(_b_) Of this solution (which is approximately 1 per cent. by weight), use 1 gallon to 20 gallons of latex.

Readers are doubtless now well aware of the corrosive action of strong sulphuric acid, and we scarcely need point out that even the dilute acid should not be kept in contact with the usual iron vessels found in factories. The mixing of solutions should be done in one of the glazed earthenware jars commonly in use.

The formula given above works out at approximately 1 part strong acid to 2,000 parts of latex (of dry rubber content 1-1/2 lbs. per gallon). The formula for using acetic acid with the same latex works out at about 1: 1,200. It will be apparent, therefore, that relatively sulphuric acid is a more powerful coagulant than acetic acid. In terms of dry rubber obtained from latex of the consistency indicated above--

1 lb. sulphuric acid will produce 300 lbs. dry rubber. 1 lb. acetic acid will produce 180 lbs. dry rubber.

With both acids selling at the same rate, sulphuric acid would be more economical in use; when its cost is less than that of acetic acid, which is the normal condition, the economic advantage in favour of sulphuric acid is augmented still further.

It may be found that the standard formula for sulphuric acid will not always give a perfectly clear remaining serum, even though an attempt is made daily to work to a uniform consistency for all latices. It is inevitable that the manipulation of the latices should be slightly in error on occasions, or that a small mistake might occur in preparing the solution of acid. Hence a clear remaining serum after coagulation may be secured less often than a slightly turbid serum. This is as it should be. The minimum quant.i.ty of acid may be adjusted so closely as to give such results. If a clear serum is obtained always, that should be an indication of continual excess of coagulant. Naturally, if a milky serum is always obtained, the reverse is the case.

As a last word on the subject, it may again be emphasised that the use of sulphuric acid is not advised, except in an emergency; and that the greatest possible care must be exercised in the observance of the strict formula for use.

HYDROCHLORIC ACID, NITRIC ACID.--These mineral acids would prove more expensive than sulphuric acid. In addition they are much more uncertain in action. For example, the use of a certain excess of hydrochloric acid would not hasten coagulation, but would prevent it. Above all their effect, in excess, is deleterious to the rubber.

HYDROFLUORIC ACID.--This has a strong corrosive action on porcelain or gla.s.s. Hence it has to be contained in bottles of gutta-percha or lead. It is mentioned here merely because some years ago it found a use as a coagulant, chiefly in Ceylon. It was sold in the form of a 10 per cent.

solution under the name of "Purub," and was the subject of a patent.

It is effective as a coagulant, and has also an anti-oxidant action, which was its chief recommendation when cheap and harmless anti-oxidants were not commonly known. It is comparatively expensive, and, as indicated above, difficult to handle and store. In short, it has nothing to commend it, in comparison with acetic or formic acids.

ALUM.--This substance has been used for years by native rubber producers as a coagulant. It fulfils the desired purpose, and its popularity was maintained because of the ease with which it could be stored and handled.

Unfortunately, this facility often led to the use of an excess, and native sheets were often criticised as being brittle. Investigations have shown that alum, even in minimum proportions, has an appreciably harmful effect upon the quality of the rubber prepared by its use as a coagulating agent.

Its employment by native rubber producers has now been largely superseded by acetic acid in some form.

PYROLIGNEOUS ACID.--This is otherwise known under the names of "crude acetic acid" and "crude wood vinegar." Owing to the shortage of acetic acid during the War, attention was directed towards the possibility of making an effective coagulant locally by what is termed the "dry distillation of wood"--_i.e._, the wood is not burned but heated in a retort. The enquiries could be placed in two cla.s.ses:

1. Those which aimed at making the pure, strong acid of commerce.

2. Those which sought information concerning a crude coagulant (pyroligneous acid) on estates.

Regarding the first cla.s.s, we can do no better than reproduce our remarks published in the April local report of the Rubber Growers' a.s.sociation for 1916--with the reservation that, on account of a threatened shortage of timber, a local scheme might not now be feasible:

"Probably the most common enquiry encountered since the rise in the price of acetic acid is concerned with the possibility of making acetic acid in this country. It may be stated that the proposition is a feasible one, even on a fairly large scale. We have the essentials necessary for such a scheme in:

"1. A good supply of suitable timbers, the most valuable of which, possibly, is mangrove timber, locally known as 'bakau.'

Other suitable timbers are known, but as far as preliminary experiments show mangrove timber gives the best yield. At present this timber is in great demand as a fuel for steam plants, but with the extension of the local coal industry the timber may become cheaper.

"2. There would appear to be less valuable timber which would be suitable for heating the retorts. Or, local coal might be used.

"3. Supplies of lime at reasonable rates are available, as the limestone formation in the peninsula is quite considerable in extent.

"4. Supplies of sulphuric acid are available from j.a.pan, Australia, Burma, etc., even at the present time, although naturally rates are higher than normal. Under ordinary conditions, supplies from England and parts of Europe would be much cheaper than at current rates.

"For the benefit of many readers perhaps a brief and nontechnical description of the preparation of acetic acid would not be amiss, and would explain the necessity for the essentials indicated above. In brief, the process is as follows:

"(_a_) A suitable timber is heated in a closed retort. This is termed 'dry distillation,' and results eventually in the carbonisation of the wood--_i.e._, charcoal is obtained in the retort.

"(_b_) Tar, vapours and gases are distilled over during the carbonisation of the wood. These liquors and gases pa.s.s through condensers. The gases pa.s.s away, while the condensed liquors separate out into (1) wood tar, (2) a watery liquor called pyroligneous acid or crude wood vinegar.

"(_c_) The pyroligneous acid is separated from the tar, and again distilled to obtain the acetic acid present.

"(_d_) This crude acid is steam-heated with milk of lime, which fixes the acid, forming calcium acetate (or acetate of lime).

"(_e_) Eventually the calcium acetate is taken out in the form of a thick paste, which is spread to dry. When dry this 'grey acetate' is the main source of all glacial acetic acid now made.

"(_f_) The acetic acid is released from the 'acetate of lime' by the action of sulphuric acid. It is then distilled several times, and under various conditions, in order to increase its strength.

In the past copper tubes were used for this purpose, but owing to the fact that traces of copper were found to be injurious to rubber, some works instal tubes of glazed earthenware for the distillation.

"Such is the process in outline, and it will be seen that no proposal to manufacture _glacial acetic acid_ on an estate could be considered feasible, although it would not present any great difficulty on a large scale and under skilled direction. Furthermore, the cost of the plant would be far too great for any estate."

Although it is clear that pure acetic acid is beyond the scope of an estate, crude pyroligneous acid has been produced on a varying scale in this country and in Ceylon. In the latter country some success was obtained by the distillation of coconut sh.e.l.ls with comparatively inexpensive plant.

In this country, wood-distillation was practised on a few estates, but improved facilities for obtaining pure acetic led to a termination of the experiments, although sufficient crude acid could then be made at a reasonable cost.

The pyroligneous acid obtained, is generally clear, after nitration, and of a dark brown colour. It has a peculiar odour reminiscent of smoked sheet-rubber, or of creosotic substances in general.

Its acid content depends chiefly upon:

(_a_) The kind of timber heated in the retort.

(_b_) The efficiency of the apparatus.

(_c_) Condition of the timber as to moisture.

(_d_) The temperature employed, and rate of working.

(_e_) The point at which distillation ceases (_i.e._, the duration of interval between commencement of heating and cessation of collection).

Samples received from estates for testing purposes were found to contain equivalents varying from 2 per cent. to 10 per cent. of acetic acid.

They were all suitable coagulants when used in quant.i.ty calculated from the discovered acidity, but produced rubber darker than ordinary when air-dried. This effect was not of much importance in the preparation of smoked sheets, but to produce a pale crepe it was necessary to employ sodium bisulphite as an anti-oxidant.

This darkening in colour is to be ascribed to the presence of traces of phenols,[24] which are stated to exert an effect upon the rubber during and after vulcanisation.[25] This subject will be discussed in another section.

[24] Whitby, _Journal Soc. Chem. Industry_, vol. x.x.xv., No. 9, 1916.

[25] See also "Preparation and Vulcanisation of Plantation Rubber" (Eaton, Grantham, and Day), Bulletin No. 27, F.M.S. Department of Agriculture, April, 1918.

With this provision the crude pyroligneous acid which can be produced on estates, could be employed as a coagulant until such time as the price of glacial acetic acid was so low as to make the production of the crude acid non-profitable. This point would be determined from a knowledge of the cost of production per gallon, and the percentage of acetic acid per unit. For example, if the cost of production (including cost of timber for distillation, cost of fuel for heating the retort, cost of labour, etc.) was 60 cents per gallon of crude acid containing 9 per cent. of acetic acid, that would be equivalent approximately to buying glacial acetic acid at $30 per demijohn of 44 lbs.

SMOKED WATER.--A weak solution of pyroligneous acid may also be obtained by pa.s.sing smoke through water. With this object in view, a machine was designed by the Federated Engineering Company of Kuala Lumpur. In this the principle of retorting was not employed. Smoke was produced by ordinary combustion in a compartment of the apparatus, and was drawn through water by the action of a high-speed fan worked by hand. A solution, equivalent in effect to a 2 per cent. solution of acetic acid, could be obtained at a comparatively cheaper cost than crude pyroligneous acid produced by dry distillation as it was then being practised. This was chiefly because of the wasteful methods of fuel combustion, in the latter process, in the heating of the retort.

CHINESE VINEGAR.--This agent was found to be a satisfactory coagulant, and, _a priori_, there is no reason why it should not be suitable, as it is essentially a dilute solution of acetic acid.

The qualities sold were generally colourless, and were probably the result of acetic fermentation of rice.

Samples tested showed a varying content of acetic acid, ranging roughly from 3 per cent. to 8 per cent.; but on this basis of valuation it was found generally that the price bore no relation to the degree of efficiency.

It was advanced not only that the vinegar was an efficient subst.i.tute for glacial acetic acid, but that it was also cheaper. This latter claim was proved to have no foundation in fact, even at the high price of acetic acid prevailing during the period of stress. It is not likely, therefore, that vinegar can displace acetic acid, except as an expedient.