Tanks are at present constructed either of hard timber or of brick and cement faced with glazed tiles; both types have inherent drawbacks. The wooden tanks are difficult to keep clean and in “sweet” condition. The glazed tiles, unless extremely well laid, allow the acid serum (from which the rubber is removed) to percolate between the interstices. Thus “pockets” of liquid collect beneath the tiles, and in process of the decomposition of certain constituents dissolved in the serum evil-smelling gases are set free.

It should not be a matter of difficulty for manufacturers to make sheets of thick glass sufficiently large to form the bed-plate and side-pieces necessary in the lining of a tank. If such adjuncts could be secured, the disabilities indicated above would be perhaps wholly removed. Unless there is a demand from estates, however, it is idle to expect a supply to be forthcoming.

An even greater improvement would take the form of unit tanks cast in glazed white-ware with the necessary slots incorporated in the sides. At present no known firm makes such tanks of sufficient size. A unit could measure (internally) 6 feet by 4 feet by 1 foot deep, with slots 11/2 inches apart, and 3/8 inch in width. The tanks might be reinforced with iron bars, so that they could either be used alone or embedded in the usual brick structure. The junctions of the bed-plate and side-pieces could be finely rounded so as to facilitate cleaning, and at one end a draining-hole could be made, say, 1 inch in diameter.

Meantime both the hard-wood tanks and those of glazed tiles find their particular applications. The former is generally employed in smaller factories, or where future large increases of crop preclude the present installation of a fixed system. The latter find use in large factories, or where no new areas remain to come into bearing.Modern Installation.—As an example of a modern installation of coagulating tanks, we can do no better than offer reproductions of the system now in use on Pataling Estate.

A warning must be given against employing all tanks of stone-ware or cement unless well glazed. Almost without exception, irrespective of the material used in the construction of coagulating tanks, wooden partitions are employed. In the few exceptional cases the partitions are either of glass or of aluminium. The former would appear to be the ideal substance, were it not for initial cost and loss by breakage. These disabilities may possibly be overcome in course of time.Care of Tanks.—The use of aluminium would have been wider had it not been for lack of supplies and the question of cost during the War. A novel method of employing aluminium partitions was introduced in the factory of Tremelbye Estate. There were no slots in the sides of the glazed-tile tanks, but the necessary slots were very ingeniously created by means of aluminium “distance-pieces,” the two long edges of which were turned at right angles to the face of each piece to a depth of about 1/4 inch. The ends of the thin aluminium partition moved in the slot thus formed between two adjacent “distance-pieces.” The friction between the surfaces was sufficient to allow all the partitions, when in position, to be raised well above the floor of the tank, so that a uniform level of latex was obtained. Slight hand-pressure only was then required to push the partitions down.

Naturally the cleansing of glass or aluminium partitions presents no difficulty, but in the case of wood failure to ensure thorough cleanliness leads to possible defects in the finished dry rubber. Provided the wood could be made waterproof, no trouble would ensue, and hence various measures have been tried with that object in view. When new the boards have been surface-waxed or varnished, and the treatment has been repeated on occasions. But in course of time the surface film of waterproof material has disappeared, partially or wholly, and the trouble recurs. When partitions become sodden with serum, the surfaces are liable to be coated with a slime, consisting largely of organic growths which have an effect upon the latex, causing “pitting” on the surface of the coagulum and enclosed bubbles within.

It is recommended, therefore, that wooden tanks, after ordinary cleansing daily, should be swabbed out with a 5 per cent. solution of sodium bisulphite. Wooden partitions should receive the same treatment, and once a week at least (or every day if possible) they should be placed in the sun for an hour or two, care being taken that both sides of a partition are exposed in turn. Before being placed in the latex, all wooden partitions should be made wet on the surfaces.

Some years ago the writers had made a partition of vulcanite, which apparently would have proved of great service but for the initial cost. The advent of the War put the matter out of the question, but it is possible now that such a material would be worthy of extended trial. Except in the matter of cost, it would appear to have advantages over any substance yet tried; and if it were possible for estates to supply their own lower grade rubbers direct, the cost might be reduced considerably.

Standard Latex.—Enough has been written (see Chapter VII.) to familiarise the reader with the use of this term for the description of latex diluted daily to a level of dry rubber content. Whatever may be the practice elsewhere, it is now fairly general on estates in Malaya to reduce all latices to a uniform “strength” for the preparation of sheet rubber. It is claimed that only in this manner can uniformity of product be achieved.The selection of a standard has been the outcome of general experience. It has been found that if too high a standard is taken difficulties arise, such as (1) unsatisfactory and uneven coagulation, (2) too thick a coagulum for easy working in general, (3) too extended a period of drying and smoke-curing, and hence too dark a colour in the finished rubber.

On the other hand, too low a standard also brings trouble in its train. The coagulum is too porous, will not stand handling, and the resultant sheet is too thin unless an abnormal thickness of coagulum is prepared. Furthermore, over-dilution means an increase in the number of tanks required for any original volume of latex. This involves an increase in floor area, and perhaps in the size of the building. The soft sheets, when rolled, may spread to such a width as to cause the edges to be squeezed under the cheek-blocks of the machines, etc.For all practical purposes, whether sheets are prepared in pans or in tanks, it has been found that the optimum results are obtained by the adoption of a standard approximating and not exceeding 11/2 lbs. dry rubber per gallon. Primarily this standard has a direct connection and interdependence with the distance between the partitions (or between the slots) in coagulating tanks. The distance found most practicable is 11/2 inches. This thickness of coagulum, when prepared from latex not exceeding a standard of 11/2 lbs. dry rubber per gallon, is found to yield a very satisfactory sheet in all respects.

It will be seen that we have two possible main factors of variation:

The effect of variation in (a) will be clear. Even when latex of a standard of 11/2 lbs. per gallon is employed the resulting sheet may be either too thin or too thick.

Similarly, as already argued, the use of too low or too high a standard of dilution (when the factor of distance between partitions is not allowed to vary) is capable of causing much difficulty. While this is correct, broadly, it is found in the experience of some estates that their requirements are satisfied by a slightly lower standard than 11/2 lbs. per gallon. Thus it is not uncommon to note the adoption of a standard equivalent to 1 lb. 4 ozs. or 1 lb. 6 ozs. dry rubber per gallon. Experience dictates, however, that for the recognised standard measurements of modern tanks the practical limits of satisfactory density of latex lie between 11/4 lbs. and 11/2 lbs. per gallon.Standardising Instruments.—For standardising latex by dilution all that is required is an instrument which will preserve a perpendicular position while floating in latex, will be sufficiently sensitive to indicate fairly small differences in density of latex, and has one mark on its aerial portion accurately indicating a density corresponding to the required standard. On scientific grounds it can be demonstrated that such an instrument as employed in common practice would not be strictly accurate.[6] It is not proposed, in this section of the book, to discuss such considerations.

Instruments of this nature are represented by the “Metrolac” (originating from the Rubber Growers’ Association) and other similar recorders. They generally consist of a submersible bulb with a projecting stem which is graduated. The “Metrolac” differs from others in that the bulb is of torpedo form (thus reducing “skin friction”), and the graduations on the stem indicate actual weight of dry rubber per gallon instead of the ordinary specific gravity figures.

Theoretical considerations to the contrary, it is found in actual practice in Malaya and Ceylon that, although such instruments are naturally delicate and require careful manipulation, they are of considerable practical value and satisfy a definite requirement. Until an instrument of greater accuracy and equal simplicity can be discovered all estates should regard the possession of a few “Metrolacs” as essential.

The nature of their construction and the average conditions under which they are used (and abused) make it impossible to rely upon their accuracy indefinitely or for any long period. It is always recommended, therefore, that there should be at least two instruments available, one of which may be in daily use, while the other is kept in safe custody and only employed, say, once a week for purposes of checking the accuracy or degree of inaccuracy of the other. This can be done with reasonable approximity by placing both instruments in a tall vessel containing well-mixed and diluted latex. Instruments showing a marked degree of inaccuracy should not be preserved; but in cases of necessity “Metrolacs” from estates belonging to company members of the Rubber Growers’ Association may be sent to the laboratories for repair and adjustment.[7]

Where field coagulating stations have been instituted on estates, it is strictly necessary that instruments should be provided in all cases; and it should be a rule that these are tested and corrected weekly by a standard instrument employed for that purpose only. This need was well recognised by many estates when, during the War and the consequent shortage of supply of “Metrolacs,” a demand arose which was met in some degree by crude instruments of local manufacture, such as that commonly known as the “Castlefield bobber,” contrived and made by the enterprising manager of the estate of that name. The demand for the more accurate instruments can now be met.Methods of Using the Instruments.—The “Metrolac” was devised and introduced by the writers on behalf of the Rubber Growers’ Association, and directions for its use were given. Tables were prepared by means of which simple calculations for the dilution of any given latex could be made. These did not find an extended application, inasmuch as in the majority of cases native workers only were in charge of the processes of rubber preparation. In point of fact, such calculations are not strictly necessary, as the operation of standardising the latex can be done quite simply and skilfully by a trained native. Latex as it reaches the store in average weather from any particular division or field does not vary greatly in density. The trained coolie or foreman, basing his practice on experience, adds to the latex a quantity of water, and then makes a first test with the standardising instrument. Several additions of water (with thorough stirring) may have to be made before a test indicates that the correct density has been obtained, but it is surprising how quickly a skilled worker will arrive at the desired standard. Extreme or absolute accuracy is not insisted upon or desired, as avoidable delay is to be deprecated, and the result in any case is sufficiently exact for practical purposes.Skimming.—During the gravitation of the latex from the reception vessels (in which the standardising of the latex is effected) to the coagulating tanks, much surface froth is usually caused. This is best removed by means of a thin board of a width slightly less than the breadth of the tank. The skimmings are sometimes placed in pans and subsequently made into a second grade of sheet rubber, or they receive treatment with a small proportion of sodium bisulphite and eventually appear as fine pale crepe. The practice varies usually according to the form in which the general No. 1 grade is prepared.On some estates a great deal of the frothing is avoided by placing in position at the receiving end of the tank a perforated partition. This partition may be made of wood, or of stout zinc (or aluminium) carrying ten circular holes to the inch. Through this the latex percolates, while the froth is retained on a small area. The froth is removed prior to the addition of the acid. After stirring in the acid solution most estates again skim the surface of the latex; but if the stirring has been performed properly there should be little froth. This, when it collapses, in any case will appear only on the upper edge of the strip of coagulum, and after rolling should not be visible. It would appear, therefore, that the second skimming is not necessary.Style of Sheet.—Within the last few years the custom of making plain sheet—i.e., sheet having a plain surface—has gradually given place to the preparation of ribbed sheet—i.e., sheet having a pattern marked on the surface. It would probably be correct to say that plain (smooth) sheet is now only prepared by natives or by some estates just come into bearing. Even in the latter case there is no reason why smooth sheet should be made, as hand machines are sold which will do all the work required. It will be evident to anyone acquainted with rubber preparation that in the matter of actual quality of rubber the question of smoothness or a pattern can have no bearing on the result. One advantage claimed for ribbed sheet which may entirely justify the preference exhibited by consumers, relates to the question of packing. When rubber arrives at home it is frequently found to be in an almost solid block, due to the pressure of the sheets superimposed in the case. The smoother the surfaces of the rubber in contact the greater the adhesion and the denser will be the mass, and consequently the greater the difficulty in separating individual pieces. Under such circumstances it is plain that the difficulty is diminished if the sheets have a raised pattern on them. It is noted also that the liability to mildew-growth is greater the smoother the surfaces of the rubber.

On these grounds the “marking” of sheet rubber is to be commended. These reasons apart, it is really astonishing the difference made in the appearance of the sheets by impressing upon them a ribbed pattern, and it is highly probable that the market value of the rubber is slightly increased. It is not our duty to attempt to reason why this simple operation should increase the market value of sheet rubber; it is sufficient to recognise that it is so, and that money may be thrown away by neglecting to cater for the taste of the market. Of the patterns impressed upon sheet rubber there is a variety, but the general style is that known as the “spirally close-cut ribbing.”Standard Sheet.—Leaving for the present the question of pattern of mark, one cannot do better by way of introduction than to reproduce the instructions[8] given generally to estates.

Rolling and Marking of Sheet Rubber.—Working with standard latex it is found that strips of coagulum 11/2 inches in thickness require little rolling to produce sheets of desirable thickness.

When to Work the Coagulum.—Before proceeding to discuss other points the question remains to be settled as to how long it may be necessary or advisable to allow the coagulum to remain in the serum before rolling it. For reasons of practical economy in factory working, it is usual to allow sheet rubber to remain over night, and the coagulum receives attention early next morning. During the interval (averaging about eighteen hours), the coagulum consolidates, leaving an almost clear serum if the correct quantity of acid has been added to the latex. Any but the very slightest trace of milkiness in the serum indicates an insufficiency of coagulant. If the serum is always definitely clear, there may be grounds for believing that an excess is being used. If the quantity of coagulant has been calculated to an average nicety, the serum should be just dubiously free from milkiness.

The firmness gained by the coagulum on standing in the serum overnight should enable it to be handled next morning without any marked stretching, and in some estates the rubber is put direct through the first pair of smooth rolls without a preliminary consolidation by means of hand-rolling.

Some estates prefer to handle the coagulum while rather softer, as it is claimed:

In such cases all latex must reach the store comparatively early in the day—e.g., before noon. Three hours is allowed for coagulation, and the working of the rubber is then commenced. As a general rule this means that the operations of rolling and marking must be completed, a short interval given for dripping, weighing must be done, and the rubber placed in the smoke-house before night falls (as a rule about 6.30 p.m.).

Unless factories dealing with a large crop are well equipped with artificial light, such a course is not open to them; in any case it remains true that night work should be avoided if possible. If, however, it can be arranged without increasing the cost of production, there would appear to be no objection to the early working of the coagulum as described above.Hand-Rolling.—As already indicated, some few estates do not give the strips of coagulum any preliminary hand-rolling, as the rubber is considered to be sufficiently firm to be handled into the first machine.

On most estates hand-rolling is found necessary, owing to the tendency of the long strips to stretch unduly, giving badly shaped sheets. This hand-rolling should be done carefully, and is best effected on a specially constructed table. This consists essentially of an inch-thick hard-wood plank about 2 inches wider, and 4 or 5 feet longer, than the strip of coagulum. Along the edges of the plank, and at right angles to its upper flat surface, may be fastened strips of wood about 1/2 inch square in section, thus forming a shallow tray open at either end. These strips serve two purposes:

(a) As the wooden roller is wider than the plank, they prevent the coagulum being rolled too thin and too firm.

(b) They prevent the coagulum being squashed too wide, and tend to keep the edges straight.