The Preparation of Plantation Rubber :: CHAPTER XVIII DEFECTS IN SHEET RUBBER

CHAPTER XVIII DEFECTS IN SHEET RUBBER

Before proceeding to deal with defects in the rubber as it is put upon the market a brief account will be given of faults which may be noted in the preparatory stages.Milky Residue or Serum.—If the serum is not clear after the ordinary period allowed for coagulation, it indicates one of the following possible causes:

(a) Failure to obtain complete mixture by thorough stirring.

(b) Insufficiency of acid solution. This may be real or indirectly due to the presence of an excess of anti-coagulant such as formalin or sodium sulphite.

(c) In cases where other coagulants than acetic or formic acids have been employed the failure may be due to an excess of, or an unsuitable, coagulant—e.g., hydrochloric acid.

Coloured Surface Blotches and Unpleasant Odour.—Sometimes the surface of the coagulum exhibits yellowish or bluish streaks and patches. It will be found generally that the yellowish colour is possessed by a slimy substance, of offensive odour, which may be scraped from the surface. Either insufficient acid has been used, or the mixing of latex and coagulant has been at fault.Dark Discoloration of the Rubber.—This may be stated to be a natural process when fresh rubber is exposed to the atmosphere. It is usually described as “oxidation,” and it will be noted to be absent, or to occur to less degree, on those portions of the rubber which are protected from the atmosphere by being below the surface of the remaining liquid. This surface change may be prevented (see Chapters VIII. and IX.) by the use of small quantities of sodium sulphite (for preference) or bisulphite.Soft Coagulum, Spongy under-surface, Tearing of Coagulum.—If the whole mass of coagulum is too soft, while coagulation appears to be complete, over-dilution of the latex has occurred. This may apply also to the case in which the under-surface only is spongy and soft. If coagulating-tanks are employed, the upper edge may be comparatively hard, while the lower is soft and weak. Often the spongy portion may adhere to the partitions. This prevents the natural rise of the coagulum, due to retraction, as the mass “sets.” The pull between the free upper portion and the adhering lower edge causes splitting and tearing of the coagulum, with marked porosity (spongy appearance). The two factors to receive attention are the standard of dilution and the condition of the surfaces of the partitions. If these have minute cracks into which latex can penetrate, and in which coagulation takes place, the boards should be discarded. Given the conditions indicated above, the tearing and splitting of rubber in coagulating tanks is sometimes augmented by the practice of flooding the tanks when coagulation is judged to be complete. The surface water finds its way downwards between strips of coagulum and the partitions, thus increasing the upward tension between the free and adhering portions. The main idea governing the practice of flooding the tanks is to prevent “oxidation” (darkening) of the upper edges. If a small quantity of sodium sulphite is employed as an anti-oxidant and to retard coagulation, it is not necessary to flood tanks.“Pitting” of Surfaces.—In pan coagulation this “pitted” appearance is usually limited to the under-surface, while coagulum prepared in tanks may exhibit the defect on both faces. The existence of these numerous “pits,” or small depressions, points to the presence of bubbles of gas which have been unable to escape freely. As the formation and retention of gas-bubbles is not a normal occurrence in coagulation, we are led to infer that some special conditions must have arisen. These may be supplied by one or more of the following contributory causes:

(a) The latex had begun to “sour” before arrival at the factory or while waiting to be treated. This premature coagulation is usually checked or diminished by the employment of anti-coagulants (see Chapters VIII. and IX.). It is generally accompanied by the appearance of enclosed gas-bubbles in the dry rubber.

(b) There may have been a slight insufficiency of coagulant, or the admixture was not thorough, thus allowing a slow putrefactive change to take place in the incompletely coagulated areas.

(c) The wooden partitions may not have been effectively cleansed. The existence of a thin slime, of bacterial origin, is sometimes noted. This is accountable for putrefactive effects in the surfaces of the coagulum, or in the serum, giving rise to the formation of gases. If these cannot escape freely, by reason of adhesion between the coagulum and the partitions, “pitting” occurs.

Thickened Ends or Edges, after Rolling.—As a rule these defects may be ascribed to the employment of too rich a latex, or faulty manipulation. Even if the standard of dilution should be correct it sometimes happens that, in the preliminary rolling of a long strip of rubber, coolies begin in the middle, rolling with a forward pressure and tension towards the ends of the strip. This is generally not so much the fault of the coolie as being due to the lack of proper facilities for preliminary rolling. The table should be about 3 feet in height, so that ease of working is obtained merely by natural pressure due to the position in which the worker stands. The use of a heavy wooden roller would contribute towards this result, inasmuch as it obviates the use of force, and the pressure is almost entirely in a vertical direction.Mis-shapen Sheets.—It is sometimes noted that sheets may be wider and thicker at the ends than in the middle. Manipulation alone, as indicated above, is not solely responsible. The primary cause is to be traced to over-dilution of latex, giving a very soft coagulum which responds too readily to tension and pressure. Faulty treatment in rolling exaggerates the tendency for the strip of sheet to become narrow and thin in the middle, wider and thicker at the ends.Thickened Patches, Torn Sheets, “Dog-Ears,” Creases.—These elementary defects are all due to careless working. While occasional errors cannot be avoided, there is no real excuse for the continuance of trouble to any degree, under average supervision.

Thickened patches are often caused in conjunction with torn sheets, and the trouble may be ascribed to faulty practice in allowing too heavy a pile of wet strips to accumulate before machining. Or a comparatively small pile may have been transported some distance. It is difficult to separate the strip, and occasionally the separation is only effected at the expense of two sheets, one of which is torn and the other has a portion of the first strip adhering to it.

“Dog-ears” due to the folding over of corners of the sheets, and creases due to the rumpling of the coagulum, are generally the result of haste and lack of average care. Machine coolies, more often than not, will not be at any pains to straighten out folds before passing the coagulum through the rolls.Greasiness before Smoking.—Under ordinary methods of working this should never be encountered. It may be taken to show that the machined rubber has been allowed to remain, either hanging or in piles, far too long before entering the smoke-house. The appearance is most marked if the rubber has remained in a cool and moist atmosphere—e.g., if it has been hanging over-night in a closed and badly-ventilated factory. In a marked degree this is to be observed in the preparation of air-dried sheets, unless they are exposed, when freshly prepared, to the action of the sun for a period. This period, in the case of rubber prepared on native small-holdings, generally extends over several days—until the sheets are more than half dry.

In the preparation of smoked sheet, the greasy appearance and the cause outlined contribute to a defect which is eventually described as “stretching rusty.”Surface Blemishes.—The coagulum, during coagulation and subsequently, can be contaminated in various ways. In most cases a little intelligence or increased care would prevent the occurrence of these defects.

When the coagulum remains over-night, in the absence of a cover, it is not uncommon to note the presence of dirt (from the roof above, or blown in from the outside), the droppings of mice and rats, flies and small insects. In theory these should be seen and removed by the factory hands. In practice, except while under immediate supervision, the extraneous matter is often rolled into the soft coagulum.A fairly common cause of this surface contamination is the exhaust from the power-unit; generally the worst offender is a steam-engine. Grit and smuts continually find their way into the factory, alighting on the tables, in the latex, in the water, and on the freshly prepared rubber. They are rolled into the soft rubber and lead to marked depreciation in the selling value. The radical remedy seems obvious, but is often beset with many difficulties not unconnected with financial considerations.

Other superficial blemishes, such as those due to the presence of rust marks, oil or grease patches, etc., are self-explanatory, if a little thought is brought to bear upon them; and it is not proposed here to discuss such defects more fully.

Having now dealt with certain defects which are visible in wet rubber, we come to the discussion of others which are only perceptible either during or after the drying period. As far as is known no plantations of any size now prepare sheets other than in the form of smoke-dried rubber, with the exception of a few which make a special form of thick and partially air-dried product known as “slab” rubber.

It is not proposed, therefore, to treat in any detail with air-dried sheet rubber. Certain obvious defects are common to both air-dried and smoke-cured sheets, and these will be first discussed.Unevenness of Appearance.—This lack of uniformity may refer either to size or colour, or to both. Apart from any other contributory causes, this variation is due, in pan sheet, to a neglect to standardise the dilution of all latices, or to lack of uniformity in the quantity of standardised latex placed in each receptacle.

Where tanks are employed all sheets from the same tank should be of the same size before rolling, and any subsequent disparity in thickness and length must be attributed to some alteration in the width of the gap between the rolls of the machines.

Unless all latices are standardised by means of an instrument, it is of course probable that the content of one tank may be found to differ from that of another.

In a general sense, whether air-dried or smoke-cured sheets are considered, a thin strip will dry more quickly than a thick one, and should be paler in colour when viewed by transmitted light—i.e., when the rubber is held between the eye and the source of light.

It is necessary, therefore, to guard against the possibility of variations in thickness caused by faulty manipulation. The distance between the squeezing rolls (smooth) and between the marking rolls (patterned) should be adjusted and should remain set until the conclusion of work. In a factory having nothing beyond average requirements in equipment of machines it should not be necessary to have to interrupt the work of the smooth rolls or “markers” by having to make adjustments. This is, however, inevitable if there is only one smooth-roll machine, as it is always desirable to reduce the thickness of the coagulum by at least two stages through even-speed smooth rolls. In some factories there are three light power-driven smooth-roll machines, the gaps between pairs of rolls being set so as to obtain a gradual thinning effect upon the fresh coagulum, which is then passed once between patterned rolls. With such equipment it is found possible, in some cases, to omit the preliminary hand-rolling, and the strips of coagulum from the tank are passed direct through rolls set with a wide gap. This work demands much care, as it is necessary to avoid any distortion of the coagulum which may be caused by its own weight and length.Variation due to Oxidation.—The subject of oxidation has been mentioned in the opening paragraphs of this chapter. It will have been learned that oxidation is a natural process, and that it may be prevented by the employment of anti-oxidants such as the sulphite or bisulphite of soda. In earlier days it was sometimes prevented by steeping the thin rubber in very hot water.

In the absence of an anti-oxidant the degree of oxidation may vary daily and in different batches of latex on any one day, so that there is always the possibility of a lack of uniformity due to oxidation effects. This would be more evident in air-dried sheets than in smoke-cured rubber, as in the latter case the darkening of the surface would be masked by the colour induced by the smoke-drying process.

To obviate this variation anti-oxidants are used on most estates, but the accidental or misinformed abuse of these chemicals may lead to further lack of uniformity. Hence it is necessary to follow carefully the formulÆ prescribed by experience.Colour of Smoked Sheets.—It may be of interest to note that the effect known as oxidation is attributed to the presence of micro-organisms called enzymes (ferments) in the latex. It can also be produced artificially in various ways—e.g., by the use of the crude product of wood-distillation (pyroligneous acid) as a coagulant, or by the addition to the latex of small quantities of a phenol such as carbolic acid. It is thus possible to prepare in sheet form a rubber which has the appearance of having been smoke-cured, although it may never have been in a smoke-house.

It will be clear, therefore, that apart from other causes, the colour of the cured sheets may be influenced by oxidation of the fresh coagulation, and by the constituents of the smoke. It follows that the smoke from timbers which are richer than others in certain chemical bodies set free by combustion will produce a rubber darker in colour.

There is thus no real connection between colour and period of cure, although in a general sense the longer the interval the darker the colour.

Similarly it is now plain that when anti-oxidants are employed in excess the paleness of the rubber is in no degree truly indicative of the period during which the rubber has been smoke-cured.

The influence of the effect of the hypsical condition of the wet rubber upon the final colour must be thoroughly grasped. One may take two sheets of apparently the same thickness, and smoke-cure them in juxtaposition within the same house, only to find that one dries much more rapidly than the other. As a consequence, the first, when fully cured, will be of a medium golden brown colour; while the other, owing to protracted smoking, will be dark. Evidently there must be some distinct difference between the two in physical condition prior to the smoking. Here the factor involved is the rubber-content of the latex. Given two pieces of coagulum of identical thickness, but prepared from latices of different dry rubber content, it will be obvious that to reduce them to similar thickness, more pressure will be necessary in one case—i.e., that piece of coagulum will be much more dense (more consolidated)—while the other will be comparatively soft and porous. Into the latter warm smoke can penetrate much more easily, and the internal moisture can escape more rapidly. The full cure may be made, say, within twelve days, while the tougher sheet may demand up to twenty days.

To attain uniformity in colour, therefore, the following points must be studied and controlled as far as is possible:

(a) Uniform dilution of all latices.

(b) Uniform dimensions of coagulating receptacles.

(d) Uniform quality and quantities of chemicals.

(e) Uniform methods of manipulating the coagulum.

(f) Uniform conditions of fuel and accommodation during smoke-curing.

Surface Gloss.—In the choice of fuel there is room for control if one has good timber available. This point opens up a discussion on the vexed question of “over-smoking,” as the term is sometimes applied to a pronounced dry glossy appearance of the surface of sheets.

Three main factors are involved:

(1) The nature of the fuel.

(2) The ratio between furnace capacity and the capacity of the smoke-house.

(3) The rate of combustion.

Obviously any fuel which yields an excessive quantity of tarry matter or creosotic substance would conduce to the formation of a heavy glaze on the rubber. Such fuel, therefore, should at most only be employed as the smaller portion in a mixture with “dead” timber.

It is impossible to lay down any general rules for the guidance of estates, as the timber available varies so widely in nature. Experience must be the only guide, and it should not be difficult to obviate the defect. Even so, there must be minor differences between the results obtained on estates, so that it is not possible to make strict estimations of the smoke-curing period by an examination of the surface appearance of rubber, even under the best of conditions. Some estates find that the rubber has a distinct gloss in ten days, while others may smoke-cure for twice that period without difficulty. Evidently, therefore, the question of available fuel is of prime importance. It may be remarked that very satisfactory results are always obtained from the use of fairly dry timber obtained from thinned rubber trees, mixed with the “dead” timber of old logs and stumps found on the estates.

Obviously if a smoke-house has a superabundance of furnaces, producing more heat and smoke than is required, glazing will result. The point is tested by the average temperature maintained and the average rate of drying. The result of a high temperature would be the possibility of volatile tarry matter being driven in excess to the upper chamber. That this effect is eventually produced even at optimal temperatures is evident from an examination of the wood-work within the upper room.

It is clear, also, that the rate of combustion exerts an influence. In a general sense a rapid consumption of fuel would augment the quantity of tarry matter passing into the upper chamber over any given period, and the possibility of glazing would be increased.

On the other hand, it is possible that a surface glaze might be formed if the temperature were uniformly too low, especially if the rubber were rather thick. The rate of drying would be so slow, that if a timber rich in tarry matter were employed, the rubber might be exceedingly glossy.

In order to guard against the appearance of a heavy glaze, then, the following points must be observed:

1. The fuel must be carefully selected by experience.

2. The sheets must not be thick. No sheets should be thicker than 1/8 inch measured in average section across the ribs.

3. The temperature must not be too high. An average working temperature of 120° to 125° F. should be ample.4. If the sheets are fairly thick, a low average temperature should be avoided. No lower average than 110° F. should be allowed.

Dull, Black Surface.—This is the opposite of the previous case, and generally is accompanied by a fairly heavy darkening of the surface due to “oxidation” effects. The fuel used is too “dead,” and needs the addition of some substance containing a fair amount of creosotic matter. The appearance of the rubber does not justify the assumption that it has been over-smoked. As a matter of fact, this type of rubber often becomes affected by mildew fairly rapidly, thus showing that the smoking has been inefficient.

It may happen that an estate is in the habit of using a fuel which gives even to a thin sheet a heavy glaze in a comparatively brief period. The general custom is to soak such sheets in cold water, and then to scrub the surfaces, sometimes using soap, in order to cleanse the rubber and free it from the glaze. This operation may be done too well, in which case the rubber will have a dull appearance, and may be rather more liable to develop surface mildew after a time.Moist Glaze, Greasiness of Surface.—This describes the condition of sheet rubber when taken from the smoke-house. Sometimes the greasiness does not develop until the rubber has been out of the smoke-house for a day or two.

As far as experience shows at present it may be due to two causes:

(a) The use of an excess of sodium bisulphite or sodium sulphite. The use of sodium bisulphite is not recommended generally for sheet-making. It may cause the rubber to be too pale in colour, and the abuse of it may delay the drying unduly. In the latter case, a trace of the salt may remain within the rubber or upon the surface. If so, as the substance remaining is fairly hygroscopic, it will take up moisture from the atmosphere and may cause the surface of the sheets to have a moist and shiny appearance. The moist surface deposit comes away upon the hand when the sheets are touched, and is difficult to remove entirely. On some estates a very small quantity of the bisulphite is employed, as it is found to be of service in the prevention of bubbles, but in unskilled hands the method is open to abuse, and is, therefore, not recommended for general use.A large number of estates now use sodium sulphite in very small quantities as an anti-coagulant and a preservative for latex in the field. The abuse of this very useful substance carries its own penalty. The substance is hygroscopic; and if an excess is present the drying period will be protracted, and the sheets will have a very moist surface film.

It may be found sometimes that only some of the sheets are affected. This indicates that, whereas uniform quantities of a solution of sodium sulphite have been served out in all fields, the proportion may have been excessive in the case of fields giving a latex of comparatively low rubber content. What suits the latex from old trees may be excessive probably for the latex of younger trees. This is not an infallible rule, as in the case of older fields in which immature bark is being tapped, it is to be noted that the dry rubber content of latex may be less than that of latex obtained from younger trees.

This type of moist glaze is not easy to remove. Ordinary surface washing had but a temporary effect, and the trouble recurs. The only way of dealing with the difficulty is to soak the sheets for days in running water (or in successive changes of water) and to re-smoke until dry.

(b) The second type of moist glaze is not so difficult to deal with, and may be removed as a rule by washing the sheets when the rubber is otherwise apparently dry.

It appears to be mainly a matter of unsuitable fuel for smoking and of failure to provide adequate ventilation. A large number of estates have been “thinning-out” or are doing so systematically. The logs thus obtained are often used as fuel in the very green stage. The smoke thus generated must be moist, and if the building is entirely closed, this moisture must be deposited eventually upon the rubber and racks. Some estates have surmounted the difficulty by opening up the roof-ridge slightly so as to allow the moisture to escape with some of the smoke; but if the logs from rubber-trees are to be used, they should be stacked in the sun for some time. Even then, preferably, they should not be used alone. A judicious admixture of dead and rotting jungle-timber appears to give very satisfactory results.

Virgin Spots and Patches.—If the description really indicates the defect it simply means that portions of the sheets are not dry. When cut they exhibit the typical whitish, opaque appearance described as “virgin.” It is doubtful whether any rubber put upon the market as No. 1 product nowadays can have this complaint levelled at it. In the extreme case it points to gross negligence on the part of the packer.

Sometimes what are taken to be small spots of “virgin” are really patches of tiny air or gas bubbles. The point can be easily determined by cutting through the patch and examining the cut edges.Surface Moulds or Mildew.—During the last two years, complaints regarding the incidence of “mouldy rubber” (i.e., relating chiefly to the presence of mildews on the surface) have become increasingly common.

To judge by the comments of producers, who as a rule never again see their rubber after it leaves the estate, one would infer that the defect is imaginary, and that the complaints are made solely with a view to repudiation of contracts or the general cheapening of an article of commerce. They can often point out, with a certain amount of truth, that there has been no change in the methods of preparation or curing, and that previously there were no complaints.

It is forgotten, however, that in former years the smaller output of rubber was taken into consumption more rapidly than of recent years. That is to say, the interval between smoke-curing and the employment of the rubber in the manufacture of goods did not demand such a long period of storage. Hence the effects of smoke-curing are now more likely to vanish.

Going still further back in the history of plantation rubber, we can point to the time when smoked sheets were allowed, or had, to remain in the curing-sheds for very extended periods. Loose specimens of rubber prepared during that decade still exhibit no signs of mildew growth.

In later years a demand arose for sheets paler in colour than the old type, and in order to meet that demand, a change had to be made in methods. This led to a system of working whereby it was possible to smoke-dry sheets thoroughly in from twelve to fourteen days. This interval was further reduced on many estates, until some were producing rubber which appeared to satisfy all requirements after only five or six days’ curing. This does not refer to the case of estates having smoke-houses of “continuous-working” type, but to those on which smoking was confined practically to the hours of night. Under former conditions of rate of production and consumption, this short period of smoke-curing would possibly have been ample; but even this is very doubtful, as often the rubber would not stand the relatively short journey from the estate to Singapore without mildew-growth being incipient. We have often received specimens of rubber sent from estates for criticism, and have noted that within a comparatively brief period mildew was to be seen.

The whole matter resolves itself into a question of thorough efficiency of smoking. This is not dependent on duration of smoking alone, but involves other factors, such as the kind of fuel employed, the rate of combustion of fuel, the average temperature sustained, the ventilation of the smoke-house, and the situation of the building. Other occasional contributory factors are contemporary adverse climatic conditions and the possible abuse of an anti-coagulant such as sodium sulphite.

It has been shown that after a time, given suitable conditions involving the presence of moisture, moulds may appear on sheets which were apparently fully smoke-cured, and that under the same conditions other and older samples were unaffected. It is argued that the latter sheets had evidently been smoked more efficiently than the others. Hence it is fair to assume that, except under very special conditions, which do not apply to the ordinary procedure in the shipping, storage, and sale of rubber, moulds will not develop upon sheets which have been properly smoked. The term “properly smoked” signifies efficient smoking for all practical purposes under ordinary procedure, and implies or includes all the advantageous factors which have been discussed or alluded to in preceding paragraphs.

Without discussing in wearisome detail conditions which may give rise to the incidence of mildew on properly smoked rubber, it may be pointed out that the following are favourable to the growth of moulds:

(a) Storing sheets in a damp place before packing.

(b) Packing sheets in wooden cases which are not thoroughly dry.(c) Piling up cases of rubber in a badly ventilated store-room.

(d) Placing the cases on a cement floor.

(e) Wetting of cases by sea-water or by rain during transport, etc.

Black Streaks, Spots or Patches.—The origin of these is not difficult to trace. They are caused by drippings from the roof, and contain condensation products from smoke plus moisture. The ventilation of the roof-ridge should receive attention, and if the trouble persists it will be necessary to place some absorbent screen below the sloping roof. Sackcloth is sometimes used, but leads to a worse state of affairs unless changed frequently. In most modern smoke-houses having an iron roof there is an inner lining of soft timber.

There scarcely seems a necessity to discuss the case in which an iron roof has become perforated by the action of smoke. The remedy is too obvious to describe.Whitish or Grey Streaks.—This is a very uncommon defect, and is generally to be traced to a building in which fairly new galvanised sheets have been employed. The zinc surface becomes oxidised, and the whitish powder which is formed “flakes,” or is carried away by drops of moisture condensing on the surface of the iron sheets.Rust.—Sometimes if a sheet is stretched forcibly and allowed to retract quickly, the hitherto clear surface will be seen to be marred by a “rusty” deposit. The rubber is then described as “stretching rusty,” and its value is depreciated.This defect has caused more trouble during recent years than any other. It is not proposed here to argue the question as to whether the presence of this film, which appears when some rubbers are stretched, is detrimental to the physical qualities of the product on vulcanisation. With the mere statement of opinion that it could do no apparent harm, we may pass to the aspect of the case as it affects the buyer and consumer. If one were to judge by the attention drawn to the appearance of smoke sheet-rubber after it has been stretched and allowed to retract, one would imagine the defect to be of comparatively sudden and recent incidence. This is not so. The peculiarity must have existed for years, and perhaps became more marked as so many estates abandoned the former common practice of allowing varying quantities of water to be placed in the collecting cups. As the substances which cause the defect to be visible are partially soluble in water, it would follow that when working with the very dilute latices which were characteristic of the earlier years of the plantation industry, the remaining liquid in the pan after coagulation would contain an appreciable quantity of soluble substances which would otherwise have been retained in the coagulum.

Conversely, the richer the latex, the greater the percentage of protein matter retained in the coagulum. In the case of very rich latex, it must be within the knowledge of every manager that the quantity of remaining liquid in the pans would be almost nil. We may assume that the greater part of these soluble proteins would be enclosed in the structure of the rubber, but as the fresh coagulum must retain a quantity of liquid amounting to from 60 to 70 per cent. by weight (we are now referring to rich latices), it follows that some of the soluble protein matter must be removed when the coagulum is placed under pressure. Even after the pressure is released more of the contained liquid will exude from the surface of the rubber; and from experience it is easy to imagine that this exudation, becoming progressively feebler, will continue until the rubber begins to dry. Then, with the evaporation of the surface moisture, the protein matter, either in original form or as a degradation product, remains on the surface of the rubber as a thin, solid film or crust. As drying continues, the interior moisture escaping through the pores of the rubber evaporates, leaving behind the substances hitherto held in solution. Should, however, the sheet be thick and/or the temperature of drying low, the rubber may dry first on the outside, forming a thin skin of dry rubber, which delays further drying indefinitely.

It will be seen, therefore, that sheets which have been prepared from rich latex or from too deep a layer of comparatively dilute latex will have a surface film of dry protein matter. Moreover, these sheets will be slow in drying, and in all probability will have a surface gloss and a dark colour. Hence it is not difficult to understand that some brokers regarded the presence of the so-called “rust” as an indication of over-smoking.

To show that this is not so, and further that the presence of rust has nothing whatever to do with smoke-curing, it may be stated that the presence of this protein film may be seen on unsmoked sheets which have been prepared from rich latex, from too deep a layer of more dilute latex, or from some thick sheets which have been rolled only very lightly. In fact, the presence of the protein film was noted on unsmoked sheet in 1910, when it was seen to resemble a thin yellowish glaze which could be scraped off with a pen-knife. Later, sufficient of this substance was removed from some very thick air-dried sheets, or thin slabs, to fill a small test-tube.

When the sheets were bent or twisted, the apparent surface of the rubber (i.e., the protein glaze) cracked in all directions. In the case of sheets prepared from less rich latex, the surface film naturally is extremely thin, and no cracking is observed.

If the fresh sheets are placed in a smoke-house, the drying film will take up colour from the constituents of the smoke, and it will be invisible. Somewhat analogous to the instance of a transparent glass giving a visible and opaque powder when crushed, so the transparent film on stretching breaks up into a visible powder which is lighter in colour than the rubber on which it is superimposed.

It will be noted that since the introduction of standard methods of preparation, involving uniform dilution of latex, say, to a content of 11/4 or 11/2 lbs. dry rubber per gallon, complaints as to “rust” have decreased considerably.

It is to be further noted as a peculiar fact that while two estates may be apparently working on identical lines, both as regards manipulation of latex and subsequent treatment of the coagulum, the rubber of the one may always be free from rust, while that of the other is often, if not always, condemned for the alleged defect. Obviously, in such a case, there must be an initial difference between the two latices as regards the percentage of proteins present; or there must be some small unrecognised difference at some stage of working.It will now be clear that “rust” is caused by a film of matter which is formed on the surface of the pressed coagulum, being there deposited by the exudations from within the rubber and through the pores. It is, therefore, necessary to avoid any conditions which will favour the formation of this deposit—e.g., allowing sheets to remain too long in a moist atmosphere before placing in the smoke-house.At present there would seem to be only two methods which are successful in the prevention of a “rusty” appearance in the dry rubber. Singularly enough, the two methods appear to be directly opposed in principle. They are:

A. The Hot-Water Treatment.—This method has been in constant use on estates which have old trees giving rich latices. These latices are always diluted to a uniform standard daily. Some estates which formerly suffered from the defect now experience no difficulty, and in other instances, where no complaint has yet been received, the treatment has been followed consistently.

(1) After the sheets have been through the marking rolls, it is the general custom to allow them to drip for about three hours. This interval is really excessive for the mere draining away of the surface water, but as a rule it is just sufficient to allow a portion of the liquid retained in the rubber to exude. It has been shown that this liquid may contain some protein matter in solution. Sometimes in the case of thick sheets which have been subjected to pressure so much of this matter is exuded as to form a thin surface slime which is distinctly evident to the touch. If the sheets are allowed to hang overnight, the presence of the exuded matter may be detected also by its odour.

(2) Obviously, any method which will remove this surface film should be of great benefit. It is found that the best results are obtained by allowing sheets to drip for about two hours, and then placing them in hot water for five or ten minutes. The water should be hot as the hand can conveniently bear, and it need hardly be pointed out that the same water should not be used for the whole day’s output. For preference there should be three or four vessels, each capable of holding a fair proportion of the total number of sheets, and frequent changes of hot water.

(3) After remaining in the hot water for the period mentioned, the sheets are removed singly, each one being surface washed or swilled as it is taken out.(4) It is important to see that the sheets are now well washed or scrubbed under running cold water, or in frequent changes of water. The reason for this procedure is plain. If the sheets are merely hung again to drip after removing from the hot water, some moisture is bound to remain on the surface of the sheet. As this surface moisture contains some protein matter in solution, it is evident that, as the water evaporates, the solid protein is again deposited on the surface of the rubber. This would explain why some estates were unsuccessful with the hot-water treatment. It is not essential that the running water should be cold; it may be conveniently lukewarm if drawn from the cooling tanks of the engines. But it is essential for the best results that there should be running water, so that the substance in solution is carried away. If the sheets are merely washed in a large vessel, which has been filled with clean water, it must be obvious that, by the time some scores of sheets have been washed, the protein matter in solution on the surface of the sheets has been transferred to the washing water, so that the later sheets of the batch are liable to show the defect again on drying.

B. The second method is much more simple, and entails no extra labour such as is demanded by the first method. A successful issue, however, is rather more uncertain, and the method appears to give the best results with sheet-rubber prepared on young estates or from more dilute latex.

In this method, the sheets after rolling are allowed to drip for a very short interval, so that the surface water is mainly removed. The sheets are then placed in the smoke-house, and smoking is commenced at once. In some cases where the defect had appeared continuously for a long period, it was found to vanish entirely as soon as the method was adopted; but when tried on some of the older estates, the results were very doubtful, and a return was made to the hot-water treatment.

The explanation of the action which takes place is rather obscure, but two theories may be advanced.

(a) It may be assumed that the interval given for dripping is too brief to allow for the exudation of the internal moisture containing dissolved protein matter.

In such case, the rubber is still in a highly porous condition, and it might be advanced that the heat of the smoke may help to maintain that condition. Thus the contained liquid might evaporate so quickly as to leave behind the dissolved substances in the minute cellular structure of the rubber. In other words, instead of the internal moisture exuding slowly to the surface in liquid form, it may leave the rubber, even in the first stages, in an evaporated condition, just as it does in the subsequent stages of drying. Thus no dissolved protein matter would be brought to the surface of the sheet and be deposited there.

(b) The other theory also demands the first assumption propounded in the preceding theory, but subsequently perhaps is less feasible as it assumes a chemical action of which we have no definite knowledge.

The idea is that as the rubber is in a porous condition, and is placed quickly in an atmosphere of smoke, the heat may maintain that condition to such a degree, that some constituents of the smoke may enter the rubber and cause the precipitation in situ of the protein matter held in solution by the contained water or other liquid. The contained liquid would be water which has in solution possibly a very slight trace of the coagulant employed, of sugars, of protein matter, and of inorganic salts. Of these the substances which would evaporate would be probably the water and the coagulant in most cases. If a salt had been used as a coagulant, the dissolved trace would be deposited within the rubber in this case, whereas if a rich latex had been employed or a thicker sheet made from more dilute latex, some of the salt would be brought to the surface and there deposited together with the protein matter. This has actually been experienced in practice, and it has been possible to remove minute crystals from the edges of the rubber so prepared.

It will be evident that in order for either theory to contain an element of probability, the rubber must be soft (porous) when placed in the smoke-house, and must also be fairly thin. It is observed in all cases where the method has been successfully employed that both these conditions are generally fulfilled—at all events the rubber is fairly thin. When thicker sheets are made, either from rich latex or from a deeper layer of comparatively dilute latex, the method is not uniformly successful.Other Views on “Rust” Causation.—Later experimental work on “rust” formation by Hellendoorn[22] leads to the observation that “rustiness” is caused, not actually by the deposition of original serum-substances, but by the decomposition thereof, under the action of aerobic micro-organisms.

[22] “The Cause of Rustiness in Sheet-Rubber,” H. J. Hellendoorn, Archief voor de Rubbercultuur, October, 1919 (Communication from the Central Rubber Station, Buitenzorg, Java).

Without going into a full discussion of the subject, the following points noted in the experimental work may be quoted:

1. Rustiness could apparently be produced at any time merely by keeping freshly rolled sheets for periods varying from twenty-four to forty-eight hours in a moist atmosphere.

2. Sheets placed immediately in a temperature of, say, 110° to 130° F. never showed “rust“; but if air-dried at ordinary room temperature, “rust” might appear.

3. “Rust” can be prevented by soaking freshly prepared sheets in dilute solutions of disinfectants—e.g., formalin, sodium bisulphite, or chinosol.

If subsequently the sheets are hung for any length of time in a moist atmosphere, the protective effect of the disinfectant gradually vanishes and “rustiness” may be produced.

The same disinfecting effect may be obtained by the use of steam or hot water. It was found that there was less liability to the formation of “rust” when sheets were immersed in water at a temperature of 95° to 120° F., whilst steeping at 140° F. gave complete freedom.

4. It was shown that the micro-organisms which cause decomposition of the serum-products flourish only in the presence of air—i.e., they are aerobic in character. It is not uncommon to find, therefore, that “rust” may be incident only on those parts of a sheet which have been exposed for some time to air and moisture before being placed in a warm smoke-room.

5. The optimal temperature for development of the particular organisms appeared to be about 100° F., in a moist atmosphere.

6. Soaking the sheets in water (except the short immersion in hot water, which we recommend), even for a period extending over a week, does not hinder the formation of “rust.”

7. Rustiness may be prevented by placing the sheets in a sufficiently warmed smoke-house as long as there is adequate ventilation and a moist atmosphere does not persist.

The simplest means of prevention is to soak the sheets first for a short period in water, and then to hang them to drip for a few hours in a well-ventilated place, outside the factory and under cover.[23]

[23] We advise and practise hanging sheets in the open, without shade or cover.

It will be gathered that, although there may be a slight difference between our previous views and those advanced by Hellendoorn as to the exact cause of formation of the “rusty” film, the general conclusions are identical with those given by us in preceding paragraphs and previously advised in the Malayan reports of the Rubber Growers’ Association.Bubbles.—The presence of bubbles in sheet-rubber has for years been the bane of some managers’ existence, and the bone of contention between sellers and buyers. Taking the argument down to bed-rock, producers urge that the presence of bubbles has no influence upon the ultimate quality of the rubber on vulcanisation. They assert that the alleged defect is merely a peg upon which to hang an unreal grievance, serving the purpose of the buyer under the existing conditions of sale. All this may be true, but as long as the present system continues, it must be recognised that “kicking against the pricks” is a futile recreation.

The sympathy of the writers is certainly on the side of the producers, inasmuch as they realise how extremely difficult, and even impossible at times, it is for the most careful individual to prepare sheet-rubber free from this blemish.Much has been written, and many have been the discussions, on this vexed subject; and it is recognised that sometimes, in spite of all precautions, there may suddenly be an incidence of bubbles in rubber which is ordinarily free from them. It must be allowed that climatic conditions and physiological variations affecting the metabolic functions of the trees exert an influence which is difficult at times to combat, and often beyond human control.

The contributory causes are many and varied. It should be premised that, although the defect is described as “air-bubbles,” it is seldom that the appellation is strictly correct. Rarely do the bubbles contain air. In the vast majority of cases they contain gases in minute quantity. These gases may be considered to arise, broadly, from some decomposition of substances (other than rubber), contained either in the coagulum or in the serum. In a general way, if this decomposition is evidenced by an unpleasant odour, it is described under the term of “putrefaction.” We are not concerned here with the question as to how far such decomposition may be ascribed to a purely chemical action, or to the indirect result of the presence of certain bacteria or other micro-organisms. Suffice it to state that, at least as far as field operations influence the result, the decomposition is generally to be attributed to the work of micro-organisms. Conditions favourable to the incidence and development of these are to be found when absolute cleanliness in all details is not aimed at.

With this preamble we may proceed to classify possible causes of the formation of bubbles into two groups:

(a) Those originating in field operations.

(b) Others which may arise after the arrival of the latex at the factory.

In the Field.—Decomposition may be caused by:

(1) Spouts, buckets, and cups being dirty. Regular cleaning is necessary. If the buckets are allowed to be taken to the lines by tappers, trouble may ensue. Within the writers’ experience it has been shown that an otherwise baffling case of premature flocculation of latex was traced to the presence of acid substances in the buckets, which had been used by coolies for preparing their food.

(2) Delay in commencing work. This means similar delay in collecting the latex which is exposed to greater heat than under ordinary circumstances.

(3) Exposure to the sun’s rays. The heating of the latex may provide improved conditions favourable to the development and action of micro-organisms.

(4) Allowing latex to stand too long before collection. This usually is the result of giving tappers too great a task.

(5) The addition of water to the latex, either purposely or accidentally, in the form of rain. The water may be slightly acid in character, or it may carry micro-organisms from the bark into the latex.

(6) Tapping trees at too great a height. The latex generally has an abnormal distance to travel before reaching the cup.

(7) Sometimes the latex from old trees, or from trees after wintering (just prior to full renewal of leaf), contains more than the usual proportion of substances (e.g., sugars), which are capable of effecting flocculation or coagulation.

(8) Too great a distance for transport. The trouble arising from this cause is likely to be much increased if the journey has to be made over bad roads. In such case the physical action augments the effect likely to be produced by long standing.

The foregoing do not include all possible causes, but serve to indicate the directions from which trouble may be mainly anticipated. It will be plain that any latex which exhibits symptoms of premature coagulation (or minute flocculation) should be regarded as a potential source of bubbles in sheet-rubber.

It will be equally obvious that the employment in the field of any harmless substance of an anti-coagulant nature is to be encouraged. This point is discussed in detail in Chapter V.In the Factory.—As a general rule it may be understood that the mischief has been done before the latex is handled at the factory. Sometimes it is perceptible from the peculiar appearance of the latex, and in such case the batch should not be used for the preparation of sheet-rubber. Often it is found that only the last to arrive at the store is visibly affected. This should not be mixed with other apparently normal latex, as it is capable of acting as a “leaven” to the bulk.

Contributory factors in the store are:

(1) Lack of cleanliness of utensils, particularly of coagulating dishes or tanks.

The trouble becomes acute sometimes where wooden tanks are employed. Unless the tank and the partitions are thoroughly and regularly cleansed, the wood may become coated with a bacterial slime, which is capable of causing what may be termed “fermentation” of the latex layers in contact.

The tank should be thoroughly cleaned occasionally with a weak (5 per cent.) solution of sodium bisulphite. The partitions should be scrubbed and placed in the sun twice or three times a week.

(2) Allowing latex to stand too long before treatment. This point needs no further expansion.

(3) The use of a latex of too high a rubber content. Such latices are difficult to handle in order to secure uniform mixture with the coagulant.

(4) The use of too concentrated a solution of coagulant. In conjunction with (3) there may be a rapid and irregular coagulation, giving rise not only to decomposition in parts (and subsequent formation of gas), but also to the formation of true “air-bubbles” by inclusion of air during stirring.

(5) The use of insufficient coagulant. Coagulation is slow and incomplete.

(6) Defective straining and skimming. Small flocculated particles of rubber may pass, or be rubbed through, the strainer. If allowed to remain, they act as local points of danger.

(7) The proximity of the coagulating latex to some source of heat, or exposure to sunlight.

(8) Any delay of drying in the preliminary stages, either before or after the rubber enters the smoke-house.

Blisters.—This description aptly fits the case in which sheet-rubber in the smoke-house exhibits large bubbles of gas which distend the surface of the rubber. When subjected to pressure, small “balloons” are formed, which burst with a perceptible report. It was formerly the belief that this defect was occasioned solely by an abnormally high temperature. That such is not the case can be shown by the experience of estates which have had only the rubber of a particular day or short period affected under normal factory conditions.

At the same time it is not disputed that the heat of the smoke-house exerts an influence (causing expansion and distension), but it is advanced that the gases had begun to form before the rubber entered the house.

The view held is that decomposition had supervened or was taking place—probably from one or more of the causes enumerated in the preceding paragraphs. The heat of the smoke-house only serves to exaggerate the effect. It is acknowledged that the degree of decomposition must be initially greater than in the ordinary incidence of “bubbles.”

Beyond this point we are not in a position to put forward any definite supposition as to the apparently haphazard occurrence of the phenomenon.

It is to be noted, fortunately, that the defect is comparatively rare, and seldom appears on estates which employ an anti-coagulant in the field. While we have examined persistent cases, one of which led to a temporary discontinuance of the preparation of smoked sheet rubber, we have not yet been able to arrive at any satisfactory idea of the exact conditions governing the incidence of “blisters.” Our investigations only lead us to two observations:

(a) That blisters have appeared on the rubber of some estates after wintering, and during the period of new leaf-development.

(b) That the defect has been noted on other estates during a period when there were frequent but not heavy rains, and when there was a comparatively low average temperature.

In either case, as the factors are beyond human control, it would be expected that without any change being made in estate procedure, the trouble would vanish as mysteriously as it appeared. This is our experience; but as showing the possible intensive effect of a high temperature in the smoke-house, it may be remarked that very infrequently, in a batch of sheets exhibiting ordinary bubbles, a few hanging directly above the furnaces show signs of a slight blistering effect.“Spot” Disease in Sheet Rubber.—That “spot” disease may appear in air-dried sheets was evident at the beginning of the outbreak in the spring of 1911. The first cases noticed took the form of pink and bluish “blushes” spreading over the whole of the sheets. Later, fungoid spots began to appear. These mainly took the form of red or black blotches, and were very unsightly. As “spot” disease cannot develop in smoked rubber, the obvious and simple course to adopt was to smoke-cure the sheets. When it is stated that “spots” do not develop in smoke-cured rubber, it is understood that the smoke-curing must be efficient and must commence as soon as the rubber has been rolled, and the surface water has drained away. If the sheets are allowed to air-dry for a few days, the disease may develop, and then smoke-curing will not get rid of the coloured patches. The operation of smoke-curing will not get rid of the coloured patches. The operation of smoke-curing may tone down the colour, but the spots would still remain evident.Support Marks.—It frequently occurs that one sees across the middle of smoked sheets a wide mark. This is where the wooden support in the smoking-chamber has been. As a rule, even in the most careful cases a faint mark may always be seen, but in many instances this mark is exaggerated to such an extent as to point to lack of care on the part of the store supervision. If bays of racks remain empty over-night, they may possibly become covered with a light sprinkling of fine wood-ash and tarry deposit. Wet rubber placed upon these racks will pick up and retain the impurities, and more often than not they cannot be washed out. It is incumbent upon the manager to see that empty racks are thoroughly cleansed before placing wet rubber upon them. The better plan is to arrange that the bars can be removed easily from sockets. There should be in stock sufficient “spares” for, say, two days’ rubber. When the dry rubber is removed, the bars should likewise be taken away, to be cleansed and kept in the factory until again required. This will ensure that fresh rubber always rests upon a clean support.

On some estates, in order to guard against a pronounced “bar-mark,” sheets are moved and turned daily.

In other smoke-houses the upper surface of the bar is chiselled in concave form, so as to admit of the passage of smoke below the surface resting on the bar.Stickiness.—This is not to be confounded with “tackiness,” from which the rubber does not recover. Stickiness is only temporary, and may be remedied. As a general rule, it is due to packing sheets, which have not a good raised “ribbing,” and which may have been coated with light tarry deposits (see Glaze). This surface film may be removed by washing the sheets, or scrubbing them, with cold water. Usually a further two days’ air-drying will make the rubber fit for packing; and if the smoke-curing has been efficient, there should be no need to anticipate trouble from mildew. Some estates adopt this practice daily with success, as a form of insurance against complaints of surface deposits.Ribbing, Surface Pattern.—While we know that the passing of sheets of rubber between rolls, causing a particular raised pattern to appear, has no effect upon the actual quality of the rubber, there is a great deal of practical advantage gained.

The practice ensures an increase of superficial area which is an aid in drying, improves the appearance of the rubber for selling purposes, and is of distinct advantage in enabling the rubber when packed to travel in better condition. Sheets do not become so closely packed; sampling and general handling are easier on delivery.

As long as the plane surfaces are sufficiently and regularly distorted, there would seem to be no limits to the type of pattern or “mark” which may be placed upon the rubber. But in actual practice the variety is small. The most popular type of “ribbing” is that best described as a small diamond effect, produced by a pair of rolls cut with closely placed narrow grooves running spirally. The spirals travel in the same direction on both rolls, producing close-cut ribbing running in opposite directions on the surfaces of the sheet. On sheets of standard thickness, the result approaches a diamond effect.

A few other patterns are employed, notably that producing longitudinal stripes of varying thickness. On the whole, the type of pattern would seem to be immaterial, if the points already indicated are achieved.

It is seldom one encounters a case nowadays in which the “marking” is unsuitable, but a few estates may be using an old type of patterned roll on which the full diamond grooving is cut. As this appears on both sides of the sheet of rubber, and as the ribbing does not coincide, a blurred effect is seen when the sheet is viewed against the light.Thick Ends, “Sheet Clippings.“—It rarely happens, even with good equipment and average supervision, that the preparation of smoked sheet is unaccompanied by slight defects. For instance, in spite of rules and regulations regarding manipulation of the coagulum, it is not uncommon to find that some sheets, after rolling, have slightly thickened ends. In the ordinary course of events these might delay drying considerably. It is the practice on some estates to cut off these thickened ends while the rubber is still wet. The pieces are then machined into crepe form, but as no sodium bisulphite may have been used, the resulting rubber cannot be classed as No. 1 Standard Crepe.

The other alternative is to trim the ends when the bulk of the rubber is thoroughly smoke-dried. The moisture containing portions are then returned to the smoke-house until dry, and are subsequently packed without further treatment as “smoked-sheet clippings.” It will be plain that, except in the particularity of form, these clippings differ in no degree from the original sheets; and, owing to extra smoke-curing, may arrive in even better condition. One must be prepared, however, to find that a slightly lower price is offered. Whether the price obtained would be comparable with that commanded by the crepe made from wet sheet slipping would depend upon general ruling market conditions, and the degree of care exercised in guarding against the inclusion of any inferior pieces of rubber. In ordinary factory practice, there could be no room for abuse under the latter clause.Other Infrequent Defects.—This chapter will be closed with a reference to other small defects which, although infrequent, cannot be classed as minor complaints. In point of fact, when they occur, they assume an importance, in the eyes of the consumer, which is not, perhaps, sufficiently appreciated by producers.Dirt.—Trouble from this source should be absent, but carelessness on the part of packing coolies may be responsible for occasional complaints. How the dirt is incident may remain a mystery, but it has been noted that sheets have at times been thrown upon a cement floor. A certain amount of loose dust may thus adhere to the rubber.Ash.—The source of this surface deposit scarcely needs indication. Where open-hearth furnaces are employed, and the wire-mesh floor screens are not perfectly sound, fine ash may find its way into the upper chamber. If this trouble is persistent in spite of precautions, the sheets should be surface-washed and air-dried before packing.Bark.—Complaints of the presence of particles of bark in sheet rubber used to be fairly frequent, but are now less common. The trouble may be traced to the use of defective straining sieves when the latex is being handled.Splinters.—The use of packing-cases of unplaned soft timber is responsible for complaints of this nature on delivery. Without here discussing the larger question of the ideal packing case, it is sufficient to emphasise that the interior surfaces of wooden chests should be planed. The cases are often so damaged in transit, that splinters of wood may be found throughout the contents. The device adopted on some estates may go far to prevent this. The cases are first lined with loose sheets, and finally other sheets are arranged to overlap at the top of the case. The bulk is thus enclosed in a wrapping of sheets, and any splinters or other deleterious substances are confined to the surface of the mass.

Clyx.com