This section is from the "Henley's Twentieth Century Formulas Recipes Processes" encyclopedia, by Norman W. Henley and others.
For earth cables and exposed strong current wires:
Melt 2 parts of asphalt together with 0.4 parts of sulphur, add 5 parts of linseed-oil varnish, linseed oil or cottonseed oil, keep at 320° F. for 6 hours; next pour in oil of turpentine as required.
Maintain 3 parts of elaterite with 2 parts of linseed-oil varnish at 392° F. for 5 to 6 hours; next melt 3 parts of asphalt, pour both substances together, and again maintain the temperature of 392° F. for 3 to 4 hours, and then add 1 part of linseed-oil varnish and oil of turpentine as required.
An insulating material which contains no caoutchouc is made by dissolving natural or coal-tar asphalt in wood oil, adding sulphur and vulcanizing at 572° F. The mixture of asphalt and wood oil may also be vulcanized with chloride of sulphur by the ordinary process used for caoutchouc. Before vulcanizing, a solution of rubber scraps in naphthalene is sometimes added and the naphthalene expelled by a current of steam. Substitutes for hard rubber are made of natural or artificial asphalt combined with heavy oil of tar and talc or infusorial earth.
Most of the insulating materials advertised under alluring names consist of asphalt combined with rosin, tar, and an inert powder such as clay or asbestos. Some contain graphite, which is a good conductor and therefore a very undesirable ingredient in an insulator.