When acetylene is employed, the yield is
pure carbon, for the only by-product is water vapour; but if the carbide
process is adopted, the carbon remains mixed with calcium oxide. Possibly
such a material as Frank's carbide process would give, viz., 36 parts by
weight of carbon mixed with 56 parts of quicklime or 60 parts of carbon
mixed with 112 parts of quicklime, might answer the purpose of a pigment
in some black paints where the amount of ash left on ignition is not
subject to specification. Naturally, however, the lime might be washed
away from the carbon by treatment with hydrochloric acid; but the cost of
such a purifying operation would probably render the residual pigment too
expensive to be of much service except (conceivably) in the manufacture
of certain grades of printers' ink, for which purpose it might compete
with the carbon obtainable by the Hubou process already referred to.
Acetylene tetrachloride, or tetrachlorethane, C_2H_2Cl_4, is now produced
for sale as a solvent for chlorine, sulphur, phosphorus, and organic
substances such as fats. It may be obtained by the direct combination of
acetylene and chlorine as explained in Chapter VI.
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