The Manufacture of Paper / With Illustrations, and a Bibliography of Works Relating to Cellulose and Paper-Making

by R. W. Sindall

Soluble Colours.

(A) Natural Dyes. These colouring matters are now seldom used.

Yellow and Brown.—The vegetable extracts, such as fustic, quercitron, cutch, turmeric, have practically all been replaced by aniline colours.

Red.—Madder (Turkey red), Brazilwood, cochineal (a dye obtained from dried cochineal insects). Safflower.

Black.—Logwood, used in conjunction with an iron salt. Cutch, used with an iron salt.

(B) Coal Tar Dyes. The dyeing and colouring of paper pulp by means of the artificial organic substances has become a matter of daily routine, the expensive natural dyes and the ordinary pigments having been almost completely superseded. The numerous colouring matters available may be classified either by reference to their chemical constitution or simply on general lines, having regard to certain broad distinctions.

If the latter classification is taken, then the dyes familiar to the paper-maker may be divided into—

(a) Acid dyes, so called because the full effect of the colouring matter is best obtained in a bath showing an acid reaction.

(b) Basic dyes, so called because the colour is best developed in an alkaline solution, without any excess of mordant.

(c) Substantive dyes, which do not require the use of a mordant, as the colour is fixed by the fibre without such reagents.

Some of the most frequently used colouring matters are shown in the accompanying table on page [202].

The distinction between acid and basic dye-stuffs is largely due to certain characteristics possessed by many of them. Thus magenta, which is the salt of the base known as Rosaniline, belonging to the basic colouring matters, a group of dyes which do not possess the fastness of colour peculiar to acid dyes, has a limited application. But by treatment with sulphuric acid magenta is converted into an acid magenta, and this dye has wider application than the basic salt. Similarly the basic dye called aniline blue is insoluble in water, and therefore has only a limited use, but by treatment with sulphuric acid it is converted into alkali blue, soluble blue and so on, which dissolve readily in water and are good fast colours. The acid dyes generally have a weaker colouring power than the basic dyes, but they produce very even shades.

Colour.	Acid.	Basic.	Substantive.
Yellow	Metanil yellow.	Auramine.	Cotton yellow.
and	Paper yellow.	Chrysoidine.	Chrysophenine.
Orange.	Orange II.
	Naphthol yellow S.
	Quinoline yellow.
Red.	Fast red A.	Rhodamine.	Congo red.
	Cotton scarlet.	Paper scarlet.	Benzopurpurin.
	Erythrine.	Safranine.	Oxamine red.
	Ponceau.	Magenta.
Blue	Water blue 1 N.	Methylene blue.	Azo blue.
and	Fast blue.	Victoria blue.
Violet.	Acid violet.	New blue.
		Indoine blue.
		Methyl violet.
		Crystal violet.
Brown	Naphthylamine brown.	Bismarck brown.
		Vesuvine.
Black	Nigrosine.	Coal Black B.
	Brilliant black B.
Green		Diamond green.
		Malachite green.

The difference in the composition of the basic and acid dyes is taken advantage of in the dyeing of paper pulp to secure a complete distribution of the colouring matter upon the pulp, with the result that the intensity of colour is increased, its fastness strengthened, and the process of dyeing generally rendered more economical. This is effected by the judicious addition of a suitable acid dye to the pulp already coloured with the basic dye.

The direct colouring matters have but a very limited application for paper dyeing owing to their sensitiveness to acids and alkalies.

In the colouring of paper pulp, attention is given to many important details, such as:—

Fading of Colour.—Some loss of colour almost invariably occurs even with dyes generally looked upon as fast to light. The shade or tint of the paper is affected not only by exposure to light, but by contact of the coloured paper with common boards on which it is often pasted. The alkalinity of straw boards, for example, is frequently one source of serious alteration of colour, and the acidity of badly made pastes and adhesives another.

In all such cases, the dyes must be carefully selected in order to obtain a coloured paper which will show a minimum alteration in tint by exposure to light or by contact with chemical substances. This is particularly necessary in coloured wrapping paper used for soap, tea, cotton yarn, and similar goods.

Unevenness of Colour.—The different affinity of the various paper-making fibres for dyes is apt to produce an uneven colour in the finished paper. This is very noticeable in mixtures of chemical wood pulp or cellulose and mechanical wood pulp. The ligno-cellulose of the latter has a great affinity for basic dyes, and if the required amount of dye is added to a beater containing the mixed pulps in an insufficiently diluted form, the mechanical wood pulp becomes more deeply coloured than the cellulose. If the former is a finely ground pulp, the effect is not very noticeable, but if it is coarse, containing a large number of coarse fibres, then the paper appears mottled. The defect is still further aggravated when the paper is calendered, especially if calendered in a damp condition. In that case the strongly coloured fibres of mechanical wood are very prominent.

When dyes have been carelessly dissolved and added to the beating engine without being properly strained, unevenness of colour may often be traced to the presence of undissolved particles of dye.

Irregular Colour of the two Sides.—Many papers exhibit a marked difference in the colour of the two sides. When heavy pigments are employed as the colouring medium, the under side of the sheet, that is, the side of the paper in contact with the machine wire, is often darker than the top side. The suction of the vacuum boxes is the main cause of this defect, though the amount of water flowing on to the wire, the “shake” of the wire, and the extent to which the paper is sized are all contributory causes. By careful regulation of these varying conditions the trouble is considerably minimised.

The under surface of the paper is not invariably darker than the top surface. With pigments of less specific gravity the reverse is found to be the case. This is probably to be explained by the fact that some of the colouring matter from the under side is drawn away from the paper by the suction boxes, and the pigment on the top side is not drawn away to any serious extent, because the layer of pulp below it acts as a filter and promotes a retention of colour on the top side.

It is interesting to notice that this irregularity sometimes occurs with soluble dyes, as for example in the case of auramine. The decomposition of this dye when heated to the temperature of boiling water is well known, and the contact of a damp sheet of paper coloured by auramine with the surfaces of steam-heated cylinders at a high temperature brings about a partial decomposition of the dye on one side of the paper. Generally speaking, acid dyes are more sensitive to heat than basic dyes.

The presence of china clay in a coloured paper is also an explanation of this irregular appearance of the two sides. China clay readily forms an insoluble lake with basic dyes, and when the suction boxes on the machine are worked with a high vacuum the paper is apt to be more deeply coloured one side than another.

The Machine Backwater.—Economy in the use of dyes to avoid a loss of the colouring matter in the “backwater,” or waste water from the paper machine, is only obtained by careful attention to details of manufacture on the one hand and by a knowledge of the chemistry of dyeing on the other. The loss is partly avoided by regulating the amount of water used on the machine, so that very little actually goes to waste, and further reduced by ensuring as complete a precipitation of the soluble dye as possible.

The acid dyes generally do not give a colourless backwater, and all pulps require to be heavily sized when acid dyes are used.

The basic dyes are more readily precipitated than the acid dyes, particularly if a suitable mordant is used, even with heavily coloured papers. The addition of an acid dye to pulp first coloured with a basic dye is frequently resorted to as a means of more complete precipitation.

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:—

(a) The colour is red; the dye is an oxazine, thiazine, azine, or acridine dye, e.g., safranine. (b) It is orange or yellow; the dye is as in (a), e.g., phosphine. (c) It is green; the dye is as in (a), e.g., azine green. (d) It is blue; the dye is as in (a), e.g., Nile blue, new blue, fast blue, or methylene blue. (e) It is violet; the dye is as in (a), e.g., mauveine. If the original colour does not reappear on drying, but does so if padded with a 1 per cent. solution of chromic acid, we draw the following conclusions:—

(a) The colour is red; the dye is rhodamine or fuchsine, or one of their allies. (b) It is green; the dye is malachite green, brilliant green, or one of their allies. (c) It is blue; the dye is night blue, Victoria blue, or one of their allies. (d) It is violet; the dye is methyl violet, crystal violet, or one of their allies.

If the original colour does not reappear even with chromic acid, it was in most cases a yellow or a brown, referable to auramine, chrysoidine, Bismarck brown, thioflavine, or one of their allies.

If the tannin reagent produces no precipitate, reduce with hydrochloric acid and zinc, or ammonia and zinc, and neutralise and filter as in the case of a basic dye. The solution when dropped on to white filter paper may be bleached (a), may have become a brownish red (b), may have been imperfectly and slowly bleached (c), or may have undergone no change (d).

(a) If the colour quickly returns the dye is azurine, indigo-carmine, nigrosine, or one of their allies. If it returns only on padding with a 1 per cent. solution of chromic acid, warming, and holding over ammonia, some of the dye is dissolved in water mixed with concentrated hydrochloric acid, and shaken up with ether. If the ether takes up the dye, we have aurine, eosine, erythrine, phloxine, erythrosine, or one of their allies. If it does not, we have acid fuchsine, acid green, fast green, water blue, patent blue, or one of their allies. If the colour never returns, heat some of the dye on platinum foil. If it deflagrates with coloured fumes, the dye is aurantia, naphthol yellow S., brilliant yellow, or one of their allies. If it does not deflagrate, or very slightly, dissolve a little of the dye in one hundred times its weight of water, and dye a cotton skein in it at the boil for about fifteen minutes. Then rinse and soap the skein vigorously. If the dyeing is fast with this treatment we have a substantive cotton yellow or thiazine red; if it is not, we have an ordinary azo dye. (b) The dye is an oxyketone, such as alizarine. (c) The dye is thiazol yellow, or one of its allies. (d) The dye is thioflavine S., quinoline yellow, or one of their allies.

If the dye is not stripped by alcohol and water, it is either inorganic or an adjective dye, such as logwood black, cutch, fustic, etc.; and we proceed according to the colour as follows:—

If it is red or brown, the dyed fibre is dried and divided into two parts. One is boiled with bleaching powder. If it is bleached entirely or to a large extent, the dye is cutch. If the bleach has no action, incinerate some of the dyed fibre in an iron crucible and heat the ash on charcoal before the blowpipe. If a globule of lead is formed, we have saturn red. The second portion is boiled with concentrated hydrochloric acid. If there is no action, we have Cologne umber; if there is partial action, we have real umber; if the dye dissolves completely to a yellow solution, we have an ochre; if the solution is colourless instead of yellow, and chlorine is evolved during solution, we have manganese brown.

If the colour is yellow or orange, boil with concentrated hydrochloric acid. If we get a green solution and a white residue, we infer chrome yellow or orange. If we get a yellow solution, we boil it with a drop or two of nitric acid and then add some ammonium sulphocyanide. A red colour shows an ochre or Sienna earth.

If the colour is green, boil with caustic soda lye. If the fibre turns brown, we have chrome green. If no change takes place, boil with concentrated hydrochloric acid. A yellow solution shows green earth; a red colour logwood plus fustic.

If the colour is blue or violet, boil with caustic soda lye. If the fibre turns brown, we have Prussian blue. If no change takes place, boil with concentrated hydrochloric acid. A yellow solution shows smalts. If the colour is destroyed, and the smell of rotten eggs is developed, we have ultramarine.

If the colour is black, warm with concentrated hydrochloric acid containing a little tin salt. If the black is unchanged, we have a black pigment. If we get a pink to deep red solution we have logwood black.

By means of the tests above detailed at length the group to which the dye belongs is discovered, and often the actual dye itself. Once the group is known it is generally easy, by means of the special reactions given in many books, e.g., in Schultz and Julius's “Tabellarische Übersicht,” to identify the particular dye.

When one has to deal with a single dye and simply desires to determine its group, the following table, due to J. Herzfeld, will suffice. Originally intended for textiles, it will serve, with some modifications here made in it, for the rapid testing of paper.