It is not uncommon to combine the slow drying of rubber with a process of smoke curing, and for some time past rubber prepared in this way has commanded a higher price than the unsmoked variety. Many buyers believe that the creosote and other substances contained in the smoke exercise a preservative and strengthening effect upon the rubber. The curing of the wet rubber after sheeting or craping is a process essentially similar to the curing of hams or of herrings. The strips of rubber are hung up in a drying chamber which is impregnated with smoke from a fire fed with green wood or coconut husks. It is usual to have the smoking house separate from the main factory in order to avoid danger from fire.
Various methods of preparing rubber by the direct action of smoke upon the latex have been suggested in imitation of the preparation of Hard Para rubber in Brazil; and a variety of machines have been devised for this purpose. Such methods of preparation are at present only tentative. Before they can be definitely recommended, further reports are required both on the quality of the rubber prepared in this way and on the facilities for adopting the method on a large scale in factories. It may be added that the last named necessity is one which inventors frequently seem to lose sight of.
In a machine devised by Mr H. A. Wickham the latex is allowed to flow upon the inner surface of a rotating cylinder, where it forms a thin film and is exposed to a jet of smoke obtained by burning coconut shells in a special stove. The rotation of the cylinder is so arranged that as each film of latex sets, another film is spread over its inner surface and is exposed in its turn to the smoke fumes. When a certain thickness of semi-solid rubber has been obtained, this is scraped out of the cylinder and pressed into a block. Rubber prepared in this way contains all the constituents of the latex, including a considerable percentage of water, and may be expected to resemble the Brazilian product closely. The chief point which still requires to be settled is how such rubber will compare in its physical properties with hard-cured Para on the one hand and with the usual forms of plantation rubber on the other hand. Rubber prepared by a somewhat similar method at the Singapore Botanic Gardens has been reported on very favourably by the manufacturers, but one or two experiments are not sufficient to establish the value of the method as compared with other processes already in operation on plantations.
Blocks of rubber are prepared in special presses by combining several layers of sheet or crêipe rubber, either smoked or unsmoked. Block rubber has the advantage of being very convenient for transport, and is also less liable to undergo damage during transit. From the point of view of the manufacturer, large blocks of rubber possess two disadvantages. It is necessary to cut up the blocks into smaller pieces before the rubber can be further dealt with in the factory. The presence of impurities or adulterations also cannot be detected simply by external inspection. Both these objections can be overcome by making flat blocks, which should not be more than one or two inches in thickness. Such blocks however are somewhat more troublesome to make than thick ones.
The highest grade of rubber prepared from the strained latex is known as First Latex Rubber. Other grades are prepared from the strainings of the latex, from the scraps of rubber which have dried on the bark of the trees or in the collecting cups, from the shavings of bark removed in paring, and even from the latex which has dried upon the ground beneath the trees. The scrap from small estates is sometimes sold as such, but on large estates the whole of it is turned into crêpe of various grades, according to the colour and the amount of impurity remaining after the washing process. After passing repeatedly through the macerating machines, the greater part of the impurities present are washed away, and the value of the rubber thus produced is only 20 or 30 per cent, less than that of First Latex Crêpe.
The rubber is packed in wooden cases containing generally between 1 and 1½ cwts. It is most important that the inside of the cases should be smoothly planed and perfectly clean, and that no packing material of any kind should be used. Anything sticking to the rubber at once detracts from its value. The presence of the smallest amount of impurity may necessitate the addition of an elaborate process of purification prior to the other processes of manufacture. Just as in the case of tea and other products which are paid for by the pound, it should be the object of the packer, when making up the cases, to include such an amount of rubber as will weigh a few ounces more than an even number of pounds on arrival at its destination. This is owing to the fact that fractions of a pound are neglected in favour of the buyers, with the result that 100J lbs. are paid for as 100, whilst 99} lbs. are paid for as only 99 lbs. With rubber at four shillings a pound the matter is worth some attention. Some experience is required for determining just the right amount that should be included, owing to the fact that rubber is liable to lose a certain amount of weight in transport.