The principle of the process was the treatment of coagulum, in either sheet or thick crepe form, with vapours produced by the volatilisation of two special fluids. This treatment was undertaken in comparatively small wooden sheds, in which the coagulum was placed. The “smoke” was conducted into the curing sheds from furnaces outside the building. The sheds were covered externally with “felt” material to prevent leakage of the vapours, and a very dense smoke was obtained.
The furnaces were specially designed, and consisted essentially of a “hot-plate” heated by a powerful kerosene blast-flame. On top of the machine were two reservoirs controlled by taps. In these were placed the special fluids which were released in definite proportion. The composition of the fluids was not divulged, but it is assumed that the principal ingredients were (a) wood tar products, (b) crude pyroligneous or acetic acid. The mixture of these, dropping on the hot plate at the correct temperature, spontaneously volatilised, to form dense whitish fumes, having an intense and not disagreeable odour of wood combustion. A duct led from the back of the machine into the curing-shed, where the vapours were distributed through perforations in the pipe.
The coagulum usually remained under treatment in the shed for three to four hours, and then was removed for ordinary air-drying. When taken from the curing-shed it had a pinkish colour, which later developed into a dark brown by a natural process of oxidation. The exterior of the rubber, on shipment, resembled the appearance of smoked sheets; while the interior, on cutting, was seen to be still white. As packed for shipping, the rubber contained from 10 to 15 per cent. of original moisture, for the usual sheet form, and even more when “slab” rubber was prepared.
Originally either crepe or sheet rubber was made, but later the preparation of the crepe form was displaced largely by “slab” rubber. These “slabs” were really very thick sheets, which had been subject to only slight pressure.
Still later the preparation of the “slab” form was displaced by “loaf” rubber. This form was built up by winding together ordinary thin sheets which had been subject to the “cure.” Only slight tension was needed, during the operation of winding, to cause close adhesion of the component wet layers, and the final result was a “loaf” or roll dark in colour, and apparently dry when examined superficially. On being cut, even after an interval of months, the middle portion was still so moist as to be quite white.
In course of time it was discovered that all the claims made for the process could not be substantiated, and for various reasons (which need not be detailed) most of the estates which had adopted the scheme reverted to ordinary methods of preparation. At the time of writing few, if any, continue to work the process. It appears to be agreed, as the result of investigations, that in no degree does the process yield advantage over ordinary methods.
Freezing Process.—A patent was secured a few years ago to cover a process whereby coagulation was effected by refrigeration.
Latex remained for several hours in the refrigerating chambers of an ordinary ice-making plant. The resulting solid mass, on being thawed, yielded a coagulum appearing in no way to differ from that obtained by ordinary methods of coagulation.
Provided the process exerted no influence for good or evil upon the quality of the resulting dry rubber, the value of it would appear to depend upon the relative cost of working, plus considerations of capital expenditure and depreciation on the plant. At the present time it would be difficult to imagine that the cost of preparation alone would compare favourably with that sustained by ordinary coagulative methods.
Furthermore, beyond the expensive refrigerating plant, the usual machinery of a factory would still be required if the ordinary market demands are to be met.