The bleached half-stuff as it leaves the steeping chests usually contains an excess of bleaching liquor, which can be removed in two ways, viz. by washing or by decomposition with an “antichlor.” The first method has the advantage of not only removing the bleach, but of also eliminating the chloride of calcium, partly existing ready formed, and also that resulting from the decomposition of the calcium hypochlorite originally present in the bleach. On the other hand, this method takes some time, and consumes a large amount of water, which in some mills is a matter of considerable importance. For this purpose, many beaters are provided with one or more drum washers (see Fig. 36). An additional objection to this method lies in the fact that a certain quantity of fibre passes through the meshes of the wire-cloth covering the washers, and is thus lost.

The more usual plan is to remove the bleach by decomposing it with an “antichlor.” The substance generally employed for this purpose is sodium hyposulphite, or thiosulphate as it is now called, which, in presence of calcium hypochlorite, is oxidized to sodium sulphate, the latter being reduced to calcium chloride. Double decomposition then takes place between these salts, with the formation of calcium sulphate and sodium chloride. The reactions which take place may be expressed by the following equation:—

The above decomposition does not accurately represent the action of bleach upon sodium thiosulphate. If the solutions employed are very dilute, the decomposition may take place in another direction, viz.:—

At the particular degree of dilution which occurs in a beater, the bleach is decomposed almost entirely according to the first of the equations; from which, on calculating, it will be seen that 158 parts of sodium thiosulphate are equivalent to 286 parts of calcium hypochlorite. As commercial sodium thiosulphate contains 36·3 per cent. of water, and bleaching powder 70 per cent. of calcium hypochlorite, on the basis of 35 per cent. available chlorine it follows that 248 parts of the former are required to neutralize 409 parts of the latter.

Within the last few years other forms of “antichlor” have been introduced, such, for example, as the various sulphites. The most important of these is sodium sulphite, which has been manufactured by a patent process at a cheap rate, by Gaskell, Deacon, & Co., Widnes. Their product contained as much as 75 per cent. of Na₂SO₃, ordinary crystallized sodium sulphite containing only 50 per cent.

Sulphites are converted by the action of bleach into sulphates, thus:—

From this equation, it will be seen that 252 parts of sodium sulphite will neutralize 143 parts of calcium hypochlorite, or 204·3 parts of bleaching powder. Assuming that crystallized sodium sulphite contains 50 per cent. Na₂SO₃, the same amount of bleach would require 504 parts. As Messrs. Gaskell, Deacon & Co.’s sulphite contains 75 per cent. Na₂SO₃, only 336 parts are needed. Comparing these numbers with {129} those given above for sodium hyposulphite, it will be seen that 204·5 parts of bleach require for neutralisation 129 parts of sodium thiosulphate and 504 parts of crystallised sodium sulphite, or 336 parts of the stronger product.

Sodium sulphite is preferred to sodium thiosulphate by some paper makers, notwithstanding the fact that even in its most concentrated form, nearly three times as much is required to produce a certain result. It is said that when it is used the wire cloth of the machine is preserved for a longer time than if sodium thiosulphate is employed. This may be due to the fact that with the latter a certain amount of free acid is always formed, which of course would act injuriously on the wire; whereas, when sodium sulphite is used, the products of decomposition are neutral salts without any action upon metals. (See the above equations.)

A very cheap “antichlor” may be prepared by boiling together lime and sulphur. One hundred and sixty-eight parts of lime, made into a milk with water, are heated to boiling in an iron vessel. Three hundred and eighty-four parts of flour of sulphur or ground sulphur are then added in small quantities at a time, and the boiling continued until the whole is dissolved. The liquid which is now of a deep yellow colour, is allowed to settle and cool and is then ready for use. It contains a mixture of calcium thiosulphate and calcium pentasulphide, the latter compound giving to it its deep yellow colour. The following equation represents the action which takes place between the lime and the sulphur:—

The decomposition which takes place when the mixture is acted upon by calcium hypochlorite may be represented as follows:—

From this it is seen that a large quantity of free sulphur is formed, which is precipitated as a fine yellowish powder in the fibre. This not only affects the colour of the pulp, but is objectionable on account of its liability, in contact with the hot cylinders of the paper machine, or by the action of moisture over a long period of time to become oxidised to sulphuric acid, which undoubtedly, even if present only in very small quantities, has an injurious effect upon the finished paper. (See hydracellulose.) This property of sulphur and its action upon cellulose may be shown by the following experiment. Take a piece of water-leaf paper, rub into it a mixture of flour of sulphur and water. If the paper thus treated be rapidly dried, it will be found to be weakened or even slightly charred in those portions which have been in contact with the sulphur. (See also Chem. Soc. Journ., p. 249, 1879.)

Whichever variety of “antichlor” is used an excess should be carefully avoided, as all act more or less upon the size and colouring matter added to the pulp subsequently. The proper method is to run in small quantities of a solution of the antichlor at a time, testing the pulps, after allowing a few minutes for complete admixture after each addition. The test is made by immersing a piece of iodide of potassium and starch paper, which will be turned blue so long as any calcium hypochlorite is present. These papers are made as follows:—

Three grms. of starch are ground up with a small quantity of water and poured into 700 cc. of boiling water, and to the solution 1 grm. of iodide of potassium and 0·5 grm. carbonate of soda are added. Sheets of white paper, water-leaf in preference, are now soaked in the solution and dried. In presence of calcium hypochlorite, iodine is liberated from the potassium iodide, and acting upon the starch forms with it a characteristic blue colour.

China clay or kaolin is sold in the form of large lumps of a white or yellowish-white colour. It is formed by the gradual disintegration of felspar by means of the action of air and water, and consists essentially of a silicate of aluminium. Its quality depends upon its whiteness and its freedom from coarse micaceous particles. It is usually prepared for admixture with the pulp by making it into a fine cream with water in a vessel provided with stirrers; it is then passed through a fine sieve in order to remove any impurities it may contain, and is then run into the beater. The clay or other filling material is usually run into the beater as soon as the latter is charged with pulp, so that by the time the beating operation is concluded, a perfect admixture of pulp and clay is effected.

Sulphate of lime, or “pearl-hardening,” is usually sufficiently pure to put direct into the engine. It is made by decomposing a solution of calcium chloride, with sulphate of soda, and is precipitated as a fine brilliantly white powder, consisting of CaSO₄ + 2 H₂O.

Two distinct forms of precipitated calcium sulphate are met with in commerce, differing from each other by their microscopical features, the one consisting of flat tabular crystals (Fig. 41), the other of fine needles (Fig. 42). Another form, erroneously called precipitated pearl-hardening, is also sold: it consists of the finely ground native mineral.

Some of the finer qualities of paper are made without addition of any loading material whatever, though such papers are of course the exception. The proportion of clay or {132} other material that can be put into a fibre depends to a certain extent upon the nature of the fibre, and upon the degree of fineness to which it is reduced in the beater. The amount added by different makers varies considerably, from {133} two or three per cent. to twenty, and even in rare cases to thirty per cent.

A new loading material called “agalite” has been lately introduced, possessing certain advantages over china clay, {134} or calcium sulphate. Agalite is a mineral of the nature and chemical properties of asbestos: it consists of nearly pure magnesium silicate. Its structure is more or less fibrous, like that of asbestos, which, as is well known, can be spun and woven and even made into paper, and it therefore, when added to a paper, forms a part of the fabric itself. It is even claimed that it assists in keeping back some of the finer fibres that invariably find their way through the meshes of the wire cloth, and it is said that 90 per cent. of the amount added to the engine is found in the paper. In the case of china clay it is well known that only from 40 to 60 per cent. is actually “carried” by the pulp. Figs. 40, 41, 42, and 43 show the appearance of china clay, pearl-hardening, and agalite when viewed under the microscope, magnified 200 times. The nature of agalite is such that it assists the paper in taking a high finish. This is probably due to its “soapy” nature, a feature which is characteristic of asbestos, French chalk, “soap-stone,” and other magnesium silicates.

When papers contain such excessive quantities as 15 or 20 per cent. of clay, it cannot be to the advantage of the consumer, and should be looked upon as an adulteration. It is a matter of some importance to be able to determine rapidly and accurately the amount of mineral matter in a paper. The usual method is to ignite a weighed quantity of the paper in a platinum crucible until the ash so obtained is either white or a very pale grey. From the weight of the ash, the percentage of mineral matter is easily calculated. The following is a very convenient plan in cases where a platinum crucible or dish is not obtainable:—Take a weighed piece of the paper to be examined, from 2 to 4 in. square, according to the thickness, roll it into a narrow hollow cylinder. Round this wind a weighed piece of platinum wire about ¹?/?₅₀ in. thick, as in Fig. 44. Hold this by means of a pair of crucible tongs in the flame of a Bunsen burner until it is completely burned. If the wire is carefully wound round, and especially if the roll of paper is made conical, the ash will be securely held in {135} position. Those who do not possess a chemical balance of the ordinary form will find a convenient substitute, which will answer the purpose of weighing the paper and ash with sufficient accuracy, in the spiral balance invented by Prof. Jolly11 illustrated in Fig. 44. It consists of a spiral of hard wire A, which is suspended in front of a mirror B, upon which millimetre divisions are marked. A small float D, dipping under the surface of the water in the vessel E, is provided for the purpose of steadying the spiral and allowing it to come quickly to rest. The balance is provided with a light pan made of a thin plate of mica, and suspended by very thin platinum wires. For the present purpose, however, the pan is not necessary, and it can be replaced by the roll of paper and platinum coil as shown in the drawing.

The method of using is exceedingly simple, as the increase in the length of the spiral is in direct ratio to the increment of weight. The position of the spiral is ascertained by placing the eye in a direct line with the small glass bead C and its image in the mirror, and noting the corresponding division on the scale. The position of the bead can be altered so as to bring it to any desired point on the scale by raising or lowering the upright rod F, which is kept in position by the screw G. The balance stands on a foot provided with levelling screws.

It is evident that where proportional weights only are required it is not necessary to know the value of the spiral, but if the balance is to be used to ascertain actual weights, the coefficient of the spiral must first be determined. This is done by noting the increase in length after the addition of a one gram weight. The spirals are made of different thicknesses of wire, which of course give varying degrees {136} of sensibility: the most useful is one which gives with one gram an extension of 100 millimetres: one mm. being therefore equivalent to one mgrm.

The following experiment will illustrate the method of using the balance and of calculating the results of a determination of the amount of mineral matter in a paper:—

In order to ascertain the percentage of mineral matter actually added to a paper it is necessary to deduct from the amount of ash obtained a certain quantity due to the mineral matter in the fibre of which the paper is composed. This amount varies with each particular fibre, and with the method by which it has been prepared. The following table gives the percentage of ash yielded by the various pulps in a perfectly bleached state. In all cases the fibres were previously treated with a dilute solution of hydrochloric acid, in order to remove any carbonate of lime or other bodies which might have been introduced in the boiling and bleaching processes: the percentages are calculated on the dry substances:—

If the paper contains calcium sulphate the ash obtained may consist partly of calcium sulphide, due to the reducing action of the carbon found on ignition, and the amount will {137} therefore not represent the true amount added. The ash should be moistened with a few drops of sulphuric acid, and again ignited, in order to reconvert it into calcium sulphate. It should also be borne in mind that the sulphate of lime as present in the paper is combined with two atoms of water CaSO₄ + 2 H₂O, and therefore that every part of calcium sulphate obtained represents 1·26 parts of “pearl-hardening” actually added.

Rosin (colophony) consists of a mixture of pinic and sylvic acids, which, when heated with a solution of carbonate of soda, combine with the alkali to form a soap, carbonic acid being evolved. The soap is prepared in the following manner:—

Ordinary rosin, the quality depending upon the quality of paper, is boiled in a jacketed pan for from two to three hours with a solution of carbonate of soda, until a sample of the soap formed is completely soluble in water. The quantity of carbonate of soda (Na₂CO₃10 H₂O) required is about 30 per cent. of the weight of the rosin. It is usual to employ crystallised carbonate of soda (soda-crystals), but {138} soda-ash of good quality, or even caustic soda, might very well be employed, in which case of course the proportion would be different.

A very convenient form of carbonate of soda has lately been introduced by Messrs. Gaskell, Deacon & Co., which goes by the name of “Crystal Carbonate.” It is obtained in the form of minute crystals of the mono-hydrate, Na₂CO₃ + H₂O. It contains 50 per cent. of alkali (Na₂O), and possesses this advantage over soda-ash, that is dissolves readily in water without forming a hard cake. It is much cheaper in proportion than soda crystals, as the expense of crystallisation and carriage is saved. The latter contain only 21·68 per cent. Na₂O, 62·93 per cent. of their weight consisting of water of crystallisation. One hundred parts of soda crystals are equivalent to 43·36 parts of the “Crystal Carbonate.”

An excess of soda should be carefully avoided, as it consumes an equivalent quantity of alum (see below): on the other hand, it is very essential that the rosin should be completely dissolved, otherwise small particles are sure to find their way into the pulp, and would form clear transparent specks in the finished paper.

The boiling being completed, the charge is run off into iron tanks and allowed to settle; the soap forms a semi-solid mass, while a dark-coloured liquor, containing the impurities of the rosin, rises to the surface, and can thus be removed. The soap so purified is next dissolved in hot water containing a small quantity of carbonate of soda, in case complete solubility has not been attained, and is then mixed with a quantity of starch paste prepared in a separate vessel by dissolving starch in boiling water. The mixture is then carefully sieved and is ready for use.

The proportion of starch to rosin differs in nearly every mill, and also the quantity of size to be added to the beater varies according as the paper is required to be soft or hard-sized. About 3 parts of starch to 1 of rosin, and between 3 or 4 lbs. of the mixture to 100 lbs. of pulp may be considered an average quantity. Some manufacturers prefer to add the {139} starch and rosin size to the engine separately; others again do not dissolve the starch in water, but merely make it up to a thin paste, in which state it is added to the pulp. This method is employed by makers of very fine papers, as it is said to give a certain feel to the paper which cannot be obtained in any other way. The method is costly, as only a small proportion of the starch added is actually found in the paper.

Some papers, which are not intended to be sized, such as blotting and filter-papers, are made with the addition of starch only, this being used to bind the fibres together to some extent. The presence of the starch does not prevent the paper from being absorbent. Papers which are intended to be tub-sized only are usually made up with a small quantity of starch, and are sometimes sized to a small extent with rosin also. The addition of a certain amount of ordinary soap to the rosin soap is said to give good results; it enables the paper to take a higher finish when calendered. Sizing with animal size (gelatine), or Tub-sizing as it is called, will be described in Chapter X.

The mixture of size and starch may be added directly to the engine, or it may be previously dissolved in water; the latter method is perhaps preferable. After allowing it to mix thoroughly with the pulp, a solution of alum is run in. It is made up with boiling water in either lead or copper tanks. Iron or zinc vessels must be avoided, as the solution acts rapidly on these metals.

Alum consists of a double sulphate of aluminium and potassium or ammonium, Al₂ 3 SO₄, K₂SO₄ + 24 H₂O; its function being to form with the rosin acids insoluble soaps which are precipitated in intimate mixture with the pulp, the sulphate of potash taking no part in the reaction. The choice of a suitable alum is a matter of very great importance; it should be free from excess of sulphuric acid and from iron. The former is deleterious on account of its action upon the colouring matter used to tone the paper, some colours being completely discharged by it; and because {140} of its effect upon any metal work with which it may come in contact, especially upon the brass wire-cloth on which the paper is made. The iron is objectionable as it forms a dark red precipitate of oxide of iron.

Alum as supplied by makers of repute is generally sufficiently pure for even the best classes of paper.

Many years ago a substitute for alum was introduced, called Pochin’s aluminous cake. It consisted principally of sulphate of alumina Al₂ 3 SO₄, and was, provided it was pure, as suitable for the papermaker as the more expensive potash alum. It was more economical at equal price, as it contained about 14 per cent. of alumina, whereas potash alum, even if absolutely pure, could only contain 10·85 per cent. Pochin’s aluminous cake however, was liable to contain a considerable amount of free sulphuric acid and soluble sulphate of iron. Of late years, a number of different varieties of sulphates of alumina have been introduced into commerce, of so pure a nature, and at such a low price, that it is a matter of wonder that they have not entirely superseded the more expensive alum. They are prepared either from very pure native alumina (bauxite), or from China clay of good quality. Methods have been discovered of eliminating almost the whole of the iron and free sulphuric acid. The following analyses of different specimens sufficiently indicate their purity.12

Sulphate of alumina possesses this advantage over alum, that it is more soluble in water, and thus a stock solution of considerable strength can be prepared. It is soluble in {141} 2 parts of cold water, alum requiring 18 parts for complete solution.

The amount of alum or sulphate of alumina added to a pulp is largely in excess of the quantity necessary to precipitate the rosin soap; as a matter of fact, in the case of esparto or straw pulps, for the bleaching of which considerable quantities of bleach have been employed, and which therefore contain a certain amount of basic lime, together with calcium chloride, complete precipitation of the size is effected without the addition of alum. A certain amount is also required to precipitate the starch. The excess of alum appears to be necessary, however, not only to brighten the colour of the paper, but also to render it capable of resisting the action of ink. From experiments made by the authors, it appears that one part of rosin requires 2·9 parts of alum for complete precipitation from its solution in soda. One part of starch requires 0·40 part alum.

In choosing an ultramarine, attention should be directed not only to its tinctorial power but to its behaviour with a solution of alum, inferior qualities being entirely discharged by it. All ultramarines are readily decomposed by free acids, hence the necessity of employing an alum as free as possible from this impurity.

Smalts on the other hand resists the action of acids, but it is only used for the finer qualities of paper, owing to its high price. The aniline blues are perhaps scarcely so brilliant in colour as the above-mentioned colours, and they {142} are moreover somewhat readily discharged by the action of light; they have however the advantage of cheapness.

The pink colouring matters are sold either as lakes, that is, compounds of the colouring matter with alumina or as solutions. They are also frequently to be met with mixed with starch in varying proportions. Those made from cochineal are superior to those containing aniline colours.

Ultramarine can be detected by means of its reaction with hydrochloric acid, the colour being discharged and sulphuretted hydrogen evolved, which can be recognised by its odour. Smalts is not acted upon by acids. The latter contains cobalt, the presence of which can be determined by its giving a blue bead with borax in a blowpipe flame.

The aniline blues are soluble either in hot water or hot alcohol, the liquid being intensely coloured. Most of them are discharged by treatment either with strong acids or alkalis, the colour being restored by the reverse treatment.

Paper of any desired colour may be made either by using rags already dyed the necessary colour or by adding to the bleached pulp in the beater such dyes or pigments as will produce it. Any colouring matter which can be obtained in the form of a fine powder or as a solution can be used. Blue papers are usually coloured with ultramarine; the dark blue papers used for wrapping sugar and other purposes are coloured with Prussian blue, either added directly to the beater or produced in it by the action of potassium ferrocyanide on ferric sulphate.

The so-called toned paper is produced by adding to the pulp a solution of pernitrate of iron, from which a fine precipitate of oxide of iron is deposited on the fibres; thus the slightly brown shade characteristic of this kind of paper is produced. The same effect may be produced by the addition of yellow ochre or some similar pigment. The following are a few of the materials which are used for producing coloured papers:—Methyl violet, eosine, chrome yellow, venetian red, catechu. Useful receipts for the preparation of coloured papers will be found in Dunbar’s ‘Practical Paper-Maker.’ {143}

During the time that the loading, sizing and colouring processes have been going on, the pulp has been continually acted upon by the roll, and if these operations have extended over a considerable time, it is probably in a proper condition for making into paper. The amount of “beating” depends, as has been stated before, upon the nature of the fibre, and also to some extent on the nature of the paper for which it is intended. The “beaterman” examines the pulp from time to time by taking a portion from the engine and placing it in a hand-bowl containing water: from its appearance when so diluted he is able to judge of the time during which it may be necessary to continue the disintegration. As soon as this is completed, the pulp is ready to be let down to the stuff-chests, usually placed at a lower level than the beaters, so that the pulp can flow into them by gravity. For this purpose the valve at the bottom of the engine is opened: to remove the last portion of pulp it is necessary to rinse out the engine with water.