_PREPARATION OF SHEET RUBBER_
PALE SHEET.--The first form in which plantation rubber was prepared was as
"biscuits" or sheets. This form remained in favour for some years. The
first biscuits or sheets were rather dark in colour owing to the natural
oxidation which followed. Then it was discovered that by diluting the latex
the degree of oxidation was diminished, and later it was found that if the
soft coagulum were placed in almost boiling water for a short time the
resulting rubber was pale. Thus there arose gradually a demand for pale
sheet. With our present knowledge we are in a position to state that the
pale biscuits were not in any way superior to the darker ones, and they
were in most cases actually inferior.
It was found also as time progressed that sheet rubber, on air-drying,
became covered with external surface moulds, and that, more often than not,
the smell of the drying rubber was the reverse of pleasant. Even when dry
the sheets had to be continually brushed free from moulds, and by the time
the rubber reached the market it was again usually mouldy. Such are, even
now, the handicaps under which those who prepare pale sheets have to
labour. Few, however, are the estates making pale sheets, and they are
confined almost entirely to native holdings.
To those accustomed only to the preparation of crepe rubber, where
coagulation can be effected in large batches, the preparation of sheet
rubber always seems to demand much more labour. As a matter of fact,
although the preliminary operations certainly do demand more care and
labour than in crepe-making, there are compensating advantages in the
machining stage. For the preparation of sheet of the highest quality on any
but the largest scale, elaborate installations of machinery are quite
superfluous, as equal results can be obtained with pairs of rolls worked by
hand.
UNIFORMITY OF PRODUCT.--There will be no need to enter again into a
discussion of the preliminary operations of receiving and straining latex
for sheet-making. They have been fully dealt with in Chapter VII. It used
to be the general custom to mix the acid and latex in each individual dish,
and in some small or non-progressive factories that is still the procedure.
Quite apart from the question of labour entailed, the process is quite
unnecessary. Even if comparatively small volumes of latex are handled,
standardisation by dilution should be the rule, and the acid solution
should be added to the bulk. It is possible to stir in the acid and to
ladle out uniform quantities in each pan or dish from a bulk volume of up
to 40 gallons if the organisation is efficient.
On any but a small scale the labour entailed in the handling and cleaning
of pans is excessive, and shallow tanks are now employed on most estates.
The reception and standardisation of latex by dilution has already been
discussed in Chapter VII. The combination of this practice with the
employment of shallow coagulating tanks has simplified working and reduced
the cost of labour. It is not intended to enter into any lengthy discussion
relative to the merits of sheets made in pans as against those made in
tanks. It is granted that it is possible to make a "pan" sheet superior in
appearance to the general average of "tank" sheets; but from an economic
standpoint the introduction of the use of tanks into all but the smallest
factories is only a matter of time, if the demand for this class of rubber
persists.
THE IDEAL TANK.--Even the most modern installations of sheet-coagulating
tanks must be regarded as merely temporary devices, as, given facilities,
the room for improvement is so wide.
The first tanks made erred in being too large, and as the result of
experience the size of units has now been reduced to a maximum of 12 feet
by 4 feet by 1 foot deep.
[Illustration: UNIT MODERN COAGULATING TANK (TWO VIEWS).
Construction of brick and cement with lining of glazed tiles. Note slots
incorporated in side tiles. Partition boards in piles in the background.]
Tanks are at present constructed either of hard timber or of brick and
cement faced with glazed tiles; both types have inherent drawbacks. The
wooden tanks are difficult to keep clean and in "sweet" condition. The
glazed tiles, unless extremely well laid, allow the acid serum (from which
the rubber is removed) to percolate between the interstices. Thus "pockets"
of liquid collect beneath the tiles, and in process of the decomposition of
certain constituents dissolved in the serum evil-smelling gases are set
free.
[Illustration: ANOTHER BATTERY OF TANKS, WITH DILUTION TANKS, RAISED, ON
THE RIGHT.
Note drainage cocks, chute, and sieve in position.]
It should not be a matter of difficulty for manufacturers to make sheets of
thick glass sufficiently large to form the bed-plate and side-pieces
necessary in the lining of a tank. If such adjuncts could be secured, the
disabilities indicated above would be perhaps wholly removed. Unless there
is a demand from estates, however, it is idle to expect a supply to be
forthcoming.
An even greater improvement would take the form of unit tanks cast in
glazed white-ware with the necessary slots incorporated in the sides. At
present no known firm makes such tanks of sufficient size. A unit could
measure (internally) 6 feet by 4 feet by 1 foot deep, with slots 1-1/2
inches apart, and 3/8 inch in width. The tanks might be reinforced with
iron bars, so that they could either be used alone or embedded in the usual
brick structure. The junctions of the bed-plate and side-pieces could be
finely rounded so as to facilitate cleaning, and at one end a draining-hole
could be made, say, 1 inch in diameter.
[Illustration: CLOSER VIEW OF FOREGOING.
Note partitions in position and coagulum being removed.]
Meantime both the hard-wood tanks and those of glazed tiles find their
particular applications. The former is generally employed in smaller
factories, or where future large increases of crop preclude the present
installation of a fixed system. The latter find use in large factories, or
where no new areas remain to come into bearing.
MODERN INSTALLATION.--As an example of a modern installation of coagulating
tanks, we can do no better than offer reproductions of the system now in
use on Pataling Estate.
A warning must be given against employing all tanks of stone-ware or cement
unless well glazed. Almost without exception, irrespective of the material
used in the construction of coagulating tanks, wooden partitions are
employed. In the few exceptional cases the partitions are either of glass
or of aluminium. The former would appear to be the ideal substance, were it
not for initial cost and loss by breakage. These disabilities may possibly
be overcome in course of time.
CARE OF TANKS.--The use of aluminium would have been wider had it not been
for lack of supplies and the question of cost during the War. A novel
method of employing aluminium partitions was introduced in the factory of
Tremelbye Estate. There were no slots in the sides of the glazed-tile
tanks, but the necessary slots were very ingeniously created by means of
aluminium "distance-pieces," the two long edges of which were turned at
right angles to the face of each piece to a depth of about 1/4 inch. The
ends of the thin aluminium partition moved in the slot thus formed between
two adjacent "distance-pieces." The friction between the surfaces was
sufficient to allow all the partitions, when in position, to be raised well
above the floor of the tank, so that a uniform level of latex was obtained.
Slight hand-pressure only was then required to push the partitions down.
Naturally the cleansing of glass or aluminium partitions presents no
difficulty, but in the case of wood failure to ensure thorough cleanliness
leads to possible defects in the finished dry rubber. Provided the wood
could be made waterproof, no trouble would ensue, and hence various
measures have been tried with that object in view. When new the boards have
been surface-waxed or varnished, and the treatment has been repeated on
occasions. But in course of time the surface film of waterproof material
has disappeared, partially or wholly, and the trouble recurs. When
partitions become sodden with serum, the surfaces are liable to be coated
with a slime, consisting largely of organic growths which have an effect
upon the latex, causing "pitting" on the surface of the coagulum and
enclosed bubbles within.
[Illustration: ANOTHER BATTERY OF TANKS, WITHOUT DILUTION TANKS OR MEANS OF
GRAVITATING LATEX.]
It is recommended, therefore, that wooden tanks, after ordinary cleansing
daily, should be swabbed out with a 5 per cent. solution of sodium
bisulphite. Wooden partitions should receive the same treatment, and once a
week at least (or every day if possible) they should be placed in the sun
for an hour or two, care being taken that both sides of a partition are
exposed in turn. Before being placed in the latex, all wooden partitions
should be made wet on the surfaces.
Some years ago the writers had made a partition of vulcanite, which
apparently would have proved of great service but for the initial cost. The
advent of the War put the matter out of the question, but it is possible
now that such a material would be worthy of extended trial. Except in the
matter of cost, it would appear to have advantages over any substance yet
tried; and if it were possible for estates to supply their own lower grade
rubbers direct, the cost might be reduced considerably.
[Illustration: A SHEETING TANK CONTAINING COAGULUM FOR CREPE PREPARATION.
Behind wall in background are the tanks in which latex is standardized.
Note vent, to the left, through which latex flows and wooden "stopper" on
edge of tank.]
STANDARD LATEX.--Enough has been written (see Chapter VII.) to familiarise
the reader with the use of this term for the description of latex diluted
daily to a level of dry rubber content. Whatever may be the practice
elsewhere, it is now fairly general on estates in Malaya to reduce all
latices to a uniform "strength" for the preparation of sheet rubber. It is
claimed that only in this manner can uniformity of product be achieved.
The selection of a standard has been the outcome of general experience. It
has been found that if too high a standard is taken difficulties arise,
such as (1) unsatisfactory and uneven coagulation, (2) too thick a coagulum
for easy working in general, (3) too extended a period of drying and
smoke-curing, and hence too dark a colour in the finished rubber.
[Illustration: A "BATTERY" OF SHEETING TANKS (PATALING ESTATE). DILUTION
TANKS, RAISED, ON THE LEFT.]
On the other hand, too low a standard also brings trouble in its train. The
coagulum is too porous, will not stand handling, and the resultant sheet is
too thin unless an abnormal thickness of coagulum is prepared. Furthermore,
over-dilution means an increase in the number of tanks required for any
original volume of latex. This involves an increase in floor area, and
perhaps in the size of the building. The soft sheets, when rolled, may
spread to such a width as to cause the edges to be squeezed under the
cheek-blocks of the machines, etc.
For all practical purposes, whether sheets are prepared in pans or in
tanks, it has been found that the optimum results are obtained by the
adoption of a standard approximating and not exceeding 1-1/2 lbs. dry
rubber per gallon. Primarily this standard has a direct connection and
interdependence with the distance between the partitions (or between the
slots) in coagulating tanks. The distance found most practicable is 1-1/2
inches. This thickness of coagulum, when prepared from latex not exceeding
a standard of 1-1/2 lbs. dry rubber per gallon, is found to yield a very
satisfactory sheet in all respects.
It will be seen that we have two possible main factors of variation:
(_a_) Distance between partitions, causing visible differences in
thickness of coagulum.
(_b_) Dry rubber content of latex, causing differences in the density
(_e.g._, hardness or softness) of the coagulum.
The effect of variation in (_a_) will be clear. Even when latex of a
standard of 1-1/2 lbs. per gallon is employed the resulting sheet may be
either too thin or too thick.
Similarly, as already argued, the use of too low or too high a standard of
dilution (when the factor of distance between partitions is not allowed to
vary) is capable of causing much difficulty. While this is correct,
broadly, it is found in the experience of some estates that their
requirements are satisfied by a slightly lower standard than 1-1/2 lbs. per
gallon. Thus it is not uncommon to note the adoption of a standard
equivalent to 1 lb. 4 ozs. or 1 lb. 6 ozs. dry rubber per gallon.
Experience dictates, however, that for the recognised standard measurements
of modern tanks the practical limits of satisfactory density of latex lie
between 1-1/4 lbs. and 1-1/2 lbs. per gallon.
STANDARDISING INSTRUMENTS.--For standardising latex by dilution all that is
required is an instrument which will preserve a perpendicular position
while floating in latex, will be sufficiently sensitive to indicate fairly
small differences in density of latex, and has one mark on its aerial
portion accurately indicating a density corresponding to the required
standard. On scientific grounds it can be demonstrated that such an
instrument as employed in common practice would not be strictly
accurate.[6] It is not proposed, in this section of the book, to discuss
such considerations.
[6] De Vries, "Archief voor de Rubbercultuur."
Instruments of this nature are represented by the "Metrolac" (originating
from the Rubber Growers' Association) and other similar recorders. They
generally consist of a submersible bulb with a projecting stem which is
graduated. The "Metrolac" differs from others in that the bulb is of
torpedo form (thus reducing "skin friction"), and the graduations on the
stem indicate actual weight of dry rubber per gallon instead of the
ordinary specific gravity figures.
Theoretical considerations to the contrary, it is found in actual practice
in Malaya and Ceylon that, although such instruments are naturally delicate
and require careful manipulation, they are of considerable practical value
and satisfy a definite requirement. Until an instrument of greater accuracy
and equal simplicity can be discovered all estates should regard the
possession of a few "Metrolacs" as essential.
The nature of their construction and the average conditions under which
they are used (and abused) make it impossible to rely upon their accuracy
indefinitely or for any long period. It is always recommended, therefore,
that there should be at least two instruments available, one of which may
be in daily use, while the other is kept in safe custody and only employed,
say, once a week for purposes of checking the accuracy or degree of
inaccuracy of the other. This can be done with reasonable approximity by
placing both instruments in a tall vessel containing well-mixed and diluted
latex. Instruments showing a marked degree of inaccuracy should not be
preserved; but in cases of necessity "Metrolacs" from estates belonging to
company members of the Rubber Growers' Association may be sent to the
laboratories for repair and adjustment.[7]
[7] This applies to the gilt brass instruments. As the result of experiment
the Rubber Growers' Association are now introducing glass instruments.
These are necessarily more fragile, but while unbroken can be relied on to
give correct readings.
Where field coagulating stations have been instituted on estates, it is
strictly necessary that instruments should be provided in all cases; and it
should be a rule that these are tested and corrected weekly by a standard
instrument employed for that purpose only. This need was well recognised by
many estates when, during the War and the consequent shortage of supply of
"Metrolacs," a demand arose which was met in some degree by crude
instruments of local manufacture, such as that commonly known as the
"Castlefield bobber," contrived and made by the enterprising manager of the
estate of that name. The demand for the more accurate instruments can now
be met.
METHODS OF USING THE INSTRUMENTS.--The "Metrolac" was devised and
introduced by the writers on behalf of the Rubber Growers' Association, and
directions for its use were given. Tables were prepared by means of which
simple calculations for the dilution of any given latex could be made.
These did not find an extended application, inasmuch as in the majority of
cases native workers only were in charge of the processes of rubber
preparation. In point of fact, such calculations are not strictly
necessary, as the operation of standardising the latex can be done quite
simply and skilfully by a trained native. Latex as it reaches the store in
average weather from any particular division or field does not vary greatly
in density. The trained coolie or foreman, basing his practice on
experience, adds to the latex a quantity of water, and then makes a first
test with the standardising instrument. Several additions of water (with
thorough stirring) may have to be made before a test indicates that the
correct density has been obtained, but it is surprising how quickly a
skilled worker will arrive at the desired standard. Extreme or absolute
accuracy is not insisted upon or desired, as avoidable delay is to be
deprecated, and the result in any case is sufficiently exact for practical
purposes.
SKIMMING.--During the gravitation of the latex from the reception vessels
(in which the standardising of the latex is effected) to the coagulating
tanks, much surface froth is usually caused. This is best removed by means
of a thin board of a width slightly less than the breadth of the tank. The
skimmings are sometimes placed in pans and subsequently made into a second
grade of sheet rubber, or they receive treatment with a small proportion of
sodium bisulphite and eventually appear as fine pale crepe. The practice
varies usually according to the form in which the general No. 1 grade is
prepared.
On some estates a great deal of the frothing is avoided by placing in
position at the receiving end of the tank a perforated partition. This
partition may be made of wood, or of stout zinc (or aluminium) carrying ten
circular holes to the inch. Through this the latex percolates, while the
froth is retained on a small area. The froth is removed prior to the
addition of the acid. After stirring in the acid solution most estates
again skim the surface of the latex; but if the stirring has been performed
properly there should be little froth. This, when it collapses, in any case
will appear only on the upper edge of the strip of coagulum, and after
rolling should not be visible. It would appear, therefore, that the second
skimming is not necessary.
STYLE OF SHEET.--Within the last few years the custom of making plain
sheet--_i.e._, sheet having a plain surface--has gradually given place to
the preparation of ribbed sheet--_i.e._, sheet having a pattern marked on
the surface. It would probably be correct to say that plain (smooth) sheet
is now only prepared by natives or by some estates just come into bearing.
Even in the latter case there is no reason why smooth sheet should be made,
as hand machines are sold which will do all the work required. It will be
evident to anyone acquainted with rubber preparation that in the matter of
actual quality of rubber the question of smoothness or a pattern can have
no bearing on the result. One advantage claimed for ribbed sheet which may
entirely justify the preference exhibited by consumers, relates to the
question of packing. When rubber arrives at home it is frequently found to
be in an almost solid block, due to the pressure of the sheets superimposed
in the case. The smoother the surfaces of the rubber in contact the greater
the adhesion and the denser will be the mass, and consequently the greater
the difficulty in separating individual pieces. Under such circumstances it
is plain that the difficulty is diminished if the sheets have a raised
pattern on them. It is noted also that the liability to mildew-growth is
greater the smoother the surfaces of the rubber.
On these grounds the "marking" of sheet rubber is to be commended. These
reasons apart, it is really astonishing the difference made in the
appearance of the sheets by impressing upon them a ribbed pattern, and it
is highly probable that the market value of the rubber is slightly
increased. It is not our duty to attempt to reason why this simple
operation should increase the market value of sheet rubber; it is
sufficient to recognise that it is so, and that money may be thrown away by
neglecting to cater for the taste of the market. Of the patterns impressed
upon sheet rubber there is a variety, but the general style is that known
as the "spirally close-cut ribbing."
STANDARD SHEET.--Leaving for the present the question of pattern of mark,
one cannot do better by way of introduction than to reproduce the
instructions[8] given generally to estates.
[8] "Handbook on Preparation of Rubber," Rubber Growers' Association, May,
1917, p. 28.
ROLLING AND MARKING OF SHEET RUBBER.--Working with standard latex it is
found that strips of coagulum 1-1/2 inches in thickness require little
rolling to produce sheets of desirable thickness.
(1) The sheets or strips are first given a preliminary rolling with a
heavy hand-roller made of hard wood. The roller is passed once in one
direction, and once in the reverse direction.
(2) The coagulum is then passed through smooth machines twice, once
with the rolls fairly open, and once with a narrower space. It is not
found advisable to close the smooth rolls so tightly that the rubber
is made too hard.
(3) The sheets or strips are then passed once through a pair of
marking rollers. Various types of patterns are used, but the one which
appears to give the most satisfactory results is that known as the
"close-cut spiral." This produces the semblance of a small diamond
pattern on the rubber. The surface of the sheet is raised in
well-defined ridges, which appear to present the maximum drying
surface exposed to the atmosphere of the smoke-house. Thus, not only
is the appearance of the sheet rendered attractive, but also the
period of drying is reduced. Starting with standard latex and
following the procedure here described for rolling and marking, sheets
should be ready for packing in ten or eleven days. If the period is
longer, it is possible that the design or structure of the smoke-house
is at fault.
WHEN TO WORK THE COAGULUM.--Before proceeding to discuss other points the
question remains to be settled as to how long it may be necessary or
advisable to allow the coagulum to remain in the serum before rolling it.
For reasons of practical economy in factory working, it is usual to allow
sheet rubber to remain over night, and the coagulum receives attention
early next morning. During the interval (averaging about eighteen hours),
the coagulum consolidates, leaving an almost clear serum if the correct
quantity of acid has been added to the latex. Any but the very slightest
trace of milkiness in the serum indicates an insufficiency of coagulant. If
the serum is always definitely clear, there may be grounds for believing
that an excess is being used. If the quantity of coagulant has been
calculated to an average nicety, the serum should be just dubiously free
from milkiness.
The firmness gained by the coagulum on standing in the serum overnight
should enable it to be handled next morning without any marked stretching,
and in some estates the rubber is put direct through the first pair of
smooth rolls without a preliminary consolidation by means of hand-rolling.
Some estates prefer to handle the coagulum while rather softer, as it is
claimed:
(_a_) That the coagulum is easier to work, and sheets of improved
appearance can be made.
(_b_) That there is greater freedom from "bubbles."
(_c_) That the incidence of "rust" is lessened.
These claims are substantiated in practice; but in the case of the
third, it only holds provided that the rubber can be finished and
placed in the smoke-house almost as soon as the last sheet has been
machined.
In such cases all latex must reach the store comparatively early in the
day--_e.g._, before noon. Three hours is allowed for coagulation, and the
working of the rubber is then commenced. As a general rule this means that
the operations of rolling and marking must be completed, a short interval
given for dripping, weighing must be done, and the rubber placed in the
smoke-house before night falls (as a rule about 6.30 p.m.).
Unless factories dealing with a large crop are well equipped with
artificial light, such a course is not open to them; in any case it remains
true that night work should be avoided if possible. If, however, it can be
arranged without increasing the cost of production, there would appear to
be no objection to the early working of the coagulum as described above.
HAND-ROLLING.--As already indicated, some few estates do not give the
strips of coagulum any preliminary hand-rolling, as the rubber is
considered to be sufficiently firm to be handled into the first machine.
On most estates hand-rolling is found necessary, owing to the tendency of
the long strips to stretch unduly, giving badly shaped sheets. This
hand-rolling should be done carefully, and is best effected on a specially
constructed table. This consists essentially of an inch-thick hard-wood
plank about 2 inches wider, and 4 or 5 feet longer, than the strip of
coagulum. Along the edges of the plank, and at right angles to its upper
flat surface, may be fastened strips of wood about 1/2 inch square in
section, thus forming a shallow tray open at either end. These strips serve
two purposes:
(_a_) As the wooden roller is wider than the plank, they prevent the
coagulum being rolled too thin and too firm.
(_b_) They prevent the coagulum being squashed too wide, and tend to
keep the edges straight.
To avoid "thick ends" it is sometimes considered advisable to insert, at
either end of the rolling table, shallow wedges about 6 inches long, of the
same width as the table (between the edge-strips), and with the sharp end
of the wedge pointing in the direction of the length of the table. The ends
of the coagulum are drawn up and finished on these inclined planes.
These points may appear to be extreme refinements, but as long as rubber is
valued on such grounds we must endeavour to meet the system imposed upon
us.
SMOOTH-ROLLING.--It is advised that, after hand-rolling, the coagulum
should be passed through at least two machines having smooth-rolls. On some
estates three such machines are employed. The purpose of this procedure is
to reduce the thickness of the coagulum gradually. The same could be
effected, of course, on one machine; but obviously the distance between the
rolls would have to be readjusted at each operation and for each piece of
coagulum. Apart from the time thus wasted, there is the certainty, in view
of the rough adjustment of the machines, that the chances of obtaining
uniformly thick sheets would be slight.
The machines should be arranged as a battery, with the marking rolls at one
end, so that the operations are consecutive. It is erroneous to imagine
that heavy machines (such as those used in crepe preparation) are required.
Light machinery only is necessary for sheet-making; but any available heavy
smooth-roll machines in a crepeing battery may serve admirably for the
purpose.
MARKING.--Heavy machines are unnecessary for the purpose of putting a
pattern on sheet rubber. If the rubber has been properly prepared a light
pair of rolls is capable of exerting sufficient pressure to cause a good
upstanding pattern.
Rolls are cut in various designs: some with "diamond" grooves on both
rolls; some with grooves of varying width and depth encircling the
circumference of the rolls, thus creating a "stripe" effect on the rubber;
and some with diagonally-cut spiral grooves placed closely together. The
last has the greatest vogue at present, while the first has almost gone out
of favour. An objection lodged against the second design is that the edges
of the grooves sometimes cut through the rubber, so that the dried sheet
divides in strips. It would appear in such instances that either the
coagulum was too thin and soft, or that the grooves had been cut too deeply
and sharply. In any case the choice of a design is an arbitrary matter, and
should depend upon the effect produced on the rate of drying and the
general appearance.
The popular "close-cut spiral" roll is machined with varying measurements,
but the usual design has grooves 1/8 inch wide by 1/8 inch deep and 3/16
inch apart.
Many estates have a particular "brand" cut in the middle of the rolls for
purposes of identification. If this is done it is advised that the main
grooving of the rolls be carried into the "branding" strip; otherwise grip
will be lacking on this portion, and a certain amount of "cockling" of the
sheets will result.
Sheets are often seen in which the potential effect of the grooving is
reduced to a comparatively flat pattern in place of the desired ridges. The
fault is generally attributed to the shortcomings of the marking rolls.
While it is true that the grooving often deteriorates by friction-wear when
the rolls are running "free," experience generally decides that the
deficiency in the appearance of the rubber should be attributed to faulty
previous preparation rather than to the marking rolls. Sets of rolls have
been changed often without justification or an improved result. It would
always be well to be certain first that the trouble did not emanate from
the fact that the coagulum had been previously rolled so thin and hard that
the rubber could not be squeezed so as to fill the grooves. This has been
found to be a common fault, and the general effect is to delay drying in
spite of the thinness of the rubber.
Again, the trouble may have been due to an incorrect standardisation of the
latex, generally in the direction of too heavy a density (too rich a latex)
being employed. The original thickness of the coagulum would be normal, but
owing to the abnormal rubber-content the effect of passing through the
smooth rolls would be the production of a strip thicker and firmer than
ordinary. If this firmness is appreciable the resistance of the rubber to
the squeezing action of the marking rolls will result in a flat
pattern--_i.e._, the grooves cannot be filled, and the ridges are low.
It is advised that all rolls used in the preparation of sheet rubber should
be at least 18 inches wide, in order to avoid the appearance of thickened
edges which delay drying.
Working with the correct standard of dilution of latex, and following the
procedure indicated in the foregoing paragraphs, the dry sheet should not
exceed an average thickness (over ridges and depressions) of 1/8 inch.
PREPARATION FOR SMOKE-CURING.--It used to be the custom to allow sheet
rubber to air-dry first for periods varying from one to several days.
Naturally moulds were soon formed, and when the sheets were quite
smoke-cured a mass of the dead moulds could be seen, if not over the whole
sheet, at least in the corners of each diamond mark. It has been
demonstrated in practice that there is no advantage in allowing sheets to
air-dry partially before smoking. In fact, to obtain the greatest benefit
from smoke-curing, sheet rubber should be placed in the smoke-house as soon
as possible. The same effect of mould-growth may be noted if the wet sheets
are placed in a smoke-house insufficiently heated.
Other defects may arise which can be traced to faulty treatment of the
marked coagulum prior to hanging in the smoke-house and subsequent to
rolling. These will be enlarged upon in a subsequent section of the book,
and at present it will suffice to indicate the procedure which experience
directs as likely to give the best results.
When the lengths of coagulum leave the marking machine they are usually
laid in piles containing two dozen or more strips. The piles are then cut
into the required lengths, the exact length generally being determined by
the available perpendicular distance between the supports in the
smoke-house. It is necessary to remark that the piles of sheets should not
be allowed to accumulate, but should be dealt with in subsequent treatment
progressively. If for some reason this is not possible, then all piles of
sheets should be turned on edge so as to assist the draining away of the
serum or "mother-liquor," which continues to ooze from the rubber for some
time after the squeezing in the machines.
Where hot water is available the freshly cut sheets should be passed into
it as soon as possible, and given a thoroughly good swilling. The caution
must be given that the hot water should be changed very frequently and, if
possible, after every batch, say, of a hundred sheets.
The sheets should then be carried immediately to racks on which they are
hung to drip. Generally these racks are situated under cover, but there is
no reason why they should not be placed in the open air without cover or
shade. From continued experience of this practice over a period of years it
is found advantageous and to be preferred to the usual method of allowing
sheets to drip under cover.
While the sheets are fresh and loaded with internal moisture, the effect of
sun-heat upon the surface, when exposed for, say, two hours, is nil; and
the safety of the process can be guaranteed, provided the stated limit is
not exceeded to an appreciable extent.
[Illustration: THE OLD METHOD OF "DRIPPING" FRESHLY ROLLED SHEETS WITHIN
THE FACTORY.]
After dripping for an hour or so, the sheets should be placed in the
smoke-house. If it is a bright sunny day, no extra precautions need be
taken; but on cool, dull days it would be advisable to light the fires
earlier than usual, consistent with the work required to be done in the
house--_e.g._, in the removal of dry rubber. There would appear to be no
reason why the dry sheets should not be first removed, so that on dull or
wet days smoking can be commenced as soon as the wet rubber has been hung.
On a few estates where the smoke-houses are worked continuously, except for
a few hours in the morning, a portion of the building is separated by a
partition for the reception of the wet rubber. The sheets are taken
directly from the marking rolls and placed in the chamber, beneath which a
fire is started. The sheets thus drip in a warm and smoke-laden atmosphere
until next morning, when they are weighed and removed to the smoke-house
proper. It is claimed that freedom from "rust" is thus obtained.
It will be clear that in the treatment of the rubber preparatory to smoking
the whole process should be continuous, and delay should be avoided if the
best results are to be obtained.
[Illustration: THE NEWER METHOD OF HANGING IN THE OPEN AIR.]
SMOKING OF RUBBER.--The assumption may have been noted above that the sheet
is to be smoked. As far as our knowledge extends, none but small native
estates now prepare sheet rubber of any other type, with the exception of
certain patent processes. Air-dried sheets are generally made on
small-holdings, and are bought in the market chiefly for the purpose of
macerating and making into blanket crepe. We have no intention, therefore,
of discussing the possibilities or qualities of air-dried sheets, as the
output of sheet-rubber from our estates is always in smoked form. The
drying (or, properly, smoking) stage will be discussed in Chapter XI.