Complaints are not uncommonly made of the appearance of spots and discoloured patches on the rubber turned out of the factory. In fairness be it remarked that such complaints are much less common than formerly. Petch states that the chief organisms concerned in the spotting of rubber biscuits appear to be bacteria and yeasts. Blood red spots sometimes appear. These have been attributed to the action of Bacillus prodigiosusy the microbe responsible for the so-called bleeding miracles. Brown and black patches may be due to the attacks of other bacteria. Discolouration may usually be prevented by attention to perfect cleanliness at every stage of preparation. All utensils which are brought in contact with the latex should be frequently scalded with boiling water, and the factory itself should be kept scrupulously clean. The hackneyed comparison with the conditions of an up-to-date dairy may profitably be borne in mind. The practice of frequent cleaning should extend to the tapping tools and collecting cups. It has been recommended that the latter should be of glass or earthenware, since proper cleansing is difficult with any form of metal cup. Glass or earthen cups are unfortunately very liable to be stolen on small estates.
The growth of mould fungi on the rubber sometimes causes trouble in wet weather. As a rule little damage is done in this way, the action being entirely superficial. Moulds seldom appear in large factories where the preparation is rapid and the storing and packing rooms are kept dry. Smoked rubber is practically immune from the attacks of moulds, owing to the antiseptic properties of the smoke constituents.
Rubber is said to become tacky when the surface turns soft and sticky. In some cases the softening may proceed so far that whole sections of rubber fall to pieces and dissolve into a liquid form. Tacky rubber is useless for the ordinary purposes of manufacture, and can only be sold at a low price as a subsidiary product. The condition is rightly dreaded as the worst defect which can possibly arise.
One certain cause of tackiness is exposure to sunlight, and it is to guard against tackiness that drying and storing rooms must be carefully guarded from excessive light. According to Petch, the assertion that tackiness can also be caused by the action of bacteria has not been conclusively proved. It is quite possible however that tackiness exists in different forms, and may be due to different causes. The use of too strong acid in coagulation may lead to tackiness, and it is generally believed that the condition arises more readily if the acid employed in coagulation is not thoroughly washed out of the rubber. Heat has also been mentioned as a cause, although the softening due to a high temperature is usually temporary, and passes off when the rubber is cooled. The fact is that the conditions which lead to tackiness are by no means fully understood, and further study is desirable.