Partly owing to its nature, and partly owing to the tanning process, leather is susceptible to the action of the dyes used to colour it in degrees varying according to their composition and penetration. We will not attempt to enumerate all the dyes that may be employed or to describe their composition. Excellent results can be obtained with ready made preparations, and some brief remarks on the subject are all that is necessary for the purpose of this work.

Practical experience alone is a reliable guide in the selection of dyes, as, whatever their base may be, they all vary greatly in use. It may be remarked that there are many aniline dyes which are more durable than vegetable ones, although there are others which merit their reputation for instability. The colour of dyed leather cannot be expected to remain unaffected by constant exposure to direct sunlight, but it should not alter with ordinary use.[11]

It is obvious that, in proportion as the colours sink into the leather, they will lose in brilliancy, but on the other hand, this very absorption, which allows colours to be superposed and appear translucent, gives a depth rarely obtainable in materials of less capillarity.

The following notes are intended merely as a general indication of the manner of treating the various substances used in colouring leather.

The substance sold by druggists under the name of potassium is very violent in its action and would burn the leather as well as the brushes used to apply it. It should only be employed, and that very sparingly, for such purposes as black lettering. Pearl-ash should be used in preference, but also with great caution, or it will not only destroy the surface, but rot the substance of the leather.

Potash is soluble in water to saturation point, that is to say, a given quantity of water will not dissolve more than a given quantity of potash.

It is very difficult to measure the exact strength to use; that which is barely sufficient to penetrate and colour a surface artificially made smooth by rolling, will sink in where the leather has been cut or punched and immediately darken it considerably. It is a good plan therefore to experiment on a spare piece of the leather. Potash lye is more easily used in the right proportion, and may be employed for staining very light coloured leather.

Soda has the same effect as potash, except that it is a little less strong; both substances have always been much employed in dyeing skins. Permanganate of potash is supposed to dye a mouse-grey colour, but it is most uncertain in its action, as it depends entirely on the porosity of the leather. Xanthin and catechol browns are pleasing in appearance, but their effect is less rich than that obtained with potash. They become improved by hard rubbing with encaustic.

Sulphate of iron has a strong and persistent odour and is also somewhat injurious. It is useful, however, and produces a colour varying from a light shade to an intense violet black. When used over potash, a dead black is obtained.

REDS

Alizarin, the colouring principle of madder, which can also be obtained from coal-tar, is employed in many colour combinations and gives a fine durable red. Hydrochloric acid changes it to violet. Carthamine and cochineal also produce good reds; the latter is turned orange by the action of hydrochloric acid.

Hydrochloric acid turns ammoniacal cochineal pink, but merely discolours the red made from archil. Direct extracts of red wood are unaffected by it.

The colouring matter of logwood, extracted by treating the powder obtained from an aqueous decoction of the wood with spirit, is sold as hematoxylin. Metallic salts convert it to a violet black, salts of tin change it to pink, alkalis to blue, and acids to yellow. Durable pinks are obtained directly from carthamine, the colouring matter of Carthamus Tinctorius.

YELLOWS

The base of most fine yellows is picric acid. The chromates of zinc and baryta give very permanent yellows, which are particularly useful in mixing greens.

The vegetable yellows are less durable. They include turmeric, which is especially fugitive, and quercitron; the former is turned red, and the latter brown, by the action of hydrochloric acid.

Orange tints are easily obtained by using red over yellow, first allowing the yellow to dry completely; or they can be bought ready prepared with alizarin. Mandarine and nasturtium tints can also be bought ready prepared, or may be made by a mixture of vegetable colour, such as saffron and arnotto.

BLUES

Good and durable blues are obtained from indigo and alizarin.

GREENS

Greens of every composition are fugitive, especially those sold ready mixed. Better results are obtained by applying yellow and blue one over the other than by the direct application of the colour, but when the range of tints thus afforded is too restricted, recourse must be had to some of the numerous dyes varying in tint from greenish yellow to deep olive. Successive applications of sulphate of iron and picric yellow will produce the latter colour, and a vegetable green, which, however, is not very durable, is made from buckthorn berries.

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All the dyes which have been just enumerated, which form, however, but a very small proportion of those in existence, are employed in dyeing textiles. These materials have first to be treated with mordants such as tannin, chrome or sumach, in order that they may take the dye, whereas leather is already prepared to receive it by the tanning process.

Ordinary water-colours based on glycerine or albumen can be used in the decoration of leather, but are recommended only for colouring small parts of a design, partly because liquid colour is more easily applied over large surfaces, and partly because, being less transparent, they do not lend themselves to the process of rubbing which plays so important a part in patine. Oil or spirit colours, on the other hand, possess a transparency which allows of their being used over a ground coloured with a different medium. When the leather on which they are painted is undyed it should first receive a coat of paste.

Colours sold under the name of patines grasses will produce a groundwork of great brilliancy, if well rubbed before they are quite dry.

The use of gouache should be limited to the hollow parts of incised or punched leather.

Gold and bronzes are also used with good effect.

DIFFERENT METHODS OF COLOURING LEATHER

Colours obtained from potash and sulphate of iron—dark green, dull red, etc.—are much used for groundwork. Potash produces a warm brownish-red, and tints, varying in colour from a pale to a dark slate grey, are obtained with sulphate of iron. As already stated, a coating of picric yellow over the latter will give a dull green.

In colouring large surfaces, it is best to use soft sponges, or small pads of cotton-wool covered with soft linen. The pads can be held by the edges of the linen and the sponges may be mounted on handles, but the best protection for the hands against staining is a pair of very soft rubber gloves like those used by surgeons.

The leather is first damped all over and the colour then applied in a succession of even washes. Care must be taken not to unduly darken any part of the leather by using too much colour, bearing in mind that different parts of the skin will vary greatly in porosity. In colouring groundwork with various superposed tints, the actual results are often entirely different from those one has had reason to expect. So far as is possible, in order to prevent this from happening, it is advisable not to mix the colours of different makers on the same piece of work without knowing their composition, or first experimenting with them on a spare piece of the leather.

A second colour should never be added until the underneath one is dry, or a muddy effect will result. If the colour becomes too dark, or if the second stain predominates unduly instead of merging into the underneath one, the whole work, fixed firmly on a board, should be dipped in water and scrubbed with a very soft brush and some soft soap. It should then be allowed to dry in the sun. This method of making the colour lighter can only be used when the ornament is not in relief; it has been found particularly successful in treating potash and sulphate dyes.

Ox-hide, light or fawn-coloured calf, pigskin and white morocco are leathers suitable for colouring. They are, however, very absorbent, and will require an application of paste before staining. This will also be found indispensable in colouring the white split sheepskins used in mosaic.

In staining leather for mosaic, several successive applications of colour will be required, especially in the case of greens which sink in quickly.

In addition to plain dyeing, there are various methods of producing shaded groundwork, marbling, sprinkling, etc.

SHADED GROUNDWORK

For shaded groundwork diluted colour is applied with a sponge or pad worked over the leather with a circular movement, and the uniform tint thus obtained is darkened as required by further applications with stronger colour. In shading a ground from one colour to another, as for example from dark olive to malachite green, orange red and pale yellow, the edge of each colour, as it is put on, is covered with a piece of thick paper, which must be firmly held in place, in order to prevent the colours mixing where they come together.

MARBLING

A sponge with large holes is dipped in potash, sulphate of iron, or some other deep coloured dye, and lightly applied over the surface of the leather so that parts of it corresponding to the holes in the sponge are left untouched; or the first application of colour may be made evenly over the leather and the marbling worked, as described, in a second application. Many varieties of pattern can be obtained by moving the sponge in different ways. Leather may be bought ready marbled by a gelatine process.

TREE MARBLING

The leather is first lightly pasted over, and drops of water, more or less large, are scattered upon it by jerking the handle of a small couch-grass whisk full of water against the left wrist held a few inches from the surface of the leather. A very little colour is then taken in the brush and jerked in the same manner over the drops of water, with which it will partially mix and give the effect of roots branching out in all directions. Variety can be obtained by applying the water with a sponge instead of the brush, and the colour in large drops over the whole or certain parts only of the field, or by using dyes of different colours which give curious effects in combining. After applying the dye, the surplus moisture is absorbed with blotting-paper.

The leather may be bleached to a light tint by means of an oil-paint brush dipped in decolorant[12]; the bristles of the brush are pushed back with the finger and when released jerk the acid in fine drops over the surface. The leather must be freshly and thoroughly damped so that the outline of the bleaching shades off into the ground.

SPRINKLING

Sprinkling is done with a rather close iron grating and a long narrow brush like a plate-brush. The dye is prepared on a flat plate and the brush dipped in it, the superfluous liquid being removed against the edge of the plate. The brush is rubbed backwards and forwards on the grating held horizontally a few inches above the leather over which it scatters a fine shower of the dye. Should the colour froth on the grating, the brush must at once be rubbed upon oiled paper. Sprinkling may also be done with a vaporiser for fixing drawings, but it is difficult to get a sufficiently fine shower so that the drops do not merge into each other.

The above process is a useful one when parts of the leather are to be “reserved,” i.e., to remain untouched by the dye. For this purpose a piece of strong paper is cut to the shape of the part to be reserved and pasted lightly to the leather during the process of sprinkling, after which it is damped and removed. Another method is to paint the reserved parts with one of the two preparations given by Mons. Belleville in Le Cuir dans la DÉcoration Moderne, afterwards removing it with benzine or petroleum spirit:—“Eighty parts of colophony, 4 of yellow wax, 6 of spermaceti, 4 of tallow, 60 of spirit of turpentine and 10 of alcohol.”

Or, “One hundred parts of virgin wax, and the same of castor oil, with 25 of borax and 25 of copal varnish.”

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Instead of reserving the parts of the leather, the dye may be removed subsequently by means of reagents called decolorants. Sulphuric, hydrochloric, oxalic, nitric, or muriatic acid diluted with water in the proportion of one in five, can be used for this purpose, but their effect should first be tried on a spare piece of the leather, as it will vary according to the nature of the dye on which they are used. It should be noticed that, in mixing the acid, the right quantity of water must be measured out and the acid carefully added drop by drop. If water is poured into acid it causes an explosion.

The outline is traced rather deeply on the leather, so that it is still visible after the groundwork has been dyed. When the dye has sunk in, the decolorant can be easily applied with a paint-brush. On modelled leather dyed bronze green, a rag or piece of cotton-wool dipped in decolorant and quickly passed over the relief, will lighten the colour and produce the effect of bronze with the exposed parts rubbed lighter by use. Should the effect of the acid be too strong, the colour may be again deepened by washing the leather over with a yellowish gold stain, which will only take effect on the relief, since it will merge into the darker colour of the rest of the work. The same process may be employed on white calf with a light olive dye to obtain the effect of old ivory. Even when much diluted, however, the action of these acids must in the end be injurious to the leather. They should be used with great care and never undiluted. Where possible, washing the leather over afterwards with water will, to some extent, diminish their harmful effect.

Stencilling is another process by which portions of the leather forming the ornament are reserved from the dye used in colouring the ground. The design is cut out of a very thin sheet of copper or tin. A large short-haired brush[13] is used to take the dye from a plate, or pad, and is then rubbed very lightly over the stencil firmly fixed on the leather. Only the parts unprotected by the stencil will take the colour. The brush must not be too full or the dye would have a tendency to run under the stencil. The most suitable dyes are glycerine inks and marking inks containing nitrate of silver. A variety of colours may be mixed with glycerine for this purpose. The ink thus obtained is transferred to a cloth pad, such as is used by printers, from which the brush will only take the required quantity.

When the leather is dry a lighter tint, transparent enough to allow the stencilled design to be visible through it, may be washed over the whole. Excellent effects of considerable richness can thus be produced. The depth of colour is further increased by the process of rubbing, which is necessary to the proper patine of leather. If done with wax, encaustic or varnish, it is an effective protection for the surface of the leather against deterioration from outside causes. With certain soft leathers, if the colour has not sunk in deeply, rubbing with the finger alone will give it great brilliancy; with other leathers the rubbing will produce an effect in proportion to the degree of porosity of the skin. It should always be done before the colour is dry, but not until it has penetrated sufficiently into the leather not to return to the surface with the pressure. The middle finger is used. A certain amount of polish can be given to the leather by rubbing it only with the finger (moistened with a little oil to allow it to slide easily), or, for a large surface, with the side of the hand instead of the finger. When a medium is used, it is applied with a circular movement by means of flannel or rag; a flannel brush consisting of a series of little flannel rolls, which can be bought at a surgical instrument-maker’s, is very useful for a large surface requiring much rubbing. Pure wax, which must be softened in the hand to make it spread evenly, encaustic and spirit varnish are all efficacious in preserving the colour and increasing its transparency.

Another method giving excellent results consists in applying opaque colour, gouache or bronzes in the hollows of a design and removing the surplus of the colour by rubbing. In this way an incised design can be boldly outlined in light on a dark ground, and may afterwards be bleached within the outlines if desired. After cutting the design on the leather, the whole surface is washed over with potash or any other dark stain, a coat of paste is then applied and immediately wiped off, so that some of it only remains in the hollows. When the paste is dry, some rather liquid gouache is washed over and run into the lines, any remaining on the surface being removed by vigorous rubbing with a damp rag. The same process may be applied to the hollows of punched leather. In using bronzes or gold in powder, some spirit varnish or specially prepared varnish is necessary to make them adhere. After applying the varnish, the surface is wiped over to remove all except that which is deposited in the interstices, and when this is sufficiently dry to hold the powder, the latter is put on with a very soft, short-haired brush and adheres to the varnish. The surface is then rubbed in the manner already described. All powder colours do not mix equally well with varnish; green and blue bronzes often change colour when brought in contact with it. Gold and silver powder give good results, used either dry or mixed with gum on shells. Chinese vermilion may also be employed with varnish, but other colours are more suitable for this purpose when prepared in the form of gouache.

GILDING

Special manipulation is required for gold leaf applied to the decoration of leather. For gilding over a whole surface, as for instance an aureole round the head of a saint, the following is the best method. Several layers of starch paste are painted with a brush over the part of the leather to be gilded, and followed when dry by a little gelatine paste. A coating of some special preparation is then applied and allowed to dry to the right condition to retain the gold leaf, which varies according to the nature of the preparation; the degree of moisture can be tested with the finger. Gold leaf is sold in small books and is exceedingly thin. A flexible knife, like a palette knife, is used to manipulate it, and a single leaf is laid on a cushion of deerskin stuffed with wadding, and cut to the required size. It is then placed over the preparation on the leather and lightly pressed down with a very soft, round, short-haired brush.

For gilding with hot tools a specially prepared powder made of resin or gum is used. This powder melts with the heat of the tool and causes the gold leaf to adhere to the leather only where it comes in contact with the parts of the tool cut in relief. The process is as follows: Some powder is lightly spread over the part of the leather to be gilded and a piece of gold leaf laid over it, the tool is heated sufficiently to melt the powder without burning the leather and is carefully applied on the right spot, the surplus gold being afterwards removed with the short brush. The tools used by bookbinders are slightly convex, so that firm and even pressure of the whole tool can be exerted by first inclining the handle towards the gilder, and then raising it while still maintaining the pressure and inclining it at the opposite angle.

Gold can also be applied with a brush in the form of powder suspended in liquid gum or spirit varnish. If it does not spread evenly, owing to there being too much liquid in proportion to the powder, dry powder may be added where necessary with a short brush when the liquid is almost dry.

Shell gold is applied with a slightly damped brush and, when dry, may be burnished with an agate or punch; the design of the punch will stand out in bright gold on a ground of dull gold. The pressure of the wrist is sufficient to work the punch and a hammer should not be used.

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There is an interesting field for the artist’s enterprise in the discovery of new processes for the decoration of leather by means of colours and bronzes. It is not possible within the limits of a single chapter to give a complete list of the many methods that may be employed.[14]

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