Oct. 1, 1903.] THE TROPICAL AGRICULTURIST. 231 
THE WASHING OF CRUDE 
RUBBER , 
The washing of crude rubber serves the purpose of 
removing from it the soluble organic and inorganic 
impurities, as alao to free it from such mechanicar 
impurities as all kinds of nondescript dirt, sand, 
wood, barkj and vegetable fragments of almost every 
description. The soluble impurities in the rubber 
very rarely exceed 1 or 2 per cent,, but the mechanical 
impurities reach very high figures indeed. 
The readiness with which any particular kind of 
crude rubber can by the washing process be converted 
into technically pure rubber depends not only upon 
the nature and amount of the impurities present, but 
is also very largely determined by the nature and 
condition of the india-rubber itself. Very firm and 
tough grades of rubber which soften only slightly at 
temperatures approaching the boiling point of water, 
and which contain only small quantities of resinous 
matter, are always very easily washed, whatever the 
nature of the impurities they contain. But soft and 
sticky grades of rubber, containing generally consider- 
able proportions of resinous matter, offer almost 
invariably very great difficulties in the washing pro- 
cess. 
In carrying out this process the rubber is cut into 
pieces of suitable size, which are then subjected to a 
treatment in hot or boiling water. Rubber grades 
which occur in small or moderate-sized balls are 
generally soaked without each ball being cut open. 
After this soaking the rubber is worked upon the well- 
known rubber washing machines in a current of clean 
water, when it finally assumes the form of very porous 
or honeycombed sheets, which are free from dirt if 
the operation has been successful. The rollers of 
these washing machines are occasionally smooth 
rollers, or they are provided with slightly oblique 
knife-edged corrugations, or these corrugations cover 
the rollers in the form of a heliz. As the rollers of the 
washing machines revolve, moreover, at different 
speeds, it follows that if they are corrugated they not 
only tear and cut in a vertical, but also in a horizontal 
direction. The corrugated rollers also produce much 
more porous sheets than the smooth rollers, and the 
former are therefore distinctly superior to the latter. 
It is, of course, true that the corrugated rollers suffer 
much greater waar and tear, but this very fact, to my 
mind, already amounts to an obvious demonstration 
of their greater efficiency as compured with the smooth 
rollers. 
As already mentioned, the preparation of the rubber 
for the washing proce^<s upon the rollers consists in its 
treatment with hot water, and it is my opinion that 
the success of the washing operation depends much 
more upon this particular treatment thau upon the 
construction of the washing machine. I am further 
of opinion that in a great many rubber factories this 
particular stage of the washing process is treated in a 
rather summary fashion, receiving but scant attention, 
with the result that the efficiency of the washing pro- 
cess, as a whole, sufiers more or less considerably. The 
mistakes committed in this process consist eith.rr in 
the rubber being subjected to the treatment in the 
tank without being cut into sufficiently sm.xU pieces, 
or the temperature of the water is too high, or the 
soaking treatment is not oonti-jued long enough. 
But the ecror most commonly to be observed is that 
all these faults occur together. 
For the recoguition of the correct mode of carrying 
out the soaking, or softening opemtioa oi the rubber 
in water, we eh ill first hnve to arrive at a clear under- 
standing as fo the pu' pose of this particular operation, 
from the description of which by practical workers, or 
in the literature on indi-i-rubbpr, one would be inclined 
to infer that nothing more was Rimed at than the 
softening of the rubber through the heat of the water. 
If this were correct, this operation might be equally 
well carried out by subjecting the rubber to the heat 
of. say, a rubber-drying stove. But this is certainly 
not the case. It i.s, of course, quite possible to soften 
a piece of crude rubber in the heat of a rubber atove, 
but if we tnke a piece thus softened upon the washing 
rollers we shall find at once that rubber softened in 
this manner cannot bo washed at all, and that the 
bulk of the dirt and woody debris it contains is simply 
ground to a fine powder, which remains cenented in 
the mass of the rnbber. 
In my studies on the coaguhtion of the rnbber latex 
I made the observation that even a very impure rubber 
if taken upon the rubber-washing machine imme- 
distely after coagulation, can with the most surprising 
rapidity be washed into a product entirely free from 
every trace of mechanical impurity. Especially par- 
ticles of wood, bark, and similar vegetable fragments 
fly from the rubber as if propelled by a spring, so that 
the man operating the machine becomes covered all 
over with these fragments, and I think that this 
obseivstion contains the key for the explanation of the 
function of the soaking process in the washing of crude 
rnbber. It seems to me that the remarkable ease and 
completeness with which in this case the separation 
of the mechanical impurities takes place is due to the 
fact that the rubber itself contains the maximum of 
water it is capable of absorbing. In consequence, this 
rubber is devoid of all adhesiveness, as may sufiiciently 
be judged already from the circumstance that even 
freshly-cut sections of this rubber cannot be made 
to adhere. It is ibis lack of adbesivity which pre- 
vents the before-named mechajuical impurities from 
getting cemented to the rubber substance. They are 
merely loosely embedded in the rubber, and float off 
with the water running upon the washing rollers as 
soon as they become exposed. It now becomes per- 
fectly clear that what we must aim at in the soaking 
process of the crude rubber is not so much a softening 
of the rubber by the heat of the water, but a complete 
saturation of it with water. This water-absorption of 
the rubber amounts to about 25 per cent, of its weight, 
and it is accompanied by a very considerable increase 
of its volume. This increase in volume is in 
itself alrea.dy equivalent to a very marked softening 
of the rubber, and should the degree of softness thus 
obtainable not be found sufficient, very short im 
mersion in warm — but not hot—water will give the 
desired result. 
The water-absorption of rubber to the above-named 
degree is. however, a rather slow process, which re- 
quires the longer time the greater the thickness of the 
pieces treated. Hence it follows that it is desirable to 
cut the rubber into pieces of very moderate thickness 
only. Practical experiments have shown that this is 
possible by the use of a rotary knife, by means of which 
the rubber caa be out into shives not exceeding | in. 
in thickness in much less time than it takes at present 
to cut up the rubber into irregular pieces by hand. Oa 
immersing the rubber in the water, the present 
method of simply letting it float upon the water is 
about as ineffective as it well could ba. Care mast be 
taken to have the whole of the rubber actually below 
the surface of the water. If the water used be cold, 
the complete saturation of the rnbber with it requires 
a period of at least twelve days but if the water is 
steadily kept at 30 deg, C, the saturation is already 
complete at the end of three days. Iq treating rubber 
qualities high in resinous matter, it is particularly im- 
portant to prevent the temperature of the water from 
ever reaching 40 deg. C', as at this temperature the 
great majority of the rubber resins already become 
very soft and sticky. Experiments I made to carry 
oat this process of w>iter-;i,bsorption under higher pre- 
sure, as in vacuo, showed th.it no saving of time, nor 
any other a-ivintage, is to be gained thereby. The 
ease with which rnbber chns savurated with w^tter can 
be w.'.shed completely free of even the worst forms of 
mineral and V 'getible impurities is quite surprising, 
but especially in dealing with rubbers containing large 
amounts of wool, it is most important to ensure the 
complete saturation of the rubber with water. 
At the present time the soaking operation almost 
everywhere ia carried out with the rubber being cut in 
