Bark and Grit.—With ordinary machines and the usual process of working, it would seem impossible to wash and macerate some of the scrap 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 scrap-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 quantity 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 quantity 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 class 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 massed 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 pass 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 class 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 pass unheeded. Sometimes bark-shavings are brought in with the latex, and if a broken sieve is being used, these, with other impurities, pass into the jar, and are embedded in the coagulated rubber. This must be classified 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 pass the whole mass 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 quantity 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 scrap-crepes of the higher grade this distinct yellow colour is often visible in streaks which indicate the presence of tree-scrap, 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 mass, 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.