The latex is generally brought in from the field by coolies in enamelled iron buckets. On very large estates wheeled tanks are sometimes employed, and in some cases a system of small tram lines or monorails has been resorted to. Where the bulk of latex to be dealt with is very great, coagulation is often carried out in sheds in the rubber fields, and the wet rubber thus obtained is transported to the central factory for further treatment
The principal medium employed for effecting coagulation is acetic acid. Other coagulants are sometimes employed. The merits of hydrofluoric acid have been widely advertised, but the powerfully corrosive effect of this substance is a disadvantage. We do not know of any exhaustive comparison between th£ effects of hydrofluoric acid and those of acetic acid. Mixtures containing tartar emetic, formaldehyde and other substances have also been recommended.
Fresh latex is slightly alkaline. The chemical and physical nature of the process of coagulation is not altogether understood, but it is generally supposed that the phenomenon consists primarily in the precipitation of the proteids of the latex which are soluble in an alkaline medium. The coagulated proteids form a network in the meshes of which the globules of rubber are entangled, and the whole then contracts into a clot. An excess of acid leads to renewed solution of the proteids, so that either too much or too little acid produces incomplete coagulation. The special merit of acetic acid lies in the wide range of proportions in which it can be added to the latex, whilst still ensuring complete coagulation. Thus Parkin states that the acid can be added either in quantities four times below the proper amount or nine times above it, with very little waste of rubber. Acetic acid has also less destructive effect on the finished rubber than most of the mineral acids. It is generally recognised however that an excess even of acetic acid is harmful to the manufactured product It is therefore important that only just sufficient acid should be added to ensure complete coagulation. The exact amount required appears to vary considerably under different circumstances. Parkin found that complete coagulation was produced by the addition of .09 per cent of pure acetic acid, or one part in about 1100 of pure latex. Working with latex from the old Henaratgoda trees, we have found that this amount is often insufficient, whereas on many estates a smaller proportion is generally found to be effective. It is to be recommended that in large factories the bulked latex should frequently be tested on a small scale in glass vessels, in order to ascertain the smallest amount necessary. If the latex has been kept for some time, its alkalinity may be reduced by the action of putrefactive bacteria, and a smaller amount of acid will then be required.
The correct amount of acid, well diluted with water, is added to the latex, which is then thoroughly stirred and allowed to stand. When crêipe is being manufactured the coagulation usually takes place in enamelled buckets. From these the spongy rubber is removed after an interval of half an hour or less and transferred at once to the washing machines. For the manufacture of sheet rubber the latex must be set in shallow pans. In these it is allowed to stand for some hours until a firm clot is formed, which can be lifted out in one piece and rolled into sheet. The pans vary in depth from 2 to 4 inches, according to the thickness of the sheet required, and a common size is 9 by 18 inches.
As opposed to coagulation, a centrifugal method of separating the rubber globules from the latex was suggested some years ago by Biffen. An electrolytic method of separation has recently been patented by Cockerill. In addition to these methods, various forms of mechanical churns and separators have been recommended for use in connection with acid coagulation. The ordinary method has a considerable advantage over all the last-named processes in the matter of simplicity, and it seems likely for the present to hold its ground. The centrifugal method has however a special use in dealing with Castilloa. At present the method of crdping is decidedly the most convenient and the most rapid for dealing with large quantities of latex.