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.
[12] "Preparation of Plantation Rubber," Morgan, 1913, chapters xii. and xiii.
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 pa.s.sed through three sets of rollers, and the stages may be described as:
(1) Rough rolling.
(2) Medium rolling.
(3) Smooth rolling.
In the first the coagulum is broken down by pa.s.sing through the machines until a thick rough crepe is formed. This pa.s.ses 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. Pa.s.sing 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 pa.s.sed 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 pa.s.sed 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.
[Ill.u.s.tration: DRYING GRAPH. PALE CREPE (THIN).]
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 circ.u.mstances 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.
[13] Rubber Growers" a.s.sociation, Malaya Local Report, No. 2, 1914.
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 04 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 circ.u.mstances 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 a.s.serted 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.
[14] "Preparation of Plantation Rubber," Morgan, 1913, chapter x.
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.
It is presumed that the smoke-houses are identical in type and efficiency, and that smoking is in force for the same length of time each day. There need be no discussion of these points; the effect of each is so obvious.
The thinner the sheet, the quicker the rate of drying; the better the sheet has been rolled, the shorter the period of drying; the higher the temperature, the more rapid the drying.
It has been shown in Chapter IX. that the condition of the sheet after rolling depends primarily upon the standard of dilution of the latex and the original thickness of the coagulum. If these factors are correctly controlled, the rolling should give a sheet which is fairly soft and porous--_i.e._, it should not have been subjected to such pressure as to make it both thin and hard. An average sheet of rubber which has been well rolled should be smoke-dried at a temperature of 120 F. in about ten days.
If sheets take appreciably longer to dry, then the three foregoing factors must be examined.
On the other hand, it is often found that thin sheets made from very dilute latex dry so quickly that they are considered to be fully smoke-cured in from five to seven days. It frequently happens in such cases, however, that the smoking is insufficient, and by the time the rubber reaches home it has begun to show signs of surface moulds. It is evident, therefore, from this discussion that:
(1) If smoked sheet develops surface moulds within a short period after smoking, the duration of curing has been insufficient, or the quality of the smoking is at fault.
(2) The actual time taken to smoke-dry rubber may be insufficient to smoke-cure it.
(3) The rate of drying of smoked sheet depends upon--
(_a_) The relative thickness of the rubber.
(_b_) The preliminary treatment of rolling.
(_c_) The temperature of the smoke-house, and
(_d_) The type of smoke-house used. This point will be treated in a subsequent chapter.
FUELS FOR SMOKING.--The general idea formerly held was that the beneficial effects of smoking were to be attributed to the const.i.tuents of the smoke, and chiefly the creosotic substances. This is not now the opinion of the writers, who attribute the effect largely to the temperature of drying and const.i.tuents of the smoke other than creosotic substances. There can be no doubt that the presence of creosotic bodies is responsible largely for the absence of moulds and the existence of the typical odour, but it is becoming increasingly known that the employment of substances rich in creosote is not required or desirable.
Estates used to be put to considerable expense in the purchase of "bakau"
(a mangrove timber rich in creosote and creating much heat), under the idea that it was the best material and almost indispensable. Most estates now restrict themselves to the consumption of timber obtained from their own areas. Thinning-out programmes are largely responsible for the supply, but the local authorities are much concerned regarding future supplies; and consideration has been given in some quarters to the question of the development of quick-growing trees on estates with a view to safeguarding the future. This seems to be desirable, as it is difficult to imagine that the place of timber can be taken by any other material in the smoke-curing of rubber. Unless some such precautions are taken it is not difficult to predict that, in course of time, some estates will be able to continue the preparation of smoked sheets only at considerable expense in obtaining suitable fuel from a distance.
It is not true that _any kind of timber_ is suitable as a fuel to be used in a smoke-house. All timbers are suitable, either alone or in mixture with others, provided that the wood is not too green.
Naturally an absolutely dead and crumbling wood will smoulder, but does not develop sufficient smoke. A green timber will give an acrid and moist smoke, but demands the consumption of a certain amount of dry timber in addition if it is to be used.
Rubber-tree prunings and sawn rubber trees obtained by thinning-out may be used in mixture with dead wood, provided the logs are stacked to dry in the sun for some weeks before use. If the timber is too green, steam is formed as well as smoke, and the sheets of rubber may have a moist surface glaze.
SUN-DRYING SHEET RUBBER.--Among the first curious sights which impress the visitor or newcomer to this country is the spectacle of sheet rubber hanging in the sun on native holdings. From what one has learned of the extraordinary care which must be exercised in all the processes of rubber preparation, one fails to understand how such rubber reaches the market without becoming tacky. That some of it does become slightly tacky is certain, but on the whole native rubber, though crudely prepared, is usually sound. The native idea of giving sheet rubber a preliminary drying in the sun is to hasten the total period of drying. That the period is curtailed would seem to be the case, but it is open to doubt, as the effect of sun-drying, if unduly prolonged, is to create a thin surface film of dry rubber which r.e.t.a.r.ds the drying of the rubber below the surface. Working with wet crepe rubber, the writer found that, to all external appearances, there was no effect upon the rubber when it was allowed to sun-dry for four or five hours. With periods of from six to ten hours the crepe becomes slightly sticky, chiefly on that portion across the support. When removed to the air-drying house this tackiness developed further, and the rubber, on the line of support, became so weak that it stretched and broke.