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.