This is usually achieved by making a very thin crepe and rolling together two lengths when wet. The resulting crepe may be slightly thicker than ordinary, and the method employed may be usually detected by the appearance of the edges unless these are trimmed.
GREENISH AND TACKY STREAKS.--Occasionally one meets cases in which pale crepe exhibits streaks varying in colour from a decided green to an almost black in which the greenish tinge is scarcely perceptible. Experience indicates that these streaks are much more dangerous than they appear superficially, inasmuch as they contain traces of bra.s.s from the "liners"
of the bearings. The presence of the copper in bra.s.s is responsible for a gradual disintegration of the rubber, commonly recognised as "tackiness."
In fact, copper may be said to be a "poison" to rubber, and every effort should be made to avoid possible sources of contamination. The effect may be proved easily and perceptibly by fastening together several pieces of crepe rubber by means of a bra.s.s "paper-fastener." In course of time a salt of copper, green in colour, will be formed, and it will be found that the portions of rubber in contact with the fastener have "perished" and become tacky.
This contamination of crepe rubber may take place in two ways:
(1) By the ejection of actual particles of bra.s.s from the bearings of machines, due to eccentric grinding of the "standards" of the rolls upon the bra.s.s "bushes." These particles are carried by exuded oil or grease into trays, and thence to the rubber.
(2) By the action of an acid lubricant upon the bra.s.s, with the formation of a metallic soap which has a decided green colour, unless obscured by the dark colour of the oil or grease. It is transferred to the crepe rubber in the manner indicated above.
The inevitable effect, apart from the superficial defect, is incipient tackiness. The extent to which this may develop will depend upon the amount of the copper compound present, but it should be remembered that an exceedingly small trace is capable of causing a disproportionately large amount of damage. This effect is further magnified if the "tacky" piece of rubber is packed in close contact with previously unaffected rubber.
When the defect is discovered, the affected portions should be cut out, and the cuttings should be burned. To mix them with the lowest sc.r.a.p grades, as may be done thoughtlessly, is only inviting further trouble.
Besides the source of danger already indicated, it may be found, but far less frequently, that contamination may arise from the presence in the rubber of small pieces of the bra.s.s mesh which is generally used for straining latex.
The view appears to be held in some quarters that these tacky streaks and patches in crepe rubber may arise from contamination with oil or grease alone. This does not agree with our experience. An experiment was made to test the point using fresh oil and grease drawn from drums in stock, specimens of the same lubricants to which traces of a copper salt were added, and samples of lubricants taken from the bearings of several machines. The treated pieces of rubber were placed in contact with untreated pieces of crepe which served as "blanks."
Notes were made at intervals extending over a period of two years. The conclusions arrived at were:
(1) Although there was surface discoloration, no tackiness had been caused by fresh (unadulterated) lubricant; neither were the "blanks"
affected.
(2) In the majority of specimens upon which had been smeared a small streak of lubricant taken from the bearings of machines, tackiness had supervened, and had developed likewise in the contact "blanks."
(3) In all cases where a trace of copper salt had been used to adulterate the fresh lubricant, tackiness was to be noted in the course of a short period (a week upwards) after the rubber was dry.
Development was slow, but progressive, over the full period of experiment, and the "blanks" in contact were affected. The degree of affection was determined by the proportion of copper salt employed. In the worst cases the affected strip of rubber had deteriorated and disintegrated to such a degree as to cause a distinct longitudinal gap, the edges of which appeared to consist of a moist gummy substance of a deep syrup colour. The adjacent blanks in some cases exhibited a similar appearance in lesser degree, or were merely affected by a characteristic brownish stain.
These observations regarding the possibility of damage to crepe rubbers from the existence of bra.s.s "liners" or "bushes" in the bearings of the machines lead to the natural query as to whether the use of bra.s.s is necessary. Experience shows that it is not necessary. Machines in use for years have been running with plain bearings of iron or other metallic substances. Satisfaction is obtained without the use of bra.s.s.
COTTON AND OTHER FIBRE.--One of the most frequent complaints made against low grade crepes is the presence of fibre--generally cla.s.sed in a wholesale fashion as "cotton-waste."
It is true that some years ago most of the complaints were genuine in referring the cause to cotton-waste. The defect arose chiefly owing to the careless use of this material in the factory. Lumps of waste when discarded were often thrown to the ground, and became mixed with the heaps of sc.r.a.p rubber and bark-shavings awaiting attention. The fault was one of sheer negligence, and nothing can be advanced in extenuation. Even when the soiled waste was thrown into the external drains, it often returned to the factory mixed up with the sc.r.a.ps of rubber recovered by means of the drain-screens.
As far as the complaint concerns itself with cotton-waste only, the remedy is plain, and lies in the power of the management by reason of the ability to restrict the use of "waste" only to the engine-drivers and mechanics.
In the vast majority of cases, however, the defect arises from circ.u.mstances beyond the direct control of the factory, and under conditions which make it difficult to check the evil. Although against instructions, and for the purpose of fulfilling other orders, some coolies persist in using pieces of cloth for cleaning cups. In course of time, unless the practice is detected, this cloth becomes coated with rubber.
Careless coolies throw it away, when it may be collected by the individuals who gather earth-sc.r.a.p; or it may be brought into the factory in the tappers" sc.r.a.p-bag.
Cases have been known in which the fibrous matter observed in the dry crepe rubber was of such a nature as to indicate that the source might be attributed to leaf-stalks which had pa.s.sed through the sc.r.a.p-washer. It is an easy matter to condemn the sorting as being careless, but it is another matter to instil into the mind of factory coolies such a respect for easy and sane precautions that the practice of them will be continued when the eye of the supervisor is not fixed upon the workers.
It will be clear that contamination by fibrous matter should be limited practically to the lowest grades of rubber.
The appearance of cotton-waste in high-grade crepes must be most unusual, and the writers have not yet seen a case in a drying-house. That it does occur, however, seems to be evident from brokers" reports. It is extremely difficult to imagine how the waste enters the rubber. One possible explanation is that a coolie may have been cleaning the rolls surrept.i.tiously with waste, which may have pa.s.sed later into the rolls together with rubber. Another explanation was offered in one factory by the observed fact that coolies engaged in cutting up coagulum, ready for pa.s.sing into the machines, kept a wad of waste for the purpose of keeping the knife-blade clean. This may have found its way into the rolls. It must be recorded that in the course of many years of experience no case has been seen in any drying-house of contamination of the higher grades of crepe by fibrous matter.
BARK AND GRIT.--With ordinary machines and the usual process of working, it would seem impossible to wash and macerate some of the sc.r.a.p rubbers sufficiently to free them entirely from bark. This applies specially to the grade of rubber prepared from bark-shavings. Specimens have been handled in which it was practically impossible to detect bark, but in such instances the amount of working necessary would be such as to interfere seriously with the regular working of the factory. Even with the employment of special sc.r.a.p-washing machines, complaints of the presence of bark in dry crepe have been received, but it is certain that this mode of operation reduces the quant.i.ty of bark to a minimum. While fully realising that the amount of working it is possible to give in proportion to the existing machinery and the output per day is limited, it must be recognised that the working of lower grades of rubber is usually insufficient, and that where possible it is the duty of estates to pay more attention to these lower grades. A considerable improvement in this direction has been noticed of recent years. It is not uncommon to encounter managers who fail to appreciate that complaints regarding the presence of bark in the lower grades are founded on legitimate grounds, and that they are not frivolous objections put forward for the purpose of depressing the price of the article. The sooner such an idea is jettisoned the better. There would appear to be a good future demand for the lower grades, and it is only natural that consumers will be willing to pay the best price only for an article which is clean.
The same arguments apply to the complaints regarding the presence of sand and grit. The quant.i.ty of the latter found in low-grade crepes from some estates is surprisingly high. Its presence can often be shown by the simple device of spreading a piece of crepe over the upturned and hollowed palm of one hand, while striking the rubber with the other hand.
The incidence of bark in higher grades of crepe may be due to inadvertence or to gross negligence. In the former cla.s.s one might put those occasions on which pieces of bark are embedded in lumps of naturally coagulated rubber. A piece of bark-shaving may fall unnoticed into latex and be partially responsible for the coagulation which takes place. This piece of coagulated lump may be ma.s.sed with others, and hence, unless each small piece is cut up, the bark is not perceived. Or again, by some unknown means, a piece of shaving may drop into a jar of latex, and so become embedded in the coagulum. Sometimes this becomes evident on cutting up the rubber, but it is quite as likely to pa.s.s unseen. On the whole, the presence of bark in first-grade rubber is most unusual, and should be seen before the rubber is packed.
In the cla.s.s due to negligence may be included cases in which careless coolies place the cup upon the ground before tapping. Pieces of shavings fall into the cup, and coolies are too lazy to pick them out. More often than not coagulation in the cup is caused. As it is impossible for the European staff to supervise each individual tree tapped, some cases must continue to pa.s.s unheeded. Sometimes bark-shavings are brought in with the latex, and if a broken sieve is being used, these, with other impurities, pa.s.s into the jar, and are embedded in the coagulated rubber. This must be cla.s.sified as negligence, for no manager would willingly allow the use of a broken sieve. Again, naturally coagulated lump rubber on arrival at the factory sometimes contains evident pieces of bark, leaves, and stems of leaves. For lack of supervision the average coolie would not think of picking out these obvious impurities, and would pa.s.s the whole ma.s.s into the machines.
OXIDATION STREAKS.--Since the introduction of sodium bisulphite defects due to streaks, caused by portions of the coagulum becoming oxidised, have practically ceased to exist. In the usual course, and without the use of an antiseptic agent, the freshly coagulated rubber has a surface darkened by oxidation. Unless this dark surface were carefully cut off, there would result a crepe containing dark streaks caused by the mixture of the oxidised surface portion with the bulk of the paler coagulum. The presence of oxidation streaks in No. 1 crepes, now being made, would imply either that no anti-oxidant substance was in use, or that the quant.i.ty necessary to prevent this surface oxidation is exceedingly small. Although the price obtained would appear to be influenced by the presence of oxidation streaks, no evidence can be obtained that the actual quality of the rubber suffers to the same degree as does the appearance--_i.e._, there is no evidence to show that a pale rubber, in which surface oxidation has been inhibited, is intrinsically superior to one in which slight natural oxidation has been incident.
"YELLOW LATEX" STREAKS.--This appearance of "yellow-latex" streaks in not common, and may be accounted for by incomplete mixture of two different latices. It is a fact of common observation that, when a new portion of bark is being tapped for the first time, there is a distinct yellow tinge in the latex excluded. As tapping progresses, this colour vanishes; usually it may persist for a period varying from two weeks to more than a month.
Should this latex be poured into ordinary latex without thorough mixing, it is sometimes found that, when the crepe rubber is dry, there are distinct yellow streaks. It should be remembered that, as the rubber content of the latex from first tappings is high, this latex is lighter than latex which is more dilute, so that the mixed latices must be well stirred with a broad paddle to obtain intimate mixture. It would be much better to keep yellow latex apart, and coagulate it separately, if at all possible. In such case the resultant crepe may be of a distinct canary yellow in tint.
In sc.r.a.p-crepes of the higher grade this distinct yellow colour is often visible in streaks which indicate the presence of tree-sc.r.a.p, etc., obtained from recently opened tapping areas.
BISULPHITE STREAKS.--These, again, arise from defective mixing. In the dry rubber it is seen that there are streaks of colourless rubber in a general ma.s.s, which may be of varying shades of yellow; or, a length of exceedingly pale rubber is apparently streaked in patches with a darker shade of colour. A solution of sodium bisulphite is heavier than latex, and there would be a tendency, therefore, for the chemical to sink in the large mixing jar. Unless stirring is thorough it is possible that portions of the latex would not be in contact with sodium bisulphite while others receive more than a fair share. Especially would this effect be seen where coagulation takes place quickly, and experience bears out the truth of the suggestion. Another factor which has some bearing on the point is the strength of solution in which sodium bisulphite is used. In the ordinary course of working, the acid coagulant is added immediately after sodium bisulphite has been stirred in. Should a strong solution of the bisulphite be used, and if coagulation takes place quickly, it is easy to see that the possibilities of obtaining a uniform and intimate mixture are small.
Probably in no factory is the sodium bisulphite now added to latex in powder form, but it has been found that if care is not taken to see that all the bisulphite has dissolved before the solution is added to latex streaks may result in the dry rubber. The undissolved particles sink to the bottom of the coagulating jar or tank, and there slowly dissolve, forming local strong solutions. The effect upon the rubber in the vicinity of these strong solutions is much more marked than in the bulk of the coagulum, and hence lighter streaks or patches appear in the dry rubber. In spite of apparently complete mixture by good stirring, it will be seen that it is possible, therefore, to have failed in this direction if any undissolved powder remains in the solution of sodium bisulphite.
"SPOT" DISEASE.--Few managers of estates preparing pale crepe rubbers are unacquainted with this defect. It is manifested by the appearance of small coloured spots varying in density (_i.e._, number to a unit area) and differing in hue. The most common colours are black and orange, but "spots"
of brick-red, yellow, violet and ruby and green tints have been noted, the last named very seldom. Sometimes in place of definite "spots," or colonies, the colour is spread over practically the whole surface of the rubber as a "flush."
These coloured spots, or "flushes," indicate infection by minute fungi, which are present in the latex prior to coagulation. The infection of the latex takes place in the field by means of spores, which are only visible with a microscope.
It is not feasible to discuss any method of preventing this infection of latex by air-borne spores, as the eventual preventive measures are so simple. But it may be believed that under ordinary weather conditions most latices are infected before reaching the factory. It is likewise true that even fine pale crepes shipped in perfect condition may contain possibilities of trouble in the form of "dormant" spores, the development of which may commence and continue if favourable conditions arise.
The subject of ""Spot" Diseases" has been treated fully in previous publications,[19] and it is not proposed here to enter into any lengthy discussion.
[19] "Preparation of Plantation Rubber," Sidney Morgan, 1913. "Spotting of Plantation Rubber," Keith Bancroft, 1913; Bulletin No. 16, F.M.S.
Department of Agriculture. "Spotting of Prepared Plantation Rubber," A.
Sharpies, 1914; Bulletin No. 19, F.M.S. Department of Agriculture.
If any reader is desirous of producing the defect experimentally, all that is necessary is to prepare a piece of crepe rubber of rather more than ordinary thickness, roll it up while wet, and place aside for some days.
This experiment reproduces the conditions favourable for the development of the spores, and spots of various colours may result. It will be clear that the chief factor influencing the result is the continued presence of plenty of moisture.
This condition may be created inadvertently in the course of factory practice, if piles of crepe rubber are allowed to remain for any appreciable period before hanging to dry. For this reason batches of wet crepe should always be placed on edge, to allow free drainage of surface moisture, if the rubber cannot be taken at once to the drying-sheds.
[Ill.u.s.tration: THREE SPECIMENS OF FINE PALE CREPE SUFFERING FROM "SPOT"
DISEASE.]
The condition also is provided if the thickness of the crepe is excessive. In some factories, having no smooth-roll finishing machines, the crepes may have a distinct raised pattern upon them. It is usual to note that if "spot" disease appears in such crepes, it is incident to much greater degree in the thicker portions of the rubber--_i.e._, upon the raised pattern.
The direct connection between the rate of drying and the appearance of coloured spots or flushes is thus established, and it only remains to adopt precautionary measures which will lead to an avoidance of delay (1) between machining and hanging, (2) in drying.
It is indicated, therefore, that, if spot disease is to be avoided, the prime consideration is the preparation of a thin crepe which will dry quickly under average conditions. It may sometimes happen that even very thin crepes will sometimes be found affected on some estates. In such instances, it will be found that the design or situation of the drying-house is at fault, and that specially favourable conditions for the development of the fungi have been created by excessively wet weather.
Should the trouble persist in spite of the preparation of the thinnest crepe, it would be advisable either to abandon this form of No. 1 product or to consider the installation of artificial aids to drying.
We have not yet encountered any case in which it was found necessary to treat the latex with an antiseptic or disinfectant substance for the prevention of "spot" disease. There appears to be an idea held in some quarters that sodium bisulphite may be so employed as a fungicide. This does not agree with our experience, which is confirmed by Sharpies (Bulletin No. 19, F.M.S. Department of Agriculture).
In 1913 experiments with chinosol were undertaken at the Pataling Laboratory of the Rubber Growers" a.s.sociation, and an account of the method of treatment was given in a printed report issued to subscribers. Dr. P.
Arens,[20] of the Malang Experimental Station (Java), has also recommended the use of chinosol. The substance is expensive, but is effective in very small quant.i.ty. On the whole, given average conditions in factory practice, such aids should not be necessary, and where keen supervision is not available may lead to other difficulties.
[20] "Guide to the Preparation of Rubber," Arens, 1918; Communications from the Experimental Station (Malang, Java).