on the character of the wood-soot from which it is made ; sometimes this contains much tarry matter which is not completely extracted from the bistre by the washing operation; the larger the quantity present in the bistre the more fugitive is the pigment. The tarry matter oxidises on exposure to light and air, and the tint becomes, in consequence, paler.

ULMIN BROWNS are pigments made by heating organic matter with alkalies; they are not used, as they are too fugitive.

ASPHALT or BITUMEN OF JUDEA was used as a pigment by many of the older artists; but, as time has brought out its many defects, artists have ceased to use it.

It enters very largely into the manufacture of varnishes—partly as a colouring matter, partly as a resinous matter. It is described in the section on varnish materials.

[blocks in formation]

NEARLY all the black pigments in use, certainly all those which are most used, are composed either of carbon itself or have that element as their colouring principle. Although carbon exists naturally, yet its native form is not used as a pigment in painting, as it lacks the properties required for that purpose ; therefore all the black carbon pigments are made artificially.

Such carbon-blacks are known under a variety of names; lampblack, vegetable black, carbon-black, are almost, especially the last two, pure carbon; animal-black, bone-black, ivory-black, drop-black, Frankfort black, are blacks prepared from animal and vegetable matters, and contain various other constituents besides carbon.

Besides the pigments just named, and which are specially prepared for use as pigments, carbon is also obtained in other forms, such as coke, charcoal, soot, &c. Some of these are more or less black, and they have been proposed for use as pigments after being subjected to grinding and washing, but they do not make good pigments, and it is doubtful whether they are so used at the present time. Certain natural minerals, such as coal and carbonaceous shale, have also been proposed to be used as pigments, but, as with those just noted, the proposal has probably never been put into practical use.

Carbon is an elementary body belonging to the group of nonmetals; its chemical symbol is O, and its atomic weight 12. It is a combustible body in all its forms; in burning it combines with oxygen to form carbon dioxide (carbonic acid gas), CO2, whence it follows that all the black pigments of which carbon forms the principal or only constituent are combustible; a fact which sometimes makes itself apparent in a disagreeable form during the process of manufacture. It is a perfectly stable element, and will remain unaltered by exposure to the atmosphere for any length of time.

Acids and alkalies have no action on carbon. All forms of

carbon-blacks are perfectly permanent pigments, and are unsurpassed in permanency by any other pigment. They can be mixed with all other pigments without bringing about any alteration.

Some of the blacks have slight peculiarities, as will be noticed in the descriptions of them.

LAMP AND VEGETABLE BLACKS. These two pigments are closely allied as regards the method of their preparation and their composition; as a matter of fact they are made together at the same time and by the same operation. Lamp-black is probably the most common and most used of the black pigments. Essentially it is a kind of soot. Whenever a combustible body, such as an oil, or fat, or grease, is burnt under such conditions as to preclude complete combustion, then a large volume of smoke is produced, and this deposits a black soot on any surface it may come in contact with ; such soot has a strong black colour and is highly prized as a pigment. Owing to the fact that the earliest convenient means of producing this black was by burning the oil in a lamp under conditions, easily attained, which would ensure that the combustible would not be completely burnt, the black has derived its name of lamp-black. Very little lamp-black is now made by burning oil in a lamp, partly because materials are now used in its preparation which cannot be burnt with good results in a lamp.

The materials used in the manufacture of lamp and vegetable blacks are very varied, and comprise all kinds of oils, fats, coaltar oils, and greases; in fact, anything that will yield a great deal of black smoke while burning, preference being given to those which are cheapest and least available for any other purpose. There are some differences in the quality of the blacks yielded by the different kinds of materials used; the fatty oils and greases yield the best blacks; the hue is better and the black is finer and less greasy than that from any other kind of grease. The greases from coal-tar give fair blacks; they are rather browner in hue than the blacks from the fatty oils, and more inclined to be oily from some of the material volatilising at the high temperature at which it is burnt. The residues from the distillation of shale give fair blacks, but are liable to contain traces of volatile unburnt matter. This oily volatile matter in the blacks from coal-tar and shale greases has the effect of causing the black to be a bad drier when used as an oil-paint.

The process of manufacture of lamp-blacks consists essentially in burning the material and collecting the soot. The plant

required consists, then, of two parts-1st, a lamp or furnace in which the material is burnt; 2nd, chambers in which the black collects.

1st Method.-An old method (not much used now as it is only capable of making lamp-black from liquid oils which are comparatively costly) consisted essentially in burning oil in lamps and collecting the soot. One of the most modern forms of the plant used is shown in Fig. 24, from which it will be seen to consist of a lamp, A, constructed on the bird-fountain principle; the shape of the lamp varies in different places, but the essential features consist of a wick-holder, a, constructed to burn a short, wide, or thick wick, to which a liberal amount of oil is supplied

[blocks in formation]

Fig. 24. —Plant for making lamp-black, by the pipe, b, communicating with the bottom of the oilcontainer, c; by thus giving a plentiful supply of oil a very smoky flame results, which is the condition necessary to produce the largest amount of lamp-black. There should be a cup, d, at the bottom of the burner to catch

any oil which may overflow from the top of the wick-tube. The oil-container, c, is so constructed that it is quite air-tight when placed in position, and can only be supplied with air by an air-tube which connects the wick-tube with the oil-container; this air-tube is small, and is so arranged that when the wick-tube is full of oil the opening into the tube is closed, thereby excluding the air, and so stopping the flow of oil from the oil-container, c; when, owing to the consumption of oil during the burning, the level of the oil in the wick-tube falls below the opening of the air-tube, then air passes into the oil-container and causes some oil to flow out into the wick-tube through the tube or pipe, b. By this means the supply of oil to the wick can be kept very uniform during the process of

manufacture, which uniformity is a necessary condition in the successful making of lamp-black. The collecting chambers consist of a series of strong cylindrical jute or linen bags, B, B, B, B, which alternately communicate with one another at the top and bottom, as shown in the drawing; these bags are suspended by means of chains from a hook in the ceiling of the shed or room in which the operation is carried on. Over the burner of the lamp is placed a large funnel, C; this opens into a large pipe, D; from this proceeds a pipe passing into the top of the first bag; the soot from the lamp passes up the funnel and into the large pipe, D; here some of the unburnt oily matter (which nearly always accompanies the soot) collects; then the soot passes on into the bags, the heaviest black collecting in the first bags, while the finer black passes on into the last bags; the heavier portions are sold as lamp-black, while the finer portions are sold as vegetable black. The bottoms of the bags are made to open, so that the black can be shaken from the bags into barrels placed underneath. The bags may be made to communicate with one another by means of large curved tubes; or they may communicate by means of short, straight tubes placed near the top of the bags (see Fig. 24). The collecting bags are about 12 to 15 feet in length and about 3 feet in diameter. The last bag communicates with a chimney, so as to secure the necessary draught and ensure the black being drawn through the bags. It is usual to place a flue between the bag and the chimney; in this flue are placed a number of gauze frames on which collect the last portions of th black. Usually a number of these apparatuses are placed side by side.

The black in the first two bags is kept separate from the black in the other bags, as it is liable to contain unburnt oil, which is a frequent cause of the black entering into spontaneous combustion. To prevent this occurring it is customary to calcine this oily black in a closed furnace, thereby destroying this oil and making the lamp-black better for use as a pigment.

One of the objections to this form of plant is the inflammable character of the material with which it is made ; and as it is liable to take fire its use is becoming obsolete, especially as a great deal of waste greases from coal-tar, shale-oils, &c., are used in the preparation of these blacks, which greases are not suitable for burning in lamps.

2nd Method.--The method most commonly used for the production of lamp and vegetable blacks, especially where heavy oils (such as creosote or anthracene oils from coal-tar and the residues of shale-oil distilling) are used, which will not burn

« VorigeDoorgaan »