Continuous Board Machine.

This machine differs from the single board machine in that the finished board can be produced from the pulp at one operation. It is used principally for cards and boards of moderate thickness which can be wound up in the form of a reel at the end of the machine.

The mixture of pulp and water is pumped into two or more vats and formed into a number of thin sheets, which are all brought together between squeezing rolls and passed through heavy press rolls which compress the several layers into a compact mass. The thick sheet obtained is dried over steam-heated cylinders which are placed at the end of the press rolls, and calendered. The whole process, indeed, resembles that of ordinary paper-making, the main difference being the method of producing the wet sheet or card.

Some machines are constructed with six or seven vats and forty to fifty drying cylinders, and are capable of turning out a large quantity of finished material.

The board can be made of uniform quality and texture throughout, or be finished off with high-grade paper on one or both sides. In the latter case the constituents of the “middle” part are waste papers and raw material of inferior quality, the outer surface of wood pulp, white or coloured according to circumstances. The variety of papers and boards which can be produced is due to the fact that the several vats of pulp are independent of one another and can be filled with any kind of paper stock. The combined sheets forming the ultimate board are dried on the ordinary cylinders, calendered, and reeled up at the end of the machine.

[CHAPTER VII]
SPECIAL KINDS OF PAPER

There are many varieties of paper products obtained by submitting finished paper to a number of special processes. Of these only a few of the more important will be described.

These products can be divided approximately into three classes:—

(1) Papers coated on one side or both sides with various substances, such as “art,” photographic papers, etc.

(2) Papers impregnated with chemicals, such as blue print, medicated, and cheque papers.

(3) Paper pulp converted into modified products by chemical treatment, such as vulcanised board, viscoid, etc.

Of the first class, the coated papers used for art and chromo illustrations are the most important.

Of the second class, the blue prints and papers impregnated with chemicals, chiefly employed for the production of engineers' drawings, may be regarded as typical.

In the third class, vegetable parchment and vulcanised board are the most familiar.

Parchment Paper.—This is produced by the action of sulphuric acid upon ordinary paper, the most suitable for this purpose being made from unsized cotton rag, free from such additions as mechanical wood pulp. The presence of the latter substance should be avoided, as it is liable to char or burn, so that in the finished product it shows itself in the form of small holes. The process depends upon the power of sulphuric acid to change the surface of the paper into a gelatinous mass, which has been shown to consist of a substance called amyloid.

The best parchment is made from pure cellulose such as rag or chemical wood pulp. The quality of the parchment depends upon attention to the strength of the acid, the temperature of the acid bath, the period of immersion, the complete removal of the acid, and the careful drying of the wet parchment.

Fig. 43.—Apparatus for making Parchment Paper.

The acid is employed at a strength of 1·71 specific gravity, being prepared by diluting the commercial concentrated acid in a leaden vessel, with a sufficient quantity of water.

The parchment is generally prepared by passing a continuous sheet of paper through a bath of acid of the proper strength at a speed which ensures the correct period of immersion. As the treated paper leaves the bath it passes through squeezing rolls which remove the excess of acid, and the paper is then led through a series of tanks containing fresh water, the last traces of acid being neutralised by small additions of ammonia, or some alkali, to the last washing tank. The wet parchment is then passed through suitable rollers and carefully dried over cylinders heated internally by steam. The paper is kept perfectly stretched as it dries, because it shrinks enormously, and would otherwise become cockled and uneven.

Thick sheets of parchment paper are frequently made by passing three sheets of paper through the acid bath and bringing them together between the rollers before washing. The sheets unite when pressed together; the remainder of the process being the same as that employed for single sheets.

The parchment exhibits remarkable differences to the original paper, the strength being increased three or four times, the density about 30 per cent., the latter being shown by the shrinkage, which amounts to at least 30 per cent.

Vulcanised Paper.—Zinc chloride has the property of parchmentising paper in a manner similar to sulphuric acid. The product obtained when this reagent is used is generally termed vulcanised fibre. The paper is passed as a continuous sheet into a bath of strong zinc chloride, having a density of 160-170 Twaddell, which causes the cellulose to swell up and partly gelatinise. A very large excess of strong zinc chloride is necessary, and the process is only rendered commercially possible by careful recovery of the zinc from the washing waters, which are submitted to chemical treatment.

The vulcanised product is subsequently treated with nitric acid or with a mixture of nitric and sulphuric acids to render them waterproof. Dextrin is frequently employed to retard the chemical action to permit of the necessary manipulation of the material before it is finally washed. The complete removal of the excess of zinc and acid is a necessary feature of the whole operation.

Willesden Paper.—When paper is passed through an ammoniacal solution of copper oxide, a superficial gelatinisation of the surface takes place, so that the paper when washed and dried is impregnated with copper oxide, which helps to preserve it, and it becomes waterproof. Such material is well known as Willesden paper.

Blue Print or Cyanotype Papers.—This name is usually given to the process by means of which blue prints of engineers' and architects' plans can be reproduced. It was discovered in 1842 by Sir John Herschel. It is a useful method of reproducing drawings, and incidentally is of great value to the amateur photographer because of the facility with which it can be applied for getting proofs from negatives quickly and easily without special baths and chemicals. The process is based upon the reduction of a ferric salt to the ferrous condition by light, and the formation of Prussian blue by the action of potassium ferricyanide. The negative cyanotype gives white lines on a blue ground. Various formulæ are in common use.

— Herschel. Clark. Watt. Rockwood.
Solution 1.
Potassium ferricyanide16274810
Water100100100100
Ammonia—2·3——
Saturated solution of oxalic acid—20——
Solution 2.
Ammonia-citrate of iron20305030
Water100100100100
Boric acid——0·5—
Dextrin———5

Equal parts of the two prepared solutions are mixed when required and spread evenly over well-sized paper. The paper is hung up, dried, and preserved in a dark dry place.

The positive cyanotype gives blue lines on a white ground, being the reverse of the ordinary blue print. That is, no image is formed where the light acts, and the reaction is the formation of blue due to the union of a ferrous salt with ferrocyanide of potassium.

Pizzighelli in 1881 gave the following formula:—

— Solution 1. Solution 2. Solution 3. Solution 4.
Water100100100100
Gum arabic20———
Ammonia-citrate of iron—50——
Ferric chloride——50—
Potassium ferrocyanide———20

Mix the first three solutions in the following order in the proportions stated:—

Solution 1. 20parts.
Solution 2.8„
Solution 3.5„

As soon as the solution, which at first gets thick and cloudy, is clear and thin, it is spread over the surface of well-sized paper, which is then dried in a warm room.

The print, which appears yellow on a dark yellow ground, is treated with the developer (solution 4) by means of a brush dipped in the solution. When the image is deep blue in colour, the print is washed in water and then placed in dilute hydrochloric acid (1 part of acid to 10 parts of water) till the ground is quite white. A final washing with water is then necessary.

Waterhouse gives the following formula:—

— Solution 1. Solution 2. Solution 3. Solution 4.
Water650150—100
Gum arabic170———
Tartaric acid —40——
Ferric chloride solution 45° Baumé ——150—
Ferrocyanide of potassium ———20

Solutions 1 and 2 are mixed and No. 3 added gradually with constant stirring. The mixture is left twenty-four hours, and diluted with water to a specific gravity of 1·100.

The paper is coated with the solution and used as already directed, being developed in ferrocyanide of potassium solution and washed with water, treated with weak hydrochloric acid, and then finally cleaned from all traces of acid.

Black Lines on a White Ground.—This modification of the ordinary blue print is arrived at with the following formula:—

Water96·0 parts.
Gelatine1·5„
Perchloride of iron (in syrupy condition) 6·0„
Tartaric acid6·0„
Sulphate of iron1·5„

The paper is coated with the solution. After printing, the image is developed with a solution containing

Gallic acid 1 part.
Alcohol10 parts.
Water50„

A final washing of the print with water completes the operation.