THE MANUFACTURE OF PAPER FROM RAGS
[Illustration: FIG. 6.--A Rag Sorting House.]
The word rag is used to designate a very wide range of raw material
suitable for conversion into paper. In the case of high-class hand-made
writing papers only the best qualities are employed, such as new linen
and cotton cuttings from factories, or well-sorted rags of domestic
origin. The usual classification adopted by merchants who supply the
paper mills is somewhat as follows:--
New white linen cuttings (from textile factories).
New white cotton cuttings (from textile factories).
Fine whites (domestic rags).
Outshots (a quality between fines and seconds).
Seconds (a grade inferior to fines).
Thirds (inferior and dirty well-worn rags).
Coloured prints (of all grades and colours).
Fustians and canvas.
Manila and hemp rope.
Baggy, gunny, and jute.
The total amount of rag used in England for paper-making is not known.
The only figures available refer to rags _imported_; and these cannot
be regarded as a measure of consumption, which could only be arrived at
by first ascertaining the quantity of _home rags_ used. The imports of
rag at stated periods are given in the appended table:--
RAGS IMPORTED INTO ENGLAND.
--------------+----------+----------+----------+----------+----------
-- | 1872. | 1882. | 1892. | 1902. | 1905.
--------------+----------+----------+----------+----------+----------
Weight (tons) | 22,254 | 21,200 | 23,032 | 18,692 | 23,681
| | | | |
Value | £373,035 | £303,349 | £214,065 | £173,732 | £224,232
--------------+----------+----------+----------+----------+----------
_Sorting and Cutting._--All rags on arrival at the mill are carefully
sorted. This process is conducted entirely by women, who sort and cut
up the rags at special tables provided with cutting knives curved in
shape similar to a scythe. These are fixed at an angle in the centre
of the table, with the back towards and in front of each work-woman.
The top of the table is made of thick coarse wire so that some of the
dirt and foreign impurities may fall through. All buttons, hooks and
eyes, pins, leather, pieces of rubber, and other articles are carefully
removed, while seams and hems are also opened out. The rags are cut
into slips 3-5 inches long and then recut crosswise, and thrown into
suitable baskets or receptacles standing round the table, by which
means the _sorting_ operation is effectually carried out. The care and
attention given to the sorting is an important item in the manufacture
of papers of uniform quality, and in the best mills the sorting is
carried out to such an extent that twenty or twenty-five grades are
obtained.
[Illustration: FIG. 7.--A Rag Duster.]
_Dusting._--The rags are next passed through a machine which removes
dirt. This is a hollow cylindrical or conical drum having an external
covering of coarse wire cloth, which rotates inside a wooden box.
The shaft is provided with projecting spikes, so that the rags are
violently agitated in their passage through the machine. The dirt and
other impurities fall through the wire on to the floor of the room,
while the clean rags are discharged from the lower end of the drum. The
loss in weight varies according to the condition of the rags. With good
materials the loss may only be 1-2 per cent., while with dirty common
rags the loss during cleaning and dusting may amount to 10 per cent.
[Illustration: FIG. 8.--A Rag Cutter.]
_Boiling._--The further purification of the rags is effected by a
chemical treatment, viz., boiling at a high temperature with alkaline
substances, which process removes fatty, glutinous, and starchy matter
from the material.
[Illustration: FIG. 9.--Interior of Paper Mill for Hand-made Paper (R.
Batchelor & Sons).]
For this purpose a spherical digester is used, generally 7-9 feet
diameter, and capable of holding 2-2½ tons of rag. The boiler or
digester is filled with dusted rags, and the requisite amount of
alkaline solution added. The manhole is then closed, and steam admitted
through the hollow trunnions until the pressure reaches 20 or 30 lbs.,
at which pressure the boiling is continued for three to six hours
according to requirements, the digester rotating slowly the whole time
in order that the rags may be evenly and thoroughly boiled.
[Illustration: FIG. 10.--View of a Rag Boiler, showing connections.]
The liquor employed for boiling is a solution of caustic soda,
carbonate of soda, or milk of lime. In the case of caustic soda the
amount required varies from 5 to 10 per cent. of the weight of rag.
Caustic soda is preferable to lime, because it acts upon the grease
and other fatty matters, forming a soluble compound which is freely
removed in the subsequent process of washing. Many paper-makers,
however, use milk of lime, carefully strained through fine cloth,
almost exclusively. Considerable experience and skill are necessary
in this operation in order to avoid injury to the fibre not only as
regards its strength, but also its colour.
_Washing._--When the rags have been sufficiently boiled, the steam
is turned off and the pressure allowed to fall. This can be effected
quickly by blowing off from a valve fixed at the bottom of the boiler
opposite to the manhole. The cover is removed from the boiler and the
boiler slowly rotated in order that the contents may be discharged
into a tank placed below. The "black liquor," as it is called, is
then drained away from the rags, which are immediately subjected to a
preliminary washing. The process of washing must be carried out in a
thorough manner in order to remove all soluble compounds, which if left
would cause an unnecessary waste of bleach in the subsequent stages of
purification. There are many schemes employed for washing, most of them
being devised with the idea of using a minimum quantity of water.
The most general practice, in the absence of special machinery, is the
preliminary treatment in the tank below the digester, followed by a
more complete washing process in a machine known as a breaking engine.
This apparatus is a shallow oval-shaped vessel with circular ends,
divided lengthwise by a partition called a mid-feather, which, however,
does not extend the full length of the apparatus. In one of the two
channels into which the vessel is thus divided a heavy roll is fitted,
which is provided with a number of steel knives. On the floor of this
channel there is fixed a "bed-plate," also provided with projecting
knives which are parallel with the knives in the roll. The distance
between the knives in the roll and those in the "bed-plate" may be
altered as required by means of an adjusting screw. In the other
channel of the breaking engine there is fitted a "drum-washer," which
serves for the removal of the dirty water from the machine. This drum
is divided into sections by means of partitions which reach from the
centre to the circumference. The surface of the "drum-washer" consists
of a fine brass wire cloth supported by a coarser material placed
underneath.
[Illustration: FIG. 11.--A Breaking and Washing Engine.]
The breaking engine is half filled with clean water, and the rags are
thrown into the engine until it is suitably filled. The rotation of the
heavy roll causes the mixture of rags and water to circulate round the
vessel, the floor of which is so constructed that the pulp is drawn
between the roll and "bed-plate" and discharged over the "backfall,"
which is that portion of the sloping floor behind the "bed-plate."
The "drum-washer" rotates with its surface in contact with the mixture
in the engine, so that the dirty water passes through the wire cloth
and is caught in the curved sections or buckets inside the drum and
discharged into a trough adjacent to the centre, and thereby conveyed
away from the engine. Clean water is allowed to run into the vessel
at one end while the dirty water is discharged by means of the
"drum-washer." At the same time the rags are broken up by means of the
knives on the roll, so that when the rags are sufficiently washed, a
process which usually occupies four hours, they are also partially
disintegrated.
_Bleaching._--The clean disintegrated rag is next bleached by means of
ordinary bleaching powder solution. Bleaching powder is a substance
prepared by the action of chlorine gas on dry slaked lime, resulting
in the formation of a compound which has the property of bleaching or
"whitening" vegetable matters. The clear solution obtained by treating
the powder with water is utilised by the paper-maker for bleaching the
rag pulp.
Various methods are used for this purpose. Sometimes the requisite
volume of clear bleach liquor is added to the pulp in the breaker,
and the material kept in constant circulation until the operation
has been completed. In other cases the broken pulp is transferred to
a "potcher," which is a vessel similar in shape to the breaker, but
merely provided with paddles for keeping the pulp in circulation, and
bleached by the addition of chloride of lime solution.
Another method frequently adopted is to discharge the pulp from the
breaker, immediately after the addition of the bleach, into brick or
cement tanks, allowing the bleaching action to proceed spontaneously
without prolonged agitation.
In some instances the process is hastened by adding dilute sulphuric
acid to the pulp after the bleach liquor has been run in, or by
heating the mixture with steam. For high-class papers such devices as
this are seldom resorted to, as experience shows that the colour of
pulp bleached by drastic methods does not maintain a high standard.
The pulp is then thoroughly washed in order to remove every trace
of residual bleach, and also the soluble compounds which have been
formed during the operation. Very large quantities of water, clear
and free from suspended dirt, are necessary. In some mills any excess
of bleach is neutralised by the use of an "antichlor" such as sodium
hyposulphite, or sodium sulphite, but the best results are undoubtedly
obtained when the quantity of chemicals used is kept at a minimum.
If the pulp is bleached in a breaker or potcher, the washing is
effected by the aid of the drum-washer. With pulp treated in steeping
tanks, fresh water is allowed to percolate or drain slowly through the
mass.
ELECTROLYTIC BLEACHING.
The substitution of a sodium hypochlorite solution for the ordinary
calcium hypochlorite solution obtained from common bleaching powder has
been the aim of specialists for many years. As early as 1851 a patent
was taken out by Charles Watt for decomposing chlorides of the alkali
metals and the formation of hypochlorites. It was not until 1886 that a
practical method was devised for producing an electrolysed solution of
salt, but in that year Hermite introduced a continuous process in which
an electrolysed solution having a strength of three grammes chlorine
per litre was passed continuously into the potcher.
Many patents for the electrolysis of salt have been taken out during
the last twenty years, of which the Bird-Hargreave process is in
operation in England, the Rhodin process in America, the Siemens and
Halske in Norway, and the Oettel and Haas apparatus in Germany. The
figures relating to the latter apparatus may be mentioned as typical of
the present condition of electrolytic bleaching. The apparatus consists
of a narrow rectangular trough divided into a number of chambers
through which a solution of brine flows at a constant and steady
rate. The electric current is passed through the solution by suitable
electrodes, the temperature being kept down by means of a cooling coil.
The cost of producing the bleach liquor as given by the inventors of
the apparatus from the results of actual working are shown in the
following table:--
TABLE GIVING ANALYSIS OF COST FOR PRODUCING BLEACH LIQUOR.
Capacity of tank 750 litres = 166 gallons.
Strength or density of brine 1·5 Baumé, or 23 Twaddell.
286 lbs. of common salt required for 166 gallons.
--------------------+-------+-------+------+-------+--------+--------
Hours worked | 2 | 4 | 6 | 8 | 10 | 12
Grammes of chlorine | | | | | |
per litre produced| 4·35 | 7·38 | 9·9 | 12·42 | 14·31 | 16·20
Temperature C. of | | | | | |
brine during | | | | | |
operation | 20 | 21 | 20 | 21 | 20 | 20
Ampères of 110 volts| 55 | 50 | 46 | 52 | 47 | 43
Power in h.p. hours | 16 | 31 | 45 | 61 | 75 | 89
Cost of the h.p. at | | | | | |
·22_d._ per h.p. | | | |1_s._ |1_s._ |1_s._
hour | 3½_d._| 6¾_d._|10_d._| 1½_d._| 4½_d._| 7½_d._
Cost of salt |1_s._ |1_s._ |1_s._ |1_s._ |1_s._ |1_s._
| 6_d._| 6_d._| 6_d._| 6_d._ | 6_d._ | 6_d._
Total cost |1_s._ |2_s._ |2_s._ |2_s._ |2_s._ |3_s._
| 9½_d._| 0¾_d._| 4_d._| 7½_d._| 10½_d._| 1½_d._
Total chlorine | | | | | |
obtained in kilos.| 3·262 | 5·535 | 7·425| 9·315 | 10·732 | 12·150
Cost of chlorine per| | | | | |
kilo. |6·6_d._| 4½_d._|3¾_d._|3·4_d._| 3·2_d._| 3_d._
Salt used per kilo. | | | | | |
chlorine | 35 | 20 | 15 | 12 | 10 | 9
--------------------+-------+-------+------+-------+--------+--------
The above costs have been estimated on prices as follows:--
Coal 10_s._ per ton.
Salt 12_s._ per ton.
After 12 hours the 166 gallons (750 litres) are converted into
electrolytic bleach liquor containing 26¾ lbs. of active chlorine
(12·15 kilos.).
_Beating._--Although the rags are reduced by the breaking engine to a
condition of fibrous lint, called "half-stuff," they are not fit for
conversion into paper. They have to be _beaten_ in special machinery
until a complete separation of the single fibres has been effected,
and this process is rightly regarded by many paper-makers as the most
important stage of manufacture.
[Illustration: FIG. 12.--Oettel and Haas' Apparatus for the manufacture
of Electrolytic Bleach Liquor.]
The beating engine is similar in construction to the breaking engine,
but there are certain essential differences in arrangement and
manipulation. There is usually no drum-washer; the roll contains a
large number of knives which are fixed in clumps or sets of three
round the circumference; the lowering of the roll upon the bed-plate
is carefully watched and controlled, and the desired effects are only
obtained by strict attention to the condition of the pulp during the
whole process.
[Illustration: FIG. 13.--The "Hollander" Beating Engine.]
The beater is first partially filled with water, and the drained
half-stuff added gradually until the "furnish," a convenient term
applied to the contents of the engine, has the proper consistency,
which varies according to the nature and quality of paper required.
The mass is circulated steadily round the engine by the action of the
beater roll, which is lowered from time to time until the distance
between the knives on the roll and those on the bed-plate has been set
to the desired adjustment. This lowering of the roll and its proper
adjustment call for the greatest care.
_Influence of the Beating._--The importance of this operation can
easily be judged from one or two specific examples. In the case of rag
papers the two extremes of variation are represented by the ordinary
blotting paper on the one hand and a hard strong writing paper known
as a _loan_ on the other. Now the great difference in these papers may
be traced to the careful selection of the rag and the treatment in the
beater as the two primary causes of the final results.
For blotting papers it is essential that the rags should be old and
tender. In the beating operation subsequent to the usual boiling and
bleaching processes the half-stuff is beaten quickly with sharp knives,
the roll being lowered soon after the engine is filled, so that the
beating is finished in about one to one and a half hours.
For the strong writing paper new strong rags are selected. In the
beating process the knives used are dull, the roll is lowered slowly
and cautiously, and the beating goes on for eight to ten hours.
The effect of such difference in treatment is easily seen by
examination of the fibres of the papers under the microscope. In
the first case the fibres appear short with clean cut ends, the
shape little distorted, the structure well defined, bearing a strong
resemblance to the unbeaten material. In the case of the well-beaten
paper the ends of the individual fibres appear to be drawn or frayed
out, the fibres do not possess the sharp well-defined outline
characteristic of blotting paper; they are partly split up into
fibrillæ which lie together in a confused mass.
In the _blotting_ paper these effects are produced because the knives
being sharp cut up the material quickly, and in the _writing_ paper
because the dull "tackle" tends to draw out the fibres and tear them up
lengthwise.
The practical result is a spongy, soft, and bulky blotting and a hard,
strong, heavy writing paper. Of course the great difference between a
blotting and a writing paper is not all due to this one operation,
but is obtained by a series of operations, of which one of the most
important is, however, the beating.
_Colouring the Paper._--The pulp is brought to any desired tint by
the addition of mineral pigments or aniline dyes to the contents of
the engine. The latter soluble dyes, however, are seldom used for
high-class rag papers. Prussian blue, ultramarine, and smalts are
chiefly used for this purpose, giving toned blue, azure, and blue laid
papers.
[Illustration: FIG. 14.--The Hand Mould showing Frame and Deckle.]
_Making the Paper._--The beaten pulp, when duly prepared, is run from
the engine into store tanks known as stuff chests, ready for the
actual manufacture. The pulp properly diluted with water is strained
through special screens to remove any insufficiently beaten material
and any impurities present, after which it is run off into the vat, a
square-shaped vessel built of wood or stone.
The apparatus used in forming the sheets is called a _hand mould_.
The mould is a rectangular frame of mahogany upon which is stretched
tightly a fine wire cloth, the surface of the latter being kept flat
by a coarser wire cloth fixed underneath, supplemented by wedge-shaped
pieces of wood. A second frame called the _deckle_ fits on to the mould
in such a manner as to form a shallow tray, the bottom of which is the
fine wire cloth.
The vatman takes up the mould with both hands and dips it into the
vat full of pulp in a slanting position, drawing it through the stuff
towards him in a peculiar manner and lifting it out from the vat with
a definite quantity of the mixture in the frame. As the water drains
away from the pulp, through the wire cloth, he imparts a shaking motion
to the mould in order to cause the fibres to "felt" properly, this
felting or interlacing of the fibres being an essential feature in the
manufacture of a good sheet of paper. When the water has drained away
sufficiently from the pulp, the vatman removes the deckle from the
mould and passes the latter over to the coucher, who takes the mould,
reverses it, and presses the contents, which may now be described as a
wet sheet of paper, down on to a damp piece of felt, by which means the
paper is transferred to the felt. He returns the mould to the vatman,
who meanwhile has made another sheet with a duplicate mould, and then,
having laid a second felt upon the wet sheet of paper, he proceeds to
transfer the next sheet of paper to the second felt. This process is
continued until a pile is formed consisting of wet sheets of paper
alternated with pieces of felt.
The pile is at once submitted to great pressure in the hydraulic press,
and the excess water slowly forced out, while at the same time the
sheets are compressed and thus "closed up," as it is termed. When all
the excess water has been removed as far as possible, the pile is taken
away and the sheets of damp paper taken out, the felts being placed in
one pile ready for further use, and the sheets of paper in a second
ready for the next process.
The papers are put back into the press without felts between the sheets
and left for some time. In most cases the sheets are turned round or
mixed in with the sheets of another pile, before pressing. In this way
any unevenness or irregularity in the sheets is counteracted and a more
uniform result obtained.
When these changes are repeated several times the paper acquires an
even texture and becomes firm and hard.
_Drying the Paper._--The sheets are hung up in the _loft_, as the
drying room is called, upon poles or ropes. The moisture gradually
evaporates, and the paper is thus dried by exposure to air. In winter
it is necessary to warm the air in the loft, as the air is then
saturated with moisture. In lofts of limited capacity the air is heated
in order to hasten the process, but the best paper is allowed to dry
naturally, as by this means the shrinkage is gradual and a maximum
strength is attained.
[Illustration: FIG. 15.--Apparatus for Sizing Paper in continuous
Rolls.]
_Sizing the Paper._--The dried paper as it leaves the loft is termed
_Waterleaf_ because, being unsized, it readily absorbs water, and
therefore before it can be used it must be sized. For this purpose
it is dipped into a solution of gelatine, an operation described as
_tub-sizing_ or _animal-sizing_, the former term being used on account
of the tub in which the size is kept, and the latter on account of
the fact that the gelatine is made from animal matter such as hides,
cartilage, hoofs, and other refuse.
_Animal Size._--This is prepared from hide pieces, skins, and the
like by a simple process, which, however, requires a good deal of care
in order to obtain the best results. The material is first thoroughly
washed in plenty of clean water, and then heated with a definite
quantity of water in a steam jacketed copper pan. The pieces slowly
dissolve until a solution of gelatine is produced, and after the dirt
and impurities have settled to the bottom of the pan the clear liquid
is drawn off into store vessels. There are many details of a technical
character to be attended to in the manufacture of good gelatine, and as
the process is expensive, considerable attention is demanded at this
stage in the completion of a sheet of paper.
The dry sheets of paper are sized by the simple expedient of dipping,
or by the passage of the paper through a long trough. In the first
case the workman takes up a number of sheets and dips the bunch into a
vat of size at the proper temperature, about 100° Fahrenheit. He then
allows the surplus size to drain off, and the sheets are submitted to a
slight pressure in order to remove the excess of gelatine that will not
drain off.
In the second case a different method is adopted in that the sheets
of paper are carried by travelling felts through a bath of heated
size, the excess gelatine being removed by the action of rubber or
wooden rollers through which the papers are passed before leaving the
apparatus. The papers are quickly and evenly sized by this method,
which is now most generally used.
_Glazing._--When the sheets of paper are quite dry they are ready for
glazing, a process which turns the dull rough surface of the sized
sheet into a highly polished smooth surface fit for use. The sheets are
placed singly between copper or zinc plates, and a pile of these passed
several times through heavy iron rollers, great pressure being applied
to the latter during the operation.
[Illustration: FIG. 16.--A Supercalender.]
The amount of polish imparted by this plate-glazing process, as it
is termed, can be varied considerably. With a light pressure and
few rollings, the sheet of paper can be turned out having a fairly
smooth surface, and without a conspicuously shiny appearance. By
employing a great pressure and repeated rolling a much higher surface
is attainable. If the plates are hot a still higher finish is
possible. Machine-made rag papers are glazed usually by means of the
supercalender, which is a stack of alternate steel and paper rolls
placed one above the other in a vertical position. The reel of paper
passes between these rolls and becomes highly surfaced.
This operation effects many changes in the paper, besides imparting
a good finish. The thickness of the sheet is reduced by about 40 per
cent., the fibres being compressed much closer together. The tensile
strength of the paper is also materially increased, and in every
way the paper is improved. Moderation is essential in this as in
everything, because excess of glazing weakens a paper, rendering it
brittle and liable to crack when folded.
_Laid and Wove Papers._--When certain papers are held up to the light
and carefully examined it will be noticed that they appear to contain
delicate transparent lines running parallel with one another at equal
distances of about an inch, and that these are intersected by similar
transparent lines running at right angles, which are much closer
together. Such papers are known as _Laid_ Papers, and the peculiar
formation of the transparent lines is due to the construction of the
mould used in the making. The wire surface of this mould consists of a
number of somewhat stout wires placed about one inch apart, interwoven
with finer wires running across and at right angles, which are threaded
much closer together. When the mould is dipped into the vat and
withdrawn, the water drains away from the under surface of the wire,
and the moist pulp settles down on the upper surface; but since the
coarser wires project a little from the finer threads, the paper is
slightly thinner along those wires, though to an almost infinitesimal
extent, with the result that on drying the sheet appears to contain
transparent lines.
_Wove_ papers are so called from the nature of the mould used. The
surface of the mould in this case consists of fine wires equally
distributed, being woven in such a manner that the wires are
equidistant from one another, as in ordinary wire gauze. A wove
paper, on being examined in the light, simply shows a number of small
diamond-shaped spaces, which in the majority of instances are difficult
to detect.
_The Watermark._--The transparent device observed in many papers when
held up to the light is known as the watermark, a term probably derived
from the conditions existing at the time the sheet of paper is made on
the mould. The effect is produced by means of a raised design sewn or
soldered to the surface of the mould, the design being fashioned out of
fine wire.
[Illustration: FIG. 17.--The First Watermark in Paper.]
When a mould thus fitted with the design is dipped into a vat of pulp
and lifted out, the water falls through the wire, and the pulp sinks
down on to the surface of the mould, forming a replica, so to speak,
of the design, which is easily seen when the dry paper is held up to
the light, because the paper is thinner just at those points where the
wires forming the design come into contact with the wet pulp.
Some of the watermarks are very elaborate and interesting. A familiar
illustration of a beautiful design of this description is to be found
in the Bank of England notes. As a general rule the ordinary watermark
consists of a mere trade term such as "Vellum," "Zenobia," or of the
name of the manufacturer, such as "J. Whatman," "R. Batchelor," and
so on. In the earlier days of paper-making many highly interesting
designs were used, and some of these are still extant. In fact many of
the names by which certain standard sizes of paper are known owe their
origin to the watermarks employed.
The earliest known watermark bears the date A.D. 1301, being in the
form of a globe and cross, as shown. Of equal interest are those
designs from which certain papers are called foolscap, crown, pott,
post, royal, columbier, and so on. The watermarks are now little used,
but the terms are still retained, as indicating the size of the sheet.
MICROSCOPIC FEATURES OF COTTON AND LINEN FIBRES.
The _cotton_ fibre is about 30 mm. long, with an average diameter of
·025 mm. of tube-like shape, and having a prominent central canal.
There are no cross markings on the cell walls, and the ends of the
fibre are rounded off into a somewhat blunt point. It exhibits a marked
tendency to twist itself, especially if dry, and this peculiarity is
readily observed with the raw material.
The process of paper-making alters the characteristic structure of the
fibre very greatly. The ends of the fibre are seldom to be seen; the
curious twist is less prominent, and the fibres are torn and destroyed.
The effect of the beating process, for example, on cotton is easily
to be noticed by comparing the fibres of a blotting paper under the
microscope with the fibres of a _bank_ or _loan_ paper.
The distortions produced by prolonged beating renders the
determination of the exact percentage of cotton in a rag paper rather
difficult, but the features to be looked for are the absence of pores,
cross markings, the existence of a central canal, striations produced
in many cases on the cell walls parallel to the length of the fibre.
The structural features are more readily observed when the fibres are
stained with a suitable reagent. (See page 71.)
[Illustration: FIG. 18.--Cotton.]
The _linen_ fibre has an average length of 27 mm. with a diameter of
·02 mm. The raw flax is very different from raw cotton and is easily
distinguished. The fibre is slender in shape, having thickened knots
at regular intervals throughout its length, the general appearance of
which may be compared to a stick of bamboo. The central canal of the
fibre is extremely narrow, running like a small thread through the
length of the fibre. The cell walls are further marked by numerous
pores, which appear as small dark lines running from side to side, but
not meeting in the centre.
[Illustration: FIG. 19.--Linen.]
In the treatment necessary for making paper these characteristics are
largely destroyed, and while it is quite easy to ascertain that a paper
is of linen, or of cotton, or that a paper is mainly cotton with a
small percentage of linen, yet there are conditions under which it is
difficult to determine the exact percentage of cotton or linen in a
rag paper. If, for example, a paper contains nearly equal quantities
of cotton and linen, the exact proportions cannot be determined closer
than 10 per cent., especially in well-beaten papers.
REAGENT FOR STAINING FIBRES.
_Preparation._--Dissolve 2·1 grams potassium iodide and 0·1 grams
iodine in 5 c.c. of water. Mix this solution with a solution containing
20 grams of dry zinc chloride in 10 c.c. of water. Allow the mixture to
stand; pour off the clear liquid into suitable bottles.
COLORATION PRODUCED.
Cotton, linen, hemp.--Wine red.
Esparto, straw and wood cellulose.--Bluish violet.
Mechanical wood, unbleached jute.--Yellow.
Manila hemp.--Blue, bluish grey to yellow.