Just as certain kinds of leather are difficult to treat, so also are old, long-stocked skins. If the finisher knows that such a piece of leather lies before him, he ought to rub it down on the raw side with best olive oil; it will thus receive a substitute for the lost natural fatty matter and become more supple. This oiling, of course, is only practicable with dark leathers, as light leathers nearly always become darker. A skin so treated should be rolled up and laid aside for a few days.
At any rate, such a defective leather is improved by a previous washing with size, no matter whether it be made from vellum, gelatine, or glue. It must not, however, be made from the commonest kinds of glue, as these make the leather dull.
Vellum cuttings, or gelatine, are soaked overnight in water sufficient to cover, and next day dissolved in a sort of glue-pot. This sizing must be used very thin and should not be quite cold. In using ordinary glue, two drops of medium strength to about three table-spoonfuls of warm water will suffice.
Many finishers prefer thin paste water as a size, with which they coat the whole surface; personally, however, I would only recommend it for unpolished calf—but here there is a real necessity for it. In this case it is liberally laid on in large sweeps with a sponge—not new, and free from grit—and then well rubbed into the pores of the leather with the ball of the hand. The surplus is quickly washed off with clean water.
Large surfaces left plain are also washed with gum tragacanth, because this sizing leaves no lines behind. About 10 grammes of tragacanth to 1/4 litre water are soaked in a vessel, and after swelling it is well stirred. This solution is also laid on with a sponge, but is not rubbed in, neither is it subsequently washed off.
These two methods of sizing will be found sufficient to meet all cases.
There are certain materials that do not allow the use of a liquid sizing, particularly silk and velvet. The latter is no longer finished by hand, and even blocking velvet is now considered bad style.
For gold tooling on such materials gilding powder is used; it is to be had either white or yellow, but for hand tooling the white only is required.
Although no exact rules can be given for determining the temperature of the tools used upon the various materials, yet we must try to give a general rule for each kind.
We will use the following terms to distinguish the different degrees of heat: Lukewarm, i.e., not the slightest hissing when tested; medium, i.e., just on the border of hissing; hot, slightly hissing. In the following group the method of sizing is repeated, and a scheme for the approximately exact temperature of the tools is given.