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estates of that company. Sheets, that came fresh from the smoke house without stripe showed it distinctly some days afterwards.
At first we thought that perhaps the high temperature in the stove or the smoke house could have played a role in this optical phenomenon. It is known, that not transparent crepe gets more transparent when heated. More probable however was the suggestion that the defect should be traced to the presence of hygroscopic, moisture attracting serum substances in the rubber. In the hot smoke house they loose their moisture and afterwards when brought into the fresh air they attract enough moisture to make the white stripe appear. The correctness of his suggestion can be proved by the following test.
At first the difference in moisture content was estimated between the most inner layer of a sheet and the dryer outer part by means of a simple moisture determination in a stove. The sample showed the stripe distinctly. It was cut into strips of several mm. thickness and each of these strips divided in three parts along their lengths: The middle of these three strips represented the moist layer. In this way we got 9,824 grs. from the outer- and 8,338 grs. moist inner rubber. After heating in a stove for ± 2 hours at 80° Cent. a constant weight was obtained and this figure noted. The moisture content of the dry outer layer appeared to be 0.50%, that of the moist inner layer 0.70%. The difference, two tenths of a percent is very small. Really this may be a little larger as some moist rubber may have been cut with the dry strips or the reverse and in both cases the difference in moisture content will be decreased.
The dried rubber was kept in the open air and had during the night the opportunity of absorbing moisture from the atmosphere.
The next morning the samples were weighed, and this was repeated every day. from these weights the new moisture contents could be derived. They are shown in table 1.
| Inner layer (white stripe) | Outer layer | |
|---|---|---|
| Original moisture content | 0.70% | 0.50% |
| Moisture content after drying | 0.00% | 0.00% |
| " " after one day | 0.42% | 0.45% |
| " " after two days | 0.48% | 0.43% |
| " " after four days | 0.58% | 0.45% |
| " " after six days | 0.53% | 0.40% |
