Skimmings and Washings.—The skimmings of tanks, as already shown, may be prepared sometimes as a second quality of smoked sheet; but generally they are amalgamated with the rinsings of cups and buckets, treated with sodium bisulphite and acid, and made into crepe form.
The cup-washings, as they arrive at the store, represent a very dilute latex, the rubber from which is generally of a greyish colour.
Bucket-washings should yield a good type of pale rubber if they are obtained properly. To obtain the maximum quantity of good rubber the buckets should first be rinsed. A gang should be taken, a small quantity (say a quart) of water poured into the first bucket, and this dilute latex used progressively in all the buckets of that gang of tappers. The result is a fair latex which can be added to the bulk of No. 1 latex, provided it is free from dirt. Where sheet rubber is being prepared, carefully strained cup-washings or bucket-washings may be utilised in the dilution of the latex to the required standard, thus increasing slightly the percentage of first-grade rubber.
Tree-Scrap.—As tree-scrap is a naturally coagulated rubber, it should be expected to show up well in quality. This is usually the case; but from what has been said of the effect of sun-heat it will be understood that if trees are not regularly “scrapped,” there is a danger that the crepes may be found to contain tacky streaks due to the inclusion of old scrap which has been sun-baked. In hot dry weather, on widely planted areas tapped on alternate days, it has been noticed that scrap remaining for two days often exhibits a resinous appearance, and feels sticky to the touch.
If tree-scrap is to be made as a separate grade, as used to be the general custom, care should be taken to see that it is free from bark and dirt. On some estates where scrap-rubber is paid for per pound collected, it is usually the rule to insist that scrap shall be washed free from dirt and picked free of bark. This course is to be commended, but might probably prove impracticable to the majority of estates. Theoretically, of course, the operation of machining should rid the scrap of all traces of bark; but in practice it does not do so.
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.