Some proportion of the tree-scrap is usually found to be heavily oxidised, and naturally if a crepe of uniform colour is to be obtained these dark scraps must be rejected, otherwise dark streaks will be formed. Coolies should be instructed to sort out the dark pieces before arriving at the store.
BARK-SHAVINGS.--It has been intimated in a previous section that the method of obtaining and collecting bark-shavings varies with the type of labour employed.
Where the scrap is removed from the edge of the bark on each occasion before tapping, the amount of rubber to be extracted from the dry shavings is very small--so small, in fact, that when the price of rubber is low, it is doubtful whether it pays to collect and work the material.
On the other hand, where trees are not "scrapped" before tapping, the bark-shavings and tree-scrap are collected together, and the amount of rubber derived from the mixture may be 30 to 40 per cent. upon the gross weight--depending chiefly upon the quality of the tapping (_i.e._, in this case, the thickness of the paring). Another factor influencing this figure would be the effect of using an anti-coagulant on the cuts.
Bark-shavings entail such wear upon the ordinary machines during working, especially if fairly free from rubber, that unless the factory is equipped with a special "scrap-washer" it is advised that this material should be sent for working to a factory having the necessary equipment. Whenever possible, bark-shavings should receive treatment on the day of collection.
It used to be quite common to see heaps of bark-shavings accumulating on the floor of a factory, and generating excessive heat. Yet these heaps were allowed to stand about for a day or days. Is it any wonder then that tackiness was found to develop when the rubber was dry? It is here definitely laid down that no heaps of bark-shavings should be accumulated even for half a day. Tanks should be provided in which the shavings should be submerged in water.
EARTH-SCRAP.--Of all grades of crepe this is the one most liable to become tacky in transit. This tackiness to a large extent cannot be avoided, as old pieces of earth-scrap may be brought in amongst the bulk. Probably these old pieces have been exposed to the sun for days, and have become quite resinous. It would be practically impossible to go through all earth-scrap in order to find these odd pieces, but unless this were done one could not guarantee that the earth-rubber would always be free from tackiness. The difficulty does not appear, however, on estates where earth-rubber is collected systematically at very frequent intervals.