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 pa.s.sed 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 pa.s.sed 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 formulae 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 quant.i.ties 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 const.i.tuents 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.

(_c_) Uniform volumes of standardised latex.

(_d_) Uniform quality and quant.i.ties 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 p.r.o.nounced 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 quant.i.ty 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 quant.i.ty of tarry matter pa.s.sing 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 a.s.sumption 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 quant.i.ty 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 quant.i.ties 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 quant.i.ties 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.