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After these results the direct proof, that it is the enclosed serumsubstances, which cause the appearance of the stripe, by absorbing moisture from the atmosphere is lacking. That the serumsubstances are very hygroscopic is generally known. When a small scale with the serum, left after coagulation, is evaporated until it has passed over into a varnish-like layer, this will be dissolved again when kept in the open air for several hours. The water has been absorbed from the atmosphere.
It is evident that during the drying process of sheet these serumsubstances are concentrated into the middle layer where the rubber remains moist for the longest period. To make sure however that they afterwards cause the white stripe, further proof was needed.
We chose therefore a sample of thick smoked sheet, that showed the stripe distinctly. It was dried in a stove and after it had been cooled, the stripe, that had disappeared during the drying process returned in a few days. Then the same sample was placed for a number of days in a glass filled with water, that waas changed several times thus giving the serum substances sufficient time to dissolve out and after several times thus giving the serum substances sufficient time to dissolve out and after several days one can assume that the rubber is free from serum. The rubber was dried again in a stove and the stripe did not return then, nor afterwards many days.
These tests by themselves would not be valueable, if they did not prove that the defect can be avoided by thoroughly removing the serum substances, i e by sufficiently washing the freshly prepared sheets. There is no doubt that longer washing would be the most efficient way, but we know on the other hand that loss of time at this period of the preparation of rubber should be avoided in order to keep fungi and bacteria from developing before the rubber is fully disinfected by smoke and thus prevent the appearance of air bubbles, moulds or rustiness. Washing therefore should not be prolonged but rather intensified.
Two hours of rinsing are sufficient in most cases, if the machines spray plenty of water and the sheets are kept in the water during the intervals between the passing through the different machines. One should not rinse in a Shanghai jar, packed tight with sheets, but in a wide tank with streaming water.
