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. One of the most successful of our rubber
factory superintendents, who formerly made the reclaimed rubber used
by his factory, has stated that his practice was to subject the
material to an alkaline bath after the acid treatment, not only for
the better cleaning of the rubber, but to neutralize any acid which
might remain. Considering all the points involved, it was his opinion
that, when scrap rubber is cheap, the mechanical process is the more
economical, while, if it is high priced, the acid process has the
advantage. Since this expression of opinion, however, prices of rubber
scrap have ranged constantly at higher figures than before, and there
is no indication that we shall have again what was known formerly as
"cheap" scrap. It is not surprising, therefore, that the volume of
mechanical "shoddy" should be placed by the best estimates at not
above one-sixth of the total production of reclaimed rubber in the
United States. And the acid product, with all its admitted
shortcomings, is still superior to any of the so-called rubber
substitutes.

Reclaimed rubber is not to be considered as an adulterant, except in
the same sense as fillings, like whiting, litharge or barytes, the use
of which in rubber compounds often gives to the product desirable
qualities that are unobtainable by the use of "pure gum." It lacks
some of the qualities of good native rubber, and yet it is rubber, and
fills its proper place as acceptably as any raw material of
manufacture. Rubber shoes made of new gum entirely would be too
elastic, and for that reason would draw the feet, besides being too
costly for the ordinary trade. The construction of a rubber shoe, by
the way, is well adapted for the use of different compounds for the
different parts. Rubber enters into twenty-six pieces of a rubber boot
a