(_a_) Dissolve sodium bisulphite in water at the rate of 1 lb. to 10 gallons.

(_b_) Of this solution use 1 gallon to every 10 gallons of latex.

MAKING A SOLUTION.--The making of a solution of the chemical would seem to be a simple matter, but to judge by the ill-effects sometimes observed in the dry rubber the simplicity of the operation appears to have been overrated. Great care must be exercised in preparing the solution, and the work should not be left to the few minutes preceding its actual requirement; such has been found to be the case in several factories, so that it is not surprising if the rubber is defective.

The powder should be added gradually to water with thorough stirring, which should be continued for five minutes at least. Even then there may often be seen at the bottom undissolved particles, sand, and other impurity. It is necessary, therefore, in such cases to decant the solution through a piece of cotton cloth before using. No solid particles should be allowed to enter the latex.

ABUSE OF SODIUM BISULPHITE.--It is now generally recognised that the abuse of sodium bisulphite, in the form of an excess, leads mainly to delay in the period of drying and the production of an overpale rubber.[5] It is probable that few estates, if any, now experience trouble due to this non-observance of the rules and quant.i.ties laid down for use.

[5] "The Preparation of Plantation Rubber," Morgan, 1913, p. 74.

RESIDUAL TRACES OF SODIUM BISULPHITE.--The prolongation of the drying period was attributed to the fact that traces of substances caused by the decomposition of sodium bisulphite remained in the rubber if the rubber were not sufficiently worked and washed on the rolls. These traces must have been very minute, but they were sufficient to r.e.t.a.r.d the progress of drying. That much depended on the care exercised in washing is evident from the fact that samples prepared with varying quant.i.ties of the chemical show varying results on extraction. These samples were tested for the presence of sulphates. Of the series tested that sample prepared with bisulphite in the proportion of 1 part to 600 parts latex showed only a trace of sulphate present; while the one prepared 1:2,400 gave an equal quant.i.ty.

Intermediate samples contained no trace of sulphate. On the whole, therefore, the presence of sulphate in crepe rubber is advent.i.tious, and properly washed crepe prepared with moderate quant.i.ties of bisulphite may be taken as free from any residual quant.i.ties. Meanwhile there cannot possibly be any doubt of the advantages gained by the use of sodium bisulphite, and it would not be very wide of the mark if the statement were made that, in the event of this chemical being discarded, most contracts for pale crepe could not be fulfilled.

SODIUM SULPHITE.--It would not be amiss to insist upon the point that while the nature of sodium _bisulphite_, as employed in the preparation of rubber, is anti-oxidant, sodium sulphite is employed chiefly for its anti-coagulant property. It is not used, therefore, in the making of crepe rubber, but is of service in the preparation of sheet rubber, where the aim is to keep the latex in good fluid condition as long as is necessary, and to r.e.t.a.r.d coagulation slightly so that enclosed bubbles of gas or air may escape. Formulae have been given for its use in the field when required. On some estates this practice is not found necessary, but a quant.i.ty of solution is always placed in the bottom of the reception vessels prior to the straining of latex into them. Only a small quant.i.ty is used, and as a working basis the following formula may be adopted:

_Sodium Sulphite: For Use in the Factory._

(_a_) Dissolve 2 ozs. of anhydrous sodium sulphite in a gallon of water.

(_b_) The gallon of solution, placed in the bottom of the reception jar or tank, is sufficient for the treatment of 40 gallons of standardised latex (1-1/2 lbs. dry rubber per gallon).

The warning previously given regarding the necessity for thoroughness in the preparation of solutions is here reiterated. Stirring should be thorough, say for five minutes, and if there is any sediment or undissolved matter the solution should be strained through cloth before using.

Where uniform jars or tanks are in use, the majority of which will contain uniform quant.i.ties of latex daily, the practice of using the chemical can be made almost fool-proof even in the hands of coolies. A calculation is made of the quant.i.ty of powder required for each vessel daily. The necessary number of lots is weighed out each morning and each placed in an envelope. The process is thus simplified by the fact that the contents of an envelope, neither more nor less, are required for each unit reception vessel. Even the weighing can be done by a coolie if he is given a counterpoise (of lead, for example) equivalent to the required weight.

It will not be found necessary to do any vigorous stirring of the solution with the latex, as the latter is strained into the solution and the continued addition of successive quant.i.ties is sufficient to give a good mixture.

USE OF FORMALIN.--Few estates now use formalin (formaldehyde) as an anti-coagulant. It must be acknowledged that when not abused there are points in favour of its employment in preference to sodium sulphite, but these are outbalanced by certain disadvantages. The argument may be stated thus:

_Points for_: (1) If made up freshly it is an effective anti-coagulant.

(2) Formalin being the solution of a gas in water, there is no residual substance left in the rubber to delay drying.

(3) Its use gives a bright clear rubber.

_Points against_: (1) Its cost at all times is greater than that of sodium sulphite.

(2) If the jar is not sealed there is loss by evaporation, thus increasing the cost.

(3) Its effect upon the rubber is uncertain. Even in normal quant.i.ty it is said to cause "brittleness" or "shortness."

Certain few estates, however, have continued its use, and no trouble is claimed to ensue. The following formula is stated to give satisfactory results in the preparation of sheet rubber, when applied as in the preceding paragraphs bearing on the employment of sodium sulphite:

_Formula for Use of Formalin (Formaldehyde)._

(_a_) 1 pint of formalin is diluted with 5 gallons of water.

(_b_) Of this solution 1 gallon is required for 50 gallons of standardised latex.

In noting this formula the writer gives no recommendation regarding its use. Whatever may be the actual facts regarding the effect of formalin upon the vulcanisation of rubber, when used in minimum proportions, there can be no question concerning its injurious effect if used in excess. Beyond this the factors of cost and loss militate against its wider employment.

CHAPTER IX

_PREPARATION OF SHEET RUBBER_

PALE SHEET.--The first form in which plantation rubber was prepared was as "biscuits" or sheets. This form remained in favour for some years. The first biscuits or sheets were rather dark in colour owing to the natural oxidation which followed. Then it was discovered that by diluting the latex the degree of oxidation was diminished, and later it was found that if the soft coagulum were placed in almost boiling water for a short time the resulting rubber was pale. Thus there arose gradually a demand for pale sheet. With our present knowledge we are in a position to state that the pale biscuits were not in any way superior to the darker ones, and they were in most cases actually inferior.

It was found also as time progressed that sheet rubber, on air-drying, became covered with external surface moulds, and that, more often than not, the smell of the drying rubber was the reverse of pleasant. Even when dry the sheets had to be continually brushed free from moulds, and by the time the rubber reached the market it was again usually mouldy. Such are, even now, the handicaps under which those who prepare pale sheets have to labour. Few, however, are the estates making pale sheets, and they are confined almost entirely to native holdings.

To those accustomed only to the preparation of crepe rubber, where coagulation can be effected in large batches, the preparation of sheet rubber always seems to demand much more labour. As a matter of fact, although the preliminary operations certainly do demand more care and labour than in crepe-making, there are compensating advantages in the machining stage. For the preparation of sheet of the highest quality on any but the largest scale, elaborate installations of machinery are quite superfluous, as equal results can be obtained with pairs of rolls worked by hand.

UNIFORMITY OF PRODUCT.--There will be no need to enter again into a discussion of the preliminary operations of receiving and straining latex for sheet-making. They have been fully dealt with in Chapter VII. It used to be the general custom to mix the acid and latex in each individual dish, and in some small or non-progressive factories that is still the procedure.

Quite apart from the question of labour entailed, the process is quite unnecessary. Even if comparatively small volumes of latex are handled, standardisation by dilution should be the rule, and the acid solution should be added to the bulk. It is possible to stir in the acid and to ladle out uniform quant.i.ties in each pan or dish from a bulk volume of up to 40 gallons if the organisation is efficient.

On any but a small scale the labour entailed in the handling and cleaning of pans is excessive, and shallow tanks are now employed on most estates.

The reception and standardisation of latex by dilution has already been discussed in Chapter VII. The combination of this practice with the employment of shallow coagulating tanks has simplified working and reduced the cost of labour. It is not intended to enter into any lengthy discussion relative to the merits of sheets made in pans as against those made in tanks. It is granted that it is possible to make a "pan" sheet superior in appearance to the general average of "tank" sheets; but from an economic standpoint the introduction of the use of tanks into all but the smallest factories is only a matter of time, if the demand for this cla.s.s of rubber persists.

THE IDEAL TANK.--Even the most modern installations of sheet-coagulating tanks must be regarded as merely temporary devices, as, given facilities, the room for improvement is so wide.

The first tanks made erred in being too large, and as the result of experience the size of units has now been reduced to a maximum of 12 feet by 4 feet by 1 foot deep.

[Ill.u.s.tration: UNIT MODERN COAGULATING TANK (TWO VIEWS).

Construction of brick and cement with lining of glazed tiles. Note slots incorporated in side tiles. Part.i.tion boards in piles in the background.]

Tanks are at present constructed either of hard timber or of brick and cement faced with glazed tiles; both types have inherent drawbacks. The wooden tanks are difficult to keep clean and in "sweet" condition. The glazed tiles, unless extremely well laid, allow the acid serum (from which the rubber is removed) to percolate between the interstices. Thus "pockets"

of liquid collect beneath the tiles, and in process of the decomposition of certain const.i.tuents dissolved in the serum evil-smelling gases are set free.

[Ill.u.s.tration: ANOTHER BATTERY OF TANKS, WITH DILUTION TANKS, RAISED, ON THE RIGHT.

Note drainage c.o.c.ks, chute, and sieve in position.]

It should not be a matter of difficulty for manufacturers to make sheets of thick gla.s.s sufficiently large to form the bed-plate and side-pieces necessary in the lining of a tank. If such adjuncts could be secured, the disabilities indicated above would be perhaps wholly removed. Unless there is a demand from estates, however, it is idle to expect a supply to be forthcoming.

An even greater improvement would take the form of unit tanks cast in glazed white-ware with the necessary slots incorporated in the sides. At present no known firm makes such tanks of sufficient size. A unit could measure (internally) 6 feet by 4 feet by 1 foot deep, with slots 1-1/2 inches apart, and 3/8 inch in width. The tanks might be reinforced with iron bars, so that they could either be used alone or embedded in the usual brick structure. The junctions of the bed-plate and side-pieces could be finely rounded so as to facilitate cleaning, and at one end a draining-hole could be made, say, 1 inch in diameter.

[Ill.u.s.tration: CLOSER VIEW OF FOREGOING.

Note part.i.tions in position and coagulum being removed.]

Meantime both the hard-wood tanks and those of glazed tiles find their particular applications. The former is generally employed in smaller factories, or where future large increases of crop preclude the present installation of a fixed system. The latter find use in large factories, or where no new areas remain to come into bearing.