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 |
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