Where this is not the case it must be suspected that there has been some carelessness in manipulating the latex or the chemicals. Attention has been drawn to the fact that there may be exceptional cases, when the determining factors lie beyond the control of factory processes--_e.g._, heavy rains causing over-dilution of latex, the yielding of "yellow" latex from newly opened areas, etc. But on the whole there is now no reason why the general average product from any estate should not be uniform in colour and appearance. Furthermore, it should be possible for large groups of estates, by the adoption of uniform methods, to produce similar rubber from all the plantations. Moreover, apart from some differences caused by factors which still need determination, the total product in a general sense should not only be uniform in appearance but uniform in physical and chemical properties.

BLOCK RUBBER.--This mode of preparation is employed only in comparatively few instances. The block is prepared from crepe rubber, which has been dried either in a hot-air drier or in a vacuum chamber.

There is another type of block which is made by placing layers of dry crepe under considerable pressure. This is not the true type of block, and the layers are quite distinct--_i.e._, they do not amalgamate. Usually this pressed rubber consists of lower grades of crepe, and it should not be popular, inasmuch as it leaves too wide an opportunity for the inclusion of dirt, bark particles, and other impurities, which cannot be seen generally on account of the protective colour of the rubber.

In the true type of block, the layers are in a plastic condition, due to heat, when they leave the drying-chamber; and being immediately submitted to great pressure the result is a h.o.m.ogeneous ma.s.s in which the layers disappear by amalgamation. Only the best grade of crepe is employed, and given the absence of defects in the layers there should be no complaint regarding the final block.

Prepared in slabs which are three or four inches in thickness, the product is easily handled, and should be sufficiently translucent to make it possible to distinguish the shape of the hand when held between the block and the light. This is not possible when blocks are made of greater thickness.

The only complaints which it should be possible to lodge against block rubbers are:

(_a_) The inclusion of dirt and other matter.

(_b_) The use of layers of crepe which have some defect.

(_c_) The inclusion of air-bubbles.

The remedy for (_a_) and (_b_) lies in the hands of the factory superintendent. The last ground of complaint is dependent upon the style of preparation of the original layers of crepe.

When layers of crepe are placed one upon the other, and submitted to great pressure, it is natural to suppose that air would be contained in s.p.a.ces, and would be unable to escape. To guard against this, it would seem necessary to prepare the crepe thin and with a fairly good surface finish.

It must be obvious to all acquainted with the processes involved in the preparation of block rubber, that no possibility exists for the presence of air-bells actually enclosed _in_ thin crepe. When the vacuum-dried crepe is folded preparatory to the blocking process it is apparent that between the layers there must always be a considerable volume of air, a small proportion of which is bound to be retained owing to the nature of the surface of crepe rubber.

That this has always been true of the preparation of block rubber cannot be denied. It is possible, of course, for one type of block to show the presence of air-bells more than another type, the proportion of air enclosed in blocking depending upon the nature of the crepe of which the block is composed. A block built up of layers of smooth, fine crepe would be expected to contain less air-bells than a block composed of layers of a rough crepe.

Block rubber has been seen which was free from air-bells, but this was the thin variety of block prepared for show purposes with far greater care, probably, than would be expended in commercial preparations.

CHAPTER XVIII

_DEFECTS IN SHEET RUBBER_

Before proceeding to deal with defects in the rubber as it is put upon the market a brief account will be given of faults which may be noted in the preparatory stages.

MILKY RESIDUE OR SERUM.--If the serum is not clear after the ordinary period allowed for coagulation, it indicates one of the following possible causes:

(_a_) Failure to obtain complete mixture by thorough stirring.

(_b_) Insufficiency of acid solution. This may be real or indirectly due to the presence of an excess of anti-coagulant such as formalin or sodium sulphite.

(_c_) In cases where other coagulants than acetic or formic acids have been employed the failure may be due to an excess of, or an unsuitable, coagulant--_e.g._, hydrochloric acid.

COLOURED SURFACE BLOTCHES AND UNPLEASANT ODOUR.--Sometimes the surface of the coagulum exhibits yellowish or bluish streaks and patches. It will be found generally that the yellowish colour is possessed by a slimy substance, of offensive odour, which may be sc.r.a.ped from the surface.

Either insufficient acid has been used, or the mixing of latex and coagulant has been at fault.

DARK DISCOLORATION OF THE RUBBER.--This may be stated to be a natural process when fresh rubber is exposed to the atmosphere. It is usually described as "oxidation," and it will be noted to be absent, or to occur to less degree, on those portions of the rubber which are protected from the atmosphere by being below the surface of the remaining liquid. This surface change may be prevented (see Chapters VIII. and IX.) by the use of small quant.i.ties of sodium sulphite (for preference) or bisulphite.

SOFT COAGULUM, SPONGY UNDER-SURFACE, TEARING OF COAGULUM.--If the whole ma.s.s of coagulum is too soft, while coagulation appears to be complete, over-dilution of the latex has occurred. This may apply also to the case in which the under-surface only is spongy and soft. If coagulating-tanks are employed, the upper edge may be comparatively hard, while the lower is soft and weak. Often the spongy portion may adhere to the part.i.tions. This prevents the natural rise of the coagulum, due to retraction, as the ma.s.s "sets." The pull between the free upper portion and the adhering lower edge causes splitting and tearing of the coagulum, with marked porosity (spongy appearance). The two factors to receive attention are the standard of dilution and the condition of the surfaces of the part.i.tions. If these have minute cracks into which latex can penetrate, and in which coagulation takes place, the boards should be discarded. Given the conditions indicated above, the tearing and splitting of rubber in coagulating tanks is sometimes augmented by the practice of flooding the tanks when coagulation is judged to be complete. The surface water finds its way downwards between strips of coagulum and the part.i.tions, thus increasing the upward tension between the free and adhering portions. The main idea governing the practice of flooding the tanks is to prevent "oxidation" (darkening) of the upper edges. If a small quant.i.ty of sodium sulphite is employed as an anti-oxidant and to r.e.t.a.r.d coagulation, it is not necessary to flood tanks.

"PITTING" OF SURFACES.--In pan coagulation this "pitted" appearance is usually limited to the under-surface, while coagulum prepared in tanks may exhibit the defect on both faces. The existence of these numerous "pits,"

or small depressions, points to the presence of bubbles of gas which have been unable to escape freely. As the formation and retention of gas-bubbles is not a normal occurrence in coagulation, we are led to infer that some special conditions must have arisen. These may be supplied by one or more of the following contributory causes:

(_a_) The latex had begun to "sour" before arrival at the factory or while waiting to be treated. This premature coagulation is usually checked or diminished by the employment of anti-coagulants (see Chapters VIII. and IX.). It is generally accompanied by the appearance of enclosed gas-bubbles in the dry rubber.

(_b_) There may have been a slight insufficiency of coagulant, or the admixture was not thorough, thus allowing a slow putrefactive change to take place in the incompletely coagulated areas.

(_c_) The wooden part.i.tions may not have been effectively cleansed.

The existence of a thin slime, of bacterial origin, is sometimes noted. This is accountable for putrefactive effects in the surfaces of the coagulum, or in the serum, giving rise to the formation of gases.

If these cannot escape freely, by reason of adhesion between the coagulum and the part.i.tions, "pitting" occurs.

THICKENED ENDS OR EDGES, AFTER ROLLING.--As a rule these defects may be ascribed to the employment of too rich a latex, or faulty manipulation.

Even if the standard of dilution should be correct it sometimes happens that, in the preliminary rolling of a long strip of rubber, coolies begin in the middle, rolling with a forward pressure and tension towards the ends of the strip. This is generally not so much the fault of the coolie as being due to the lack of proper facilities for preliminary rolling. The table should be about 3 feet in height, so that ease of working is obtained merely by natural pressure due to the position in which the worker stands.

The use of a heavy wooden roller would contribute towards this result, inasmuch as it obviates the use of force, and the pressure is almost entirely in a vertical direction.

MIS-SHAPEN SHEETS.--It is sometimes noted that sheets may be wider and thicker at the ends than in the middle. Manipulation alone, as indicated above, is not solely responsible. The primary cause is to be traced to over-dilution of latex, giving a very soft coagulum which responds too readily to tension and pressure. Faulty treatment in rolling exaggerates the tendency for the strip of sheet to become narrow and thin in the middle, wider and thicker at the ends.

THICKENED PATCHES, TORN SHEETS, "DOG-EARS," CREASES.--These elementary defects are all due to careless working. While occasional errors cannot be avoided, there is no real excuse for the continuance of trouble to any degree, under average supervision.

Thickened patches are often caused in conjunction with torn sheets, and the trouble may be ascribed to faulty practice in allowing too heavy a pile of wet strips to acc.u.mulate before machining. Or a comparatively small pile may have been transported some distance. It is difficult to separate the strip, and occasionally the separation is only effected at the expense of two sheets, one of which is torn and the other has a portion of the first strip adhering to it.

"Dog-ears" due to the folding over of corners of the sheets, and creases due to the rumpling of the coagulum, are generally the result of haste and lack of average care. Machine coolies, more often than not, will not be at any pains to straighten out folds before pa.s.sing the coagulum through the rolls.

GREASINESS BEFORE SMOKING.--Under ordinary methods of working this should never be encountered. It may be taken to show that the machined rubber has been allowed to remain, either hanging or in piles, far too long before entering the smoke-house. The appearance is most marked if the rubber has remained in a cool and moist atmosphere--_e.g._, if it has been hanging over-night in a closed and badly-ventilated factory. In a marked degree this is to be observed in the preparation of air-dried sheets, unless they are exposed, when freshly prepared, to the action of the sun for a period.

This period, in the case of rubber prepared on native small-holdings, generally extends over several days--until the sheets are more than half dry.

In the preparation of smoked sheet, the greasy appearance and the cause outlined contribute to a defect which is eventually described as "stretching rusty."

SURFACE BLEMISHES.--The coagulum, during coagulation and subsequently, can be contaminated in various ways. In most cases a little intelligence or increased care would prevent the occurrence of these defects.

When the coagulum remains over-night, in the absence of a cover, it is not uncommon to note the presence of dirt (from the roof above, or blown in from the outside), the droppings of mice and rats, flies and small insects.

In theory these should be seen and removed by the factory hands. In practice, except while under immediate supervision, the extraneous matter is often rolled into the soft coagulum.

A fairly common cause of this surface contamination is the exhaust from the power-unit; generally the worst offender is a steam-engine. Grit and s.m.u.ts continually find their way into the factory, alighting on the tables, in the latex, in the water, and on the freshly prepared rubber. They are rolled into the soft rubber and lead to marked depreciation in the selling value. The radical remedy seems obvious, but is often beset with many difficulties not unconnected with financial considerations.

Other superficial blemishes, such as those due to the presence of rust marks, oil or grease patches, etc., are self-explanatory, if a little thought is brought to bear upon them; and it is not proposed here to discuss such defects more fully.

Having now dealt with certain defects which are visible in wet rubber, we come to the discussion of others which are only perceptible either during or after the drying period. As far as is known no plantations of any size now prepare sheets other than in the form of smoke-dried rubber, with the exception of a few which make a special form of thick and partially air-dried product known as "slab" rubber.

It is not proposed, therefore, to treat in any detail with air-dried sheet rubber. Certain obvious defects are common to both air-dried and smoke-cured sheets, and these will be first discussed.

UNEVENNESS OF APPEARANCE.--This lack of uniformity may refer either to size or colour, or to both. Apart from any other contributory causes, this variation is due, in pan sheet, to a neglect to standardise the dilution of all latices, or to lack of uniformity in the quant.i.ty of standardised latex placed in each receptacle.

Where tanks are employed all sheets from the same tank should be of the same size before rolling, and any subsequent disparity in thickness and length must be attributed to some alteration in the width of the gap between the rolls of the machines.