Greasiness before Smoking.—Under ordinary methods of working this should never be encountered. It may be taken to show that the machined rubber has been allowed to remain, either hanging or in piles, far too long before entering the smoke-house. The appearance is most marked if the rubber has remained in a cool and moist atmosphere—e.g., if it has been hanging over-night in a closed and badly-ventilated factory. In a marked degree this is to be observed in the preparation of air-dried sheets, unless they are exposed, when freshly prepared, to the action of the sun for a period. This period, in the case of rubber prepared on native small-holdings, generally extends over several days—until the sheets are more than half dry.
In the preparation of smoked sheet, the greasy appearance and the cause outlined contribute to a defect which is eventually described as “stretching rusty.”
Surface Blemishes.—The coagulum, during coagulation and subsequently, can be contaminated in various ways. In most cases a little intelligence or increased care would prevent the occurrence of these defects.
When the coagulum remains over-night, in the absence of a cover, it is not uncommon to note the presence of dirt (from the roof above, or blown in from the outside), the droppings of mice and rats, flies and small insects. In theory these should be seen and removed by the factory hands. In practice, except while under immediate supervision, the extraneous matter is often rolled into the soft coagulum.
A fairly common cause of this surface contamination is the exhaust from the power-unit; generally the worst offender is a steam-engine. Grit and smuts continually find their way into the factory, alighting on the tables, in the latex, in the water, and on the freshly prepared rubber. They are rolled into the soft rubber and lead to marked depreciation in the selling value. The radical remedy seems obvious, but is often beset with many difficulties not unconnected with financial considerations.
Other superficial blemishes, such as those due to the presence of rust marks, oil or grease patches, etc., are self-explanatory, if a little thought is brought to bear upon them; and it is not proposed here to discuss such defects more fully.
Having now dealt with certain defects which are visible in wet rubber, we come to the discussion of others which are only perceptible either during or after the drying period. As far as is known no plantations of any size now prepare sheets other than in the form of smoke-dried rubber, with the exception of a few which make a special form of thick and partially air-dried product known as “slab” rubber.
It is not proposed, therefore, to treat in any detail with air-dried sheet rubber. Certain obvious defects are common to both air-dried and smoke-cured sheets, and these will be first discussed.
Unevenness of Appearance.—This lack of uniformity may refer either to size or colour, or to both. Apart from any other contributory causes, this variation is due, in pan sheet, to a neglect to standardise the dilution of all latices, or to lack of uniformity in the quantity of standardised latex placed in each receptacle.