If a standard of 2 lbs. per gallon is adopted, the formula given for the 11⁄2 lbs. standard will not give full satisfaction, and the quantity of acid solution must be increased slightly in order to obtain complete coagulation. Assuming that the original solution is prepared in 1 per cent. strength, the following difference would be noted:
| (a) | One part pure acetic acid to 100 parts water (theoretically 99 parts). | |
| (b) | 11⁄2 lbs. per gallon. | 2 lbs. per gallon. |
| Of the above solution use 1 gallon to every 12 gallons of standardised latex. | Of the above solution use 1 gallon to every 10 or 11 gallons of standardised latex. | |
It is not possible to lay down an exact figure governing all cases, as so much depends upon the treatment undergone by the latex before it reaches the store.
Some estates continue to use solutions of greater strength, generally 5 per cent., in crepe preparation. While such solutions may be effectively stirred in when the latex is dilute, it is advised that for intimate mixture the solution need not be stronger than 1 per cent.
In estimating the quantities of acetic acid required much depends upon the interval which is to elapse between the addition of acid and the time of working of the coagulum. If the rubber is to remain until next morning, the average formulæ will be found suitable; but if it is required to work the coagulum with an interval of less than three hours, an excess of acid must be employed. The excess need be comparatively small, unless the interval is much reduced. For instance, it is the practice on some few estates to begin the machining of the coagulum about half an hour after coagulation commences; in which case it is usual to add from a quarter to a half of the normal quantity in excess. It need scarcely be pointed out that unless this procedure is strictly unavoidable it should be discouraged on account of the waste of coagulant involved. Incidentally, the use of strong solutions of acid under such circumstances may lead to increased deterioration of the rolls.
Colour of Fine Crepe.—We are sometimes assured that manufacturers do not pay the attention to the question of colour which sale conditions would lead one to believe. As far as we are concerned, and as long as there is no direct traffic between producer and consumer, it must be recognised that in the vast majority of cases we are forced to concern ourselves only with the standards set up in the markets. This, in spite of the knowledge that, all other things being equal, the arbitrary distinctions in colour afford no indication of the intrinsic value of the rubber. Under present circumstances it is plain that if paleness is demanded it has to be supplied.
Probably without exception all estates employ sodium bisulphite as the agent for the prevention of that darkening (oxidation) which is natural in drying rubber.
Sodium Bisulphite.—A formula for use of this chemical is given in [Chapter VIII.], and is applicable to latex standardised to 11⁄2 lbs. dry rubber per gallon. If a higher standard is chosen the quantity calculated as in (b) of that formula may be increased slightly, and the exact requirements found by experience. The caution must again be given that the employment of an excess of sodium bisulphite will lead to the production of an over-pale rubber, and a prolongation of the drying period. If thick crepes are made, an excess of the chemical is sometimes made visible by a greyish powder deposited on the edges of the strips of dry rubber.
It must be emphasised that the formula in [Chapter VIII.] indicates the maximum quantities required for use with standard latex. Many estates will find it expedient to use less of the chemical; and if it is found that the desired result is not obtained from normal proportions, attention should be directed to the points discussed in the following paragraph.
Evaluation and Deterioration of Sodium Bisulphite and Sodium Sulphite.—Sodium bisulphite and sodium sulphite are both bought for our purpose in the form of a fine crystalline powder, and on analysis good specimens should contain over 90 per cent. pure substance, when packed in well-sealed vessels.