The two patents upon which the suits for infringement rested principally were No. 249,970, granted to N.C. Mitchell, in 1881, and No. 300,720, granted to the same, in 1884. About the same time the Rubber Reclaiming Company, formed in 1890 by the combination of five leading rubber reclaiming plants, and working under license from the company above named, was resolved into the original elements. There were about that time five other rubber reclaiming plants in the United States, operating either the "acid" or the "mechanical" process, besides nine general rubber factories producing their own reclaimed rubber by the "acid" process. While several of the latter - rubber shoe concerns controlled by the United States Rubber Company - have been consolidated, there has been an increase in the number of rubber manufacturers reclaiming their own rubber, since the end of the patent litigation, so that the total number of reclaiming plants now probably is twenty.

The first step in any process for reclaiming rubber is the grinding of the waste, for which purpose several machines have been designed specially, an early patent for disintegrating rubber scrap by "subjecting it to the abrading action of grindstones" having failed to meet with favor. The most usual chemical treatment is a bath in a solution of sulphuric acid in lead-lined tanks. Generally heat is employed to hasten the process, through the medium of steam, in which case the tanks are tightly closed. The next step is the washing of the scrap, to free it of acid and dirt, after which it is sheeted by being run between iron rollers and hung in drying rooms. As soon as it has become dry it is ready for sale.

In the extended litigation over the acid process patents, the points at issue related to the strength of the acid named in the various specifications and also to the methods of applying steam. Prof. Charles F. Chandler, called as an expert in one case, testified that the effects of acids, such as sulphuric or hydrochloric, upon rubber and rubber compounds, under varying strength and temperature, had been known at a period antedating all the patents then the basis of suits for infringement; also that their effect upon cotton and woolen fabrics had been equally well known. They had the same effect upon fibers, whether the latter were combined with rubber or not, but very strong acids would affect the rubber injuriously. The line of defense in this case was that "no invention was required in selecting the strength of acid; only the common sense of the manufacturer, aided by his skill and experience, was necessary to bring about the proper results." In support of this a factory superintendent testified that varied stocks required skill and judgment in their treatment and more or less variation as to the strength of acid, temperature, etc.

As to the use of steam, Prof. Henry B. Cornwall, of Princeton College, called as an expert in another case, testified that, having put to a test the specifications in all the patents involved, he had found it necessary in no case to inject live steam into the mixtures of acid and rubber scrap in order to effect the decomposition and removal of either woolen or cotton fiber. The use of the acids specified was sufficient for this, and the various high temperatures called for were not essential for the destruction of the fibers. He neglected to mention, however, that the steam served an equally important purpose in devulcanizing the rubber.

It appeared that the practice in different factories had included the use of sulphuric acid varying from a 2½ per cent. solution to the full commercial strength of the acid, but one of the defendant companies based their case upon their use of acid of the strength of 28° to 30° Baumé, whereas the patent they were charged with infringing specified a strength of 66°. Their tanks were lead-lined and provided on the interior with steam pipes running down the sides and along the bottom, the sections at the bottom being perforated and the steam admitted at a pressure of 75 to 80 pounds. The chemical treatment lasted from 2½ to 4 hours.

The sulphuric acid treatment, however, is confined mainly to scrap containing cotton fiber. Where woolen fibers occur, which is much less frequently, their disintegration is accomplished generally by the use of caustic soda.

In the mechanical process of reclaiming rubber, the rubber is separated from the fiber, after the whole has been finely ground, by means of an air blast, the method being not unlike that practiced by furriers for separating hair and fur from bits of pelt after skins have been finely divided. As the powdered waste comes from the blower, the rubber falls in a heap near the machine, while the particles of fiber, being lighter, are carried far enough away to make the separation complete. Devulcanization in this case is effected by exposure to live steam at a high temperature. No oil is used in the process, the sheeting of the product being facilitated by means of hot friction rollers.

The cost of reclaiming rubber by the acid process is less than by mechanical means, for which reason the former is now much more generally used. But some manufacturers are willing to pay more per pound for mechanically-reclaimed rubber, either (1) because it can be "compounded" more heavily than the acid product, or (2) because of certain inherent disadvantages of the latter. It is the testimony of these manufacturers that the action of sulphuric acid upon whiting (one of the most common adulterants used in rubber shoes) is to turn it into sulphate of lime - an ingredient which is far from advantageous in a rubber compound. Again, any acid which may remain in the reclaimed rubber is liable to rot thin textile fabrics with which it may be combined in manufacture. Finally, rubber recovered by the chemical process, it is claimed, is harder than that obtained by any other; so that it is usual to add, during vulcanization, in order to soften the product, the residuum obtained from petroleum manufactures, or palm or other oils. Unvulcanized rubber clippings also have been used for this purpose.