In 1870 there were imported 5,132,000 pounds at an average rate of $1 per pound; in 1880 the imports were 17,835,000 pounds, at an average price of 85 cents per pound; in 1890 31,949,000 pounds were imported, at an average price of 75 cents per pound. The present price of India rubber varies from 75 cents per pound for fine Para rubber to 45 cents per pound for the cheapest grade.
It will be seen that, notwithstanding the increase in importations, the price of the raw material remains at a comparatively high figure. Many experiments have been made to find a substance possessing the same properties as India rubber, but which could be produced at a cheaper rate.
Many of the compositions which have been invented have been well adapted for use for certain purposes and have been used to adulterate the pure rubber, but no substance has been produced which could even approach India rubber in several of its important characteristics. There has never been a substance yet recommended as a substitute for rubber which possessed the extraordinary elasticity which makes it indispensable in the manufacture of so many articles of common use.
Great hopes were at one time placed in a product prepared from linseed oil. It was found that a material could be produced from it which would to a certain extent equal India rubber compositions in elasticity and toughness.
It was argued that linseed oil varnish, when correctly prepared, should be clear, and dry in a few hours into a transparent, glossy mass of great tenacity. By changing the mode of preparing linseed oil varnish in so far as to boil the oil until it became a very thick fluid and spun threads, when it was taken from the boiler, a mass was obtained which in drying assumed a character resembling that of a thick, congealed solution of glue.
Resin was added to the mass while hot, in a quantity depending upon the product designed to be made, and requiring a greater or less degree of elasticity.
Many other recipes have been advocated at different times to make a product resembling caoutchouc out of linseed oil in combination with other substances, but all have failed to give satisfaction, save as adulterants to pure rubber.
Among the best compounds in use in rubber factories at present is one made by boiling linseed oil to the consistency of thick glue. Unbleached shellac and a small quantity of lampblack is then stirred in. The mass is boiled and stirred until thoroughly mixed. It is then placed in flat vessels exposed to the air to congeal.