Further, if 1 gallon of latex be diluted with 2, 3, or even 4 gallons of water it is still only necessary to add 16 ozs. of the acid mixture.
At dilutions beyond this limit, however, it is necessary to add a little more acid to obtain complete coagulation.
In the process of preparing sheet rubber it is very necessary to see that the minimum quantity of acid is used, otherwise visible defects are caused. But in coagulating latex intended for preparing crepe, where the rubber undergoes protracted washing on the machines, the presence of a slight excess of acid in coagulation is not calculated to cause any deterioration in the quality of the rubber. Advantage must not be taken of this statement to argue that more than a slight excess may be used without injury to the rubber, as it can be shown that the use of a large excess of acid results in an inferior rubber.
Quantities Necessary for Modern Requirements.—It may be commended to the notice of the beginner that any further experimental work as to the quantity of acetic acid necessary for complete coagulation would only involve a waste of time and energy.
The general figure given in a preceding paragraph (1 part pure acid to 1,000 parts of latex) may be accepted as the rough basis for working. In modern practice, however, undiluted latex is usually diluted to a standard which may vary on different estates from 11⁄4 lbs. to 11⁄2 lbs. dry rubber per gallon.
Latices of these strengths can be coagulated at a ratio of 1 part pure acid to 1,200 parts of standardised latex; and this quantity need not be exceeded, except in cases where an appreciable amount of some anti-coagulant is present in the latex. The proportion may then be raised to 1 in 1,000.
If considered advisable the acid may be used in a 1⁄2 per cent. solution for sheet preparation; but in any case it is advised for the sake of uniformity that a 1 per cent. solution should be employed in the preparation of both sheet rubber and crepe rubber. In most modern factories, measuring vessels of various capacities are to be found, and it is always more satisfactory to have the solution made up in approximately correct strength at the rate of 1 oz. of pure acid to 5 pints of water. Often, however, on some estates European supervision of this work is not possible, and the preparation of the acid solution has to be left in the hands of a (more or less) skilled coolie. It is thus necessary to find some less fine, but still approximately correct, method of procedure. In the East the kerosene tin is in universal favour for the carriage of water, and there is no reason why it should not be utilised as a standard measure for preparing the dilute acid solution, providing it is not allowed to become rusty. The capacity of the tin is 4 gallons (640 fluid ozs.), so that a one-hundredth part would be approximately 61⁄2 ozs. It is suggested that this quantity should be measured out by means of a glass graduated vessel, and then that an aluminium cup should be cut down so as to hold the exact quantity.
This would reduce the making of a solution, sufficiently approximate to 1 per cent. strength for all practical purposes, into a simple operation of mixing pure acid and water in the ratio of one cupful of acid to 1 kerosene tin of water.
The actual quantity of solution required for the coagulation of any volume of standardised latex can be calculated easily from the ratio 1:1,200. As the strength of solution is 1:100 it will be seen that the quantity to be taken is always one-twelfth that of the volume of latex—e.g.:
(a) If the latex tank holds 90 gallons of standardised latex, 71⁄2 gallons of dilute acid solution are required.