Air-Drying of Crepe.—It is still the prevailing custom to air-dry crepe rubbers. A few estates, it is true, have artificial driers installed, and in some necessary cases others will be erected. But in the majority of cases where money has been expended in building air-drying sheds, as long as it is only possible to ship rubber regularly air-drying is likely to remain in favour.

The great drawback to air-drying is that one is so dependent upon the weather conditions. In favourable weather the rubber dries well, but in a long period of wet weather rubber may accumulate at an alarming rate, and the accommodation is sometimes severely taxed. Of course, the rate of drying under the best conditions is mainly dependent on the thickness of the crepe, and every endeavour should be made to maintain a thin style of preparation. If this precaution is not taken, the rubber is liable to recurrent attacks of “spot” disease, and one’s troubles are very much augmented. This is a disability to which rubber treated in artificial driers is not liable. Still, air-dried rubber can be made equal, if not superior, in appearance to pale rubbers prepared by other processes.

For the lowest grades of crepe air-drying is always likely to remain the only method, as it would be extremely unsafe to submit them to heat.

It is noted in ordinary practice that the rate of drying on different estates, for the same type of rubber, may vary widely. Naturally the construction of the house has a great effect, and this subject will receive attention in a subsequent chapter.

Similarly the position of the drying-shed exerts an important influence, and the erection of the building in low-lying surroundings is always calculated to prolong the drying period appreciably. Incidentally this means that the building must be larger than a normal rate of drying would otherwise demand.

The combination of a poor type of drying-house, a low-lying situation, and a prolonged wet season, might render it advisable to abandon the air-drying of high grade crepes in favour of artificial drying.Artificial Driers for Crepe.—It is more common to find artificial driers in use in Ceylon than in Malaya, possibly because these driers have been in use in Ceylon for other products. Some time ago the question of installing artificial driers received the serious attention of a number of estates in this country, chiefly on account of the incidence of fungoid and bacterial diseases in crepe rubber. The simple treatment for the prevention of these diseases is to get the rubber dry in the shortest possible space of time. In most cases it is found sufficient to roll crepe thin for air-drying in order to prevent the appearance of coloured spots. It is found, however, that some drying-houses are so badly planned and constructed, that quick drying under even the best of conditions is a practical impossibility. Cases have been known in which the disease may disappear almost entirely during a period of freedom from rain, only to recur as soon as wet weather sets in again. There can be no doubt that, on the whole, the number of cases of “spot” disease is on the decline; but equally it is certain that a very few estates will always be liable to outbreaks as long as drying is attempted in existing houses. For these reasons it is a poor policy to temporise, and the only sound policy in extreme cases would be to give up ordinary air-drying in favour of some method of artificial drying. As regards the majority of estates preparing pale crepe for various reasons, it is not expected that any will instal artificial driers. Money has been expended in elaborate buildings which certainly do the work for which they were designed. As long, therefore, as the accommodation is sufficient, and regular shipments are the rule, it is expected that ordinary air-drying will still remain the general practice.

Of the better-known artificial driers, there are only three which merit serious consideration in these pages. They are the vacuum driers, the Colombo Commercial Company’s hot-air drier, and the Michie-Golledge process.Vacuum Driers.—The vacuum drier is so well known that only a brief description need be given. It consists of a chamber heated by steam pipes and capable of having the contained air and moisture withdrawn by a pump. This description sounds very simple, and in practice the operation of vacuum drying is really a simple one, and can well be entrusted to an intelligent coolie under efficient supervision. Indicators are fitted which show the vacuum pressure and the pressure of steam in the heating pipes which travel underneath horizontal slabs upon which trays may be placed. Still, in spite of the apparent simplicity of the process, there would appear to be a number of little details which, if overlooked, prove to be factors influencing the result to a considerable degree. Thus it is not uncommon to find complaints that the rubber is not dry when packed. The writers have seen rubber taken from a vacuum drier still containing a visible quantity of moisture. One would have imagined that continuous working of the drier would give the experience necessary to obtain dry rubber, but, apparently, such is not the case in a number of instances. Elaborate instructions are given by the makers, but often they are more honoured in the breach than in the observance. Really, there are only two points to bear in mind:

These two points presume that the vacuum drier is true to its name, and that one can obtain a maximum steady pressure. The machines are so well made now that no drier should be taken over from those responsible for its erection unless it can show a vacuum pressure of 28 inches within fifteen minutes of starting the pump; and with the pump stopped, there should not be a greater fall in pressure than 1 inch within ten minutes after stopping the pump.

One of the most frequent sources of error is the control of steam pressure which is responsible for the temperature of the drier. It is quite unnecessary and unwise to maintain any steam pressure once the drying is well under way. All that is necessary is to heat the chamber well, with a steam pressure of 5 lbs., before inserting the rubber. As soon as the maximum vacuum pressure has been obtained, steam should be shut off from the heating pipes, and it will be found that the temperature is well maintained throughout the operation with a rise of ten to twenty degrees at the end. If the drier is working at a vacuum pressure of 28 inches, and if the crepe has been prepared thin enough, the rubber should be quite dry within two hours. Should the operation have to be extended to two and a half hours at 28 inches vacuum pressure, it is a sign that the crepe is too thick. On such occasions it is often noticed that these thicker crepes are not thoroughly dry, having moist spots enclosed in them. On re-rolling, these moist patches become easily visible, and are a source of great annoyance, inasmuch as they take quite a long time to dry out.

As mentioned before, the crepe for vacuum drying should be thin. There is no necessity to give it a superfine finish, and the presence of small holes is quite permissible, as they disappear on subsequent re-rolling. The thin crepe may be folded loosely to the length (or breadth) of the tray several times, but in no other way can the drier be expected to perform its work satisfactorily. A case was noted in which thin crepe was excellently prepared, and four or five layers were rolled together for vacuum drying. Naturally this mode of procedure does not give the drier a fair chance, and it would be ridiculous to judge vacuum drying on the results. After two and a half hours at a temperature of 145° F. the rubber appeared to be only about three parts dry, and the subsequent air-drying extended well into a fortnight.

It is the common practice to screw up the door of the chamber as tightly as possible. As a rule it is found in course of time that the obtainable maximum vacuum pressure decreases. This may be attributed solely to the forcible screwing up of the door. Around the inside edges of the door are strips of rubber compound, the function of which is to form a tight joint. Should the door be screwed up too tightly, these strips become deformed in course of time, and slight leaks occur. It should be pointed out that it is only necessary to screw up the door at the beginning of the operation. When the vacuum has been obtained, the screw pressure may be released, as the atmospheric pressure outside the chamber is more than ample to keep the door in a close fitting position. This will be obvious from the fact that the difference in pressure between the inside and the outside of the door amounts to, say, 28 inches atmospheric pressure—i.e., nearly 14 lbs. per square foot. By slackening the screw handles, therefore, as soon as the indicator shows the maximum working vacuum pressure, the life of the door joints may be prolonged, and the drier will remain efficient for a longer time.

A careful consideration of the question of temperature leads one to the conclusion that the practice of placing a thermometer through the roof of the chamber does not enable one to determine the temperature correctly. In the same way a thermometer suspended behind the observation window cannot indicate the temperature of the rubber, as in both of these positions the thermometer must be influenced by radiation from the walls of the chamber. The only position in which the correct temperature could be indicated is between the folds of crepe. This can be arranged easily so as to enable one to read the temperature from the observation window.Colombo Commercial Company’s Drier.—The drier of the Colombo Commercial Company consists in principle of a number of small chambers or units in which crepe rubber is placed, and through which hot air is passed. As in the case of vacuum drying, a great deal depends upon the preliminary treatment of the rubber. If the crepe is not rolled thin enough drying will be unduly prolonged, with a possibility that the rubber will become tacky. The temperature usually obtained is about 150° F., and if the rubber is thin the production of an installation of two chambers should be at the rate of 1 lb. of dry rubber per minute. The usual period of drying is under two hours. One advantage which this drier has over the vacuum drier is that the chamber can be opened at any time for a short period to withdraw or insert trays. The thin crepe is folded several times, as in the case of vacuum-drying.

Figures obtained from the actual working of a drier in Ceylon are given below:

It will be seen, therefore, that the drier had an output in 2 hrs. 23 mins. of 1411/2 lbs., which is at the rate of 1 lb. per minute approximately.

As the rubber leaves the driers it resembles vacuum-dried rubber in being surface-sticky. This stickiness is only temporary, and is got rid of by passing the crepe through wet rolls. Opinions differ as to when this rolling should be given. On some estates the rubber is only allowed to cool a little before passing through the rolls; on others it is given a day or so before rolling. The methods of rolling also differ. In some factories the rubber has been cut to lengths before drying, and these lengths are merely rolled together by simple pressure. Other estates prefer to re-macerate the crepe while still fairly warm and soft. It is probable that little harm, if any, results from this re-maceration while the rubber is soft, as it is more easily worked in this condition. The thick rubber is then generally hung for a few days to air-dry before packing. As most of the moisture taken up by the dry rubber is surface moisture, three or four days is usually found ample for air-drying.Michie-Golledge System.—The Michie-Golledge system comprises a process of preparation and drying. The latex is diluted with two, three, or four volumes of water and coagulated with acid in a vessel which is rotated with a churning motion. In this cylinder there are curved and fixed blades. The revolving cylinder and its ribs force the latex against the curved blades so as to cause an eddy in the middle of the machine. Here the rubber coagulates and accumulates, the remaining liquor whirling round outside the blades. It can be imagined that with such dilute latex, the coagulum is very soft and spongy. This soft mass is passed through a machine which cuts it into “worms” about 3/16 inch in section. These are placed upon wire trays and dried by means of hot air. The “worms” when dry are re-macerated and built up into medium and thick crepes. The colour of the rubber prepared by this process is usually very good. When treated in a Colombo drier the “worms” usually require about two hours to dry, so that crepe rubber may be packed at latest on the fourth or fifth day, as in the case of vacuum-dried rubber.Rate of Air-Drying of Crepe Rubber.—In spite of the facts that some estates have been making thin pale crepes for years, and that so much has been written concerning the preparation of this grade of rubber, one occasionally meets with a case in which an estate seems to be unable to prepare thin pale crepe, or if it does the period of drying is much longer than obtains on most estates.

Again, when cases of infection by spot disease in fairly thin crepes are submitted, it is usually found that the particular crepes are of that type which, though fairly thin, show whitish spots of moisture when the bulk of the rubber is nearly dry. This type of crepe is to be noted for the excessive period of drying in comparison with other crepes of equal thinness. It has been advanced elsewhere[12] that a factor of the most considerable importance in the rate of drying of crepe rubber is the type of drying-house and its situation. This accounts very largely for observed differences in the rate of drying of thin crepes on different estates. Yet even where two drying-houses may be of the same type, and the situations may be comparable, one still observes that one thin crepe dries more quickly than another. It has been remarked also that a thin crepe in one old drying-house dries in a shorter period than a similar crepe in another more modern house, although the methods of coagulation and preparation exhibit no apparent diversity. In all these conflicting cases allowance is made for the weather conditions, and the observed differences seem to be inexplicable. It has always been the opinion of the writers that the actual rolling of the rubber plays an important part in determining the rate of drying of crepe, apart from the question of thinness; and it seemed possible that this factor would account for the discrepancies noted above, either partially or wholly.

With a view to determining to what degree the drying of crepe rubber was hastened by the extent to which the rubber was rolled, experiments were made. It was hoped, also, that some idea would be gained of the particular stage in crepe rolling which had the greatest effect upon the rate of drying. In preparing crepe in the estate in the ordinary way the coagulum is passed through three sets of rollers, and the stages may be described as:

In the first the coagulum is broken down by passing through the machines until a thick rough crepe is formed. This passes to the intermediate rollers, where it is worked down to a medium crepe. The rubber finally goes to the smooth running at approximately even speeds. Passing through these a number of times it emerges as a thin uniform crepe, free from “lumpiness” and free from holes, which should dry in from ten to twelve days.

In the experiment the rubber was passed through the machines with varying frequency, the number of times in each machine being progressively increased, while the working on the other machines remained constant.

It was determined that the rate of drying was affected only by the extent to which the crepe was worked in the smooth rolls. The less often the rubber passed through these rolls, the slower the rate of drying. Beyond a limit in the other direction, increased rolling did not reduce the period of drying. It follows, therefore, that crepes which have a good thin finish should dry in a minimum period.

When does Air-Drying take place?—Experiments[13] were conducted with a view to discovering, if possible, the rate at which crepe rubber dries, and the extent of drying during the night under weather conditions such as prevail ordinarily in Malaya. It is to be remembered that, during the day, most drying-houses are fairly open and that the temperature ranges from about 88° F. in the lower rooms to over 100° in the upper rooms (near the roof) when the sun shines. At night, however, there is usually a decided drop in the temperature, and unless it is a very clear night the air is generally saturated with moisture. In addition the drying-house is closed as thoroughly as possible, and we should expect the atmosphere of the house to be laden with moisture from the wet and drying rubber. It would be a just inference, therefore, that the rate of drying during the night would be much less than the rate of drying during the day, and the results of experiments confirm this very fully. One was hardly prepared, however, to find that, under certain circumstances and at a certain stage, the amount of drying is nil; not only so, but it was found under certain conditions that the amount of drying which took place was negative—i.e., the rubber weighed slightly more when taken out in the morning than it had weighed the previous afternoon.

Crepe may Increase in Weight.—As an instance of the kind of result obtained a graph is here given of the rate of drying of a batch of pale crepe. This was hung to dry in the top room of a drying-house in which rubber ordinarily dries quickly. The rubber was hung in a good position, with the bulk of output, near a window which was open for some time during the day. In order to restrict the day interval of drying to the actual period in which the sun was likely to be in evidence, the day was taken to begin at 8 a.m. and end at 4 p.m., the night interval covering the remaining sixteen hours. Thus the night interval was twice as long as the period of day drying. The lengths of crepe were weighed carefully at 8 a.m. and 4 a.m., and the average of the several weights was plotted in a graph.

The weights are placed vertically and the duration of drying horizontally. It will be seen that the rubber was quite dry and fit for packing on the sixth day, as far as could be judged in the usual way by casual inspection. Peculiarly enough at this time it weighed slightly more than had been registered on the fourth and fifth days, but the difference did not amount to more than about 0·4 per cent. In examining the graph it should be borne in mind that the steeper the slope of the curve downwards the quicker the rate of drying, and that when the curve takes an upward direction there is an addition of moisture instead of abstraction. It will be noted that when drying takes place the slopes more nearly approximating the vertical represent the extent of day drying, and that often the night drying is represented either by a very flat curve or even by an upward curve which shows the addition of moisture. A striking feature of the experiment is shown by the rapidity with which drying takes place during the first few days and the comparative slowness with which the remaining moisture is got rid of. Thus, from the graph, it may be calculated that about 80 per cent. of the total moisture content was lost in the first two days, and over 93 per cent. in two and a half days. Yet three days had to elapse before the remaining 7 per cent. of total moisture was lost—i.e., before the rubber was judged to be ready for packing. It will be seen that after this stage had been reached the rubber alternately lost and gained in weight, with a tendency to increase. This increase was attributed to the presence of surface moisture after hanging overnight, when the rains had become frequent. Some light is thus shed upon a subject which has puzzled both shippers and receivers of crepe rubber.Differences in Weight.—It will be obvious that if rubber is allowed to hang after becoming dry, and is taken down, packed, and weighed in the early morning, it will weigh more than when it reaches a drier climate. The loss in weight under such circumstances might amount to even 1 per cent. It may seem to some an unnecessary refinement to introduce, but it would appear from the graph that rubber should be packed for preference in the afternoon if the weights are to be more nearly correct.

It is extremely singular to note how quickly the curve changes its slope after the major portion of the moisture has evaporated, and it will be very plain that in the last stages any decrease in weight during the day would appear to be counterbalanced, or more than counterbalanced, by the addition of moisture during the night. It may be pointed out, however, that this increase in weight during the later stages of drying of pale crepe is mainly, if not altogether, due to surface moisture. The chief point of interest is the fact that in the case of thin pale crepe, quite 80 per cent. of the total moisture content is lost during the first two or three days, and that, owing to the negative influence of the night atmosphere, the final drying is delayed. It will be understood that the foregoing results applied to thin pale crepe. Thin lower-grade crepes appeared to dry at more uniform rates, but the differences between the rates of drying at night and during the day were similarly notable.Aids to Normal Air-Drying.—These experiments were undertaken in a drying-house, favourably situated for rapid drying, in which the average period of drying for thin crepes is nine days. It is easy to imagine that the condition of affairs as revealed would be much exaggerated in a drying-house situated on low-lying ground and surrounded by trees. In extreme cases of this nature the use of large fans and heating pipes has been advocated. It is believed that in some cases these installations have given satisfaction, but that in others the degree of improvement obtained has not been in economic proportion to the outlay incurred.Smoke-Curing of Sheet Rubber.—It will have been evident that one of the disadvantages of air-drying sheet is the incidence of moulds. Now it is found that moulds should not develop in smoke-curing; and if they do, then the smoke-curing has been insufficient or inefficient. The difference in the drying period also is a strong argument in favour of smoke-curing, so that all-round it is seen that there are many valuable advantages to be gained by smoke-curing sheet in comparison with air-drying, and no disadvantages.

The manipulation of the rubber, after it leaves the marking rolls and preparatory to smoke-curing, has been discussed in Chapter IX. It is sufficient only to allow adequate time for furnace water to drip from the sheets before transferring them to the smoke-house. As it is the general rule to roll sheet rubber in the morning, this arrangement fits in very well. The furnaces of the smoke-house are usually extinguished as soon as the sun is well risen, and the rest of the day is occupied in sorting dry sheets, etc. Towards noon the day’s wet sheets should have been admitted, and smoking may be commenced as soon as the sun is well in the west—say, at half-past four o’clock or earlier.It used to be the custom on a few estates to smoke during the daytime and to discontinue smoking at night. As the night-air in Malaya is usually heavily laden with moisture, it will be plain that such a policy was a topsy-turvy one. It is vastly more reasonable to smoke-cure at night; usually the heat of the sun during the day is quite sufficient in itself to promote the drying of rubber; but there is no reason why smoking should not be carried on in the daytime in wet weather, should it be found expedient to do so.Recording Instruments.—During the night the care of the smoke-houses is usually in the hands of natives, except for occasional surprise visits from a European superintendent. To all acquainted with the ways of the native it must be plain that means must be provided for the checking of the temperatures attained in the smoke-house. Ordinary thermometers are quite unsuitable, and even thermometers registering maximum and minimum temperatures are of little avail, inasmuch as they record only the degree of heat attained at a particular moment, and do not indicate any period during which a particular temperature was maintained.

It is evident that something more informative is required. There are many types of suitable recording instruments or “pyrometers,” some of which can be electrically connected, so as to cause the ringing of a bell, placed in the superintendent’s office or house, on the attainment of a certain temperature. The type best known in estate practice is that named the “Thermograph,” in which a pen traces a curve or graph on a plotted piece of paper carried by a rotating cylinder which is actuated by clockwork. Such instruments can be purchased through most of the local firms dealing in estate supplies. From experience it can be asserted that, given intelligent attention, these instruments yield very satisfactory results. The apparatus should not be placed always in one position in the smoke-house, but should be moved frequently so as to obtain information regarding the distribution of heat.Temperature of Smoke-Curing.—In the question of temperature of drying, it is well to be as strict as possible; not that any great harm will result from a rise of 10° above that recommended, but because the higher the temperature recorded the larger the fires must have been, and consequently the more real danger there was of the store becoming ignited. It has been shown[14] that the temperature giving the maximum benefit of drying and quality was found experimentally to be rather above the temperature usually prescribed for smoke-houses, but in the experimental work there was no danger from fire.

The figure given in previous publications as a maximum working temperature for smoke-houses was 110° F., but certainly the temperature may be as high as 130° if it is considered safe to allow fires to be so arranged. One or two estates are known to work at temperatures of 130° F. and over, in spite of the recommendations of the writers. If those estates care to risk it they may do so, with increased rapidity of drying; but no responsibility can be taken for whatever may happen in smoke-houses where the temperature is allowed to remain, as in one case, at 160° F. Naturally the range of temperature is strictly limited by the properties of the substance to be treated, and with a substance such as rubber it would be far better to err on the side of caution than to risk damage to such a commodity, apart from the consideration of the possible destruction of the building.Period of Drying.—Considerable differences are noted in the periods of drying on various estates; but, as there is more than one factor influencing the results, it is not easy at first to find why these differences should exist. Really there are three factors:

(1) Relative thickness of rubber.

(2) Extent and quality of rolling.

(3) Temperature of drying.