RETTING, BREAKING AND SCUTCHING
The retting operation is that process which converts the constituents of
the stems into that condition which will enable the bast layer, see Fig.
7, to be separated easily from the remaining parts of the stem. In all
fibrous plants of the type illustrated in Figs. 6 and 7, a retting
process is conducted in which the plants are either submerged in water,
called “water retting,” or spread on lands adjoining the cultivated
areas to undergo what is termed a weathering action, or “dew retting.”
Water retting is the more satisfactory and gives the better results,
and, in the hands of experienced operatives a more rapid production of
fibre of the better grades.
The submersion of the plants, caused by placing stones, clods or the
like on to the bundles, may be in slowly-running rivers, in which case
the bundles are kept intact in crates moored to the bank, or a similar
submersion may be conducted in a series of tanks or ponds. In the latter
case a supply of water may be allowed to enter and leave the tank, and
the plants are kept there until the operation is complete. Stagnant
water acts quicker on the plants than does running water.
The retting action is a process of fermentation, and the amount of
active bacteria can be regulated by the temperature and rate of movement
of the water. Flax retting in the river Lys, near Courtrai, is the
finest system known at present, and its value is due to the slow rate of
movement of soft water which is favourable to the production of the
retting bacteria; the adaptation of a similar system to this where the
water supply is suitable will give high-class results.
Fermentation starts soon after the plants are submerged, and the rate of
fermentation depends upon the temperature of the air and water; its
progress is identified by the presence of air bells on the surface of
the water. As the operation proceeds, the bundles have a tendency to
rise to the surface, and hence extra weights are added to keep them
submerged. When the formation of air bells ceases, the operatives
carefully examine and test the stems to ascertain the progress of the
operation; they usually strip off part of the bast layer, see Fig. 7,
from the wood or core, and their judgment of the correct stage of
retting is determined by the ease with which this separation is
effected. Great skill is required here, or rather ripe experience, for
if the retting is not complete, a portion of the woody matter goes
forward with the fibre, while if the stems are over-retted, the fibre is
weak; in both cases, a faulty judgment causes trouble in the actual
manual or mechanical processes which follow.
Other methods of retting are adopted in different countries, and even in
certain districts of those countries where the above system is in vogue.
It will be understood that the choice of any system will depend largely
upon local circumstances, and in all cases, other things being equal,
the method adopted will be that which will yield the largest quantity of
hemp fibre at the least cost.
The characteristics of the fibres are typical of the countries in which
the plants were grown, and of the processes of retting. It will be
almost invariably found that the best fibre is the result of the most
elaborate and careful methods of cultivation and retting, together with
the equally careful and efficient subsequent processes of breaking and
scutching.
It need hardly be said that the above elaborate and costly methods are
adopted only for the very finest grades of fibre; they would not be
attempted in the case of those plants which grow and ripen so rapidly in
some tropical countries, and in which a short, harsh fibre only is
obtained; for such plants the cheapest and simplest methods of
extraction are practised.
Many praiseworthy attempts have been made, and others are still in
progress, with varying degrees of success, to extract the fibre quickly.
None has yet been able to supplant the above-described costly, lengthy
and laborious process, but with modern science, machinery and
experience, one might expect that some brilliant genius will ultimately
solve the problem. Many industrial problems have been solved by the
joint action of experience and applied science, and one might therefore
hope to see a great simplification of the present hazardous operation of
retting.
The successful introduction of a machine or a system of machinery which
would pull, strip and clean hemp and allied plants and fibres on the
field of growth would not only open up new fields of cultivation, but
would increase the wealth of our country by millions of pounds; it would
do much to prevent the depopulation of the rural districts and so help
to preserve the hygienic conditions of our large towns.
The retting operation completed, the stems are washed and spread on
grass land, if available, or stooked like grain and allowed to dry
thoroughly. It is acknowledged to be advantageous to allow the stems to
remain a few days on the grass, for after this exposure the fibre is
more easily and efficiently separated from the other constituents of the
bast layer.
The ribbon-shaped layer may be about 3 ft. long in the shorter Russian
grades of hemp, but up to 15 ft. in length in the Italian grades. The
colour varies from grey and brown to a rich cream and almost white in
the finest grades.
The ultimate fibres are large and somewhat irregular in shape; they vary
in length from 0·2 to 2 in., with an average length of about 1 in.,
while the diameter is only about 1/1000 or 0·001 of an inch.
BREAKING AND SCUTCHING.--Various methods are adopted to separate the
bast layer from the central or woody part of the retted and dried stems
of hemp, but in all cases the operation thus involved is termed
“breaking.” The central woody part has to be broken into a great number
of short lengths, and this is done in some districts by exceedingly
simple apparatus, and in other districts by modern breaking machines.
Perhaps the simplest apparatus which is used for this purpose consists
of a series of Δ-shaped wooden bars arranged horizontally in the form of
a grid, and into the cavities of this row of bars fits another group or
series of similar bars but inverted. The latter group is hinged at one
end and provided with a handle at the other.
When the handle and the upper set of bars are raised, a few hemp stems
are laid across the fixed lower bars; the handle is then pressed
downwards, and this causes the stems to be squeezed and broken between
the two sets of bars. By repeated blows with the upper bars, and lateral
movements of the stems, it is evident that the woody core would be
broken, and this is done without damaging the fibrous layer. A treadle
may be attached to the handle end of the hinged grid and thus leave both
hands free to manipulate the stems and to remove that portion of the
broken wood which has not already dropped through the slots in the lower
grid but remains between the bars of the same.
The mechanical means for this purpose consist of a number of fluted
rollers between which the stems pass and by the flutes of which the wood
is broken. Sometimes scrapers are used in the same machine to help to
remove the small particles of wood. What remains in the hand after the
simple manual process is completed, or what is delivered from the
machine by the delivery rollers, are the unbroken fibrous layers to
which still adhere several particles of woody matter or shive as it is
called. A further operation, termed “scutching,” is necessary to remove
this shive and so leave the lengths of fibre as clean as possible.
SCUTCHING.--The operation of scutching may be considered in some
respects in the light of a scraping action in which the broken and
partially-clean, ribbon-like structures of fibres occupy a position
between a fixed and a movable board, and are subjected to the friction
between them. The simplest apparatus for this purpose consists of an
upright wooden board with a horizontal slot near its upper end and
through which the ends of the fibres are passed. The fibres hang
downwards, and while thus depending a flat wooden “scutching handle” or
flail--very similar in shape to a baking spit--is brought smartly with
its edge to traverse downwards against the fibres, and thus to remove
the objectionable shive but at the same time to prevent, as far as
possible, the destruction of the fibrous layer and the accumulation of
waste. The operative can expose as much of the fibrous layer as desired
to the action of the scutching handle in virtue of the slot, and after
one end of the “strick” is finished, the other end is treated similarly.
While the above hand method is largely practised and is quite
satisfactory where comparatively small quantities have to be treated, or
for very fine and expensive material where delicate treatment is
essential, the modern method of scutching is done by power. The feeding
and manipulation of the stricks are, however, still under the direct
control of the operative. In these mechanical scutchers it is usual to
employ six to twelve handles--narrower but longer than the hand
flail--and these handles project from a common centre or shaft, somewhat
after the form of the sails of a windmill. As the shaft rotates, the
handles are brought successively to act against the fibres as in the
simpler process.
Large quantities of Russian and other hemps are only partially cleaned,
and are termed “siretz” hemps, while in some districts where the most
valuable plants are grown, the bast layer is stripped from the stems,
and the material subjected in smaller quantities to the cleaning and
washing processes, thus producing a higher value fibre.
In hand scutching an operative cleans on an average about 10 to 12 lb.
of Italian hemp per hour, but such quantities can be, obviously, only
approximate, for the quantities prepared will vary greatly, depending as
they do upon the efficiency of the apparatus at command, the degree to
which the fibrous layer has to be cleaned, the quality of the material
and the skill of the operator. The better grades of fibre usually and
almost invariably receive more treatment than the lower grades.
The commercial value of hemp depends, as already stated, upon its
strength, colour, freedom from faults, and its spinning properties;
comparative values are scarcely possible unless in certain seasons,
because prices fluctuate greatly according to the demand for certain
grades of cordage as well as to the prices of other fibres which may be
used for similar goods.
Italian hemp can be spun into thinner or finer yarns than any of the
other hemps, and it is therefore a competitor with certain grades of
flax. French, Chinese and Russian hemps are also valuable, and besides
being used alone, are sometimes mixed with the coarser varieties of
Italian hemp for certain kinds of cordage and lines.
The following table shows the amount of fibre in tons for five years in
regard to Russian and Italian hemp imports to the United Kingdom.
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│ 1907. │ 1908. │ 1909. │ 1910. │ 1911.
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Russian │ 17,299 │ 15,753 │ 13,816 │ 12,576 │ 14,981
Italian │ 10,462 │ 8,133 │ 10,144 │ 10,298 │ 10,343
──────────┼─────────┼─────────┼─────────┼─────────┼────────
│ 27,761 │ 23,886 │ 23,960 │ 22,874 │ 25,324
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