Dyeing to Sample.—The matching of colours has been greatly simplified through the publication of pattern books by the firms who manufacture dyes, in which books full details as to the composition of the paper, the proportion of colour and the conditions for maximum effects are fully set out. The precise results obtained by treating paper pulp with definite proportions of a certain dye, or a mixture of several dyes, is determined by experimental trials. A definite quantity of moist partially beaten and sized pulp, containing a known weight of air-dry fibre, is mixed with a suitable volume of water at a temperature of 80° to 90° F. and the dye-stuff added from a burette in the form of a 1 per cent. solution. If preferred a measured volume of a 1 per cent. solution of the dye can be placed in a mortar, and the moist pulp, previously squeezed out by hand, added gradually and well triturated with the pestle.
The dyed mixture is then suitably diluted with water, made up into small sheets of paper on a hand mould or a siphon mould, and dried.
The effect of small additions of colour to the contents of a beating engine is frequently examined in a rough and ready way by the beaterman, who pours a small quantity of the diluted pulp on the edge of the machine wire while the machine is running. This gives a little rough sheet of paper very quickly.
The comparison of the colour of a beaterfull of pulp with the sample paper which it is desired to match is also effected by reducing a portion of the paper to the condition of pulp, so that a handful of the latter can be compared with a quantity of pulp from the engine. This is not always a reliable process, especially with papers coloured by dyes which are sensitive to the heat of the paper machine drying cylinders.
Detection of Colours in Papers.—The examination of coloured papers for the purpose of determining what dyes have been employed is a difficult task. With white papers which have been merely toned the proportion of dye is exceedingly small, and a large bulk of paper has to be treated with suitable solvents in order to obtain an extract containing sufficient dye for investigation.
With coloured papers dyed by means of pigments, the colour of the ash left on ignition is some guide to the substance used, a red ash indicating iron oxide, a yellow ash chromate of lead, and so on.
With papers dyed by means of coal tar colours the nature of the colouring matter may be determined by the methods of analysis employed for the examination of textile fibres.
The following hints given by Kollmann will be found useful:—
Tear up small about 100 grammes of paper, and boil it in alcohol, in a flask or a reflux condenser. This must be done before the stripping with water, so as to extract the size which would otherwise protect the dye from the water. Of course the alcohol treatment is omitted with unsized paper. The paper is now boiled with from three to five lots of water, taking each time only just enough to cover the paper. This is done in the same flask after pouring off any alcohol that may have been used, and also with the reflux condenser. The watery extract is mixed with the alcohol extract (if any). Three cases may occur:—(1) The dye is entirely stripped, or very nearly so. (2) The dye is partly stripped, what remains on the fibres showing the same colour as at first or not. (3) The dye is not stripped. To make sure of this the solution is filtered, as the presence in it of minute fragments of fibre deceive the eye as to the stripping action. In the first two cases the mixed solutions are evaporated down to one half on the water bath, filtered, evaporated further, and then precipitated by saturating it with common salt. The dye is thrown out at once, or after a time. It may precipitate slowly without any salt. The precipitated dye is filtered off and dried. To see whether it is a single dye or a mixture, make a not too dark solution of a little of it in water, and hang up a strip of filter paper so that it is partly immersed in the solution. If the latter contains more than one dye they will usually be absorbed to different heights, so that the strip will show bands of different colours crossing it. If it is found that there is only one dye, dissolve some of it in as little water as possible, and mix it with “tannin-reagent,” which is made by dissolving equal weights of tannin and sodium acetate in ten times the weight of either of water. If there is a precipitate there is a basic dye, if not, an acid dye. In the former case mix the strong solution of the dye with concentrated hydrochloric acid and zinc dust, and boil till the colour is destroyed. Then neutralise exactly with caustic soda, filter, and put a drop of the filtrate on to white filter paper. If the original colour soon reappears on drying, we draw the following conclusions:—