We have seen that plantation rubber is placed upon the market in a variety of different forms, among which biscuit, sheet, crêipe and block are the most familiar. Any of these forms may be either smoked or unsmoked, and the unsmoked varieties may differ much in colour; whilst the thickness and other characteristics of the different types also vary considerably. It is a curious fact that whilst one of the principal demands of the manufacturer is for uniformity in the product which he buys, the present diversity is partly due to the failure of the manufacturers to make up their minds which type they prefer. In this way a vicious circle is established, and the desire of the producer to turn out a uniform product is partly frustrated by the buyer. On the other hand, it must be admitted that similar grades of rubber, when produced on different estates and from trees of different ages, appear to differ considerably in strength and resiliency.
Light coloured rubber is valuable for certain purposes; for example transparent tubes for feeding bottles are now made from pale plantation rubber. The number of uses for which a very pale tint is necessary is however comparatively small. One obvious advantage of light coloured transparent rubber lies in the fact that its purity is unmistakable, as compared with dark commercial rubbers, and until recently a higher price was commanded by the palest crêpe rubbers. The quality of raw rubber, however, depends mainly upon what is known as nerve. This expression more or less sums up the results of tests upon the breaking strain, extensibility and resiliency, all of which should, generally speaking, be as high as possible. The resiliency of the rubber is measured by the pull exerted after a certain period of extension, and by the permanent extension shown after a certain period of stretching, followed by a certain interval of rest The most resilient rubber exerts a large pull, and shows a small permanent extension. In practice, the strength and resiliency of the rubber are estimated by the brokers simply by handling and stretching; and although expert buyers arrive at a high degree of skill and judgment, such methods cannot be regarded as infallible. It is even whispered on estates that thick crêpe is preferred to thin on account of its greater apparent strength. In this particular form of rubber, strength is a feature" particularly difficult to determine by rule of thumb methods. A simple and uniform method of testing commercial samples is therefore greatly to be desired. At the present time there is little doubt that differences in the original samples, and differences which arise during transport and storage, are often to some extent discounted by slight inaccuracies in the methods of testing. The determination of the best form of plantation rubber is thus further delayed.
It is therefore not surprising that the opinions of experts seem to differ as regards the precise effect of almost every factor which is capable of influencing the quality of the manufactured rubber. Thus rubber from young trees was formerly regarded as markedly inferior to that obtained from old trees. Recent experiments have not invariably confirmed this conclusion, although some appear to do so, and the question is not finally settled In a similar way opinions differ widely as regards the effect of smoking or the use of vacuum driers. At present the general opinion is that plantation rubber is on the whole less satisfactory and less durable than fine Para. Thus it is commonly stated that plantation rubbers are unsuitable for the manufacture of elastic thread, a use which is generally admitted to represent one of the most stringent tests of quality.
As the crops on Eastern estates increase, and larger and larger bulks of rubber are turned out, a steady increase in uniformity is to be expected. Already many samples of plantation rubber have been produced which are indistinguishable in Quality from the best Para by all ordinary tests. In fact, there is every reason for believing that in the future the highest plantation grades will share with hard Para the distinction of representing the highest standard of quality, even if they do not oust the American product from its present position of superiority.
As regards chemical purity, plantation rubber already surpasses any wild kind. It is therefore preferred by manufacturers to any wild rubber for the manufacture of rubber solutions and for waterproofing.