[178] Such mixtures may often be detected by putting a drop of their solution on blotting-paper, when the dyes form differently coloured rings according to their more or less rapid fixation by the paper, or by dusting the dry dye very thinly on wet blotting-paper, when each particle produces its separate spot.
Much of the success of practical leather-dyeing depends on proper selection and preparation of the goods. Sound uninjured grain is a matter of first importance; no satisfactory dyeing can be expected on skins which through carelessness in soaks, limes, or bates, are tainted by what is known as “weak grain,” caused by destruction or injury of the delicate hyaline layer, which forms the natural glaze and outer surface of the skin ([p. 50]). For such goods, “acid” are to be preferred to “basic” dyes, the latter having an especial tendency to dye darker and deeper where the grain is imperfect. Goods of different tannages and colours should never be dyed together, as they are certain to produce different shades in the same dye-bath. Tanned skins which have been dried, especially if they have been in stock for some time, should be thoroughly softened by soaking in tepid water and drumming, a temperature of between 40° and 45° C. being most advantageous. Skins, such as calf of mixed or bark tannage, must now be freed from all bloom by scouring with brush and if necessary with slate or stone, but great care is requisite to avoid injury to the grain. A little borax or other weak alkaline solution assists in removing bloom. Fresh sumach-tanned skins merely require setting out with a brass sleeker, but those which have been long dried often dye more evenly and readily if they are re-sumached.
Dark coloured tannages, such as Australian bazils, and East India sheep and goat tanned with cassia bark, are always improved by sumaching, and if for light colours, by first stripping a portion of the original tan by drumming for a quarter of an hour with a weak (1⁄4 per cent.) solution of soap powder or borax at a temperature of 30° to 35° C. and then passing (after well washing in warm water, but with as little exposure as possible to the air) through a weak sour of sulphuric acid of 1-2 per cent. The acid should now be as thoroughly removed as possible by washing in water, and the goods should be sumached. The process, and especially the use of sulphuric acid, is always deleterious to the skins, and is one of the causes of the early decay of coloured bookbindings and furniture leathers. Lactic, formic, or acetic acid may be substituted for sulphuric with safety, and the risk of injury from sulphuric, which generally is only apparent after the lapse of a considerable time, is a good deal lessened by adding to the sumach liquor a small quantity of potassium tartrate, sodium acetate or lactate, or some other salt of a weak organic acid, which is thus substituted for the much more dangerous sulphuric. Except in cases of absolute necessity for the production of light shades, the use of sulphuric acid should not be resorted to, and then only for goods which are not expected to possess great permanence. For light shades for bookbinding and upholstery, good sumach-tanned leathers and organic acids only should be employed. Alkaline treatment also demands great caution, as excess of strong alkalies is very injurious to the leather. Another objectionable method for the preparation of leather for very light shades, is the use of the lead-bleach described on [p. 399].
The sumaching is best done in a drum, at a temperature of about 40°. Lamb advises that 1 to 2 lb. of sumach per dozen is sufficient for calf, and recommends running in this liquor for two or three hours. The skins are then rinsed in water to free them from adhering sumach, and set out on a table with a brass sleeker, and are now ready for dyeing with “acid” dye-stuffs. If “basic” dyes are used, thorough washing in several tepid waters is necessary to free them from the loose tannin; and if deep colours are to be dyed, it is better, instead of too much washing, to fix the tannin, which then serves as a mordant for the colour. For blues, blue-greens, or violets, this is done with a solution of “tartar emetic” (antimony potassium tartrate, of 5 to 20 grm. per liter according to the amount of tannin to be fixed, often with addition of some common salt), which produces no alteration in the colour. For browns, yellows, deep reds, or yellow-greens, it is advantageous to use titanium-potassium lactate or oxalate (2 grm. per liter), which in combination with the tannin produces a very permanent yellow coloration on which the basic colours dye freely. In many cases the titanium salt is best applied after dying with one of the dyewoods (Dreher).
The basic colours usually require simple solution in hot water before adding to the dye-bath, and are used in quantities of 0·5 to 2·5 grm. per liter of dye-bath, according to their colouring power, which varies a good deal, and to the depth of shade required. The solutions should not be boiled, and some colours are injured by too high a temperature. Some colours dissolve incompletely, and require filtration through a cotton cloth. As basic colours are precipitated by calcium carbonate, it is important that “temporary” hard waters should be neutralised with acetic or lactic acid till they faintly redden litmus; and in the case of colours which, from their attraction for the leather fibre, dye too rapidly, and consequently unevenly, better dyeing is often obtained by the use of a small excess of acetic acid, which also increases the solubility of the colour. Too much acid, however, will prevent the proper exhaustion of the bath. Some few colours, now little used, require to be dissolved in the first instance in a little methylated spirit; and the addition of spirit will often assist dyeing and staining where the leather is slightly greasy, though considerations of cost generally prevent its use. Sodium sulphate is not unfrequently added to dyeing baths to improve equality of dyeing; and with some of the cotton dyes common salt is used to lessen their solubility and facilitate the exhaustion of the dye-bath.
“Acid” colours usually dye better if acid is added to the bath, to liberate their colour-acids, and for this purpose sulphuric acid is generally used in weight about equal to that of the colour used. Its use is, however, objectionable, in this case, for the same reasons as in bleaching, since it is impossible by mere washing to remove it entirely from the leather, which it ultimately rots when concentrated by exposure to a dry atmosphere or high temperature; and it is better to use formic or acetic acid to the extent of two or three times the weight of the dye-stuff. Sodium acid sulphate may also be used, but is probably more objectionable than an organic acid. Many acid colours, however, dye quite satisfactorily from a neutral bath. The acid colours are used in somewhat similar quantities to the basic, but are generally inferior in colouring power, though they dye more evenly, especially on defective grain, and are often more permanent to light.
Mention has already been made of the polygenetic or mordant dye-stuffs, which are still used to some extent for dyeing glove-leathers, and of which logwood is important in dyeing blacks. Fustic and Brazil-wood (peach-wood) are not quite gone out of use among old-fashioned dyers, even for dyeing moroccos and other coloured leathers of vegetable tannage. Peach-wood, with a tin mordant (generally a so-called “tin spirits” made by dissolving tin in mixtures of hydrochloric and nitric acid) was formerly much used in dyeing cheap crimsons, but is now quite displaced by the azo-scarlets. The acid tin-solutions were frequently very injurious to the leather.
The wood-infusion, rendered slightly alkaline with soda, ammonia or, formerly, with stale urine, is usually dyed first on the leather, and followed by the mordant “striker”; ferrous or ferric solutions, and potassium bichromate being used for dark colours, and tin salts, or sometimes alum, for the brighter ones. The mordant is sometimes added to the dye-bath towards the end of the operation, but is better used as a separate bath, as it is apt to produce a precipitate of colour-lake on the surface of the skin, which rubs off on friction. In some cases, and especially in black dyeing, the strong infusion of dye-wood, and the necessary “striker” are successively applied by brushing instead of in the dye-tray.
Logwood and Brazil wood are both Cæsalpinias closely allied to divi-divi. Logwood is Cæsalpinia (see [p. 287]) Campechianum. Its colouring matter is hæmatoxylin, a substance nearly allied to tannins, and almost colourless; which on oxidation gives hæmatin, which dyes a yellow-brown, only developing other colours by the aid of mordants. Logwood chips are extracted by boiling or heating under pressure for some time with water; and as hæmatin gives dark purplish-red compounds with alkalies, soda or stale urine is frequently added under the mistaken belief that it produces a better extraction, but really leads to waste of colouring matter by oxidation. It is best to extract with water alone, and add any necessary alkali to the infusion before use. 1-2 lb. of wood per gallon is frequently employed in making the infusion, and as this proportion of water is quite insufficient to properly extract the wood, the residue should be boiled with one or more further quantities, which are employed in turn for extracting fresh portions of wood. Logwood dyes best at high temperatures, and especially in the case of chrome leather with which a temperature of 80° C. may be safely used. The presence of a trace of a salt of lime is advantageous, and with very soft waters a little lime water or chalk may be added to the logwood liquor.
In blacking skins, the strong infusion is rendered slightly alkaline with sodium carbonate or ammonia, and brushed undiluted on the leather. If employed as a bath, a somewhat weaker infusion is used, and the leather is frequently treated first in an alkaline bath, to which a small quantity of potassium bichromate is often added. The object of the alkali is not only to assist in the formation of the colour-lake, by saturating the acid set free from the iron-salt used as a striker, and thus to prevent the colour from penetrating the leather too deeply, but, at the same time, to overcome the resistance to wetting caused by grease or oil which the leather may contain. It must thus be used more freely when stuffed leather is to be blacked, but excess should be carefully avoided, as it easily renders the leather tender and brittle. The potassium bichromate oxidises the hæmatoxylin, or the ferrous salt subsequently applied, and forms a nearly black chrome-logwood lake.