Block rubber has been seen which was free from air-bells, but this was the thin variety of block prepared for show purposes with far greater care, probably, than would be expended in commercial preparations. CHAPTER XVIII _DEFECTS IN SHEET RUBBER_ Before proceeding to deal with defects in the rubber as it is put upon the market a brief account will be given of faults which may be noted in the preparatory stages. MILKY RESIDUE OR SERUM.--If the serum is not clear after the ordinary period allowed for coagulation, it indicates one of the following possible causes: (_a_) Failure to obtain complete mixture by thorough stirring. (_b_) Insufficiency of acid solution. This may be real or indirectly due to the presence of an excess of anti-coagulant such as formalin or sodium sulphite. (_c_) In cases where other coagulants than acetic or formic acids have been employed the failure may be due to an excess of, or an unsuitable, coagulant--_e.g._, hydrochloric acid. COLOURED SURFACE BLOTCHES AND UNPLEASANT ODOUR.--Sometimes the surface of the coagulum exhibits yellowish or bluish streaks and patches. It will be found generally that the yellowish colour is possessed by a slimy substance, of offensive odour, which may be scraped from the surface. Either insufficient acid has been used, or the mixing of latex and coagulant has been at fault. DARK DISCOLORATION OF THE RUBBER.--This may be stated to be a natural process when fresh rubber is exposed to the atmosphere. It is usually described as "oxidation," and it will be noted to be absent, or to occur to less degree, on those portions of the rubber which are protected from the atmosphere by being below the surface of the remaining liquid. This surface change may be prevented (see Chapters VIII. and IX.) by the use of small quantities of sodium sulphite (for preference) or bisulphite.