The iron solution is generally either of ferrous sulphate of perhaps 5 per cent. strength, or commercial “iron-liquor,” which is a “pyrolignite” or crude acetate of iron, containing catechol-derivatives and other organic products from the distillation of wood, which act advantageously, both as antiseptics, and in preventing the rapid oxidation which occurs when pure ferrous acetate is used. Iron-liquor is generally to be preferred to ferrous sulphate (“green vitriol”), as the sulphuric acid of the latter, unless completely neutralised by the alkali employed in preparation, acts in the end disastrously on the leather. Commercial iron-liquor is often adulterated with ferrous sulphate, which may be detected by its giving a precipitate with barium chloride. Great care should be taken not to use iron in excess of the logwood or tannin present, as it otherwise takes tannin from the leather itself, making it hard and liable to crack, while any uncombined iron acts as a carrier of oxygen, giving up its oxygen to the colouring matter or tannin with which it is in contact, and again oxidising from the air, and so causing “spueing” or oil-oxidation, and other evils.
Good blacks which are more permanent than those with logwood, may be obtained by merely treating leather containing an excess of oak-bark tannin or sumach, first with an alkaline solution (not at the most stronger than 21⁄2 per cent. of liquid ammonia, or 5 per cent. of soda crystals), and then with iron-liquor. If it is not certain that the leather contains excess of a suitable tannin, a tannin-solution must be employed like the logwood infusion, or the leather must be sumached. The addition of some sumach to logwood liquor is often advantageous, and a blacker (i.e. less blue) black, especially on alumed leathers, is obtained by using a proportion of fustic. Solutions made by boiling 10 per cent. of cutch with 5 per cent. of sodium carbonate give good blacks with iron-liquor, and do not make the leather tender, and they can be used in mixture with logwood. Many commercial logwood extracts contain chestnut-wood extract as an adulterant.
Instead of dyeing in the bath, it is very common, especially for the cheaper leathers such as linings, and coloured leathers of the commoner sort, to apply the colour by brushing (commonly called “staining”). Many colours, however, which dye well with time and warmth, are inapplicable in this way, and only those should be used which have a strong attraction for the leather, and hence go on well in the cold. If “acid” colours are employed, it is essential to select those which can be used in neutral solution, or at most with addition of some mild organic acid such as formic or acetic, since, as the leather is not washed after staining, the sulphuric acid would remain in it, and would ultimately destroy it. Where leathers have a hard and repellent surface, the addition of a little methylated spirit to the dye is often very useful. The colours are used in solutions of from 1⁄4 to 1 per cent., which should be quite clear and free from sediment. Difficultly soluble colours must be used in weak solution, or the dye kept warm while in use. Dye-solutions will not generally keep for any great length of time without change.
Before staining, the leather must be carefully “set out,” or otherwise made as smooth as possible, and the staining is generally done after most of the other operations of currying or dressing have been completed. Staining is best begun with the leather in a slightly damp or “sammied” condition, and the colour is applied evenly with a softish brush in two or three coats, the leather being slightly dried after each. As a rule the more coats are applied, the more even is the work; but to save cost of labour it is common on cheap goods to be content with two, of which the first is given, preferably with a weaker solution, to the dry leather. Where the leather is “weak-grained” it is sometimes advantageous to size it first with a weak solution of gelatine, gum tragacanth, or linseed mucilage, and similar solutions are often used to fix the colour and give a higher gloss. The stearine-glaze mentioned on [p. 401] may also be used for this purpose, and a weak solution of it is sometimes employed as a vehicle for the acid colours. Acid yellows and browns may also be dissolved in the undiluted glaze where only a pale colour is required, or to heighten the colour of leather already stained. A list of suitable colours for staining is given in the Appendix, [p. 486].
It rarely happens in leather dyeing that the required colour can be given by the application of a single dye, most of the shades now required being produced by mixtures. It is, therefore, necessary to say a few words on the theory of colour combinations.
White light is of course composed of a mixture of all the spectrum-colours, and can be separated into them by the prism. It is probable, however, that the eye is only capable of three distinct colour-sensations, and that all the colours we perceive are represented by the excitement of these in different proportions, the actual colour-sensations being red, blue-green, and violet.[179] If we interpose a piece of yellow glass between the eye and white light, the violet and blue are absorbed, and the remaining red and green rays combine to produce the sensation of yellow. If pure blue glass is used, the red is absorbed, and we have blue as the result of the remaining mixture of green and violet. Red glass absorbs the whole of the green, and greenish-blue, allowing red and much of the violet to pass. Thus, if we combine blue and yellow glass, only the green is allowed to pass, and similarly with red and blue glass, green and blue is cut out, and only the violet remains. Thus red, yellow, and blue are frequently called the primary colours, and by combining all three in equal proportions all colours are cut out, and black or grey results. The blue and violet which are stopped by yellow glass are those colours which would produce the sensation of violet-blue, and hence the latter is called the “complementary colour” of yellow, and so on with the rest. It will be noted that all the colours of coloured objects are produced by absorption of a part of the light, and therefore coloured bodies are always darker than white ones, and where a colour is mixed with its complementary in suitable proportion, all colours are absorbed and black or grey is produced.
[179] The subject of colour is too complicated to be adequately treated here; and for fuller information, readers are referred to Abney’s ‘Colour Measurement and Mixture,’ S.P.C.K., London, 1891. It may, however, be pointed out that, while the true primary colour-sensations are unquestionably red, blue-green and violet, and by mixture of light of these colours, all other colours, including white, can be produced; the primary pigments or dyes are red, yellow, and blue; the effect being produced in the former case by the addition of colours, and in the latter by their subtraction.
Colours which are made by mixing two primary colours are generally called “secondary”; while the duller tints made by the addition to these of black, or of a complementary colour which produces black, are called “tertiary.” Any primary colour is complementary to the secondary colour produced by mixing the other two primaries and vice versa. The following tabular arrangement shows at once the effect of colour mixing.
| Pri- mary. | Secondary. | Tertiary. | |||
|---|---|---|---|---|---|
| Red | |||||
| - | Orange | with Black. | Brown. | ||
| Yellow | |||||
| - | Green | „ | Olive, Sage. | ||
| Blue | |||||
| - | Purple (Violet). | „ | Puce, Maroon. | ||
| Red | |||||
Theoretically, any colour may be obtained by mixture of the primaries, and that this is possible to a great extent is shown in the success of modern “three colour” printing, by which pictures are obtained in natural colours by the use of three primaries only; but in practice few colours are quite pure, and if two very different colours are mixed, it is difficult to avoid the production of tertiaries. The most brilliant colours are generally produced by dyeing with the nearest colour which can be obtained to that required, and shading with another which is near, but on the other side of the desired tint.