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 a.s.sume 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 pa.s.s 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 quant.i.ties 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 quant.i.ty 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 quant.i.ty of remaining liquid in the pans would be almost nil. We may a.s.sume 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 quant.i.ty 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. .h.i.therto 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 sc.r.a.ped 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 const.i.tuents of the smoke, and it will be invisible.

Somewhat a.n.a.logous to the instance of a transparent gla.s.s 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 1-1/4 or 1-1/2 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 a.s.sumed 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 a.s.sumption propounded in the preceding theory, but subsequently perhaps is less feasible as it a.s.sumes 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 const.i.tuents 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 h.e.l.lendoorn[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. h.e.l.lendoorn, 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.