Lampblack is an exceedingly light, dull-black powder, formed by the imperfect combustion of oils, fats, resins, etc. It may be prepared on a small scale by suspending a small tin-plate funnel over the flame of a lamp fed with oil, tallow, or crude naphtha, the wick being so arranged that it shall burn with a large and smoky flame. Dense masses of this light carbonaceous mutter gradually collect in the funnel, and may be removed from time to time. The funnel should be furnished with a metallic tube to convey the gases away from the room, but no solder must be used in making the connections. An especially fine quality of lampblack is obtained from bone-oil deprived of the ammonia with which it is always contaminated.
A process has been devised by Martin and Grafton for the preparation of lampblack from coal-tar, which affords a very good product. The coal-tar is first stirred up energetically with limewater, and the mixture is allowed to stand until the coal-tar has subsided to the bottom, when the lime-water is drawn off. The tar is then well washed by decantation with hot water, and rectified in the ordinary naphtha still. Afterwards it is run into a long iron cylinder, which is placed over a furnace, and supplied with numerous large burners. Each burner has a metallic funnel placed immediately above it, connected with a cast-iron pipe, into which all the fumes from each burner are conducted. The naphtha in the cylinder is heated almost to the boiling-point by the furnace beneath. A series of smaller pipes lead away the fumes from the main pipe into a row of chambers, and thence into a series of large canvas bags, placed side by side, and connected alternately at top and bottom. The bags vary in number from 50 to 80, the last one being left open to allow the smoke to escape, after traversing some 400 yd. since leaving the burners. The best quality of lampblack is found in the last bags, that near the furnace being much coarser and less pure.
The bags are emptied whenever they contain a sufficient quantity.
The process employed in Germany for the manufacture of lampblack is to conduct the products of the combustion of any resinous matter in a furnace into a long flue, at the end of which is placed a loose hood, made of some woollen material, and suspended by a rope and pulley. The lampblack collects in this hood, and, when a sufficient quantity has accumulated, is shaken down and removed. In this manner about 6 cwt. of lampblack may be collected in 24 hours.
In England, an inferior variety is sometimes obtained from the flues of coke-ovens. That known as " Russian lampblack " is made by burning chips of resinous deal or pine-wood, and collecting the soot formed; but it is objectionable, owing to its liability to take fire spontaneously when left for a long time moistened with oil.
The lampblack made in these ways is generally purified by calcination, in order to remove the empyreumatic oils which it invariably contains. This is effected in close vessels, and the product is called "burnt" lampblack, and is especially useful as a water-colour. The particular virtue of lampblack as a pigment lies in its state of extremely fine division, which could not possibly be attained by other means*, this quality renders it invaluable as the basis of black pigments, all of which contain it in a greater or less quantity. Indian ink and printers' ink are also composed principally of this substance.
At Petrolia the flames of several thousand natural gas-jets are made to impinge against sheets of slate, on which the smoke or fine carbon is deposited, just as a piece of glass is smoked when held over a candle flame. When a sufficient deposit of the smoke has formed on the slates, it is scraped off, packed, and sent to market. This product proves to be of a perfectly black tint, and to contain no oily matter, while on combustion it leaves only a slight trace of ash, composed of the oxides of iron and copper.
The transport of lampblack is effected in barrels or bags; when in the latter, these should be. previously soaked in water containing some clay in suspension, which stops up the pores of the sacking, and prevents loss.