Lampblack and Soot

It is well known that the luminous flame of a candle or of coal-gas deposits carbon upon a cool surface brought in contact with it. This is because the combustion of carbon particles in the flame, whose incandescence is the cause of luminosity, is prevented by cooling.

If a flame is not properly ventilated it will smoke and deposit soot without artificial cooling; and this is the more likely to happen if the proportion of carbon in the burning vapour is large. Thus the hydrocarbon vapours arising from coal burning in the grate smoke and deposit soot in the chimney, and hydrocarbons such as benzene vapour and acetylene burn with very smoky flames unless their combustion is carefully regulated.

Frank's process of obtaining soot consists in burning acetylene with carbon monoxide or dioxide according to the following reactions:

C₂H₂ + CO = 3C + H₂O; 2C₂H₂ + CO₂ = 5C + 2H₂O.

Lampblack is made by burning tar or resinous matter, or aromatic compounds such as naphthalene, in an insufficient supply of air, and collecting the smoke on cloths, metallic plates, or revolving cylinders. The lampblack is partially purified by being heated in closed vessels, but even then contains about 20 per cent, of oily impurities, which are removed by ignition in a current first of chlorine, then of hydrogen, as in the case of sugar-charcoal.

The density of lampblack is 1.78, but rises to 1.87 when the lampblack is heated for some hours at 910° C.; by the same treatment the ignition temperature rises from 371° to 476° C.

The finest quality of lampblack is used for making indian ink, and in calico-printing; common lampblack is employed as the basis of printers' ink and black paint.