SHEET RUBBER.--The commonest defect is mould.[45] This is usually of a light surface type, easily brushed off, and numbers of vulcanising tests failed to trace any reduction in rate of vulcanising or other defect due to this. In spite, however, of the harmlessness of light surface moulds, they are looked upon with suspicion by the manufacturer. Occasionally samples of smoked sheet are offered contaminated with a "heavy" type of mould. The sheet feels damp and "heavy" or flabby, and contains an excess of moisture; sometimes a moist exudation is noticeable on the surface, and "virgin" patches are present. Such sheet vulcanises more slowly than F.A.Q. samples, but does not necessarily show other defects after washing and drying. [45] Bulletin R.G.A., February, 1921, p. 97; April, 1921, p. 190; June, 1921, p. 243; November, 1921, p. 472. "Stretching rusty," as already explained, is due to a dry film on the surface of the sheet, and according to a recent investigation, this film consists, not of serum substances, but of a microscopic mould growth, which presumably grows on the serum substances. A sample of sheet which stretches rusty gives the rubber a "dry" appearance, and for a long time manufacturers mistook the surface film for resin. On the assumption that such rubber was "resinous" they rejected it, and to this day it is regarded as a defect, although it has no influence on the vulcanising properties of the rubber. It is hardly necessary to point out that defective appearance, such as is due to thickened edges, faint markings, bubbles, and so forth, have no effect on the vulcanising properties of the rubber. They only point to some irregularity or carelessness in preparation. The only justification for distinguishing between rubber of good and bad appearance is that the former bears the impress of careful preparation, and is therefore more likely to be uniform in rate of vulcanising. Similar considerations apply to the colour of smoked sheet, which may vary from a pale yellow-brown, through various shades of red-brown to dark brown. There are various factors affecting the colour, but the buyer can see but one--viz., the "degree" of smoking--and the rubber, from his point of view, may be undersmoked or oversmoked. No doubt the degree of smoking affects the vulcanising properties, but to a less extent than was at one time imagined. In a recent paper[46] it has been shown that the average breaking strain and rate of cure of a number of samples of smoked sheets were practically the same for light as for dark sheets. [46] Bulletin R.G.A., December, 1921, p. 521.