SOFT COAGULUM, SPONGY UNDER-SURFACE, TEARING OF COAGULUM.--If the whole mass of coagulum is too soft, while coagulation appears to be complete, over-dilution of the latex has occurred. This may apply also to the case in which the under-surface only is spongy and soft. If coagulating-tanks are employed, the upper edge may be comparatively hard, while the lower is soft and weak. Often the spongy portion may adhere to the partitions. This prevents the natural rise of the coagulum, due to retraction, as the mass "sets." The pull between the free upper portion and the adhering lower edge causes splitting and tearing of the coagulum, with marked porosity (spongy appearance). The two factors to receive attention are the standard of dilution and the condition of the surfaces of the partitions. If these have minute cracks into which latex can penetrate, and in which coagulation takes place, the boards should be discarded. Given the conditions indicated above, the tearing and splitting of rubber in coagulating tanks is sometimes augmented by the practice of flooding the tanks when coagulation is judged to be complete. The surface water finds its way downwards between strips of coagulum and the partitions, thus increasing the upward tension between the free and adhering portions. The main idea governing the practice of flooding the tanks is to prevent "oxidation" (darkening) of the upper edges. If a small quantity of sodium sulphite is employed as an anti-oxidant and to retard coagulation, it is not necessary to flood tanks. "PITTING" OF SURFACES.--In pan coagulation this "pitted" appearance is usually limited to the under-surface, while coagulum prepared in tanks may exhibit the defect on both faces. The existence of these numerous "pits," or small depressions, points to the presence of bubbles of gas which have been unable to escape freely. As the formation and retention of gas-bubbles is not a normal occurrence in coagulation, we are led to infer that some special conditions must have arisen. These may be supplied by one or more of the following contributory causes: (_a_) The latex had begun to "sour" before arrival at the factory or while waiting to be treated. This premature coagulation is usually checked or diminished by the employment of anti-coagulants (see Chapters VIII. and IX.). It is generally accompanied by the appearance of enclosed gas-bubbles in the dry rubber. (_b_) There may have been a slight insufficiency of coagulant, or the admixture was not thorough, thus allowing a slow putrefactive change to take place in the incompletely coagulated areas.