Drying-Houses for Crepe.—It has already been shown in the previous chapter that one type of drying-houses—viz., that over a factory—stands condemned, except for the drying of low-grade rubbers. Generally speaking, a great advance has been made in the design of crepe drying-houses during recent years, and it has been possible even to improve older ones so as to bring them into line with the more modern buildings. Houses for drying crepe rubber may be of one floor, two floors, or even three floors. Doubtless those built with three floors were designed with a view to economising the available site for factory buildings, and as long as the ventilation is good there can be no very great objection to them. It might be pointed out, however, that even with the best of ventilation the air passing successively through three layers of rubber must be fairly saturated with moisture by the time it leaves the building. The effect of this upon the rate of drying in the uppermost chamber will not be so marked as it will be in the middle floor, as the temperature of the top floor must be many degrees higher than that of the other two rooms. It would be expected, therefore, that the rate of drying in the middle storey would be slower than that in either of the other two.

In houses of two floors this objection would not have to be met, and drying-houses of this type are successful and common.How Many Storeys?—Again nothing could be urged against a building of two or three storeys in which the ground floor was occupied as a packing-room, except that, by negligence in not allowing wet crepe a preliminary dripping period, water might fall upon the packed rubber below.As a matter of experience, such a house is, taking all into consideration, the cheapest and most suitable type for any estate with an increase in output. Even at the outset there should be a separate room in which sorting and packing is undertaken. This is conveniently the lower room of a drying-house. The only stipulation to be made for a house with two storeys is that the floor of the upper room should be of an open pattern, so that the air may circulate right through the building. This is usually and very successfully attained by laying down wide slats of wood, with spaces of an inch or more between them. It is not advisable to have spaces wider than 11/2 inches, otherwise there is a certain amount of danger to the limbs of individuals who have to work or supervise in the building. In any case, it is very convenient to have pathways of planks running the whole length of the floor, so that the supervision is made more convenient. If this is done, there can be no objection to the custom of suspending the rubber of a lower chamber from the slats of the floor of the upper room. At present, in some drying-houses, this means of suspension is used, but no planks are laid down, and it becomes necessary to walk over the drying rubber. This is a detail, but it is one which does not make for the improvement of rubber, and the expenditure of a small sum would be sufficient to rectify the matter.

From every point of view, it would be desirable to have the floor of the packing-shed (or the packing-room in a combined house) raised from the ground, to a height of, say, 3 feet; or the height of a bullock-cart or motor-lorry. Not only is ventilation improved, but there would be a great saving in labour. Packed cases could be wheeled directly on a level with the cart or lorry.

A great many estates favour drying-houses of one storey. These are eminently suitable, provided that the site is suitable, and that the relative dimensions of the house are favourable to efficient ventilation. It is a common mistake to find buildings of which the breadth is out of proportion to the height. Obviously, if the height is not considerably in excess of the breadth, ventilation will be defective. For a single-storey drying-house, the maximum height should bear the ratio to the breadth of 3:2, and in a house of this type specially long pieces of crepe can be utilised. Naturally, in a house of two storeys, this factor is not likely to be neglected, and if the lower room is used for packing purposes the rate of drying should be rapid. Again, when a single-storey building is contemplated, it is well to make strict examination of local conditions. If the site is low-lying and surrounded by trees it will be clear that tall buildings are required, and that a house of more than one floor is to be preferred. Considerations of this nature would have prevented the erection of some dry-sheds which do not give a satisfactory rate of drying.Ventilation.—No matter how many floors there may be in a drying-house, the greatest attention should be given to the question of ventilation. It is an elementary point in the study of ventilation problems that the best system of natural ventilation is obtained by admitting cool air near or through the floor and providing an exit for the warmer air at the highest point in the building. It is not often that such a rule is infringed in the ventilation of rubber drying-houses, but several of the older buildings erred in this respect. In a good modern house there is a space (about 2 feet in height) all round the base of the walls merely closed with expanded metal; this admits cool air. An exit for warm air is provided in the ridge of the roof by either ventilation chimneys or by a jack-roof. The latter is preferable, as it provides for a more free and uniform escape.

In some drying-houses, besides the ridge openings, the space along the eaves is left open. This would seem to be undesirable, as it provides for the entrance of outer air, which might combat the ascending warm air and so interfere with the natural upward currents. Provided that a jack-roof or other suitable openings have been installed, there is, therefore, no necessity for the existence of open spaces at the eaves, and they probably do more harm than good.

In the tropics, on days of sunshine, there must always be an upward current of air in well-designed houses. Temperatures of 105° F. are easily recorded in the ridge space of a building, while the temperature in the lower part of the house may be at least 15° F. lower. On the floor of an upper room a temperature of 90° F. is commonly noted, and in buildings with three storeys the usual day temperature of the top room is about or over 100° F. Even, therefore, when there is no trace of a breeze, there must be a displacement of air in an upward direction, though it may not be detected without tests being applied.

It is often asked whether a temperature of 100° F., such as is obtained in the upper room, is calculated to injure the quality of the rubber. There need be no fear on this ground; the experience of many estates goes to show not only that no harm results, but also that the drying of the rubber is expedited. There would seem to be no reason why crepe rubber should not be dried at a temperature of 100° F. It must be understood, however, that higher temperatures for crepe rubber are not recommended, as it has been proved that the rubber is affected. The fact becomes obvious with continued treatment at temperatures much above 100° F., for the rubber stretches and breaks across the support.Windows.—Concerning the subject of window space in a drying-house, there has been much discussion at various times. Years ago it was common to find windows widely open with the sunshine streaming in. Naturally, tackiness developed in some of the rubber, and care was then taken to keep the windows closed. Thus the rooms were darkened and air excluded. There followed a period in which windows were fitted with ruby-coloured glass to keep out the actinic rays of the sun, which were responsible for tackiness, and excess of light, which was supposed to be responsible for the rapid oxidation of rubber. Unless special precautions were observed in the processes of coagulation and preparation, it was not proved that the exclusion of light prevented or lessened the natural oxidation of crepe rubber. Since the introduction of sodium bisulphite for the prevention of oxidation, there has been no cause to worry as to the possible effect of light, as no perceptible darkening of the rubber takes place. It follows, therefore, that no trouble need be taken to exclude light, although the necessity for excluding direct sunshine still exists. Windows may be left open as long as the sun does not reach them. This can usually be arranged in a drying-house by manipulating the windows at intervals during the day, so that those in the shady side of a building are always open, while those on the sunny side are always closed. If it is thought that this manipulation cannot be entrusted with success to the store coolies, the case may be met by having all windows constructed on the louvre pattern, so that, although the windows are closed all day, air and light are not excluded. Should it be desired to retain the existing type of windows, which open outwards, and to keep them open all day, a simple arrangement of ruby-coloured cloth on an outstanding wooden frame may be placed within the walls of the building, or the shutters of the windows may be hinged at the top to open outwards. Unless there is a pronounced breeze, or it is required to examine the rubber closely, there is no necessity to have windows open, except in the case of a house in which the bottom floor is used as a packing-room. The windows of this chamber may remain open during the day, to advantage in sorting and packing, and also to the proper ventilation of the building. Thus the direct rays of the sun are rendered harmless, while air and light are allowed to enter.Hot-Air Drying-Houses.—Mention has already been made of the existence of a system of drying in which hot air is forced into a drying-house by means of a powerful fan. Provided that the temperature of the hot air could be so regulated as not to exceed 100° F., there would be merit in the system. Such matter of regulation could be solved by having a duct in the main air passage, through which cool air could be admitted in such proportion as to modify the temperature of the hot air. As the process is worked at present, the temperature attained is often well above 100° F., and there is a danger of thin crepe placed in this house over-night being found upon the floor in the morning. Unless the crepe is prepared thick and cut into fairly short lengths, it will not bear its own weight at higher temperatures; and if it is made thick, drying is impracticably prolonged. It is probable that, with a temperature of 100° F., and a steady current of air, average thin crepe would dry in such a drying-house within six or seven days. This would be an improvement upon the usual rate of drying in most factories, although several ordinary drying-houses are known in which thin crepe will dry naturally in that period.Smoke-Houses.—No discussion of theoretical considerations regarding the process of smoke-curing will be attempted here. We are concerned only with the necessity for supplying a demand for smoke-cured sheet rubber. Broadly, the process is akin to the smoke-curing of herrings, and the objects are much the same—viz., (1) drying, (2) preservation—except that while herrings are only dried partially, rubber should be dried perfectly.

On a small scale a primitive smoke-house could be built easily and cheaply, and such a building might be fully as efficacious as the most elaborate and expensive installation. In the early days of estates it was not uncommon to see temporary smoke-houses constructed of wood, and roofed with “attaps” (palm leaves). Some of the best rubber in the market has come from wooden buildings, but naturally the risk of destruction by fire is considerable.

For imperative reasons it may be sometimes found necessary to smoke rubber when the only available building is a single-storey one. As a temporary measure, the building may be converted into a smoke-house by placing the fires in pits sunk deeply into the ground, and effectively screened above by iron baffle plates. But it is not advisable that smoking be continued in such a single-storey building, as the best effects are not obtained, and the risk of fire is far too great.Usual Types.—At first sight it would appear that the best type of smoke-house would be one consisting of a tall building, covering a comparatively small superficial area, and having a number of superimposed chambers in which the rubber could be hung to dry. In practice there are several solid objections which limit the height and the number of floors. Chief among these is the question of temperature. If smoke-curing is to be effective, a certain temperature must be attained and maintained. To obtain such results in a house of excessive height would be difficult, if not impossible, under normal conditions. It would be found that the chamber immediately above the furnace-room would be overheated if the temperature in the upper rooms was within the desired range, etc.

Until recent years smoke-houses could be classed as belonging to one of two types:

The number of the former existing at the present time must be very small, as it has been shown that the arrangement of the furnace outside the house is unsatisfactory in comparison with the other type of house. In discussing the question of smoke-houses, therefore, it will be understood that the standard type accepted is that having an internal furnace. In its original form it was known as a “Kent” drier, and consisted of a tall two-storey wooden building. The walls of the lower chamber had the form of an inverted and truncated pyramid. By this arrangement it was possible to obtain from a comparatively small fire sufficient smoke and heat to cure the product placed in the room above. This is the principle upon which many smoke-houses in Malaya are designed. On a very large scale it is not claimed that the sloping sides of the lower chamber lead to economy in the number of fires, but merely divert the smoke in an upward direction. It is acknowledged that vertical lower walls are quite effective, and it is an easier matter to fit in doors.It may be noted that the usual type of smoke-house now in general use consists of a building of two storeys, in the lower of which are situated the furnaces, while rubber is hung on racks in the upper room. Sometimes there may be a third storey, also used as a drying (curing) chamber. As a rule the drying-room is one long unit, as also is the furnace chamber; but in some cases they are subdivided by vertical partitions into smaller chambers, for ease of working and better control. This applies with some force in the case of very long houses standing in an open space. It is sometimes found in such cases that at certain seasons the prevailing winds have the effect of making drying and curing uneven in parts of the building.With these exceptions, the ordinary type of smoke-house functions very efficiently, and is capable of drying average sheet (from standardised latex) in a period ranging from seven to eleven days. Should the building not be capable of such performance, in spite of the strict observance of all rules laid down for the processes of preparation, then there is some defect in ventilation or in the distribution of heat.General Ventilation.—The ordinary rules of ventilation in drying-houses apply equally to a smoke-house. There should be a slow current of air and smoke from the lowest point to the highest point in the building.

In spite of all that has been written on this subject, it is by no means uncommon to encounter the idea that a smoke-house should be perfectly closed in order to get good results. As to what must become of the (say) 25 per cent. of moisture which the rubber contains there is no knowledge. In dozens of cases, when complaints regarding slowness of drying have been investigated, it has been necessary to point out the need for providing a rational system of ventilation.

Naturally only a slow current of air and smoke is required. The creation of an appreciable draught would have the effect of increasing the fuel consumption of the furnaces, raising dust from the ash, and of causing a temperature higher than that which is known to be desirable. It will be clear, therefore, that if there are to be any openings at the base of the walls they should be small in area, and should have some device by means of which the current of air can be efficiently regulated. In the usual case the construction of the building is not calculated to render it air-tight, and the necessity for providing special air inlets does not arise.Windows.—Windows are not strictly necessary, and are only intended to be of service during the time in which coolies are at work within the building. The operations of examining rubber, turning sheets, removing dry rubber, cleaning racks and floors, and putting wet rubber into position, usually occupy some hours daily. During this interval the windows should be widely opened if the weather is favourable, and should remain so until the fires have been lighted. It should not be forgotten that during the heat of the day quite an appreciable degree of drying is possible. Advantage can be taken of this; but there is no necessity to extend the interval unduly, and it is of greater advantage to proceed with smoke-curing when the work in the drying-chambers has ceased.Racks of Supports.—Still referring to the usual type of smoke-house, it may be remarked that in the upper room bays of racks run at right angles to a central passage down the length of the building. Narrower passages run between the bays of the racks to facilitate ease in handling and inspection. The wooden supports may be placed about 3 inches apart horizontally, and 15 or 18 inches apart vertically. A full bay of racks should contain nine or more lines of support in each of the planes which are 15 or 18 inches apart vertically. The number of these planes is governed only by the height of the room, measured from the floor to eaves. The supports should be of smooth timber, and need not exceed 11/2 inches square in section.

It is usual and advisable to smooth off the rectangular edges of the supports or bars, to avoid the incidence of splinters of wood adhering to the rubber. The bars should not be fixtures, but may either be accommodated in slots, or may rest between two nails, so that it is possible to give them a rotary motion by turning the projecting ends. This practice is followed in smoke-houses, the idea being to move the drying sheets slightly each day, with a view to the prevention of a pronounced mark across the sheets.

Care should be taken to see that the vacant racks are thoroughly cleaned before fresh rubber is placed upon them, otherwise a distinct dirty mark is caused across the middle of the sheet. This mark usually cannot be removed, even by scrubbing with water. Where this mark occurs regularly in all sheets, attention should be turned to the openings beneath the bays of racks, if open fire furnaces are employed. It will generally be found that gauze of too wide mesh has been fitted. This should be removed or covered with a finer gauze.

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 space 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 quantity 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 quantity 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 assume 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 illustration (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.