Colouring.
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.
| Percentage of K2Al2(SO4)4 + 24 H2O. | Specific Gravity. | Degrees Twaddle. |
|---|---|---|
| 1·0 | 1·0065 | 1·30 |
| 2·0 | 1·0110 | 2·20 |
| 3·0 | 1·0166 | 3·32 |
| 4·0 | 1·0218 | 4·36 |
| 5·0 | 1·0269 | 5·38 |
| 6·0 | 1·0320 | 6·40 |
{144}
CHAPTER X. PAPER MACHINES; HAND-MADE PAPER.
The pulp as it comes from the beaters is now ready to be made into paper. We will first consider briefly the manufacture of hand-made paper.
It is made on a mould of wire-cloth, with a movable frame of wood, called the “deckle,” fitting on to the outside of the mould and extending slightly above its surface.
The wire-cloth is generally supported by a much coarser wire-cloth, or by pieces of thick wire, and these again by wedge-shaped pieces of wood, the thin end being next to the wire.
To form a sheet of paper the workman dips the mould, with the “deckle” in position, into a vat containing the prepared pulp diluted with water, lifting up just so much as will make a sheet of the necessary thickness. As soon as the mould is removed from the vat, the water begins to drain through the wire-cloth, and to leave the fibres on the surface in the form of a coherent sheet of paper. The felting or intertwining is assisted by lateral motion in every direction given to the frame by the workman. The movable deckle is then removed, and the mould, with the sheet of paper, given to another workman, called the “coucher,” who turns it over and presses it against the felt, by this means transferring the sheet from the wire to the felt. In the meantime the “vat-man” is engaged in the formation of another sheet with a second mould.
The Fourdrinier Paper Machine. Plate I.
A number of the sheets thus formed are piled together, alternately with pieces of felt, and when a sufficient number {145} have been obtained, the whole is subjected to strong pressure, to expel the water. The felts are then removed, and the sheets again pressed.
They are then sized, if required, by dipping them into a solution of gelatine: again slightly pressed, and hung up on lines or poles to dry. Such paper is called loft-dried.
When dry the sheets of paper are calendered. (See Chapter XI.)
The making of paper by hand involves considerable dexterity on the part of the workman; on account of the expensive labour necessary, in comparison with paper-machines, it is comparatively little practised in the present day; certain kinds of paper, however, such as bank-notes, various drawing papers, and printing papers intended for the production of very elaborate editions are always made in this way.
Any pattern or name required on the paper is obtained by means of a raised pattern on the wire-cloth mould: consequently, less pulp lodges there, and the paper is proportionally thinner, thus showing the exact counterpart of the pattern. Such devices are known as “water-marks.” (See also p. [156].)