(6) Tapping trees at too great a height. The latex generally has an abnormal distance to travel before reaching the cup.
(7) Sometimes the latex from old trees, or from trees after wintering (just prior to full renewal of leaf), contains more than the usual proportion of substances (_e.g._, sugars), which are capable of effecting flocculation or coagulation.
(8) Too great a distance for transport. The trouble arising from this cause is likely to be much increased if the journey has to be made over bad roads. In such case the physical action augments the effect likely to be produced by long standing.
The foregoing do not include all possible causes, but serve to indicate the directions from which trouble may be mainly anticipated. It will be plain that any latex which exhibits symptoms of premature coagulation (or minute flocculation) should be regarded as a potential source of bubbles in sheet-rubber.
It will be equally obvious that the employment in the field of any harmless substance of an anti-coagulant nature is to be encouraged. This point is discussed in detail in Chapter V.
IN THE FACTORY.--As a general rule it may be understood that the mischief has been done before the latex is handled at the factory. Sometimes it is perceptible from the peculiar appearance of the latex, and in such case the batch should not be used for the preparation of sheet-rubber. Often it is found that only the last to arrive at the store is visibly affected. This should not be mixed with other apparently normal latex, as it is capable of acting as a "leaven" to the bulk.
(1) Lack of cleanliness of utensils, particularly of coagulating dishes or tanks.
The trouble becomes acute sometimes where wooden tanks are employed. Unless the tank and the partitions are thoroughly and regularly cleansed, the wood may become coated with a bacterial slime, which is capable of causing what may be termed "fermentation" of the latex layers in contact.
The tank should be thoroughly cleaned occasionally with a weak (5 per cent.) solution of sodium bisulphite. The partitions should be scrubbed and placed in the sun twice or three times a week.
(2) Allowing latex to stand too long before treatment. This point needs no further expansion.