It has been shown by the writers in previous publications that over-working of the coagulum has an effect on the vulcanisation of the rubber; and this has been confirmed by others.[10] Apart from this point, it should be recognised that over-working, beyond that necessary to produce a thin crepe of even texture, is to be deprecated, on the ground of economy, in working. [10] Bulletin No. 27, Department of Agriculture F.M.S., April, 1918, "Preparation and Vulcanisation of Plantation Para Rubber," Eaton, Grantham and Day. Owing to the existing differences in equipment and speed of drive, etc., the regular practice of any one estate may be unsuitable for another. It remains, therefore, a matter of study for each estate to discover the minimum number of times which rubber should pass through the machines, consistent with the factors indicated above. In any case it may be assumed that if any factory cannot prepare a good crepe by passing the rubber, say, twelve times through the rolls, there is some deficiency in the machines, or of speed; the coagulum may be too hard, or the rolls may be badly worn. LOWER GRADES OF CREPE RUBBER.--Even a few years ago it was plain that the lower grades of crepe (_i.e._, all grades lower than first latex rubber) were not sufficiently appreciated in the market. There was often a marked difference in price between a first-grade crepe and crepe made from naturally coagulated lump. This arose chiefly from lack of knowledge. It has since been recognised in some measure that no reason exists for such a wide difference in price, and more recently the margin between even the first-grade rubber and the lowest grade of scrap rubber has been a gradually diminishing one. Providing sufficient care is exercised in the preparation of the lower grades, one would expect to see but very small difference in prices between any two grades. It is true that adequate attention has been given to the preparation of the scrap grades only in comparatively recent years, and it is acknowledged that when high prices were ruling for first-grade rubbers sufficient attention was not generally given to the subject of the preparation of the lower grades. NATURALLY COAGULATED LUMP RUBBER.--The grade of rubber made from the naturally coagulated lump which forms in buckets and carts is usually of a mixed colour, due to the fact that the lumps oxidise very quickly. When they are allowed to remain overnight before being machined, it can be imagined that the colour of the dry crepe would be very dark, or would contain very dark streaks. Such is ordinarily the case, unless special precautions are taken.