_DRYING OF RUBBER_
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:
(1) That the rubber must be fairly thin.
(2) That the temperature be not allowed to rise too high. Some makers
advise 140 deg. F. as a maximum, but no harm results from a temperature of
150 deg. to 160 deg. as long as the interval is not prolonged.[11]
[11] These figures refer to temperatures recorded by thermometers placed in
the folds of the rubber.
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 deg. 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 deg. 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:
-------------------------------------------------+-----------------------
CHAMBER 1.--TEMPERATURE 160 deg.-180 deg.F. |
-------------------------------------------------|CHAMBER 2.--TEMPERATURE
_NO. OF | _DRYING | _WEIGHT OF | _WEIGHT OF | 150 deg.-165 deg.F.
Tray._ | Period._ | Wet Rubber._ | Dry Rubber._ |
--------+----------+--------------+--------------+-----------------------
|Hrs. Mins.| Lbs. | Lbs. |
1 | 1 22 | 7-1/2 | 6 |Worked similarly
2 | 1 42 | 7-1/2 | 6 |to No. 1. Yielded
3 | 1 57 | 7-1/2 | 6 |in 2 hrs. 23 mins.
4 | 1 57 | 7-1/2 | 6 |70-3/4 lbs. dry rubber,
5 | 1 57 | 7 | 5-3/4 |from 87-1/2 lbs. wet
6 | 1 57 | 7-1/2 | 6 |rubber.
7 | 2 0 | 7-1/2 | 6 |
8 | 2 0 | 7-1/2 | 6 |
9 | 2 11 | 6-1/2 | 5 |
10 | 2 11 | 7-1/2 | 6 |
11 | 2 11 | 7-1/2 | 6 |
12 | 2 18 | 7-1/2 | 6 |
--------+----------+--------------+--------------+-----------------------
| | 88-1/2 | 70-3/4 |
--------+----------+--------------+--------------+-----------------------
It will be seen, therefore, that the drier had an output in 2 hrs. 23 mins.
of 141-1/2 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.
[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 passed 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 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.
[Illustration: 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 deg. F. in the lower rooms to over 100 deg. 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.
[13] Rubber Growers' Association, 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 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 deg. 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 deg. F., but certainly the temperature may be as high
as 130 deg. if it is considered safe to allow fires to be so arranged. One or
two estates are known to work at temperatures of 130 deg. 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 deg. 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 deg. 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 constituents 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
constituents 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 retards 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.
Reasoning by analogy, it would appear that no apparent harm would result to
sheet rubber from sun-drying for periods up to four or five hours. From
experience (see Chapter IX.), not the slightest ill-effect is found to
result from the short interval of preliminary drying or dripping practised
on many estates preparatory to smoke-curing.
ARTIFICIAL DRIERS FOR SHEET RUBBER.--It is understood that when vacuum
driers were first applied to the drying of rubber it was thought possible
to dry sheet rubber in this way. The practice was found to be impossible,
as the length of time required and the temperature were responsible for the
destruction of the form of the rubber; it became tacky and semi-liquid.
THE "CHULA" DRIER.--Although several suggestions of devices for
artificially drying sheet have been made, only one is known to be in use at
the present time. In the original form this was used for drying other
tropical products. It consists of a large iron chamber, in which are
several compartments divided by means of baffle-plates. At one end there is
a small furnace and, by means of a fan, smoke and hot air are drawn
through the compartments. Owing to the temperature attained (140 deg. to 160 deg.
F.) sheet rubber cannot be completely dried in the chambers, and is, as a
rule, only treated in this manner for one or two days. Drying is then
completed in an ordinary air-drying house. It is claimed that drying is
expedited, and that the rubber can be packed in ten days.
In the more recent modification, the smoke and hot air which leave the
Chula drier pass through a large room in which may be hung either sheet or
crepe rubber. It would seem that all sources of danger have not been
eliminated from the process, as on one estate a wooden room containing
rubber was ignited by a spark which passed through the drier.
Yet another form exists in which the furnace is outside the main building,
and in the ordinary course of working only heats a series of open pipes
through which air is drawn by a powerful fan. By means of a valve it is
possible to allow smoke from the furnace to pass into the room with the hot
air for the preparation of smoked rubber. The hot air or smoke is
distributed in the lower room by means of main and branch pipes, and passes
through an open floor to the room above. With such an arrangement it is
possible, therefore, to prepare either air-dried or smoke-cured rubber. If
the method could be successfully applied to the drying of crepe it would be
of great assistance on some estates. There would seem to be a difficulty in
working it for the drying of sheer rubber and crepe together, as the
temperature suitable for the one is excessive for the other. Given an
efficient control over the temperature of the hot air, the house should be
successful in the drying of crepe, provided the rubber is not hung in folds
of too great length. For smoke-curing sheet rubber the period is said to be
reduced by several days in comparison with the time occupied in an ordinary
smoke-house, but it is not clear that such a system would have any
advantage over a modern smoke-house, in types of which rubber can be fully
cured in periods ranging from five to ten days.