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 h.e.l.lendoorn 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' a.s.sociation.

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 a.s.sert 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 p.r.i.c.ks" 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 quant.i.ty. 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 cla.s.sify 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 circ.u.mstances.

(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 antic.i.p.ated. 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 part.i.tions 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 part.i.tions 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 pa.s.s, 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 inc.u.mbent 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 p.r.o.nounced "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 pa.s.sage of smoke below the surface resting on the bar.