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.

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.

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.

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.

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.

Practical Data calculated from Experiments.

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.

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.

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.

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.

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.

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:—

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.

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.

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.