ESPARTO AND STRAW
ESPARTO PAPERS.
The value of Esparto for the manufacture of high-class printing and
medium quality writing paper is well known. This material has qualities
which cannot readily be obtained from other fibres, such as rag and
wood pulp. It is chiefly used in papers required for lithographic
printing, books, and art illustration, since it gives a sheet having a
good surface and one which is soft and flexible.
The grass is obtained from Spain, Morocco, Algeria, Tunis, and Tripoli,
in which countries it grows wild, requiring very little cultivation.
The condition of the crop is improved by proper treatment, and in
districts where the grass is cut for export as a paper-making material
attention is given to cultivation.
The plant grows to a height of three or four feet, and when mature the
long blades of grass curl up into the form of a cylinder resembling a
piece of wire. The leaf consists of two parts, the stalk and a sheath,
which are easily separated when harvested. The grass is pulled up by
hand and stacked into heaps in order that it may be dried by the heat
of the sun, after which process it is carefully picked over for the
removal of all extraneous matter and impurities. It is then graded,
the best sorts being kept for weaving, and the remainder being sold
for paper-making. It is packed up into large bales of about 4 cwt.
capacity, compressed into small bulk by powerful presses, and shipped
to England.
_Esparto Pulp._--The first process in the manufacture of the paper
is cleaning. The bundles of grass are opened up, shaken out, and put
through a willowing machine. This consists of a hollow conical drum,
the outer surface of which is a coarse wire cloth. Inside the drum is
fitted a shaft provided with wooden teeth, and as the grass passes
through it is tossed about and the dust removed. The clean grass is
conveyed by travelling belts to the digester house. For the production
of a high-class paper the grass is often examined by girls, who stand
on either side of the travelling conveyer and take out any coarse root
ends and foreign material not removed by the willowing machine.
_Boiling._--The object of submitting esparto to chemical treatment is
to obtain a pure paper-making fibre known as cellulose. The composition
of this raw material is shown by the following analysis:--
_Spanish Esparto._
Cellulose 48·25
Water 9·38
Aqueous extract 10·19
Pectous matter 26·39
Fatty matter 2·07
Ash 3·72
------
100·0
------
Yield of dry cellulose obtained in actual practice
from good raw material 45 to 48%
By boiling the esparto with caustic soda under pressure for a stated
time, the non-fibrous constituents are removed, leaving the cellulose
in a more or less pure form according to the severity of the chemical
treatment.
[Illustration: FIG. 20.--An Esparto Duster.]
In practice the grass is packed tightly into upright stationary
digesters and a definite quantity of caustic soda solution added,
the amount of chemical used being equal to 15-18 per cent. of the
weight of grass packed into the digester. The form of digester almost
universally employed is that known as the Sinclair's "vomiting" boiler,
which is constructed so that a continuous circulation of the liquid
is maintained by means of what are called "vomit" pipes. These are
fitted to the sides of the digester in such a manner that the caustic
soda solution circulates from the bottom of the digester, up through
the "vomit" pipes, and is discharged downwards upon the contents of
the boiler through a perforated plate fixed in the upper part of the
digester. The requisite quantity of caustic soda solution is placed in
the digester, and steam admitted into the bottom of the vessel while
the grass is being thrown in. In this way a much larger weight of grass
can be boiled at one operation, since the bulk is greatly reduced when
the grass has become thoroughly soft and wet.
[Illustration: FIG. 21.--Sinclair's "Vomiting" Esparto Boiler.]
When the boiler is loaded the inlet is closed up and steam turned on
to the full pressure of about 40 or 50 lbs., this being maintained
for a period of about four hours. The non-fibrous constituents of the
esparto are gradually dissolved out by the caustic soda, and when the
operation is completed the black liquor is run off from the digester
into large store tanks, and the esparto grass which remains in the
digester is then completely washed until the soda is almost entirely
washed out.
[Illustration: FIG. 22.--A Porion Evaporator.]
The conditions for boiling and bleaching esparto are varied by the
paper-maker as circumstances require. A maximum yield of fibre is
obtained when the least possible quantity of caustic soda is used, but
a larger percentage of bleaching powder may be necessary to ensure a
well bleached pulp. The use of an excess of caustic soda is probably
the general practice for several reasons, amongst which may be noted
the advisability of guarding against irregularities in the quality
of the esparto, and consequent insufficient boiling, as well as the
advantage of having some free caustic in the spent liquors to prevent
the furring up of the tubes of the evaporating apparatus in the soda
recovery department.
The following experiments, given by a contributor to the _Paper Trade
Review_ some years ago, are interesting as showing the effect of
varying proportions of caustic soda used per unit of grass:--
EXPERIMENTS _RE_ YIELD OF AIR-DRY BLEACHED PULP FROM ORAN ESPARTO.
Air-dry Pulp containing 10 per cent. water.
---+--------+----------------+------------------+-------+--------+-------
|Esparto.| Soda Liquor. | Conditions of | | Dry |
| | | Boiling. |Weight | Pulp |Bleach-
---+--------+-------+--------+------+-----+-----+ of |on Dry | ing
No.| Wt. |Volume,| Per |Time. |Temp.|Pres-|Air-dry|Esparto.|Powder.
| taken. | C.C. | cent. |Hours.|° C. |sure.|Pulp. | Per | Per
| Grams. | |Na_{2}O.| | | Lbs.|Grams. | cent. | cent.
---+--------+-------+--------+------+-----+-----+-------+--------+-------
1 | 200 | 800 | 1·58 | 3 | 142 | 55 | 87·30 | 43·65 | 29·5
2 | 200 | 800 | 2·13 | 3 | 142 | 55 | 80·67 | 40·33 | 18·5
3 | 200 | 800 | 2·69 | 3 | 142 | 55 | 72·00 | 36·00 | 10·5
---+--------+-------+--------+------+-----+-----+-------+--------+-------
No. = No. of Experiment.
PRACTICAL DATA CALCULATED FROM EXPERIMENTS.
---+------------+----------+------------+-------------+----------------
| Boiling. |Weight of |60 per cent.| Bleaching |For One Ton of
No.+------+-----+Esparto to|Caustic Soda| Powder |Esparto used.
|Time. |Pres-|give 1 ton|required to |required to +----------------
|Hours.|sure.| Pulp. | Digest |Bleach 1 ton | 60 per |Bleach-
| |Lbs. | Cwts. | Esparto. |Air-dry Pulp.| cent. |ing
| | | | Cwts. | Cwts. |Caustic.|Powder.
| | | | | | Lbs. | Lbs.
---+------+-----+----------+------------+-------------+--------+-------
1 | 3 | 55 | 45·8 | 4·30 | 5·26 | 210 | 260
2 | 3 | 55 | 49·5 | 6·27 | 3·39 | 282 | 156
3 | 3 | 55 | 55·5 | 8·90 | 1·96 | 358 | 79
---+------+-----+----------+------------+-------------+--------+-------
No. = No. of Experiment.
_Recovery of Spent Liquor._--As it is possible to recover 75 to 80 per
cent. of the soda originally used in digesting the esparto, the washing
of the boiled grass is conducted on scientific principles in order to
ensure a maximum recovery of soda at a minimum cost.
The recovery is effected by evaporating down the black liquor, together
with the washing waters, to a thick syrupy mass, which can be burnt.
The organic and resinous constituents of the esparto which have been
dissolved out by the caustic soda, forming the soluble soda compounds,
ignite readily, and during combustion the organic soda compounds are
converted more or less completely into crude carbonate of soda.
It is obvious, then, that the cost of recovery depends mainly on the
quantity of weak washing water which has to be evaporated. Consequently
methods are devised by means of which the grass is thoroughly washed
with as little water as possible, and some of the methods are very
ingenious.
The spent liquors and washing waters are evaporated to a small bulk in
a vacuum multiple effect apparatus, and the thick liquid mass obtained
by evaporation is burnt either in a rotary furnace or on an ordinary
hearth. Every precaution is taken to effect this operation with a
minimum quantity of coal. The burning off of this mass results in the
formation of a black substance which is taken away from the furnace
and allowed to char or slowly burn until the impure white soda ash, or
carbonate of soda, is obtained.
Two systems of recovery are in general use, which deserve a brief
notice:--
_Direct Evaporation._--The liquors may be evaporated to a small bulk
ready for incineration by treatment in long shallow pans or furnaces,
the heat necessary for the process being obtained mainly from the
combustion of the thick concentrated liquor. The most familiar type of
this form of apparatus is the Porion evaporator.
[Illustration: FIG. 23.--Scott's Multiple Effect Evaporator.]
The combustion of the concentrated liquor is started by a coal furnace
at one end of the apparatus. The thick viscous mass catches fire and
burns with a fierce flame, and the heat is utilised in evaporating the
weaker liquors which flow continuously through shallow brick troughs,
the surface of which is freely exposed to the heat and flames from the
hearth where the organic soda compounds produced in the boiling of
esparto are being incinerated and converted into soda ash.
Under suitable conditions this evaporator is most economical in its
results. It can be erected cheaply, and when all the heat is fully used
in every possible direction it can be worked at a low cost compared
with the more modern multiple effect evaporators.
_Vacuum Multiple Effect Evaporation._--Advantage is taken of the
fact that water boils at a lower temperature in a vacuum than at the
ordinary pressure of the atmosphere. There are many forms of apparatus
based on this principle, amongst which the most recent is Scott's
evaporator. The black liquor from the boilers is pumped through tubes
heated externally by high-pressure steam. The liquor is passed into a
chamber in which a slight vacuum is maintained, so that immediately on
entering, the liquor parts with a good deal of water in the shape of
steam. The steam liberated is utilised in producing further evaporation
of the partially concentrated liquor, and this operation is repeated
several times until the concentration is effected to the desired point.
In most cases the actual incineration of the thick liquor is carried
out in a rotary furnace when such an apparatus as this is used.
EVAPORATION TABLE.
Showing the volume of liquor obtained by evaporating 1,000 gallons of
weak black lye of density _d_ to a higher density D.
-------------+-----------------------------------------------------
Lower | Higher Density D (Twaddell) at 100° F.
Density _d_ +-----------------------------------------------------
(at 100° F.).| 20. | 25. | 30. | 35. | 40. | 45. | 50. | 55. | 60.
-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----
2 |100 | 80 | 66·6| 57·1| 50 | 44·4| 40 | 36·3| 33·3
3 |150 |120 |100 |85·7 | 75 | 66·6| 60 | 54·5| 50
4 |200 |160 |133·3|114·3|100 | 88·8| 80 | 72·7| 66·6
5 |250 |200 |166·6|143 |125 |111·0|100 | 90·9| 83·3
6 |300 |240 |200 |171·4|150 |133·3|120 |109 |100
7 |350 |280 |233·3|200 |175 |155·5|140 |127 |116·6
8 |400 |320 |266·6|228·6|200 |177·6|160 |145·5|133·3
9 |450 |360 |300 |257 |225 |200 |180 |163·5|150
10 |500 |400 |333·3|286 |250 |222 |200 |181·8|166·6
-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----
EXAMPLE:--1,000 gallons of weak liquor at a density of 7° Twaddell
are reduced to a volume of 200 gallons having a density of 35°
Twaddell, or to a volume of 140 gallons with a density of 50°
Twaddell, by evaporation.
_Preparation of Caustic Soda._--The crude soda ash recovered from
previous boiling operations is dissolved in large lixiviating tanks
and extracted with hot water. The clear solution obtained after
all impurities have been allowed to settle is pumped up into the
causticising tanks, where it is converted into caustic soda, the loss
due to the amount of soda not recovered being made up by the addition
of ordinary soda ash. The causticising pans are large circular iron
vessels usually 9 feet diameter and 8 or 9 feet deep, into which a
known volume of the recovered carbonate of soda solution is placed.
A weighed quantity of ordinary quicklime is then put into a perforated
iron cage which is fixed inside the causticising pan at such a level
that the whole of the lime is immersed in the solution. The liquor is
kept in constant circulation by means of an agitator and heated to
boiling point, with the result that the chemical reaction sets in,
the carbonate of soda being converted into caustic soda and the lime
being thrown out as chalk. When the operation is completed, the steam
is turned off and the chalk allowed to settle. The clear liquor is
carefully strained off and pumped up into store tanks from which the
required quantities are drawn off into the digesters as circumstances
demand.
_Washing._--The grass which has been partially washed in the digester
is dug out by the workmen and discharged through a manhole fitted on
one side of the digester near the bottom. It is then conveyed in any
convenient manner to the breaking engine, in which the grass is more
completely washed. This important machine has already been described
on page 53. The floor of the vessel slopes slightly upward towards
the front of the roll and falls suddenly behind the roll, in order to
promote a circulation of the contents of the engine round and round the
vessel.
A definite weight of boiled grass is thrown into the engine together
with a large quantity of fresh water. The circulation of the roll draws
the mixture of pulp and water between the knives, breaking it up and
at the same time discharging it behind the beater roll, and producing
a continuous circulation of the mixture in the two sections of the
vessels.
The dirty water is continuously removed from the vessel by means of a
"drum-washer." This is a large hollow drum, the outer surface of which
consists of a fine wire cloth, the interior of the washer being fitted
with specially curved scoops. The drum-washer is lowered until it is
half immersed in the mixture of pulp and water, and as it rotates the
dirty water finds its way through the wire cloth, being caught up by
the internal scoops and discharged through a pipe to a drain outside
the breaking engine. At the same time fresh water is run into the
vessel at one end, and the continuous washing of the pulp thus effected.
_Bleaching._--The clean boiled grass is bleached by means of a solution
of chloride of lime.
There are several methods used for this purpose, each of which has
special advantages of its own, though this is largely a question of
local conditions:--
(A) The pulp can be bleached in the washing engine directly the grass
has been sufficiently cleaned. In this case the flow of fresh water
is stopped and as much water as possible removed by means of the
drum-washer. The drum-washer is then raised out of the pulp and a
known volume of bleaching powder solution corresponding to a definite
weight of dry powder is added to the contents of the breaking engine.
The amount used depends on the quantity of dry grass in the breaking
engine, the usual proportion being 8 to 10 per cent. on the calculated
air-dry weight of raw grass. As the stuff circulates round the engine
the colour gradually changes from dark yellow to white.
The process is sometimes hastened by blowing a small quantity of steam
into the mixture and thereby raising its temperature. Considerable care
must be exercised in using heat, because pulp bleached quickly by this
means is liable to lose colour at the later stages of manufacture.
When the pulp has been bleached to the required extent, the drum-washer
is again lowered into contact with the bleached pulp, and the latter
is thoroughly washed so as to be quite free from traces of bleach and
other soluble impurities.
(B) Esparto is often bleached in a "Tower" bleaching engine which
consists of a tall cylindrical vessel of 9 feet diameter, and 15 or 16
feet deep, at the bottom of which is fixed a small centrifugal pump.
The boiled grass together with sufficient water and clear bleaching
powder solution is placed in the engine; the centrifugal pump draws
the mixture from the bottom of the vessel and discharges it, by means
of a large external pipe, direct into the top of the vessel, where, as
it falls, it comes into contact with a circular baffle-plate, which
distributes the pulp evenly over the surface of the mixture in the
vessel. A continuous and rapid circulation is thus maintained, and the
process is said to be very effective. The bleached pulp is subsequently
washed free from any traces of bleach.
(C) Esparto is frequently bleached by the "steeping" process. In this
case the pulp is washed in the breaking engine, mixed with the required
quantity of bleach, and at once discharged through the outlet pipes
of the engine into large brick tanks, where the bleach is allowed to
act quietly upon the boiled grass. This method produces a pulp of good
colour and is economical.
Whichever process of bleaching is adopted, it is necessary to remove
all the by-products formed during the process, as these soluble
by-products if left in the mixture produce a lowering of colour.
The presence of small traces of bleaching powder solution can be
detected by the use of starch and potassium iodide test papers. If a
handful of the pulp after bleaching, when squeezed out, does not turn
the test paper violet or blue, then the absence of any free bleach is
taken for granted. The slightest trace of bleach will turn such test
papers blue or violet according to the amount present. This is the test
usually applied by the men in charge of the bleaching operations.
_Making Sheets of Esparto Pulp._--For convenience in handling, it is
usual to work up the washed and bleached pulp into the form of moist
sheets. This is effected on a machine known as a "presse-pâte," an
apparatus which closely resembles the wet end of a paper machine. It
consists of a set of flat strainers or screens, a horizontal wire
similar to the paper machine wire, provided with deckles, the usual
couch rolls, and press rolls.
[Illustration: FIG. 24.--A Presse-pâte for Esparto Pulp.]
The pulp diluted with water is passed through the screens and on to
the horizontal wire, where it is formed into a moist sheet, the water
draining away from the wire, and also being removed by vacuum pumps.
The thick sheet of pulp is carried through the couch rolls and press
rolls, being finally wound up on a wooden roller at the end of the
machine. In this moist condition it is ready for use in the mill.
_Dry Esparto Pulp._--When the bleached pulp is intended for export
a more elaborate machine is used--to all intents and purposes a
paper-making machine--by means of which the continuous sheet of moist
pulp is dried and cut up into smaller sheets of suitable size. These
dried sheets are packed up in bales containing 2 cwt. or 4 cwt. of
dried pulp, then wrapped in hessian and bound with iron wires.
_Other Methods._--Since the yield of esparto pulp from the raw material
is less than 50 per cent. and it requires 45 cwt. of grass to make
one ton of finished pulp, methods have been devised for treating the
grass in the green state in the districts where it is grown, but so far
nothing has been done on a large scale.
_The isolation of the cellulose by alkaline treatment in the cold_ has
been suggested, but the method never passed beyond the experimental
stage. This process was indeed first mentioned by Trabut, who many
years ago considered that the removal of non-fibrous constituents from
fresh grass could be readily accomplished by the less drastic treatment
of the esparto with alkaline carbonates of soda and potash at ordinary
temperatures.
_The production of esparto pulp by bacteriological fermentation_ is an
idea of later date. According to the inventor, the grass is crushed
mechanically by means of rollers and then immersed in sea water
inoculated with special bacillus obtained from esparto, and gradually
resolved into cellulose and soluble by-products by fermentation which
is complete in about eleven days. The commercial value of this idea has
not yet been demonstrated.
ESPARTO PULP: MICROSCOPICAL FEATURES.
The pulp of esparto when examined under the microscope is easily
recognised, first by the characteristic appearance of the long slender
cylindrical-shaped fibres, and secondly by the numerous cells always
present. These cells consist of cuticular vessels with serrated edges,
and also of small pear-shaped seed hairs, the shape of which is a ready
means of identifying esparto. An examination of the transverse section
of the raw material indicates the source of these pear-shaped vessels.
_Test for Esparto in Papers._--Paper containing esparto fibre may be
tested by means of a weak solution of aniline sulphate. The suspected
paper is gently heated in the test reagent, and if esparto is present
the paper turns a rose-red or pink colour, the depth of colour being
a measure of the amount of esparto. Most of the modern book papers
are prepared from chemical wood pulp and esparto mixed in varying
proportions, and while this test can be used as a means of detecting a
small or a large proportion of esparto, a microscopical examination is
required for a more accurate estimation.
The proportions used by the paper-maker depend upon the weighing
out of the wood pulp and esparto more or less accurately, while
the microscopical test is based upon the relative proportions as
represented by the volume of fibres of each class on the glass slip
placed under the microscope. Since the wood pulp consists of a number
of broad flat ribbon-like fibres, and the esparto of small cylindrical
fibres, considerable practice is necessary in making a proper analysis
of the two constituents in paper.
[Illustration: FIG. 25.--Esparto Pulp.]
STRAW.
The use of straw for the manufacture of paper was first brought
prominently into notice about the year 1800 by Matthias Koops, who
published a book printed on paper made from straw, but it was not until
1860 that this material was used in any large quantity.
[Illustration: FIG. 26.--A Cylindrical Digester for Boiling Fibre.]
Straw is now converted into a bleached paper pulp for news and
printings, and is also utilised for the manufacture of straw boards.
The production of a white paper pulp from straw is carried out in a
manner similar to that used in the case of esparto fibre, viz., by
digestion with caustic soda under pressure and subsequent bleaching.
As the straw contains considerable quantities of siliceous matter, the
chemical treatment necessary to reduce the material to paper pulp is
more severe, a stronger solution of caustic soda being used, and the
process of digestion being carried out at a higher temperature.
For the best quality of straw cellulose, the material is cut up into
small pieces by machines which resemble an ordinary chaff-cutter, and
the knots taken out by a separating machine. In most cases, however,
the whole straw is simply cut up into small lengths of about one to
two inches long, and placed at once in the digester. When the straw is
contaminated with foreign weeds, sand, husks, and similar substances,
as is usually the case, it is carefully hand-picked by girls, who
remove these impurities, which tend to produce particles of unbleached
matter in the finished pulp. The expense of this preliminary cleaning
process is more than compensated for by the enhanced value of the
bleached straw pulp.
_Digesting._--The cut straw is boiled in rotary cylindrical or
spherical vessels, stationary upright boilers of the vomiting type
being seldom employed because the circulation of the caustic soda
liquor does not take place freely with straw packed in the latter.
As the material is very bulky, some of the liquor is first put into the
boiler and the steam admitted while the straw is being thrown in. By
this means the straw is softened and reduced in bulk, so that a larger
quantity can be added before the digester is quite full. The full
amount of caustic soda is then made up by further additions of liquor,
and the contents of the digester heated by high-pressure steam for four
to six hours.
The conditions of treatment are shown by the following trial:--
Amount of straw 5,600 lbs.
Caustic soda, 20 per cent. 1,120 lbs.
The caustic soda was added in the form of a liquor, having a volume of
2,012 gallons and a specific gravity of 1·055.
Time of boiling 5 hours.
Pressure 60 lbs.
_Washing._--The boiled straw is discharged into large tanks placed
below the digester and washed with hot water, the smallest possible
quantity being used consistent with complete washing in order to
prevent the accumulation of large volumes of weak lye. The spent liquor
and washing waters are drained off into store tanks and evaporated in a
multiple effect apparatus by the same process as that used for esparto
pulp. The last washings are usually run away because the percentage of
soda in them is too small to pay for the cost of recovery.
The final washing of the straw pulp is completed by the use of a
breaking engine or potcher. As straw pulp contains a large proportion
of cellular matter which cannot be regarded as true fibres, there
is always a danger of considerable loss in yield if the use of the
breaking engine is extensively adopted, because the short cells escape
through the meshes of the drum-washer. The washing is most economically
effected in the tanks if a good yield of pulp is required.
_Separating out Knots._--The broken pulp from the breaking engines is
diluted with large quantities of water and pumped over sand traps in
order to remove knots and weeds which have resisted the action of the
caustic soda. These traps consist of long shallow trays, perhaps sixty
to eighty yards long, one yard wide, and nine inches deep, containing
boards which stretch from side to side, sloping at an angle, and nailed
to the bottom of the trays. The dilute pulp flows through the trays,
leaving the heavy particles, knots, and foreign matter behind the
sloping boards, and finally passes over the strainers, which retain
any large coarse pieces still remaining.
_Making Sheets of Pulp._--The mixture from the strainers contains a
large excess of water which has to be removed before the pulp can be
bleached. For this purpose a wet press machine (see page 103) or a
presse-pâte (see page 85) is employed, and the wet sheets of pulp are
then ready for bleaching.
_Bleaching._--The process by which the pulp is bleached is exactly
similar to that used for treating esparto.
From 1870 to 1890 large quantities of straw were used for the
manufacture of newspaper in conjunction with esparto and wood pulp, but
the price of the material was gradually advanced so that it could not
be used with advantage, especially as the production of wood pulp gave
a material which was much cheaper, and which could be utilised at once
without chemical treatment.
In the manufacture of newspaper the tendency during recent years has
been to make the paper mill operations as mechanical as possible and to
dispense with the preliminary operations which are essential for the
manufacture of half-stuff, the chemical processes being left in the
hands of the pulp manufacturers.
The manufacture of straw cellulose is now practically confined to
Germany, but small quantities of the bleached straw cellulose are
imported because the pulp imparts certain qualities to paper which
improve it, notably in making cheap printing papers harder and more
opaque.
MICROSCOPICAL FEATURES OF STRAW.
The paper pulp obtained from straw consists of a mixture of short
fibres together with a large proportion of oval-shaped cells. The
fibres are short and somewhat resemble esparto, but the presence of
the smaller cells is a sure indication of the straw pulp. The fibres
themselves closely resemble the fibres of esparto, but as a rule the
latter are long slender fibres, while the straw fibre is very often
bent and twisted or slightly kinked.
[Illustration: FIG. 27.--Straw.]
The only method of distinguishing between straw and esparto is by
examination with the microscope. There is no chemical reagent known
which will produce a colour reaction on a paper containing straw that
will serve to distinguish it from a paper containing esparto. If such
papers are gently heated in a weak solution of aniline sulphate a pink
colour is slowly developed, the intensity of which is to some extent a
measure of the amount of straw or esparto present.
Straw and esparto are usually described in text-books under one
heading, partly because the fibres possess strong resemblances in
physical and chemical constitution, and partly because the methods of
manufacture are identical. At the same time the qualities of the two
pulps are so different that they cannot be used indiscriminately, the
one for the other. Straw cellulose cannot be utilised in the place
of esparto, particularly for light bulky papers. Hence in magazine
and book papers containing a fibre which gives a pink coloration with
aniline sulphate it is fairly safe to assume that esparto pulp is
present.