The Preparation of Plantation Rubber Part 10

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.

To avoid "thick ends" it is sometimes considered advisable to insert, at either end of the rolling table, shallow wedges about 6 inches long, of the same width as the table (between the edge-strips), and with the sharp end of the wedge pointing in the direction of the length of the table. The ends of the coagulum are drawn up and finished on these inclined planes.

These points may appear to be extreme refinements, but as long as rubber is valued on such grounds we must endeavour to meet the system imposed upon us.

SMOOTH-ROLLING.--It is advised that, after hand-rolling, the coagulum should be pa.s.sed through at least two machines having smooth-rolls. On some estates three such machines are employed. The purpose of this procedure is to reduce the thickness of the coagulum gradually. The same could be effected, of course, on one machine; but obviously the distance between the rolls would have to be readjusted at each operation and for each piece of coagulum. Apart from the time thus wasted, there is the certainty, in view of the rough adjustment of the machines, that the chances of obtaining uniformly thick sheets would be slight.

The machines should be arranged as a battery, with the marking rolls at one end, so that the operations are consecutive. It is erroneous to imagine that heavy machines (such as those used in crepe preparation) are required.

Light machinery only is necessary for sheet-making; but any available heavy smooth-roll machines in a crepeing battery may serve admirably for the purpose.

MARKING.--Heavy machines are unnecessary for the purpose of putting a pattern on sheet rubber. If the rubber has been properly prepared a light pair of rolls is capable of exerting sufficient pressure to cause a good upstanding pattern.

Rolls are cut in various designs: some with "diamond" grooves on both rolls; some with grooves of varying width and depth encircling the circ.u.mference of the rolls, thus creating a "stripe" effect on the rubber; and some with diagonally-cut spiral grooves placed closely together. The last has the greatest vogue at present, while the first has almost gone out of favour. An objection lodged against the second design is that the edges of the grooves sometimes cut through the rubber, so that the dried sheet divides in strips. It would appear in such instances that either the coagulum was too thin and soft, or that the grooves had been cut too deeply and sharply. In any case the choice of a design is an arbitrary matter, and should depend upon the effect produced on the rate of drying and the general appearance.

The popular "close-cut spiral" roll is machined with varying measurements, but the usual design has grooves 1/8 inch wide by 1/8 inch deep and 3/16 inch apart.

Many estates have a particular "brand" cut in the middle of the rolls for purposes of identification. If this is done it is advised that the main grooving of the rolls be carried into the "branding" strip; otherwise grip will be lacking on this portion, and a certain amount of "c.o.c.kling" of the sheets will result.

Sheets are often seen in which the potential effect of the grooving is reduced to a comparatively flat pattern in place of the desired ridges. The fault is generally attributed to the shortcomings of the marking rolls.

While it is true that the grooving often deteriorates by friction-wear when the rolls are running "free," experience generally decides that the deficiency in the appearance of the rubber should be attributed to faulty previous preparation rather than to the marking rolls. Sets of rolls have been changed often without justification or an improved result. It would always be well to be certain first that the trouble did not emanate from the fact that the coagulum had been previously rolled so thin and hard that the rubber could not be squeezed so as to fill the grooves. This has been found to be a common fault, and the general effect is to delay drying in spite of the thinness of the rubber.

Again, the trouble may have been due to an incorrect standardisation of the latex, generally in the direction of too heavy a density (too rich a latex) being employed. The original thickness of the coagulum would be normal, but owing to the abnormal rubber-content the effect of pa.s.sing through the smooth rolls would be the production of a strip thicker and firmer than ordinary. If this firmness is appreciable the resistance of the rubber to the squeezing action of the marking rolls will result in a flat pattern--_i.e._, the grooves cannot be filled, and the ridges are low.

It is advised that all rolls used in the preparation of sheet rubber should be at least 18 inches wide, in order to avoid the appearance of thickened edges which delay drying.

Working with the correct standard of dilution of latex, and following the procedure indicated in the foregoing paragraphs, the dry sheet should not exceed an average thickness (over ridges and depressions) of 1/8 inch.

PREPARATION FOR SMOKE-CURING.--It used to be the custom to allow sheet rubber to air-dry first for periods varying from one to several days.

Naturally moulds were soon formed, and when the sheets were quite smoke-cured a ma.s.s of the dead moulds could be seen, if not over the whole sheet, at least in the corners of each diamond mark. It has been demonstrated in practice that there is no advantage in allowing sheets to air-dry partially before smoking. In fact, to obtain the greatest benefit from smoke-curing, sheet rubber should be placed in the smoke-house as soon as possible. The same effect of mould-growth may be noted if the wet sheets are placed in a smoke-house insufficiently heated.

Other defects may arise which can be traced to faulty treatment of the marked coagulum prior to hanging in the smoke-house and subsequent to rolling. These will be enlarged upon in a subsequent section of the book, and at present it will suffice to indicate the procedure which experience directs as likely to give the best results.

When the lengths of coagulum leave the marking machine they are usually laid in piles containing two dozen or more strips. The piles are then cut into the required lengths, the exact length generally being determined by the available perpendicular distance between the supports in the smoke-house. It is necessary to remark that the piles of sheets should not be allowed to acc.u.mulate, but should be dealt with in subsequent treatment progressively. If for some reason this is not possible, then all piles of sheets should be turned on edge so as to a.s.sist the draining away of the serum or "mother-liquor," which continues to ooze from the rubber for some time after the squeezing in the machines.

Where hot water is available the freshly cut sheets should be pa.s.sed into it as soon as possible, and given a thoroughly good swilling. The caution must be given that the hot water should be changed very frequently and, if possible, after every batch, say, of a hundred sheets.

The sheets should then be carried immediately to racks on which they are hung to drip. Generally these racks are situated under cover, but there is no reason why they should not be placed in the open air without cover or shade. From continued experience of this practice over a period of years it is found advantageous and to be preferred to the usual method of allowing sheets to drip under cover.

While the sheets are fresh and loaded with internal moisture, the effect of sun-heat upon the surface, when exposed for, say, two hours, is nil; and the safety of the process can be guaranteed, provided the stated limit is not exceeded to an appreciable extent.

[Ill.u.s.tration: THE OLD METHOD OF "DRIPPING" FRESHLY ROLLED SHEETS WITHIN THE FACTORY.]

After dripping for an hour or so, the sheets should be placed in the smoke-house. If it is a bright sunny day, no extra precautions need be taken; but on cool, dull days it would be advisable to light the fires earlier than usual, consistent with the work required to be done in the house--_e.g._, in the removal of dry rubber. There would appear to be no reason why the dry sheets should not be first removed, so that on dull or wet days smoking can be commenced as soon as the wet rubber has been hung.

On a few estates where the smoke-houses are worked continuously, except for a few hours in the morning, a portion of the building is separated by a part.i.tion for the reception of the wet rubber. The sheets are taken directly from the marking rolls and placed in the chamber, beneath which a fire is started. The sheets thus drip in a warm and smoke-laden atmosphere until next morning, when they are weighed and removed to the smoke-house proper. It is claimed that freedom from "rust" is thus obtained.

It will be clear that in the treatment of the rubber preparatory to smoking the whole process should be continuous, and delay should be avoided if the best results are to be obtained.

[Ill.u.s.tration: THE NEWER METHOD OF HANGING IN THE OPEN AIR.]

SMOKING OF RUBBER.--The a.s.sumption may have been noted above that the sheet is to be smoked. As far as our knowledge extends, none but small native estates now prepare sheet rubber of any other type, with the exception of certain patent processes. Air-dried sheets are generally made on small-holdings, and are bought in the market chiefly for the purpose of macerating and making into blanket crepe. We have no intention, therefore, of discussing the possibilities or qualities of air-dried sheets, as the output of sheet-rubber from our estates is always in smoked form. The drying (or, properly, smoking) stage will be discussed in Chapter XI.

CHAPTER X

_PREPARATION OF CREPE RUBBER_

NO. 1, OR FINE PALE CREPE.--Considering first the preparation of the highest grade, fine pale crepe, it must be stated that the difficulties attached to the process are generally not sufficiently appreciated. In this pale rubber minor blemishes are so plainly apparent that their importance is highly exaggerated, and what would worthily escape notice in smoked rubber a.s.sumes disproportionate prominence in pale crepes. The very fact that such a delicate material as colourless coagulum has to be manipulated in coa.r.s.e iron rollers, with the attendant oil and grease worries, should be sufficient to deter one from criticising too harshly the occasional lapses of an estate struggling to give of its best to the market. At the same time there can be no doubt that if precautions are taken to attend to all likely sources of contamination, defects in pale crepe may be avoided to a wonderful extent; and on some estates the observance of elementary rules enables the preparation of the finest pale crepe to be made almost mechanically.

STANDARDISATION OF LATEX.--The question of the standardisation of latex has been dealt with in a general way in Chapter VII., and the reader is now familiar with the trend of the argument in its favour. It will be recognised that the necessity for standardisation exists to the same degree in the correct preparation of pale crepe as in the case of smoked sheet.

Unless the dry rubber content is invariable, and the quant.i.ties of chemicals fixed, the colour of the crepe may vary appreciably.

It may be pointed out that it is not _essential_ to adopt the same standard of dilution as for sheet preparation. Given that latices from all fields or divisions are fairly uniform, and of high rubber content, the standard may be taken at a figure equivalent, for example, to 2 lbs., or 2-1/2 lbs., or even 3 lbs. per gallon. It is wise, nevertheless, to take a lower standard for several reasons. For instance:

(_a_) The average dry rubber content varies with climatic conditions, position of the cut on the tree, general health of the tree, etc. On a rainy day the dry rubber content may be lowered too greatly by advent.i.tious circ.u.mstances.

(_b_) Recording instruments often fail to give even approximately correct readings in rich latex. Errors may thus be made easily.

(_c_) A fairly soft coagulum means easier working on the machines, less labour, and proportionately cheaper costs.

[Ill.u.s.tration: THREE GRADES OF CREPE RUBBER.

Left to right: fine pale crepe; second quality pale crepe; compound crepe.]

It is advised, therefore, that for general purposes the same standard as that found suitable for sheet rubber should be taken--viz., 1-1/2 lbs. dry rubber per gallon. At all events the standard should not exceed 2 lbs. per gallon.

COAGULATION AND COAGULANT.--Coagulation may be undertaken without any special arrangement of tanks, and is usually effected in the ordinary "Shanghai" glazed earthenware jars containing about 45 gallons. Given reasonable care, and a fairly fool-proof system of calculation for the quant.i.ties of chemicals required, no difficulty need be experienced.

[Ill.u.s.tration: A WASHING SHED.

Sheets are soaked in hot water in tanks in the background, and then scrubbed under a spray of cold water.]

On a larger scale it is advised that proper reception tanks, in which standardisation can be effected, should be installed. Where both sheet rubber and fine crepe are being prepared, the whole system of sheet-coagulating tanks may be employed with considerable advantage, even to the insertion of the part.i.tions.

If ordinary jars are used, and the coagulum is left until the following morning, the ma.s.s of rubber has to be cut up into pieces of a size suitable for the machines. The knives or saws are sometimes rusty, and the colour of the coagulum is affected. The coolies often feed into the machines lumps which are too large, with the result that portions are thrust under the cheek-blocks and become stained.

When a sheet-coagulating tank is used all labour of cutting the coagulum is obviated. The long strips are handled and fed into the rolls easily. It may be seen, likewise, that actual work is thus saved in machining.

QUANt.i.tY OF COAGULANT.--For a general discussion on the coagulant and quant.i.ties employed, the reader is referred to Chapter VIII. It is there recommended that for latex standardised to a level of 1-1/2 lbs. per gallon, the proportion of pure acetic acid should be in the ratio of 1:1,200. Directions are there given for the making of the solution, and the calculation of the quant.i.ty required for any given volume of latex.