_Colouring._—Cellulose, except in rare instances, is never sufficiently white to produce a perfectly white paper, and the manufacturer finds it necessary to complement the slightly yellow colour which most bleached pulps possess by the addition of small quantities of blue and pink colouring materials. The blues usually employed are ultramarine, smalts, and aniline blues; the pinks are generally preparations of either cochineal or aniline dyes. Ultramarine is met with in commerce as a bright blue powder; the shade of colour and the quality of which vary considerably. 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.