Its acid content depends chiefly upon:
(a) The kind of timber heated in the retort.
(b) The efficiency of the apparatus.
(c) Condition of the timber as to moisture.
(d) The temperature employed, and rate of working.
(e) The point at which distillation ceases (i.e., the duration of interval between commencement of heating and cessation of collection).
Samples received from estates for testing purposes were found to contain equivalents varying from 2 per cent. to 10 per cent. of acetic acid.
They were all suitable coagulants when used in quantity calculated from the discovered acidity, but produced rubber darker than ordinary when air-dried. This effect was not of much importance in the preparation of smoked sheets, but to produce a pale crepe it was necessary to employ sodium bisulphite as an anti-oxidant.
This darkening in colour is to be ascribed to the presence of traces of phenols,[24] which are stated to exert an effect upon the rubber during and after vulcanisation.[25] This subject will be discussed in [another section].
[24] Whitby, Journal Soc. Chem. Industry, vol. xxxv., No. 9, 1916.