STICKINESS.--This is not to be confounded with "tackiness," from which the rubber does not recover. Stickiness is only temporary, and may be remedied.

As a general rule, it is due to packing sheets, which have not a good raised "ribbing," and which may have been coated with light tarry deposits (see Glaze). This surface film may be removed by washing the sheets, or scrubbing them, with cold water. Usually a further two days' air-drying will make the rubber fit for packing; and if the smoke-curing has been efficient, there should be no need to antic.i.p.ate trouble from mildew. Some estates adopt this practice daily with success, as a form of insurance against complaints of surface deposits.

RIBBING, SURFACE PATTERN.--While we know that the pa.s.sing of sheets of rubber between rolls, causing a particular raised pattern to appear, has no effect upon the actual quality of the rubber, there is a great deal of practical advantage gained.

The practice ensures an increase of superficial area which is an aid in drying, improves the appearance of the rubber for selling purposes, and is of distinct advantage in enabling the rubber when packed to travel in better condition. Sheets do not become so closely packed; sampling and general handling are easier on delivery.

As long as the plane surfaces are sufficiently and regularly distorted, there would seem to be no limits to the type of pattern or "mark" which may be placed upon the rubber. But in actual practice the variety is small. The most popular type of "ribbing" is that best described as a small diamond effect, produced by a pair of rolls cut with closely placed narrow grooves running spirally. The spirals travel in the same direction on both rolls, producing close-cut ribbing running in opposite directions on the surfaces of the sheet. On sheets of standard thickness, the result approaches a diamond effect.

A few other patterns are employed, notably that producing longitudinal stripes of varying thickness. On the whole, the type of pattern would seem to be immaterial, if the points already indicated are achieved.

It is seldom one encounters a case nowadays in which the "marking" is unsuitable, but a few estates may be using an old type of patterned roll on which the full diamond grooving is cut. As this appears on both sides of the sheet of rubber, and as the ribbing does not coincide, a blurred effect is seen when the sheet is viewed against the light.

THICK ENDS, "SHEET CLIPPINGS."--It rarely happens, even with good equipment and average supervision, that the preparation of smoked sheet is unaccompanied by slight defects. For instance, in spite of rules and regulations regarding manipulation of the coagulum, it is not uncommon to find that some sheets, after rolling, have slightly thickened ends. In the ordinary course of events these might delay drying considerably. It is the practice on some estates to cut off these thickened ends while the rubber is still wet. The pieces are then machined into crepe form, but as no sodium bisulphite may have been used, the resulting rubber cannot be cla.s.sed as No. 1 Standard Crepe.

The other alternative is to trim the ends when the bulk of the rubber is thoroughly smoke-dried. The moisture containing portions are then returned to the smoke-house until dry, and are subsequently packed without further treatment as "smoked-sheet clippings." It will be plain that, except in the particularity of form, these clippings differ in no degree from the original sheets; and, owing to extra smoke-curing, may arrive in even better condition. One must be prepared, however, to find that a slightly lower price is offered. Whether the price obtained would be comparable with that commanded by the crepe made from wet sheet slipping would depend upon general ruling market conditions, and the degree of care exercised in guarding against the inclusion of any inferior pieces of rubber. In ordinary factory practice, there could be no room for abuse under the latter clause.

OTHER INFREQUENT DEFECTS.--This chapter will be closed with a reference to other small defects which, although infrequent, cannot be cla.s.sed as minor complaints. In point of fact, when they occur, they a.s.sume an importance, in the eyes of the consumer, which is not, perhaps, sufficiently appreciated by producers.

DIRT.--Trouble from this source should be absent, but carelessness on the part of packing coolies may be responsible for occasional complaints. How the dirt is incident may remain a mystery, but it has been noted that sheets have at times been thrown upon a cement floor. A certain amount of loose dust may thus adhere to the rubber.

ASH.--The source of this surface deposit scarcely needs indication. Where open-hearth furnaces are employed, and the wire-mesh floor screens are not perfectly sound, fine ash may find its way into the upper chamber. If this trouble is persistent in spite of precautions, the sheets should be surface-washed and air-dried before packing.

BARK.--Complaints of the presence of particles of bark in sheet rubber used to be fairly frequent, but are now less common. The trouble may be traced to the use of defective straining sieves when the latex is being handled.

SPLINTERS.--The use of packing-cases of unplaned soft timber is responsible for complaints of this nature on delivery. Without here discussing the larger question of the ideal packing case, it is sufficient to emphasise that the interior surfaces of wooden chests should be planed. The cases are often so damaged in transit, that splinters of wood may be found throughout the contents. The device adopted on some estates may go far to prevent this. The cases are first lined with loose sheets, and finally other sheets are arranged to overlap at the top of the case. The bulk is thus enclosed in a wrapping of sheets, and any splinters or other deleterious substances are confined to the surface of the ma.s.s.

PART V

GENERAL

CHAPTER XIX

_CHOICE OF COAGULANT_

Almost without exception, the agent employed in the coagulation of plantation (_Hevea_) rubber is acetic acid, or in some cases formic acid.

Under ordinary trade conditions supplies are always obtainable at reasonable prices, but during the recent War the question of possible subst.i.tutes was brought greatly to the fore. Fortunately the subject of coagulation and coagulants had been previously studied to such effect in laboratory practice, that there would have been small difficulty in prescribing agents other than acetic acid in cases of expediency. As far as our knowledge extends, all the possible substances which have the power of coagulating latex have been tested. They include mineral acids, organic acids, compounds known chemically under the general term of "salts,"

alcohols, sugars, etc.

The heading of this chapter must be seen to "beg the question," inasmuch as it leads to the a.s.sumption that a coagulant (in the popular sense) is necessary to secure coagulation. In point of fact, methods are sometimes employed which depend upon no artificial coagulant to produce the desired effect. To these methods reference will be made later.

In this section it is proposed to describe briefly the more important agents which are used, or might be used, in effecting coagulation. In the cla.s.s of those which are not in common use some could be used as expedients, while others are only of scientific interest.

ACETIC ACID.--There is no need to enter into a discussion of the merits of this agent. In practice it remains the cheapest and safest coagulant known at present.

FORMIC ACID.--This agent is equally as safe to use as acetic acid, and as easy to handle. Bulk for bulk its coagulative power is higher than that of acetic acid. Its pre-war shipping price, when taken in conjunction with its coagulative power, was slightly below that of acetic acid, but local prices put the balance in favour of the latter. If prevailing costs put it on terms of parity with acetic acid, there would appear to be no reason why formic acid should not have a widely-extended use on plantations.

CITRIC ACID, TARTARIC ACID.--The acids of the extracted juices of most tropical fruits consist, to a large degree, of citric or tartaric acids.

These can be used in place of acetic acid as satisfactory coagulants in case of emergency; but the questions of availability of supplies and of costs preclude their more general adoption.

OXALIC ACID.--This is a satisfactory coagulant as far as observed effect is concerned. It produces a rubber paler than ordinary coagulants (without the use of sodium bisulphite), as it has the nature of an anti-oxidant.

It would not be a safe agent in the hands of coolies, as it is cla.s.sed as a poison.

SULPHURIC ACID.--During the War, in a period of shortage of acetic acid and of high prices, this agent was used with success on some estates.

It scarcely need be remarked that it is a dangerous substance to handle, and that its employment must be accompanied by close European supervision.

At prevailing prices during the War it was very much cheaper than acetic acid, and even at the present reduced cost of the latter the advantage still lies with sulphuric acid.

It must be emphasised, however, that the abuse of this agent to any but the slightest degree is harmful to the resultant rubber. Hence its use would be sanctioned _only in the absence of the commoner, and much safer, coagulants_.

In view of the possible incidence of such an emergency, the following notes are given. It is impressed that strict adherence to the rules must be given.

HANDLING SULPHURIC ACID.--(_a_) Always use gla.s.s or glazed earthenware vessels.

(_b_) Pour slowly and avoid splashing. Drops finding their way to clothing or other fibrous material will destroy it locally; and if thrown upon any part of the body may cause painful burns.

(_c_) When diluting this agent always remember to pour the acid into the water (_i.e._, the lesser into the greater), and never _vice versa_. Pour the acid carefully and slowly down the side of the vessel, and stir well.

(_d_) Should strong acid be spilled, do not throw water upon it. A supply of sand or dry earth should be kept close at hand. Throw this upon the acid.

STORING SULPHURIC ACID.--(_a_) Jars or cases should be handled as seldom, and as carefully, as possible. If the acid is contained in a case, the top should be plainly indicated.

(_b_) Stocks should be stored in a detached building which should not be damp. Jars or cases should not stand on a wooden floor if possible.

(_c_) See (_d_) above.

BUYING SULPHURIC ACID.--(_a_) Commercial acid of specific gravity 184 is the best of its kind. It contains impurities which are non-injurious to rubber preparation.

(_b_) It is always advisable, if possible, to buy the acid in small jars containing not more than 100 lbs. each. Smaller jars, with a content not exceeding 50 lbs., would be preferable.

(_c_) If the acid is bought in jars, it should be stipulated that the stoppers be covered with a plaster head, and that the containing crate or case should have prominent labels or marks indicating the top of the case.

FORMULA FOR USE OF SULPHURIC ACID.--It will be understood that as this formula has been calculated for working with latex, having a consistency of 1-1/2 lbs. dry rubber per gallon, it applies in a strict degree only to such latex. In other cases, where the dilution of the latex is not known, the formula will serve as a basis for experiment until the correct quant.i.ty has been discovered.

(Sulphuric acid of specific gravity 184.)

NOTE.--The directions must be followed carefully, and gla.s.s measuring vessels should be used if procurable.

(_a_) Measure out 1 pint of strong acid, and pour it carefully and slowly _down the inner surface_ of a jar containing 20 gallons of water. Do not pour it directly into the water.

The heavy acid will sink to the bottom of the jar, and a good mixture must be obtained by stirring well.