A more effective way of dealing with the trouble, provided other precautions have been taken, is to have plenty of spare wooden bars. It should be a rule stringently enforced that, as soon as racks are emptied, the bars should be removed to the factory to be cleansed thoroughly. A spare set should enter the smoke-house with each batch of fresh rubber. The actual number of spare sets required could be limited to a two days'

supply, and the extra cost would be recouped easily.

FLOOR OF DRYING-CHAMBER.--The floor of the chamber is usually of planks, except that the s.p.a.ce under each bay of racks should be filled with expanded metal. With the use of wood fires there is always a large amount of light ash formed, which may find its way into the upper chamber. To counteract this, screens of fine mesh gauze are laid over the expanded metal. This gauze may be fitted into a movable wooden frame, so that when it becomes necessary to clean it the whole may be removed.

The difficulty is that with furnaces of the "open-fire" type the rise of dust is so great that the gauze screens soon become clogged, especially as the slight tarry matter in the smoke condenses on the gauze, causing the dust to adhere. With the better types of furnaces, the employment of gauze screens is not necessary, as there should be very little rise of dust. It is sufficient to use only expanded metal, to prevent any displaced pieces of rubber falling into the furnace chamber.

FURNACES GENERALLY.--The crudest and dirtiest method of fuel consumption in the preparation of smoked-sheet rubber is that of making a fire on the ground. This is still a common practice, and should be condemned as being both wasteful and harmful. Under prevailing conditions coolies will, in spite of instructions, heap up a pile of logs in order to save themselves the trouble of stoking the fire in small quant.i.ty and at regular intervals.

A small supply of water is kept at hand with which to quench the fire somewhat if it threatens to cause trouble. Naturally a large quant.i.ty of fine ash is thus thrown up, and the rubber above receives the deposit. If the coolie does not happen to be sufficiently awake, of course a house burns occasionally.

From this primitive type of furnace, others have been evolved. These usually take the form of more or less shallow trucks, the majority of which are similar in principle to the fire on the ground, except that the container can be withdrawn from the house for the purpose of removing the ash. Sometimes they are even more objectionable than the ground fire, inasmuch as, being raised above the ground level, an under-draught through fire-bars is caused, and consumption of fuel is so much the more rapid.

PITS.--It is clear that large fires are not desirable, and that combustion should be slow, provided that the necessary temperature can be maintained.

The lines along which the development of furnaces needed to extend are therefore plain. The simplest device adopted was the digging of pits in the ground. Sometimes these pits received the addition of an iron truncated cone which was movable. Naturally the combustion was slow, but sufficient heat was obtained if the pits were large enough or in sufficient number. An objection was that the ash had to be cleared _in situ_, and in the process the earthen pits gradually increased in size. In all cases it was necessary to suspend an iron baffle-plate above the furnaces to distribute smoke and arrest any sparks.

"POT" FURNACES.--The next development was the employment of "pot-furnaces."

These consist of iron drums, sometimes merely resting on the ground, and sometimes mounted on trucks for easy withdrawal. These drums radiate sufficient heat if present in sufficient numbers, and the fuel consumption is low. They are usually manipulated by starting a fire in the bottom and packing in logs cut to the necessary length. Some have no lids, while others are fitted with perforated caps.

It was considered necessary in some instances to punch a few small holes near the base of the drum in order to ensure a very slight upward draught.

In a few cases this perforation has been exaggerated to the form of a hinged door. Unless this can be closed with ease, and is closed according to instructions, part of the object of this type of furnace is defeated; fuel consumption is rapid, and the temperature is too high. In the original form "pot-furnaces" have been found to be effective on many estates, and are still employed with satisfaction.

IRON STOVES.--Working on exactly the same principle, on some estates one finds small iron stoves in use. Sometimes broad pipes are attached for the better distribution of the smoke; if this is the case it should be noted that the pipes should have a slight downward slope, and that the "bend" at the end should be turned downwards. In this way condensed moisture and creosotic matter falls to the ground, and does not lodge in the pipe. The life of the conduit is thus prolonged. Usually such stoves are in use where the "head-room" of a smoking chamber is insufficient for other types, or where the nature of the site does not permit of sunken furnaces being installed. They are of value likewise on occasions where the fuel supply is limited to a rich timber such as mangrove-logs ("bakau"), when it is necessary to ensure a low combustion with low cost of fuel.

HORIZONTAL DRUM-FURNACES.--To overcome difficulties inherent to drums or "pot-furnaces," the next development was that in which the drum was made to a.s.sume a horizontal position, and adapted ingeniously to a simple system of working from the outside of the building. Reference to the drawings given will explain how this is effected. In the first ill.u.s.tration (No. 2) it will be noted that the drum is supported upon brick pillars, with one end projecting through the wall of the building. At the other end a short chimney is mounted, having within it a "damper" which is adjustable from the outside. Over this chimney is suspended a simple baffle-plate, made from a Chinese iron cooking-pan. The outer end of the drum is furnished with a hinged and latched door, in which a small air-regulator is accommodated.

In the second set of drawings (No. 1) the drum is increased in size and fitted in a special manner for incorporation with a distinct type of building. Such a scheme was first put into effect by Mr. R. C. Sherar, the manager of Third Mile Estate, Seremban, F.M.S., and for ease of reference the house and furnace will hereafter be mentioned when necessary as the "Third Mile" type.

[Ill.u.s.tration: "THIRD MILE" TYPE; HORIZONTAL DRUM.

This type of furnace is suitable for adapting to existing buildings with perpendicular lower walls.]

[Ill.u.s.tration: "THIRD MILE" TYPE OF FURNACE, USED IN CONJUNCTION WITH "THIRD MILE" SMOKE-HOUSE.]

It will be seen that the furnace has at the farther end a door for the removal of ash. As this, if badly fitting, may result in too great a draught, it is well to insist upon good workmanship. Other adjustable air-inlets are provided, and the drum is enclosed in a brick chamber.

RATE OF COMBUSTION.--However successful this furnace may have proved in the hands of trained coolies, one must feel that with such a number of air-inlets (whether accidental or designed) there would always be present the possibility of obtaining too rapid a combustion. In the original forms of drums or pot-furnaces of various kinds, a very slow rate of combustion was attained. Naturally a relatively larger proportion of carbon remained unconsumed, and there was a small proportion of ash. In these respects the furnaces resembled charcoal-burners. In point of fact, some estates used this principle for the dual purpose of smoke-curing the rubber, and at the same time obtaining a supply of charcoal to provide fuel for their suction-gas engines. This is a consideration in times when managers are desirous of discovering any devices which tend towards reduction of costs.

It will be clear that, under ordinary circ.u.mstances, the condition of what remains after the combustion of the fuel gives an indication of the rate at which the wood has burned, and this test should apply to all furnaces. That in which there is the most ash and the least charcoal is the one least to be desired. In direct connection with this consideration, one must recognise that a fire which is sunk below the level of the ground exposes the least surface from which heat may radiate; and hence, in order to obtain the maximum benefit of heat from a slow-combustion furnace, it should be above ground-level, or should have a superstructure from which the heat may be dissipated.

Simple drum furnaces, with slow combustion, have the further advantages that a "charge" of fuel will need no attention for possibly eight to ten hours, and practically no ash is found to be ejected. These advantages have great practical importance. The first minimises any disabilities arising from neglect on the part of coolies, and the second makes for increased cleanliness in the drying-chamber. While these advantages would appeal to most estates, there would appear to be a further advantage to small estates which have only temporary timber smoke-houses. With a slow rate of combustion in a furnace of this type, danger from fire is diminished considerably.

Bearing in mind the slow rate of combustion, and hence the comparatively low temperature obtained, it will be plain that drum furnaces should be employed in larger number than ordinary open-hearth fires; and the drums can be so placed as to ensure the best possible uniform distribution of heat and smoke.

Large furnaces are sometimes seen, with flues of brickwork. In view of the foregoing remarks, it will be obvious that these tend to large fires and a rapid combustion, and hence must be cla.s.sed as undesirable.

BRICK STOVES.--Developing from "drum" furnaces, another type comes into existence. In principle it consists of an enclosed brick furnace, with feeding door, and a low conical dome surmounted by an adjustable cap or spark-arrester. The rate of combustion can be influenced by a suitable movement of the cap, which is operated by a screw. This type of furnace has been installed on several estates by the engineering department of Messrs.

Harrisons and Crosfield, and is understood to give satisfaction.

PATALING TYPE.--With the exception of the "Third Mile" type already mentioned, all the furnaces described are open to a strong objection, in that the coolies have to enter a room, usually filled with hot smoke, in order to attend to the fires. The mere opening of the door of the building is sufficient to fan most fires into a blaze and to raise sparks. Apart from these points, it is natural for coolies to avoid entering too often, with the result that they generally stoke with the maximum load of timber.

Even should they not sleep the danger is clearly great.

[Ill.u.s.tration: SIDE SECTIONAL ELEVATION (PATALING TYPE OF FURNACE).]

[Ill.u.s.tration: PATALING TYPE OF FURNACE.]

To obviate these drawbacks, furnaces which are fed from the outside of the building were designed. There have been various forms, but as they were first installed on Pataling Estate, in the present form, they may be known under the description of the Pataling type of furnace. They are eminently satisfactory, and have a low rate of fuel consumption. They are very safe, and in fact, if worked with average intelligence in supervision, can be regarded as being fool-proof. There is practically no ejection of fine ash, and no fine-mesh screens need be employed. They can be adapted to any building having either vertical or sloping walls of galvanised iron.

[Ill.u.s.tration: LARGE SMOKE-HOUSE OF ORDINARY CONSTRUCTION, WITH SHIELDED VENTILATORS PERMANENTLY OPEN.

In foreground, movable folding racks on which sheets "drip" in the open air. This smoke-house is equipped with brick furnaces fed from the outside (Pataling Estate).]

In essential the furnace consists of a shallow pit below ground-level, lined with brick, and having a square brick superstructure rising 4 feet above the floor of the building. On top of the brick walls rests a sheet of boiler-plate perforated with small holes. The hearth being below ground-level, and with the extra 4 feet of height above the floor, it follows that if ash is disturbed it is confined.

From the drawings it may be seen that the pit is prolonged to the side wall of the building, with steps leading up to the ground-level. The top and sides of the opening are made with galvanised sheeting, forming a kind of short tunnel in which the coolie may stand upright. The outer face of the brick furnace forms the inner end of the tunnel, and accommodates the door of the furnace. The bottom of the pit is filled up with clay and stones almost to the level of the bottom of the door. This ensures a very shallow hearth, and guards against an unduly large fire. Obviously it is not desirable or necessary to make the hearth of fire-bars, as was done in one instance, with the provision of a door below for removing the ash. This would lead only to a strong draught being created, with a high rate of fuel consumption.

[Ill.u.s.tration: BRICK AND CEMENT SUPERSTRUCTURE OF FURNACE INSIDE THE BUILDING, BUT FED FROM OUTSIDE.

On the top of the superstructure rests a sheet of perforated boiler-plate.

The actual fire-pit is below ground-level, and to the left may be seen parts of the sides and top of the downward approach, from the outside, to the door of the fire-pit.]

The openings can be screened by a narrow sloping lean-to, which serves to keep out rain, and provides shelter for the stock of fuel and the coolie.

The iron furnace-door should be well made, with an easily worked latch; but it is not necessary that it should be perfectly fitting. Any slight aperture will serve to provide the necessary air-inlet, but in any case it should not be more than slight.

CONSUMPTION OF FUEL.--Regarding this furnace, it may be said in conclusion that it is more satisfactory in general working than any other furnaces yet encountered. Obtaining information from over sixty estates, on the question of fuel consumption compared with output of rubber, it was found that, as far as ordinary smoke-houses were concerned, the Pataling type of furnace showed the lowest unit consumption of fuel.

[Ill.u.s.tration: GENERAL VIEW OF SHELTERS COVERING APPROACHES TO FURNACES.]

This was at the rate of slightly less than 1 lb of fuel per 1 lb of thoroughly cured sheet rubber. The figure on some estates mounted as high as 4-1/2 lbs. of fuel per lb. of rubber. Naturally this factor may have been affected by failure to utilise the drying s.p.a.ce to its fullest capacity, but in the main the high rate of consumption could be attributed solely to the deficiencies of the furnaces.

FLOOR OF FURNACE-ROOM.--As a rule no attempt is made to improve the natural earthen floor. Whether open-hearth fires, truck furnaces, or drums are employed, it is usual to find a floor with an inch or two of dust upon it.

Where all endeavours are directed in other directions towards cleanliness, it appears strange that this should be overlooked. In contrast, houses employing the Pataling type of furnace (or others) have concrete and cement floors, which can be kept quite clean. Cleanliness should be as zealously attempted in the smoke-house as in other departments.

[Ill.u.s.tration: NEAR VIEW OF SHELTER.

Steps lead downwards where the wall of the smoke-house has been removed.]

ROOF.--In any type of smoke-house, the roof should fit tightly at the eaves, and the only vent should be in or near the roof-ridge.

In an ordinary smoke-house, the opening should take the form either of a low jack-roof or of squat chimneys protected against rain. If a jack-roof is chosen, it may be so low as to need no scheme of adjustment, or otherwise adjustable swing shutters must be provided. The chimneys may be made with such low fitting between the cap and the body that no interior swinging flaps are required.

During the operation of smoke-curing the smoke vents must remain open to a degree which is arrived at by experience. Failure to provide a comparatively free egress for smoke and moisture will bring trouble in its train. After a house has been in use for some time, it will be noted that the timber becomes covered with a shiny tarry coating deposited by the smoke. If the rubber remained in the house for an equal period, it would take on the same appearance. During the interval between the entry and the exit of the rubber some amount of deposit does take place, and it is this mixture of creosotic substances which plays a part in fitting the rubber to withstand growths of mildew which would otherwise form.

If proper smoke-vents are not provided, the moisture evaporating from the sheets is unable to escape quickly enough, with the result that a great deal condenses at night-time upon the inner surface of the comparatively cool roof, and falls back upon the rubber in unsightly black "drips," which leave a distinct mark on the sheet. Even if vents are open, this may happen during seasons of rain. The temperature of the moist smoke in the roof-ridge may be as high as 130 to 140 F., while the outer atmosphere may have been cooled by rain to 70 F. Such a difference on the two surfaces of the roof must lead to condensation within the house, with consequent "dripping." It used to be the custom to drape sacking material above the bays of racks in order to prevent the drops of liquid falling upon the rubber; but often for want of renewal the last state was worse than the first. Modern houses have often an inner lining, a few inches below the roof. This is made of soft wood which receives any product of condensation and absorbs it.

OTHER TYPES OF SMOKE-HOUSE.--So far we have confined the arguments to smoke-houses of the usual type. There are others which vary in either design and method of working, or in the material of the structure. Mention may be made of the most prominent of these.

BRICK HOUSES.--Some houses are constructed of brick, and may have one or two storeys above the furnace chamber. The floors are sometimes made of ferro-concrete, and the furnaces may also be of this material. These brick houses give satisfaction, but there would seem to be some difficulty in obtaining and maintaining the desired temperature, although it is not quite plain why this should be so. The principle of these buildings is the same as that of the ordinary iron house, and the suggestions made in previous paragraphs apply with equal force.

"THIRD MILE" TYPE.--Reference has been made to the "Third Mile" type of furnace. This is an integral part of a smoke-house, which for clearness of distinction may be known as the "Third Mile" type of smoke-house, the original of which was erected on the Third Mile Estate, Seremban, F.M.S.