Very little thick or blanket crepe is now made on estates in Malaya, so

that beyond the mention of that type little need be written. A fair amount

of blanket crepe is sold in the Singapore market, but it should generally

be regarded as re-made rubber--_i.e._, it may have been prepared from thin

crepes, or from native pale sheets, in local rubber-washing factories. In

appearance these crepes have a rough finish, and vary in colour according

to the crude material employed.

The general preference of the market at present is for a thin,

smooth-finished crepe, with a close-knitted surface--_i.e._, free from what

is described as "laciness." What effect this looseness of finish can

possibly have upon the quality of the rubber is not understood, but the

standard type set up by the market must be comparatively free from small

holes.

Under existing conditions governing the sale and purchase of rubber,

various "standards" are set up. These really have no bearing upon the

intrinsic qualities of the rubber, and are concerned almost entirely with

superficial attributes. They are necessary in the absence of any proper

scheme of evaluation for the establishment of certain standards of

comparison, which imply that the rubber is apparently clean, free from

certain recognised defects, and has been carefully prepared--as far as can

be determined by a superficial examination. Thus the question of "finish"

has attained disproportionate importance, but must be respected when

preparing rubber for sale.

Under ordinary conditions, thin crepe rubber, as it leaves the finishing

machines, has what may be termed "deckled" edges. On many estates, in order

to comply with market conditions, the edges of the wet crepe are trimmed,

and the trimmings re-made into lengths of crepe. This is done under the

impression that the market price is influenced by the evenness of the edges

of crepe rubbers.

Again, it sometimes happens that, owing to "wear" of the rolls, the

finished dry crepe may show a faint but distinct pattern of mark--a diamond

or a horizontal bar. Since these are not accepted under the "standard"

comparisons, rubber exhibiting these characteristics does not obtain the

top market price. In other words, these innocent and innocuous marks are

regarded as defects and penalised accordingly.

Enough has been written to show how very important becomes the question of

finish. It will be acknowledged that the superficial qualities demanded in

the "standard" market type can be reproduced by any estate having adequate

machinery and ample facilities for drying and handling the rubber.

Methods of preparation and formulae for the employment of chemicals are so

well laid down that, up to the stage of machining, no difficulty need be

encountered. But the standard type of finish in the dry crepes cannot be

obtained unless the estate factory is fully equipped with the three types

of rolls necessary--_i.e._, macerators, intermediate crepers, and smooth

finishing-rolls. This subject has received full discussion in Chapter

XIII., and is here only mentioned with the view of emphasising the point

that no estate can be blamed for a lack of "finish" in crepe rubbers if the

equipment of machinery is inadequate or in poor condition.

If, on the other hand, the factory has ample machinery for requirements,

and a good finish cannot be obtained on the thin crepe, then it is time the

rolls were attended to and changed, or that the ratios of the driving

pinions were altered.

DIRTY EDGES.--It seems to be almost impossible to keep old machines clean,

and it is equally difficult to keep the edges of crepe free from oil and

dirt. Usually these dark edges are to be found on crepe which is rather

wide, and it will be noticed that where wide crepe is made, unless special

precautions are taken, the edges of the rubber often pass under the edges

of the hopper and so pick up dirt and oil. On most machines it is a great

mistake to attempt the preparation of wide crepe; nothing but narrow crepe

must be made. To obtain this it is necessary to decrease the width of the

hopper placed above the rolls. This can easily be effected by blocks of

heavy hard wood, cut to shape and fastened in position.

Sometimes the dark edges of crepe are due to another cause. Rolls may be

gradually worn in the middle, so that to obtain a good finish it becomes

increasingly necessary to tighten up the screws which regulate the distance

between the rolls. It thus happens that just at, and beyond, the edges of

the rubber the rolls grind upon each other, and fine particles of iron and

graphite are transferred to the rubber. In such a case it is evident that

either the rolls must be "turned" or that a new pair of rolls must be

substituted.

IRON-STAINS.--One of the causes of iron-stain on rubber has been mentioned

in the preceding paragraph. This particular kind of iron-stain must not be

confounded with rust-stain, and gives a dark dirty colour. It results from

the grinding together of the rolls, and is usually noticed in the finishing

of fine pale crepe. For this operation it is necessary to screw up the

rolls tightly, and it will be plain that, whenever the rolls are vacant of

rubber, there is a tendency for them to grind upon each other, thus setting

free fine particles of iron and graphite. In order to avoid this, one must

be careful to see that between the working of each length of fine crepe the

rolls should be occupied with another piece of rubber, which may be kept

for the purpose. In some factories this trouble apparently does not exist,

while in others the amount of wear on the rolls is surprisingly great, and

the damage done to the rubber is excessive. The only way in which this

difference can be accounted for is that there must be a great difference

in the quality of the roll material. Some rolls seem to be excessively

soft, and from these contamination by iron-stain is great. For this reason

rolls are sometimes rejected, and there would appear to be an objection to

any but chilled steel rolls for the final stage of finishing crepe rubbers.

RUST-STAINS.--Rust-stains, on the other hand, throw the responsibility

entirely upon the labour and supervision of the factory. Rust is formed

upon the rolls when they are at rest, and any one passing pale rubber

between the rolls before they have been thoroughly cleaned is guilty of

culpable negligence. Even when apparently clean, a piece of lower grade

rubber should be passed through the rolls several times so as to remove any

slight trace of rust remaining.

Rust-stains have also been caused in a few cases by the large knives which

are used to cut up lumps of coagulum, or by allowing freshly coagulated

rubber to come into contact with iron vessels in the factory.

A similar appearance has been traced in a few instances to contamination of

the coagulum in transit by the dust of the reddish rock (laterite) employed

in localities for road-making.

OIL-MARKS.--The origin of oil-marks in crepe has already been described in

Chapter XIII. The whole question resolves itself into one of cleanliness,

moderation in lubrication, and supervision. The machines should be

inspected every day, and once a week rolls may be swabbed down with a 10

per cent. solution of caustic soda applied by means of a piece of cloth

fastened round the end of a stick. Immediately after this operation water

should be turned on and the rolls set in motion, so that all traces of

caustic soda are thoroughly removed. If possible, lubrication by oil should

be substituted by grease lubrication through screw caps.

Particular attention should be paid to the back of the machines. None but

the individual in charge of engines should be allowed to lubricate the

machines, and he should be held responsible for any excess of lubricant.

As a rule oil-marks are restricted to the edges or the proximity of the

edges of crepe, but sometimes the streak is to be found in the middle of

the length. In such a case it is almost certain that the oil or grease has

been picked up by the rubber in the tray. It sometimes happens, if the

"liners" of the bearings are eccentrically worn, that a few drops of dirty

oil or a particle of grease are squirted out to some distance. These

usually find a resting-place in the tray, and the contamination may then

appear in any part of the rubber.

It will be clear, therefore, that all trays beneath machines should be

examined as the probable source of danger from contamination by oil and

dirt. If the trays are as wide as or wider than the effective portion of

the rolls, they should be discarded. In their place (except sometimes in

the case of the macerating machine) all that is necessary is a movable

piece of board, in width not less than from 4 to 6 inches shorter than the

width of the rolls. Any oil or grease ejected from the bearings will thus

be allowed to fall clear of the board; and defects due to oil streaks,

etc., will be very much diminished, if not entirely obviated.

This point in connection with the damage possible by the existence of wide

trays is commended to the notice of manufacturers of machines for

plantations, as it is common to find that trays are made which contravene

the rule prescribed by experience. In fact, trays on some machines have

been so designed as to act as "traps" for all dirty matter exuding from the

bearings. Not only so; they are sometimes made of such a shape and height

that oil or grease lodging upon the edges act as a "wipe" to the rolls,

thus increasing the possibility of contamination. Until this defect was

investigated, it was common to note continued contamination of pale crepes

in spite of all precautions taken in cleaning the rolls at frequent

intervals. The trouble due to this cause is intensified when the same

machines are employed for the preparation of scrap-rubber crepes and No. 1

crepe. Small pieces of scrap find their way towards the bearings and lodge

on the edges of the trays. Unless a thorough inspection is made before

proceeding with the working of the No. 1 (pale) grade, contamination may be

continuous.

DIRT.--Streaks due to the presence of dirt (as apart from oil or grease

contamination, or that due to pieces of oxidised scrap) are unusual, and

when they do appear their origin seems to be somewhat of a mystery. It

could scarcely be advanced that the dirt was picked up on the machines, as

it is difficult to imagine where such dirt could come from. In one or two

instances there has been fairly clear evidence that the dirt was contained

in the coagulum, and the only explanation fitting the case is that it fell

into the latex after straining and during the course of coagulation. On

cutting open lumps of coagulum brought in from the field division, it has

sometimes been noticed that dirt is included, and the foregoing explanation

is the only reasonable one. How it was possible for dirt to get into the

latex must be left for explanation to those better acquainted with the

conditions under which the latex was coagulated.

HOLES.--On some estates it would seem impossible, with the existing

machines, to make really good crepe. The complaint is that, if thin crepe

is attempted, it is invariably found to be "full of holes"; and as,

apparently, the presence or absence of small holes in crepe rubber is a

factor which influences buyers, this defect must be avoided at all costs.

Why this matter of small holes in thin crepe should weigh so heavily with

buyers is a matter which the writers are not in a position to explain. As a

matter of fact, the presence of small holes is most generally an indication

that the rubber has received the minimum amount of working on the rolls

consistent with good washing. Further working would only be undertaken with

the idea of so consolidating the rubber as to get rid of holes in order to

meet the market scheme of valuation.

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 brass from the "liners"

of the bearings. The presence of the copper in brass 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 brass "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 brass from the bearings of

machines, due to eccentric grinding of the "standards" of the rolls

upon the brass "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 brass, 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 scrap 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 brass 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 brass "liners" or "bushes" in the bearings of the

machines lead to the natural query as to whether the use of brass 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 brass.

COTTON AND OTHER FIBRE.--One of the most frequent complaints made against

low grade crepes is the presence of fibre--generally classed 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 scrap

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 scraps 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

circumstances 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-scrap; or it may be brought into the factory in the

tappers' scrap-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 passed through the scrap-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

surreptitiously with waste, which may have passed 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

passing 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 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.

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.

[Illustration: 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' Association, 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 quantity. 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).

It has already been remarked that it is possible for "spot" disease to

develop in dry rubber which previously gave no evidence of the presence of

fungi. The condition necessary to such an occurrence is supplied by the

presence of moisture. Thus, to state instances which are by no means

uncommon, if a box of rubber is allowed to remain exposed to rain, or is

damaged by flood-water, or by sea-water during transit, or (sometimes) if

the rubber is packed in a damp case, the crepe on arrival at its

destination may be found to be affected to a degree dependent upon the

extent of wetting and the duration of the wetting period.

No means are known by which these coloured spots, due to the growth of

chromogenic organisms, can be removed from the rubber. Naturally, although

they may be present in the darker lower grades of crepe, they are not so

easily visible as in pale crepe. It follows, therefore, that every possible

precautionary measure must be taken when pale crepe has to be prepared.

We are often asked whether it is possible for an infected piece of rubber

to affect sound rubber hanging in the same building; and whether, in case

of "spot" disease appearing, it is necessary to disinfect the drying-house.

In a general sense, the answer to both queries is in the negative. It has

not been proved possible to transmit the disease from one piece of crepe to

another, except by the closest possible contact and in the presence of an

abundance of moisture.

A dry crepe, even when in close contact with an infected dry specimen, has

not been found to be affected.

Unless, therefore, pieces of rubber are pressed together, under favourable

conditions as to moisture, there has been observed no transfer of disease.

Similarly it has not been found that the presence of spotted rubber in one

part of the drying-house has been responsible for an outbreak of disease

in another part of the same building. Furthermore, after the removal of

diseased rubber from the drying-shed, freshly prepared rubber may be hung

on the same supports without becoming affected, and without any

intermediate treatment of the wooden bars, providing the crepe is thin and

weather conditions are good. In our experience, no case has been observed

in which the disease has been communicated to freshly prepared rubber by

reason of the previous presence of affected rubber. In our opinion,

therefore, any scheme for disinfecting the interior of a drying-house, as a

preventive measure against the spread of "spot" disease, is unnecessary.

All other things being equal, it is plain that much will depend, as to the

incidence of coloured spots, upon the design and situation of the

drying-house. Sufficient has been written in previous chapters to indicate

the importance of these points as affecting the rate of drying, upon which

hinges the possibility of the appearance of "spot" disease.

In conclusion, the chief points in any discussion of this subject may be

summarised thus:

1. No coagulum should be left without working for longer than the

ordinary period. Otherwise, the prevailing conditions are very

favourable for the development of the disease.

2. Thin crepe only should be made. The quicker the rate of drying the

less possibility is there of the coloured spots appearing.

3. Crepe should never be allowed to remain folded overnight, and

batches of folded wet crepe should be placed on edge to drain off

surface moisture. The rubber should be hung to dry as soon as

possible.

4. Several species of fungi causing coloured spots have been

recognised, and it has been proved conclusively that it is possible to

infect latex and also fresh coagulum.

5. As far as our present knowledge goes, it appears that infection

takes place chiefly, if not entirely, by means of the latex in the

field-vessels. It may take place during transport also, or even during

coagulation.

6. While it is certain that infection can be caused by contact, it has

not yet been shown that infection of the finished wet rubber takes

place in the drying-houses by means of air-borne spores--at least,

under ordinary drying conditions.

7. There is reason to believe that no further infection takes place

once the rubber is well into the drying stage, and that dry rubber is

not infected even by contact. From this one might infer that, as long

as rubber remains dry, infection cannot take place during the voyage

to the port of consignment.

8. Coloured spots do not appear until the rubber is about half dry,

because that period is necessary for the development of the fungus to

that stage in its life-history when it excretes colouring matter. The

fungus in its earlier and colourless stage may have been present from

the time the latex entered the cup.

9. The natural habitat of the fungi would appear to be decaying

vegetable matter in the field.

10. Finally, if it is found impossible to be rid of fungoid-spot

disease after having exercised all care and observed all known

precautions, nothing remains but to supersede the ordinary drying

process by some system of quick drying, such as the vacuum-drying

process or a hot-air draught system, in which the rubber dries so

quickly that any possibility of appearance of "spots" is entirely

removed.

SURFACE MOULDS OR MILDEWS ON CREPE RUBBER.--Defects of this nature are most

uncommon in the higher grades of crepe rubber, but cases of affection in

the lower grades are not rare.

It will be evident from all previous discussions that the incidence of

these moulds must be due to an extremely slow rate of drying. The necessary

conditions would be supplied by one or more of the following causes:

(_a_) Making the crepe too thick.

(_b_) Hanging the crepe in a badly ventilated or badly situated

building.

(_c_) Occasionally by abnormally wet weather.

(_d_) Allowing piles of crepe to remain too long before hanging.

(_e_) Using excessive quantities of deteriorated sodium bisulphite. In

short, any factor contributing towards a retarded rate of drying may

be responsible for the appearance of surface mildews. The last

mentioned cause is of not infrequent occurrence. Knowing the chemical

to be of poor quality, relatively more is used to produce the desired

anti-oxidant effect. Unless the rubber is particularly well washed on

the rolls, there remains within it a residue of sodium _bisulphate_,

an oxidation product of the bisulphite. This is hygroscopic to some

degree--_i.e._, it takes up moisture from the atmosphere. Hence

drying is delayed, and even should mildews not develop the chemical

may sometimes be seen on the surface of the rubber as a whitish

"bloom."

The enumeration of the possible causes of mildews on crepe rubber is

sufficient to indicate the necessary precautions to be taken, and the

discussion will not be extended further.

TACKINESS IN RUBBER.--"Tackiness" is a term used to denote a deterioration

of rubber which renders it sticky, and, beyond this, implies that some

physical and chemical change in the nature of the substance has taken

place. In fact, it is no longer "rubber," but an oxidation product

containing much resinous matter. It does not behave as rubber, and hence

its value is much depreciated.

With modern ideas of erection of factories to guard against the

introduction of direct sunlight, it was hoped that this defect had

practically ceased to exist. In one grade of rubber it would be expected

that tackiness would continue to appear. Earth-rubber, often exposed to

direct sunlight for a week, would naturally become tacky, and this

tackiness cannot be avoided unless the earth-scrap is to be collected more

frequently. But in many cases even the higher grades of rubber show signs

of tackiness. Experiments have been carried out at various times and in

various places to determine the cause of tackiness. For some time the

theory of bacterial origin was in favour, but none of the experimental

results was convincing. Bacteria may be present in tacky rubber; but, on

the other hand, many cases of bacteria in rubber have been observed in

which there was no tackiness. Experiments were made by one of us some years

ago with a view to testing the bacterial theory by inoculating latex with

small pieces of tacky rubber. In opposition to the results which were

stated to have been obtained, there was no spread of tackiness. Other

investigators have obtained similar results. One writer proposed to explain

tackiness as caused by excess of moisture. This perfectly simple

explanation unfortunately displays only a profound ignorance of the

subject, and does not take into account the fact that tackiness is incident

in rubber after dryness has been reached. It need not be pointed out to

planters in Malaya that wet sheets of rubber are often exposed to direct

sunlight by workers of native holdings, with no resulting harm as long as

plenty of moisture is present in the rubber.

TACKINESS THE RESULT OF A SLOW PROCESS OF CHANGE.--As stated above,

tackiness does not appear until the rubber is dry, and even then it is to

be noted that it is possible for tackiness to appear in rubber arriving in

London, which showed no indications of tackiness when packed for shipment.

TACKINESS CAUSED BY TRACES OF COPPER SALTS.--Spence, as the result of

investigations, has pointed out that none of the various theories put

forward to account for tackiness--viz., the action of bacteria, premature

putrefaction, oxidation, excess of moisture, the action of enzymes,

etc.--have any basis in scientific proof, and believes that the cause of

tackiness cannot be directly attributed to bacteria. It has been stated

that the only known way of causing rubber to become tacky is to expose it

to sunlight or heat. While agreeing that in the ordinary way this statement

is correct as far as one rules out the employment of chemical substances,

it must be pointed out that tackiness of the worst degree may be caused by

the presence of traces of copper or copper salts. This point has already

been touched upon in a preceding paragraph dealing with the defect of

"green streaks" in pale crepe rubber.

In the course of laboratory experiments tackiness has often been induced by

the use of traces of copper salts. The rate at which tackiness is induced

appears to be dependent upon the amount of copper salt used, but once it

begins, the rubber molecule is very rapidly broken down, and resins are

formed. As the formation of resins is accompanied by the inclusion of

oxygen in the chemical constitution, it would be expected that dry rubber

becoming tacky should increase in weight. This is found to be the case, and

to give an idea of how this weight increases with the progress of

tackiness, the results below may be studied.

It will be seen that the maximum quantity of copper sulphate used amounted

to 0.025 per cent, (approx.) upon the weight of latex taken. Now it is

highly probable that only a fraction of this quantity was retained in the

rubber on coagulation, the remainder being in solution in the serum.

Furthermore, as the rubber was well washed and worked down to thin crepe,

_the total quantity of copper salt remaining in the dry crepe must have

been exceedingly small_. Yet the effect is most marked and should impress

upon all managers the necessity for guarding against any possible

contamination caused by brass or copper.

----------+-----------------+--------------------------------------------

_Sample._ | | _Weight of Rubber._

+------+ +------+--------+--------+--------+----------

| | | After |Further |Further | Percent-

|_Amount of Copper Salt._| When |Interval|Interval|Interval|age in In-

| | Dry. |of Four |of Seven|of Three|crease in

| | | Weeks. | Weeks. | Weeks. | Weight.

---+------------------------+------+--------+--------+--------+----------

| | Grms.| Grms. | Grms. | Grms. |

1 |0.02 grms. copper | | | | |

|sulphate per 100 c.c. | 430 | 441 | 482 | 488 | 13.5

|latex | | | | |

2 | Ditto | 428 | 439 | 481 | 486 | 13.55

3 |0.01 grms. copper } | | | | |

|sulphate, per 100 c.c.} | | | | |

|latex } | 962 | 987 | 1035 | 1036 | 7.7

|0.01 grms. copper } | | | | |

|acetate, per 100 c.c. } | | | | |

|latex } | | | | |

4 |0.025 grms. copper | | | | |

|sulphate, per 100 c.c. | 502 | 513 | 558 | 560 | 11.5

|latex | | | | |

---+------------------------+------+--------+--------+--------+----------

In view of the effect thus produced by the addition of traces to latex of a

copper salt, and the observed effect on rubber of contact with copper

salts, one may imagine the result produced some years ago when on an estate

smoked sheets were washed with a solution of copper sulphate as a remedy

for surface moulds!

With the exception of this chemical action we know of no other means by

which tackiness is produced, beyond those of direct sunshine and heat.

Cases governed by these two causes are common on estates. They are confined

chiefly to the lowest grades of scrap rubber, when the component raw

materials have been exposed to the sun for a period before being brought to

the factory.

It is now comparatively rare to find cases of tackiness in the higher

grades of crepe, and when they occur, one may look for evidence of gross

carelessness in the admission of direct sunshine. Usually this means the

failure of some individual to regulate window shutters according to the

position of the sun in the sky. More rarely does it happen that tackiness

may have been induced by placing thin crepe rubber too near the iron roof

of the drying-shed.

Regarding the question as to whether tackiness may be communicated by

direct contact, opinion appears to be divided. It has been stated that

sound rubber left in contact with tacky specimens was found to be

unaffected after two years. On the other hand, it is claimed that tackiness

has been induced in a sound rubber by infecting it with small pieces which

were tacky. In a preliminary article on the effect of copper and copper

salts upon pieces of dried and sound crepe[21] it was noted, after one

year, that tackiness had been communicated from the treated portion to the

"blank" in contact. There is sufficient evidence to warrant the injunction

that tacky rubber should be excluded from contact with sound rubber. If

shipped it should be packed separately.

[21] Report I., 1916 (Sidney Morgan), Rubber Growers' Association (Malaya).

Compounds have been put upon the market which assumedly claim to be cures

for tackiness. These are merely palliatives, consisting of starch, talc, or

chalk powders, which counteract stickiness.

NO CURE FOR TACKINESS.--At the present stage of our knowledge, there

appears to be no cure for tackiness. Neither do we see the necessity for a

cure when the phenomenon may be avoided by taking simple precautions, which

may be briefly summarised thus:

(1) Any permanent openings through which it is possible for direct

sunlight to enter, whether large or small, should either be totally

closed or provided with some substance which cuts off the direct

effect of the sunlight--_e.g._, ruby glass or ruby glazed cloth.

(2) Rubber should under no circumstances be placed near any source of

heat.

(3) No rubber should be hung in a drying-room in such a position

adjacent to a window or door that it is possible for sunshine to reach

it, even should coolies neglect to obey rules.

(4) Instruments or vessels of copper or brass should not be used where

acids are employed.

LACK OF UNIFORMITY IN COLOUR.--The complaint is far less real than it was

a few years ago. The introduction by the Rubber Growers' Association of the

"Metrolac" led to uniform dilution of latices varying in rubber content.

Previously the only known method of obtaining uniformity in colour and

appearance was that by which latices from all fields were mixed together in

bulk. Even so the uniformity applied only to the one bulking operation, and

any other day's results might show considerable variation from the first

standard.

This does not take into account any observed differences in shade of colour

attributable to natural oxidation which might vary in intensity from day to

day. The introduction of sodium bisulphite as an anti-oxidant exerted a

great influence upon the colour of pale crepes generally; but considerable

variation would still have been notable but for the adoption of the scheme

for uniform dilution, in addition to the use of small quantities of

anti-oxidant.

On most estates it is now possible, with slight exceptions due to abnormal

conditions, so to treat the latex that the pale crepes prepared on any one

day differ in no perceptible degree from the product of any other day.

Where this is not the case it must be suspected that there has been some

carelessness in manipulating the latex or the chemicals. Attention has been

drawn to the fact that there may be exceptional cases, when the determining

factors lie beyond the control of factory processes--_e.g._, heavy rains

causing over-dilution of latex, the yielding of "yellow" latex from newly

opened areas, etc. But on the whole there is now no reason why the general

average product from any estate should not be uniform in colour and

appearance. Furthermore, it should be possible for large groups of estates,

by the adoption of uniform methods, to produce similar rubber from all the

plantations. Moreover, apart from some differences caused by factors which

still need determination, the total product in a general sense should not

only be uniform in appearance but uniform in physical and chemical

properties.

BLOCK RUBBER.--This mode of preparation is employed only in comparatively

few instances. The block is prepared from crepe rubber, which has been

dried either in a hot-air drier or in a vacuum chamber.

There is another type of block which is made by placing layers of dry crepe

under considerable pressure. This is not the true type of block, and the

layers are quite distinct--_i.e._, they do not amalgamate. Usually this

pressed rubber consists of lower grades of crepe, and it should not be

popular, inasmuch as it leaves too wide an opportunity for the inclusion of

dirt, bark particles, and other impurities, which cannot be seen generally

on account of the protective colour of the rubber.

In the true type of block, the layers are in a plastic condition, due to

heat, when they leave the drying-chamber; and being immediately submitted

to great pressure the result is a homogeneous mass in which the layers

disappear by amalgamation. Only the best grade of crepe is employed, and

given the absence of defects in the layers there should be no complaint

regarding the final block.

Prepared in slabs which are three or four inches in thickness, the product

is easily handled, and should be sufficiently translucent to make it

possible to distinguish the shape of the hand when held between the block

and the light. This is not possible when blocks are made of greater

thickness.

The only complaints which it should be possible to lodge against block

rubbers are:

(_a_) The inclusion of dirt and other matter.

(_b_) The use of layers of crepe which have some defect.

(_c_) The inclusion of air-bubbles.

The remedy for (_a_) and (_b_) lies in the hands of the factory

superintendent. The last ground of complaint is dependent upon the style of

preparation of the original layers of crepe.

When layers of crepe are placed one upon the other, and submitted to great

pressure, it is natural to suppose that air would be contained in spaces,

and would be unable to escape. To guard against this, it would seem

necessary to prepare the crepe thin and with a fairly good surface finish.

It must be obvious to all acquainted with the processes involved in the

preparation of block rubber, that no possibility exists for the presence

of air-bells actually enclosed _in_ thin crepe. When the vacuum-dried crepe

is folded preparatory to the blocking process it is apparent that between

the layers there must always be a considerable volume of air, a small

proportion of which is bound to be retained owing to the nature of the

surface of crepe rubber.

That this has always been true of the preparation of block rubber cannot be

denied. It is possible, of course, for one type of block to show the

presence of air-bells more than another type, the proportion of air

enclosed in blocking depending upon the nature of the crepe of which the

block is composed. A block built up of layers of smooth, fine crepe would

be expected to contain less air-bells than a block composed of layers of a

rough crepe.

Block rubber has been seen which was free from air-bells, but this was the

thin variety of block prepared for show purposes with far greater care,

probably, than would be expended in commercial preparations.

CHAPTER XVIII

_DEFECTS IN SHEET RUBBER_

Before proceeding to deal with defects in the rubber as it is put upon the

market a brief account will be given of faults which may be noted in the

preparatory stages.

MILKY RESIDUE OR SERUM.--If the serum is not clear after the ordinary

period allowed for coagulation, it indicates one of the following possible

causes:

(_a_) Failure to obtain complete mixture by thorough stirring.

(_b_) Insufficiency of acid solution. This may be real or indirectly

due to the presence of an excess of anti-coagulant such as formalin or

sodium sulphite.

(_c_) In cases where other coagulants than acetic or formic acids have

been employed the failure may be due to an excess of, or an

unsuitable, coagulant--_e.g._, hydrochloric acid.

COLOURED SURFACE BLOTCHES AND UNPLEASANT ODOUR.--Sometimes the surface of

the coagulum exhibits yellowish or bluish streaks and patches. It will be

found generally that the yellowish colour is possessed by a slimy

substance, of offensive odour, which may be scraped from the surface.

Either insufficient acid has been used, or the mixing of latex and

coagulant has been at fault.

DARK DISCOLORATION OF THE RUBBER.--This may be stated to be a natural

process when fresh rubber is exposed to the atmosphere. It is usually

described as "oxidation," and it will be noted to be absent, or to occur to

less degree, on those portions of the rubber which are protected from the

atmosphere by being below the surface of the remaining liquid. This surface

change may be prevented (see Chapters VIII. and IX.) by the use of small

quantities of sodium sulphite (for preference) or bisulphite.

SOFT COAGULUM, SPONGY UNDER-SURFACE, TEARING OF COAGULUM.--If the whole

mass of coagulum is too soft, while coagulation appears to be complete,

over-dilution of the latex has occurred. This may apply also to the case in

which the under-surface only is spongy and soft. If coagulating-tanks are

employed, the upper edge may be comparatively hard, while the lower is soft

and weak. Often the spongy portion may adhere to the partitions. This

prevents the natural rise of the coagulum, due to retraction, as the mass

"sets." The pull between the free upper portion and the adhering lower edge

causes splitting and tearing of the coagulum, with marked porosity (spongy

appearance). The two factors to receive attention are the standard of

dilution and the condition of the surfaces of the partitions. If these have

minute cracks into which latex can penetrate, and in which coagulation

takes place, the boards should be discarded. Given the conditions indicated

above, the tearing and splitting of rubber in coagulating tanks is

sometimes augmented by the practice of flooding the tanks when coagulation

is judged to be complete. The surface water finds its way downwards between

strips of coagulum and the partitions, thus increasing the upward tension

between the free and adhering portions. The main idea governing the

practice of flooding the tanks is to prevent "oxidation" (darkening) of the

upper edges. If a small quantity of sodium sulphite is employed as an

anti-oxidant and to retard coagulation, it is not necessary to flood tanks.

"PITTING" OF SURFACES.--In pan coagulation this "pitted" appearance is

usually limited to the under-surface, while coagulum prepared in tanks may

exhibit the defect on both faces. The existence of these numerous "pits,"

or small depressions, points to the presence of bubbles of gas which have

been unable to escape freely. As the formation and retention of gas-bubbles

is not a normal occurrence in coagulation, we are led to infer that some

special conditions must have arisen. These may be supplied by one or more

of the following contributory causes:

(_a_) The latex had begun to "sour" before arrival at the factory or

while waiting to be treated. This premature coagulation is usually

checked or diminished by the employment of anti-coagulants (see

Chapters VIII. and IX.). It is generally accompanied by the

appearance of enclosed gas-bubbles in the dry rubber.

(_b_) There may have been a slight insufficiency of coagulant, or the

admixture was not thorough, thus allowing a slow putrefactive change

to take place in the incompletely coagulated areas.

(_c_) The wooden partitions may not have been effectively cleansed.

The existence of a thin slime, of bacterial origin, is sometimes

noted. This is accountable for putrefactive effects in the surfaces of

the coagulum, or in the serum, giving rise to the formation of gases.

If these cannot escape freely, by reason of adhesion between the

coagulum and the partitions, "pitting" occurs.

THICKENED ENDS OR EDGES, AFTER ROLLING.--As a rule these defects may be

ascribed to the employment of too rich a latex, or faulty manipulation.

Even if the standard of dilution should be correct it sometimes happens

that, in the preliminary rolling of a long strip of rubber, coolies begin

in the middle, rolling with a forward pressure and tension towards the ends

of the strip. This is generally not so much the fault of the coolie as

being due to the lack of proper facilities for preliminary rolling. The

table should be about 3 feet in height, so that ease of working is obtained

merely by natural pressure due to the position in which the worker stands.

The use of a heavy wooden roller would contribute towards this result,

inasmuch as it obviates the use of force, and the pressure is almost

entirely in a vertical direction.

MIS-SHAPEN SHEETS.--It is sometimes noted that sheets may be wider and

thicker at the ends than in the middle. Manipulation alone, as indicated

above, is not solely responsible. The primary cause is to be traced to

over-dilution of latex, giving a very soft coagulum which responds too

readily to tension and pressure. Faulty treatment in rolling exaggerates

the tendency for the strip of sheet to become narrow and thin in the

middle, wider and thicker at the ends.

THICKENED PATCHES, TORN SHEETS, "DOG-EARS," CREASES.--These elementary

defects are all due to careless working. While occasional errors cannot be

avoided, there is no real excuse for the continuance of trouble to any

degree, under average supervision.

Thickened patches are often caused in conjunction with torn sheets, and

the trouble may be ascribed to faulty practice in allowing too heavy a pile

of wet strips to accumulate before machining. Or a comparatively small pile

may have been transported some distance. It is difficult to separate the

strip, and occasionally the separation is only effected at the expense of

two sheets, one of which is torn and the other has a portion of the first

strip adhering to it.

"Dog-ears" due to the folding over of corners of the sheets, and creases

due to the rumpling of the coagulum, are generally the result of haste and

lack of average care. Machine coolies, more often than not, will not be at

any pains to straighten out folds before passing the coagulum through the

rolls.

GREASINESS BEFORE SMOKING.--Under ordinary methods of working this should

never be encountered. It may be taken to show that the machined rubber has

been allowed to remain, either hanging or in piles, far too long before

entering the smoke-house. The appearance is most marked if the rubber has

remained in a cool and moist atmosphere--_e.g._, if it has been hanging

over-night in a closed and badly-ventilated factory. In a marked degree

this is to be observed in the preparation of air-dried sheets, unless they

are exposed, when freshly prepared, to the action of the sun for a period.

This period, in the case of rubber prepared on native small-holdings,

generally extends over several days--until the sheets are more than half

dry.

In the preparation of smoked sheet, the greasy appearance and the cause

outlined contribute to a defect which is eventually described as

"stretching rusty."

SURFACE BLEMISHES.--The coagulum, during coagulation and subsequently, can

be contaminated in various ways. In most cases a little intelligence or

increased care would prevent the occurrence of these defects.

When the coagulum remains over-night, in the absence of a cover, it is not

uncommon to note the presence of dirt (from the roof above, or blown in

from the outside), the droppings of mice and rats, flies and small insects.

In theory these should be seen and removed by the factory hands. In

practice, except while under immediate supervision, the extraneous matter

is often rolled into the soft coagulum.

A fairly common cause of this surface contamination is the exhaust from the

power-unit; generally the worst offender is a steam-engine. Grit and smuts

continually find their way into the factory, alighting on the tables, in

the latex, in the water, and on the freshly prepared rubber. They are

rolled into the soft rubber and lead to marked depreciation in the selling

value. The radical remedy seems obvious, but is often beset with many

difficulties not unconnected with financial considerations.

Other superficial blemishes, such as those due to the presence of rust

marks, oil or grease patches, etc., are self-explanatory, if a little

thought is brought to bear upon them; and it is not proposed here to

discuss such defects more fully.

* * * * *

Having now dealt with certain defects which are visible in wet rubber, we

come to the discussion of others which are only perceptible either during

or after the drying period. As far as is known no plantations of any size

now prepare sheets other than in the form of smoke-dried rubber, with the

exception of a few which make a special form of thick and partially

air-dried product known as "slab" rubber.

It is not proposed, therefore, to treat in any detail with air-dried sheet

rubber. Certain obvious defects are common to both air-dried and

smoke-cured sheets, and these will be first discussed.

UNEVENNESS OF APPEARANCE.--This lack of uniformity may refer either to size

or colour, or to both. Apart from any other contributory causes, this

variation is due, in pan sheet, to a neglect to standardise the dilution of

all latices, or to lack of uniformity in the quantity of standardised latex

placed in each receptacle.

Where tanks are employed all sheets from the same tank should be of the

same size before rolling, and any subsequent disparity in thickness and

length must be attributed to some alteration in the width of the gap

between the rolls of the machines.

Unless all latices are standardised by means of an instrument, it is of

course probable that the content of one tank may be found to differ from

that of another.

In a general sense, whether air-dried or smoke-cured sheets are considered,

a thin strip will dry more quickly than a thick one, and should be paler in

colour when viewed by transmitted light--_i.e._, when the rubber is held

between the eye and the source of light.

It is necessary, therefore, to guard against the possibility of variations

in thickness caused by faulty manipulation. The distance between the

squeezing rolls (smooth) and between the marking rolls (patterned) should

be adjusted and should remain set until the conclusion of work. In a

factory having nothing beyond average requirements in equipment of machines

it should not be necessary to have to interrupt the work of the smooth

rolls or "markers" by having to make adjustments. This is, however,

inevitable if there is only one smooth-roll machine, as it is always

desirable to reduce the thickness of the coagulum by at least two stages

through even-speed smooth rolls. In some factories there are three light

power-driven smooth-roll machines, the gaps between pairs of rolls being

set so as to obtain a gradual thinning effect upon the fresh coagulum,

which is then passed once between patterned rolls. With such equipment it

is found possible, in some cases, to omit the preliminary hand-rolling, and

the strips of coagulum from the tank are passed direct through rolls set

with a wide gap. This work demands much care, as it is necessary to avoid

any distortion of the coagulum which may be caused by its own weight and

length.

VARIATION DUE TO OXIDATION.--The subject of oxidation has been mentioned in

the opening paragraphs of this chapter. It will have been learned that

oxidation is a natural process, and that it may be prevented by the

employment of anti-oxidants such as the sulphite or bisulphite of soda. In

earlier days it was sometimes prevented by steeping the thin rubber in very

hot water.

In the absence of an anti-oxidant the degree of oxidation may vary daily

and in different batches of latex on any one day, so that there is always

the possibility of a lack of uniformity due to oxidation effects. This

would be more evident in air-dried sheets than in smoke-cured rubber, as in

the latter case the darkening of the surface would be masked by the colour

induced by the smoke-drying process.

To obviate this variation anti-oxidants are used on most estates, but the

accidental or misinformed abuse of these chemicals may lead to further lack

of uniformity. Hence it is necessary to follow carefully the formulae

prescribed by experience.

COLOUR OF SMOKED SHEETS.--It may be of interest to note that the effect

known as oxidation is attributed to the presence of micro-organisms called

enzymes (ferments) in the latex. It can also be produced artificially in

various ways--_e.g._, by the use of the crude product of wood-distillation

(pyroligneous acid) as a coagulant, or by the addition to the latex of

small quantities of a phenol such as carbolic acid. It is thus possible to

prepare in sheet form a rubber which has the appearance of having been

smoke-cured, although it may never have been in a smoke-house.

It will be clear, therefore, that apart from other causes, the colour of

the cured sheets may be influenced by oxidation of the fresh coagulation,

and by the constituents of the smoke. It follows that the smoke from

timbers which are richer than others in certain chemical bodies set free by

combustion will produce a rubber darker in colour.

There is thus no real connection between colour and period of cure,

although in a general sense the longer the interval the darker the colour.

Similarly it is now plain that when anti-oxidants are employed in excess

the paleness of the rubber is in no degree truly indicative of the period

during which the rubber has been smoke-cured.

The influence of the effect of the hypsical condition of the wet rubber

upon the final colour must be thoroughly grasped. One may take two sheets

of apparently the same thickness, and smoke-cure them in juxtaposition

within the same house, only to find that one dries much more rapidly than

the other. As a consequence, the first, when fully cured, will be of a

medium golden brown colour; while the other, owing to protracted smoking,

will be dark. Evidently there must be some distinct difference between the

two in physical condition prior to the smoking. Here the factor involved is

the rubber-content of the latex. Given two pieces of coagulum of identical

thickness, but prepared from latices of different dry rubber content, it

will be obvious that to reduce them to similar thickness, more pressure

will be necessary in one case--_i.e._, that piece of coagulum will be much

more dense (more consolidated)--while the other will be comparatively soft

and porous. Into the latter warm smoke can penetrate much more easily, and

the internal moisture can escape more rapidly. The full cure may be made,

say, within twelve days, while the tougher sheet may demand up to twenty

days.

To attain uniformity in colour, therefore, the following points must be

studied and controlled as far as is possible:

(_a_) Uniform dilution of all latices.

(_b_) Uniform dimensions of coagulating receptacles.

(_c_) Uniform volumes of standardised latex.

(_d_) Uniform quality and quantities of chemicals.

(_e_) Uniform methods of manipulating the coagulum.

(_f_) Uniform conditions of fuel and accommodation during

smoke-curing.

SURFACE GLOSS.--In the choice of fuel there is room for control if one has

good timber available. This point opens up a discussion on the vexed

question of "over-smoking," as the term is sometimes applied to a

pronounced dry glossy appearance of the surface of sheets.

Three main factors are involved:

(1) The nature of the fuel.

(2) The ratio between furnace capacity and the capacity of the

smoke-house.

(3) The rate of combustion.

Obviously any fuel which yields an excessive quantity of tarry matter

or creosotic substance would conduce to the formation of a heavy glaze

on the rubber. Such fuel, therefore, should at most only be employed

as the smaller portion in a mixture with "dead" timber.

It is impossible to lay down any general rules for the guidance of estates,

as the timber available varies so widely in nature. Experience must be the

only guide, and it should not be difficult to obviate the defect. Even so,

there must be minor differences between the results obtained on estates, so

that it is not possible to make strict estimations of the smoke-curing

period by an examination of the surface appearance of rubber, even under

the best of conditions. Some estates find that the rubber has a distinct

gloss in ten days, while others may smoke-cure for twice that period

without difficulty. Evidently, therefore, the question of available fuel is

of prime importance. It may be remarked that very satisfactory results are

always obtained from the use of fairly dry timber obtained from thinned

rubber trees, mixed with the "dead" timber of old logs and stumps found on

the estates.

Obviously if a smoke-house has a superabundance of furnaces, producing more

heat and smoke than is required, glazing will result. The point is tested

by the average temperature maintained and the average rate of drying. The

result of a high temperature would be the possibility of volatile tarry

matter being driven in excess to the upper chamber. That this effect is

eventually produced even at optimal temperatures is evident from an

examination of the wood-work within the upper room.

It is clear, also, that the rate of combustion exerts an influence. In a

general sense a rapid consumption of fuel would augment the quantity of

tarry matter passing into the upper chamber over any given period, and the

possibility of glazing would be increased.

On the other hand, it is possible that a surface glaze might be formed if

the temperature were uniformly too low, especially if the rubber were

rather thick. The rate of drying would be so slow, that if a timber rich in

tarry matter were employed, the rubber might be exceedingly glossy.

In order to guard against the appearance of a heavy glaze, then, the

following points must be observed:

1. The fuel must be carefully selected by experience.

2. The sheets must not be thick. No sheets should be thicker than 1/8

inch measured in average section across the ribs.

3. The temperature must not be too high. An average working

temperature of 120 deg. to 125 deg. F. should be ample.

4. If the sheets are fairly thick, a low average temperature should be

avoided. No lower average than 110 deg. F. should be allowed.

DULL, BLACK SURFACE.--This is the opposite of the previous case, and

generally is accompanied by a fairly heavy darkening of the surface due to

"oxidation" effects. The fuel used is too "dead," and needs the addition of

some substance containing a fair amount of creosotic matter. The appearance

of the rubber does not justify the assumption that it has been over-smoked.

As a matter of fact, this type of rubber often becomes affected by mildew

fairly rapidly, thus showing that the smoking has been inefficient.

It may happen that an estate is in the habit of using a fuel which gives

even to a thin sheet a heavy glaze in a comparatively brief period. The

general custom is to soak such sheets in cold water, and then to scrub the

surfaces, sometimes using soap, in order to cleanse the rubber and free it

from the glaze. This operation may be done too well, in which case the

rubber will have a dull appearance, and may be rather more liable to

develop surface mildew after a time.

MOIST GLAZE, GREASINESS OF SURFACE.--This describes the condition of sheet

rubber when taken from the smoke-house. Sometimes the greasiness does not

develop until the rubber has been out of the smoke-house for a day or two.

As far as experience shows at present it may be due to two causes:

(_a_) The use of an excess of sodium bisulphite or sodium sulphite.

The use of sodium bisulphite is not recommended generally for

sheet-making. It may cause the rubber to be too pale in colour, and

the abuse of it may delay the drying unduly. In the latter case, a

trace of the salt may remain within the rubber or upon the surface. If

so, as the substance remaining is fairly hygroscopic, it will take up

moisture from the atmosphere and may cause the surface of the sheets

to have a moist and shiny appearance. The moist surface deposit comes

away upon the hand when the sheets are touched, and is difficult to

remove entirely. On some estates a very small quantity of the

bisulphite is employed, as it is found to be of service in the

prevention of bubbles, but in unskilled hands the method is open to

abuse, and is, therefore, not recommended for general use.

A large number of estates now use sodium sulphite in very small

quantities as an anti-coagulant and a preservative for latex in the

field. The abuse of this very useful substance carries its own

penalty. The substance is hygroscopic; and if an excess is present the

drying period will be protracted, and the sheets will have a very

moist surface film.

It may be found sometimes that only some of the sheets are affected.

This indicates that, whereas uniform quantities of a solution of

sodium sulphite have been served out in all fields, the proportion may

have been excessive in the case of fields giving a latex of

comparatively low rubber content. What suits the latex from old trees

may be excessive probably for the latex of younger trees. This is not

an infallible rule, as in the case of older fields in which immature

bark is being tapped, it is to be noted that the dry rubber content of

latex may be less than that of latex obtained from younger trees.

This type of moist glaze is not easy to remove. Ordinary surface

washing had but a temporary effect, and the trouble recurs. The only

way of dealing with the difficulty is to soak the sheets for days in

running water (or in successive changes of water) and to re-smoke

until dry.

(_b_) The second type of moist glaze is not so difficult to deal with,

and may be removed as a rule by washing the sheets when the rubber is

otherwise apparently dry.

It appears to be mainly a matter of unsuitable fuel for smoking and of

failure to provide adequate ventilation. A large number of estates

have been "thinning-out" or are doing so systematically. The logs thus

obtained are often used as fuel in the very green stage. The smoke

thus generated must be moist, and if the building is entirely closed,

this moisture must be deposited eventually upon the rubber and racks.

Some estates have surmounted the difficulty by opening up the

roof-ridge slightly so as to allow the moisture to escape with some of

the smoke; but if the logs from rubber-trees are to be used, they

should be stacked in the sun for some time. Even then, preferably,

they should not be used alone. A judicious admixture of dead and

rotting jungle-timber appears to give very satisfactory results.

VIRGIN SPOTS AND PATCHES.--If the description really indicates the defect

it simply means that portions of the sheets are not dry. When cut they

exhibit the typical whitish, opaque appearance described as "virgin." It is

doubtful whether any rubber put upon the market as No. 1 product nowadays

can have this complaint levelled at it. In the extreme case it points to

gross negligence on the part of the packer.

Sometimes what are taken to be small spots of "virgin" are really patches

of tiny air or gas bubbles. The point can be easily determined by cutting

through the patch and examining the cut edges.

SURFACE MOULDS OR MILDEW.--During the last two years, complaints regarding

the incidence of "mouldy rubber" (_i.e._, relating chiefly to the presence

of mildews on the surface) have become increasingly common.

To judge by the comments of producers, who as a rule never again see their

rubber after it leaves the estate, one would infer that the defect is

imaginary, and that the complaints are made solely with a view to

repudiation of contracts or the general cheapening of an article of

commerce. They can often point out, with a certain amount of truth, that

there has been no change in the methods of preparation or curing, and that

previously there were no complaints.

It is forgotten, however, that in former years the smaller output of rubber

was taken into consumption more rapidly than of recent years. That is to

say, the interval between smoke-curing and the employment of the rubber in

the manufacture of goods did not demand such a long period of storage.

Hence the effects of smoke-curing are now more likely to vanish.

Going still further back in the history of plantation rubber, we can point

to the time when smoked sheets were allowed, or had, to remain in the

curing-sheds for very extended periods. Loose specimens of rubber prepared

during that decade still exhibit no signs of mildew growth.

In later years a demand arose for sheets paler in colour than the old type,

and in order to meet that demand, a change had to be made in methods. This

led to a system of working whereby it was possible to smoke-dry sheets

thoroughly in from twelve to fourteen days. This interval was further

reduced on many estates, until some were producing rubber which appeared to

satisfy all requirements after only five or six days' curing. This does not

refer to the case of estates having smoke-houses of "continuous-working"

type, but to those on which smoking was confined practically to the hours

of night. Under former conditions of rate of production and consumption,

this short period of smoke-curing would possibly have been ample; but even

this is very doubtful, as often the rubber would not stand the relatively

short journey from the estate to Singapore without mildew-growth being

incipient. We have often received specimens of rubber sent from estates for

criticism, and have noted that within a comparatively brief period mildew

was to be seen.

The whole matter resolves itself into a question of thorough efficiency of

smoking. This is not dependent on duration of smoking alone, but involves

other factors, such as the kind of fuel employed, the rate of combustion of

fuel, the average temperature sustained, the ventilation of the

smoke-house, and the situation of the building. Other occasional

contributory factors are contemporary adverse climatic conditions and the

possible abuse of an anti-coagulant such as sodium sulphite.

It has been shown that after a time, given suitable conditions involving

the presence of moisture, moulds may appear on sheets which were apparently

fully smoke-cured, and that under the same conditions other and older

samples were unaffected. It is argued that the latter sheets had evidently

been smoked more efficiently than the others. Hence it is fair to assume

that, except under very special conditions, which do not apply to the

ordinary procedure in the shipping, storage, and sale of rubber, moulds

will not develop upon sheets which have been properly smoked. The term

"properly smoked" signifies efficient smoking for all practical purposes

under ordinary procedure, and implies or includes all the advantageous

factors which have been discussed or alluded to in preceding paragraphs.

Without discussing in wearisome detail conditions which may give rise to

the incidence of mildew on properly smoked rubber, it may be pointed out

that the following are favourable to the growth of moulds:

(_a_) Storing sheets in a damp place before packing.

(_b_) Packing sheets in wooden cases which are not thoroughly dry.

(_c_) Piling up cases of rubber in a badly ventilated store-room.

(_d_) Placing the cases on a cement floor.

(_e_) Wetting of cases by sea-water or by rain during transport, etc.

BLACK STREAKS, SPOTS OR PATCHES.--The origin of these is not difficult to

trace. They are caused by drippings from the roof, and contain condensation

products from smoke plus moisture. The ventilation of the roof-ridge should

receive attention, and if the trouble persists it will be necessary to

place some absorbent screen below the sloping roof. Sackcloth is sometimes

used, but leads to a worse state of affairs unless changed frequently. In

most modern smoke-houses having an iron roof there is an inner lining of

soft timber.

There scarcely seems a necessity to discuss the case in which an iron roof

has become perforated by the action of smoke. The remedy is too obvious to

describe.

WHITISH OR GREY STREAKS.--This is a very uncommon defect, and is generally

to be traced to a building in which fairly new galvanised sheets have been

employed. The zinc surface becomes oxidised, and the whitish powder which

is formed "flakes," or is carried away by drops of moisture condensing on

the surface of the iron sheets.

RUST.--Sometimes if a sheet is stretched forcibly and allowed to retract

quickly, the hitherto clear surface will be seen to be marred by a "rusty"

deposit. The rubber is then described as "stretching rusty," and its value

is depreciated.

This defect has caused more trouble during recent years than any other. It

is not proposed here to argue the question as to whether the presence of

this film, which appears when some rubbers are stretched, is detrimental to

the physical qualities of the product on vulcanisation. With the mere

statement of opinion that it could do no apparent harm, we may pass to the

aspect of the case as it affects the buyer and consumer. If one were to

judge by the attention drawn to the appearance of smoke sheet-rubber after

it has been stretched and allowed to retract, one would imagine the defect

to be of comparatively sudden and recent incidence. This is not so. The

peculiarity must have existed for years, and perhaps became more marked as

so many estates abandoned the former common practice of allowing varying

quantities of water to be placed in the collecting cups. As the substances

which cause the defect to be visible are partially soluble in water, it

would follow that when working with the very dilute latices which were

characteristic of the earlier years of the plantation industry, the

remaining liquid in the pan after coagulation would contain an appreciable

quantity of soluble substances which would otherwise have been retained in

the coagulum.

Conversely, the richer the latex, the greater the percentage of protein

matter retained in the coagulum. In the case of very rich latex, it must be

within the knowledge of every manager that the quantity of remaining liquid

in the pans would be almost nil. We may assume that the greater part of

these soluble proteins would be enclosed in the structure of the rubber,

but as the fresh coagulum must retain a quantity of liquid amounting to

from 60 to 70 per cent. by weight (we are now referring to rich latices),

it follows that some of the soluble protein matter must be removed when the

coagulum is placed under pressure. Even after the pressure is released more

of the contained liquid will exude from the surface of the rubber; and from

experience it is easy to imagine that this exudation, becoming

progressively feebler, will continue until the rubber begins to dry. Then,

with the evaporation of the surface moisture, the protein matter, either in

original form or as a degradation product, remains on the surface of the

rubber as a thin, solid film or crust. As drying continues, the interior

moisture escaping through the pores of the rubber evaporates, leaving

behind the substances hitherto held in solution. Should, however, the sheet

be thick and/or the temperature of drying low, the rubber may dry first on

the outside, forming a thin skin of dry rubber, which delays further drying

indefinitely.

It will be seen, therefore, that sheets which have been prepared from rich

latex or from too deep a layer of comparatively dilute latex will have a

surface film of dry protein matter. Moreover, these sheets will be slow in

drying, and in all probability will have a surface gloss and a dark colour.

Hence it is not difficult to understand that some brokers regarded the

presence of the so-called "rust" as an indication of over-smoking.

To show that this is not so, and further that the presence of rust has

nothing whatever to do with smoke-curing, it may be stated that _the

presence of this protein film may be seen on unsmoked sheets_ which have

been prepared from rich latex, from too deep a layer of more dilute latex,

or from some thick sheets which have been rolled only very lightly. In

fact, the presence of the protein film was noted on unsmoked sheet in 1910,

when it was seen to resemble a thin yellowish glaze which could be scraped

off with a pen-knife. Later, sufficient of this substance was removed from

some very thick air-dried sheets, or thin slabs, to fill a small test-tube.

When the sheets were bent or twisted, the apparent surface of the rubber

(_i.e._, the protein glaze) cracked in all directions. In the case of

sheets prepared from less rich latex, the surface film naturally is

extremely thin, and no cracking is observed.

If the fresh sheets are placed in a smoke-house, the drying film will take

up colour from the constituents of the smoke, and it will be invisible.

Somewhat analogous to the instance of a transparent glass giving a visible

and opaque powder when crushed, so the transparent film on stretching

breaks up into a visible powder which is lighter in colour than the rubber

on which it is superimposed.

It will be noted that since the introduction of standard methods of

preparation, involving uniform dilution of latex, say, to a content of

1-1/4 or 1-1/2 lbs. dry rubber per gallon, complaints as to "rust" have

decreased considerably.

It is to be further noted as a peculiar fact that while two estates may be

apparently working on identical lines, both as regards manipulation of

latex and subsequent treatment of the coagulum, the rubber of the one may

always be free from rust, while that of the other is often, if not always,

condemned for the alleged defect. Obviously, in such a case, there must be

an initial difference between the two latices as regards the percentage of

proteins present; or there must be some small unrecognised difference at

some stage of working.

It will now be clear that "rust" is caused by a film of matter which is

formed on the surface of the pressed coagulum, being there deposited by the

exudations from within the rubber and through the pores. It is, therefore,

necessary to avoid any conditions which will favour the formation of this

deposit--_e.g._, allowing sheets to remain too long in a moist atmosphere

before placing in the smoke-house.

At present there would seem to be only two methods which are successful in

the prevention of a "rusty" appearance in the dry rubber. Singularly

enough, the two methods appear to be directly opposed in principle. They

are:

A. THE HOT-WATER TREATMENT.--This method has been in constant use on

estates which have old trees giving rich latices. These latices are

always diluted to a uniform standard daily. Some estates which

formerly suffered from the defect now experience no difficulty, and in

other instances, where no complaint has yet been received, the

treatment has been followed consistently.

(1) After the sheets have been through the marking rolls, it is the

general custom to allow them to drip for about three hours. This

interval is really excessive for the mere draining away of the surface

water, but as a rule it is just sufficient to allow a portion of the

liquid retained in the rubber to exude. It has been shown that this

liquid may contain some protein matter in solution. Sometimes in the

case of thick sheets which have been subjected to pressure so much of

this matter is exuded as to form a thin surface slime which is

distinctly evident to the touch. If the sheets are allowed to hang

overnight, the presence of the exuded matter may be detected also by

its odour.

(2) Obviously, any method which will remove this surface film should

be of great benefit. It is found that the best results are obtained by

allowing sheets to drip for about two hours, and then placing them in

hot water for five or ten minutes. The water should be hot as the hand

can conveniently bear, and it need hardly be pointed out that the same

water should not be used for the whole day's output. For preference

there should be three or four vessels, each capable of holding a fair

proportion of the total number of sheets, and frequent changes of hot

water.

(3) After remaining in the hot water for the period mentioned, the

sheets are removed singly, each one being surface washed or swilled as

it is taken out.

(4) _It is important to see that the sheets are now well washed or

scrubbed under running cold water, or in frequent changes of water._

The reason for this procedure is plain. If the sheets are merely hung

again to drip after removing from the hot water, some moisture is

bound to remain on the surface of the sheet. As this surface moisture

contains some protein matter in solution, it is evident that, as the

water evaporates, the solid protein is again deposited on the surface

of the rubber. This would explain why some estates were unsuccessful

with the hot-water treatment. It is not essential that the running

water should be cold; it may be conveniently lukewarm if drawn from

the cooling tanks of the engines. But it is essential for the best

results that there should be running water, so that the substance in

solution is carried away. If the sheets are merely washed in a large

vessel, which has been filled with clean water, it must be obvious

that, by the time some scores of sheets have been washed, the protein

matter in solution on the surface of the sheets has been transferred

to the washing water, so that the later sheets of the batch are liable

to show the defect again on drying.

B. The second method is much more simple, and entails no extra labour

such as is demanded by the first method. A successful issue, however,

is rather more uncertain, and the method appears to give the best

results with sheet-rubber prepared on young estates or from more

dilute latex.

In this method, the sheets after rolling are allowed to drip for a

very short interval, so that the surface water is mainly removed. The

sheets are then placed in the smoke-house, and smoking is commenced at

once. In some cases where the defect had appeared continuously for a

long period, it was found to vanish entirely as soon as the method was

adopted; but when tried on some of the older estates, the results were

very doubtful, and a return was made to the hot-water treatment.

The explanation of the action which takes place is rather obscure, but

two theories may be advanced.

(_a_) It may be assumed that the interval given for dripping is too

brief to allow for the exudation of the internal moisture containing

dissolved protein matter.

In such case, the rubber is still in a highly porous condition, and it

might be advanced that the heat of the smoke may help to maintain that

condition. Thus the contained liquid might evaporate so quickly as to

leave behind the dissolved substances in the minute cellular structure

of the rubber. In other words, instead of the internal moisture

exuding slowly to the surface in liquid form, it may leave the

rubber, even in the first stages, in an evaporated condition, just as

it does in the subsequent stages of drying. Thus no dissolved protein

matter would be brought to the surface of the sheet and be deposited

there.

(_b_) The other theory also demands the first assumption propounded in

the preceding theory, but subsequently perhaps is less feasible as it

assumes a chemical action of which we have no definite knowledge.

The idea is that as the rubber is in a porous condition, and is placed

quickly in an atmosphere of smoke, the heat may maintain that

condition to such a degree, that some constituents of the smoke may

enter the rubber and cause the precipitation _in situ_ of the protein

matter held in solution by the contained water or other liquid. The

contained liquid would be water which has in solution possibly a very

slight trace of the coagulant employed, of sugars, of protein matter,

and of inorganic salts. Of these the substances which would evaporate

would be probably the water and the coagulant in most cases. If a salt

had been used as a coagulant, the dissolved trace would be deposited

within the rubber in this case, whereas if a rich latex had been

employed or a thicker sheet made from more dilute latex, some of the

salt would be brought to the surface and there deposited together with

the protein matter. This has actually been experienced in practice,

and it has been possible to remove minute crystals from the edges of

the rubber so prepared.

It will be evident that in order for either theory to contain an element of

probability, the rubber must be soft (porous) when placed in the

smoke-house, and must also be fairly thin. It is observed in all cases

where the method has been successfully employed that both these conditions

are generally fulfilled--at all events the rubber is fairly thin. When

thicker sheets are made, either from rich latex or from a deeper layer of

comparatively dilute latex, the method is not uniformly successful.

OTHER VIEWS ON "RUST" CAUSATION.--Later experimental work on "rust"

formation by Hellendoorn[22] leads to the observation that "rustiness" is

caused, not actually by the deposition of original serum-substances, but by

the decomposition thereof, under the action of aerobic micro-organisms.

[22] "The Cause of Rustiness in Sheet-Rubber," H. J. Hellendoorn, Archief

voor de Rubbercultuur, October, 1919 (Communication from the Central Rubber

Station, Buitenzorg, Java).

Without going into a full discussion of the subject, the following points

noted in the experimental work may be quoted:

1. Rustiness could apparently be produced at any time merely by

keeping freshly rolled sheets for periods varying from twenty-four to

forty-eight hours in a moist atmosphere.

2. Sheets placed immediately in a temperature of, say, 110 deg. to 130 deg. F.

never showed "rust"; but if air-dried at ordinary room temperature,

"rust" might appear.

3. "Rust" can be prevented by soaking freshly prepared sheets in

dilute solutions of disinfectants--_e.g._, formalin, sodium

bisulphite, or chinosol.

If subsequently the sheets are hung for any length of time in a moist

atmosphere, the protective effect of the disinfectant gradually

vanishes and "rustiness" may be produced.

The same disinfecting effect may be obtained by the use of steam or

hot water. It was found that there was less liability to the formation

of "rust" when sheets were immersed in water at a temperature of 95 deg.

to 120 deg. F., whilst steeping at 140 deg. F. gave complete freedom.

4. It was shown that the micro-organisms which cause decomposition of

the serum-products flourish only in the presence of air--_i.e._, they

are aerobic in character. It is not uncommon to find, therefore, that

"rust" may be incident only on those parts of a sheet which have been

exposed for some time to air and moisture before being placed in a

warm smoke-room.

5. The optimal temperature for development of the particular organisms

appeared to be about 100 deg. F., in a moist atmosphere.

6. Soaking the sheets in water (except the short immersion in hot

water, which we recommend), even for a period extending over a week,

does not hinder the formation of "rust."

7. Rustiness may be prevented by placing the sheets in a sufficiently

warmed smoke-house as long as there is adequate ventilation and a

moist atmosphere does not persist.

The simplest means of prevention is to soak the sheets first for a

short period in water, and then to hang them to drip for a few hours

in a well-ventilated place, outside the factory and under cover.[23]

[23] We advise and practise hanging sheets in the open, without shade or

cover.

It will be gathered that, although there may be a slight difference between

our previous views and those advanced by Hellendoorn as to the exact cause

of formation of the "rusty" film, the general conclusions are identical

with those given by us in preceding paragraphs and previously advised in

the Malayan reports of the Rubber Growers' Association.

BUBBLES.--The presence of bubbles in sheet-rubber has for years been the

bane of some managers' existence, and the bone of contention between

sellers and buyers. Taking the argument down to bed-rock, producers urge

that the presence of bubbles has no influence upon the ultimate quality of

the rubber on vulcanisation. They assert that the alleged defect is merely

a peg upon which to hang an unreal grievance, serving the purpose of the

buyer under the existing conditions of sale. All this may be true, but as

long as the present system continues, it must be recognised that "kicking

against the pricks" is a futile recreation.

The sympathy of the writers is certainly on the side of the producers,

inasmuch as they realise how extremely difficult, and even impossible at

times, it is for the most careful individual to prepare sheet-rubber free

from this blemish.

Much has been written, and many have been the discussions, on this vexed

subject; and it is recognised that sometimes, in spite of all precautions,

there may suddenly be an incidence of bubbles in rubber which is ordinarily

free from them. It must be allowed that climatic conditions and

physiological variations affecting the metabolic functions of the trees

exert an influence which is difficult at times to combat, and often beyond

human control.

The contributory causes are many and varied. It should be premised that,

although the defect is described as "air-bubbles," it is seldom that the

appellation is strictly correct. Rarely do the bubbles contain air. In the

vast majority of cases they contain gases in minute quantity. These gases

may be considered to arise, broadly, from some decomposition of substances

(other than rubber), contained either in the coagulum or in the serum. In a

general way, if this decomposition is evidenced by an unpleasant odour, it

is described under the term of "putrefaction." We are not concerned here

with the question as to how far such decomposition may be ascribed to a

purely chemical action, or to the indirect result of the presence of

certain bacteria or other micro-organisms. Suffice it to state that, at

least as far as field operations influence the result, the decomposition is

generally to be attributed to the work of micro-organisms. Conditions

favourable to the incidence and development of these are to be found when

absolute cleanliness in all details is not aimed at.

With this preamble we may proceed to classify possible causes of the

formation of bubbles into two groups:

(_a_) Those originating in field operations.

(_b_) Others which may arise after the arrival of the latex at the

factory.

IN THE FIELD.--Decomposition may be caused by:

(1) Spouts, buckets, and cups being dirty. Regular cleaning is

necessary. If the buckets are allowed to be taken to the lines by

tappers, trouble may ensue. Within the writers' experience it has been

shown that an otherwise baffling case of premature flocculation of

latex was traced to the presence of acid substances in the buckets,

which had been used by coolies for preparing their food.

(2) Delay in commencing work. This means similar delay in collecting

the latex which is exposed to greater heat than under ordinary

circumstances.

(3) Exposure to the sun's rays. The heating of the latex may provide

improved conditions favourable to the development and action of

micro-organisms.

(4) Allowing latex to stand too long before collection. This usually

is the result of giving tappers too great a task.

(5) The addition of water to the latex, either purposely or

accidentally, in the form of rain. The water may be slightly acid in

character, or it may carry micro-organisms from the bark into the

latex.

(6) Tapping trees at too great a height. The latex generally has an

abnormal distance to travel before reaching the cup.

(7) Sometimes the latex from old trees, or from trees after wintering

(just prior to full renewal of leaf), contains more than the usual

proportion of substances (_e.g._, sugars), which are capable of

effecting flocculation or coagulation.

(8) Too great a distance for transport. The trouble arising from this

cause is likely to be much increased if the journey has to be made

over bad roads. In such case the physical action augments the effect

likely to be produced by long standing.

The foregoing do not include all possible causes, but serve to indicate the

directions from which trouble may be mainly anticipated. It will be plain

that any latex which exhibits symptoms of premature coagulation (or minute

flocculation) should be regarded as a potential source of bubbles in

sheet-rubber.

It will be equally obvious that the employment in the field of any harmless

substance of an anti-coagulant nature is to be encouraged. This point is

discussed in detail in Chapter V.

IN THE FACTORY.--As a general rule it may be understood that the mischief

has been done before the latex is handled at the factory. Sometimes it is

perceptible from the peculiar appearance of the latex, and in such case the

batch should not be used for the preparation of sheet-rubber. Often it is

found that only the last to arrive at the store is visibly affected. This

should not be mixed with other apparently normal latex, as it is capable of

acting as a "leaven" to the bulk.

Contributory factors in the store are:

(1) Lack of cleanliness of utensils, particularly of coagulating

dishes or tanks.

The trouble becomes acute sometimes where wooden tanks are employed.

Unless the tank and the partitions are thoroughly and regularly

cleansed, the wood may become coated with a bacterial slime, which is

capable of causing what may be termed "fermentation" of the latex

layers in contact.

The tank should be thoroughly cleaned occasionally with a weak (5 per

cent.) solution of sodium bisulphite. The partitions should be

scrubbed and placed in the sun twice or three times a week.

(2) Allowing latex to stand too long before treatment. This point

needs no further expansion.

(3) The use of a latex of too high a rubber content. Such latices are

difficult to handle in order to secure uniform mixture with the

coagulant.

(4) The use of too concentrated a solution of coagulant. In

conjunction with (3) there may be a rapid and irregular coagulation,

giving rise not only to decomposition in parts (and subsequent

formation of gas), but also to the formation of true "air-bubbles" by

inclusion of air during stirring.

(5) The use of insufficient coagulant. Coagulation is slow and

incomplete.

(6) Defective straining and skimming. Small flocculated particles of

rubber may pass, or be rubbed through, the strainer. If allowed to

remain, they act as local points of danger.

(7) The proximity of the coagulating latex to some source of heat, or

exposure to sunlight.

(8) Any delay of drying in the preliminary stages, either before or

after the rubber enters the smoke-house.

BLISTERS.--This description aptly fits the case in which sheet-rubber in

the smoke-house exhibits large bubbles of gas which distend the surface of

the rubber. When subjected to pressure, small "balloons" are formed, which

burst with a perceptible report. It was formerly the belief that this

defect was occasioned solely by an abnormally high temperature. That such

is not the case can be shown by the experience of estates which have had

only the rubber of a particular day or short period affected under normal

factory conditions.

At the same time it is not disputed that the heat of the smoke-house exerts

an influence (causing expansion and distension), but it is advanced that

the gases had begun to form before the rubber entered the house.

The view held is that decomposition had supervened or was taking

place--probably from one or more of the causes enumerated in the preceding

paragraphs. The heat of the smoke-house only serves to exaggerate the

effect. It is acknowledged that the degree of decomposition must be

initially greater than in the ordinary incidence of "bubbles."

Beyond this point we are not in a position to put forward any definite

supposition as to the apparently haphazard occurrence of the phenomenon.

It is to be noted, fortunately, that the defect is comparatively rare, and

seldom appears on estates which employ an anti-coagulant in the field.

While we have examined persistent cases, one of which led to a temporary

discontinuance of the preparation of smoked sheet rubber, we have not yet

been able to arrive at any satisfactory idea of the exact conditions

governing the incidence of "blisters." Our investigations only lead us to

two observations:

(_a_) That blisters have appeared on the rubber of some estates after

wintering, and during the period of new leaf-development.

(_b_) That the defect has been noted on other estates during a period

when there were frequent but not heavy rains, and when there was a

comparatively low average temperature.

In either case, as the factors are beyond human control, it would be

expected that without any change being made in estate procedure, the

trouble would vanish as mysteriously as it appeared. This is our

experience; but as showing the possible intensive effect of a high

temperature in the smoke-house, it may be remarked that very infrequently,

in a batch of sheets exhibiting ordinary bubbles, a few hanging directly

above the furnaces show signs of a slight blistering effect.

"SPOT" DISEASE IN SHEET RUBBER.--That "spot" disease may appear in

air-dried sheets was evident at the beginning of the outbreak in the spring

of 1911. The first cases noticed took the form of pink and bluish "blushes"

spreading over the whole of the sheets. Later, fungoid spots began to

appear. These mainly took the form of red or black blotches, and were very

unsightly. As "spot" disease cannot develop in smoked rubber, the obvious

and simple course to adopt was to smoke-cure the sheets. When it is stated

that "spots" do not develop in smoke-cured rubber, it is understood that

the smoke-curing must be efficient and must commence as soon as the rubber

has been rolled, and the surface water has drained away. If the sheets are

allowed to air-dry for a few days, the disease may develop, and then

smoke-curing will not get rid of the coloured patches. The operation of

smoke-curing will not get rid of the coloured patches. The operation of

smoke-curing may tone down the colour, but the spots would still remain

evident.

SUPPORT MARKS.--It frequently occurs that one sees across the middle of

smoked sheets a wide mark. This is where the wooden support in the

smoking-chamber has been. As a rule, even in the most careful cases a faint

mark may always be seen, but in many instances this mark is exaggerated to

such an extent as to point to lack of care on the part of the store

supervision. If bays of racks remain empty over-night, they may possibly

become covered with a light sprinkling of fine wood-ash and tarry deposit.

Wet rubber placed upon these racks will pick up and retain the impurities,

and more often than not they cannot be washed out. It is incumbent upon the

manager to see that empty racks are thoroughly cleansed before placing wet

rubber upon them. The better plan is to arrange that the bars can be

removed easily from sockets. There should be in stock sufficient "spares"

for, say, two days' rubber. When the dry rubber is removed, the bars should

likewise be taken away, to be cleansed and kept in the factory until again

required. This will ensure that fresh rubber always rests upon a clean

support.

On some estates, in order to guard against a pronounced "bar-mark," sheets

are moved and turned daily.

In other smoke-houses the upper surface of the bar is chiselled in concave

form, so as to admit of the passage of smoke below the surface resting on

the bar.

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 anticipate 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 passing 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

classed 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 classed as minor

complaints. In point of fact, when they occur, they assume 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 mass.

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

substitutes 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 assumption 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

class 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 classed 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 glass 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 1.84 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 quantity

has been discovered.

(Sulphuric acid of specific gravity 1.84.)

NOTE.--The directions must be followed carefully, and glass 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.

(_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 quantity 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

glass. 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 classes:

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 class, we can do no better than reproduce our remarks

published in the April local report of the Rubber Growers' Association 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: