Cultivation and Chem Istry Rubber Botany

The only tapping tools in general use on the plantations are a gouge (straight or bent) and the ordinary farrier's knife (or its modification).

It is the usual practice to

make the first cut at between two and four feet from the ground. A shaving is then taken at definite intervals of time from the lower edge of the exposed bark. The thickness of the shaving removed is so arranged that the consump tion of bark is between half an inch and one inch per month, some districts, such as Ceylon, preferring thin shavings while others, such as Malaya, prefer thicker shavings.

The length of the shaving varies from one-half to one-quarter of the circumference, some estates employing a single spiral cut at an angle of about 3o° to the horizontal and some a V cut. In Ceylon it is the practice to change the tapping panel every six or twelve months, but elsewhere it is customary to continue tapping on one panel down to the ground before commencing a fresh panel. Tap ping is generally carried out every alternate day, one portion of an estate being rested while the other is tapped.

After the bark has been cut away, it is regenerated complete with latex vessels in a few years. With the most conservative sys tem it is eventually necessary to tap on renewed bark. As the replacement of bark becomes more difficult each time a panel is tapped, there is a limit to the economic life of the tree, but the industry is too voune for definite information on this noint.

Tapping operations are always carried out in the early morning when the flow of latex is greatest. The latex from each tree is col lected in a cup, transferred to a pail and taken to the factory as quickly as possible. Sometimes sodium sulphite is added to pre vent premature coagulation. When the latex has ceased to drip there remains on the bark a thin film which dries in the sun leav ing a strip of rubber. This is collected, washed in the factory and sold as "tree scrap"—an inferior grade.

Preparation.

The details of the methods employed in the preparation of plantation rubber depend upon the shape and appearance of the product to be put on the market, but in nearly all cases the outlines of the procedure adopted are the same. The sieved and diluted latex containing 15-25% rubber is treated with a coagulant such as acetic or formic acid or sodium silicoflu oride. This causes the rubber to rise to the surface as a wet, white,

doughy coagulum leaving in solution a small quantity of mineral and organic matter. The coagulum is then pressed between rollers until it is the required consistency, thickness and shape. After that it is hung to dry and eventually packed in wooden cases and shipped to its destination.

The two most important forms of plantation rubber are sheet and crępe. Sheet is generally dark brown in colour because it is dried in smoke, whilst crępe is a straw colour and is dried in air.

Sheet is obtained from latex coagulated in shallow tanks divided into compartments of suitable dimensions, each piece of coagulum being pressed by light machinery to the required thickness.

In the preparation of crępe the coagulum is machined much more drastically, heavy rollers being necessary. During this proc ess the coagulum is torn and pressed until it is sufficiently thin to dry in air without artificial heat. The rubber is dried by hanging on racks in well ventilated sheds for about a week.

Sheet is thicker than crępe and requires artificial heat to dry it in a reasonable time. It is therefore exposed to the smoke of a wood fire for about 14 days. It was at one time presumed that smoking had a beneficial effect on quality because the highest grade rubber was obtained from the Amazon where the latex is coagulated with smoke, a wooden paddle being repeatedly dipped into the latex and exposed to the smoke of burning Urucuri nuts until a fairly large sized ball of rubber is built up. Experimental evidence does not support this view however and it is probable that as firewood becomes more difficult to obtain other means of drying sheet rubber will be developed on estates.

Both crępe and sheet are sold largely on appearance. It is cus tomary therefore to add sodium bisulphite to latex when preparing crepe so that the rubber may be as pale as possible, and p-nitro phenol to latex in the preparation of sheet so that the devel opment of mould may be prevented. Sheet rubber contains more mechanically enclosed serum substances than crepe and is there fore a more suitable medium for the growth of mould. Not even drying in smoke entirely prevents this when conditions are favour able, but p-nitrophenol is an effective safeguard.