worm, and keep that intermediate condenser at a suitable constant temperature, so that all the less volatile part of the vapour is recondensed and sent back to the still. An excellent apparatus of this kind was constructed and worked successfully by Coupier. His apparatus consists of three parts, viz.:—(1) a still heated by means of a coil of close steam pipes; (2) a columnar rectifier—“colonnen-apparat” as the Germans call it,—which communicates with the still, and which is divided into many compartments by horizontal septa so contrived that the vapour in passing from a compartment to the next higher one must bubble through the liquid condensed there from preceding vapour,—an overflow pipe, trapped below by condensate, hindering accumulation of the liquid in any compartment beyond a certain level; (3) a constant temperature still-head, consisting of a succession of communicating ring-shaped tubes, which are immersed in a bath of water or molten paraffin kept at a prescribed constant temperature. Only the most volatile part of the vapour survives as such in the columnar rectifier, the degree of its volatility depending, of course, other things being equal, on the rate at which we distil. This most volatile part suffers partial condensation at the prescribed temperature in the still-head; the condensed parts are sent back to compartments of the “column” by pipes bent into the shape of a U at the point where they join the “column," so as to prevent vapour from entering them. The uncondensed vapour goes to the worm, and is condensed as usual.
To prepare benzene, the still-head is kept at 60° to 70° C. At first a mixture of low-boiling bodies and benzene goes over, which is rejected, but soon pure benzol follows and continues until almost all this component has distilled over. The benzol obtained boils between 80° and 82° C., and consequently is practically pure. In order now to extract the toluene, (C7H8, we raise the temperature of the still-head to 100° C. A small quantity of a mixture of benzene and toluene follows, which is rejected. After it comes a continuous distillation of almost pure toluene, boiling at 110° to 112° C. In a similar manner (relatively) pure xylene, (C8H10, boiling point 137° to 140°, and tri-methyl-benzene, (C9H12, boiling point 148° to 150°, can be extracted successively; but the process becomes troublesome with anything above toluene on account of the high temperatures involved for still and still-head. Coupier’s apparatus is now superseded by other constructions, but they all work on the same principle,—that of the Coffey still, as used for the rectifying of spirit of wine.
Pure benzene, toluene, and xylene are used largely for the manufacture of tar-colours. The following (and other) mixtures are produced directly from the light oil or first runnings:—
| (1) 90 per cent, benzol initial boiling point 82° C. | initial boiling point | 82° C. |
| (2) 50 per cent, benzol | ,,„ | 88 |
| (3) ‘‘Toluol" | ,,„ | 100 |
| (4) Carburetting naphtha | ,,„ | 108 |
| (5) Solvent naphtha | ,,„ | 110 |
| (6) Burning naphtha | ,,„ | 138 |
No. 4 serves for enriching coal-gas and adding to its luminiferous power, No, 5 for varnishes, No. 6 for feeding primitive lamps used in the open air, where smoke is no objection.
The following percentage table for the tar from the Berlin gas-works (given in Chemische Industrie for 1879) gives an idea of the quantitative composition of this most complex material:—
| Benzol (including toluol, &c.) | 0.80 | |
| Higher benzols | 0.60 | |
| Crystallized carbolic acid | 0.20 | |
| Cresol for disinfecting purposes | 0.30 | |
| Naphthalene | 3.70 | |
| 5.60 | ||
| Creosote oil | 24.00 | |
| Anthracene (pure) | 0.20 | |
| Pitch | 55.00 | |
| Water and loss | 15.20 |
- ↑ For wood tar, see Wood Spirit and Vinegar.
- ↑ It is described in Lunge’s Treatise on the Distillation of Coal Tar, London, 1882. to which this article is largely indebted.