We distinguish between blocking in blind, gold, colour, and relief. Blocking in blind, both on leather and cloth, should not be attempted upon articles which have become very dry. A good plan is to store them in a cellar the night before and to bring out only as many as can be finished in the next half-hour's work. Blocking in blind can only be done on matt or grained material, as its effect is nil upon a smooth polished surface.
The finish can be considerably heightened in special cases by going over the impression with a brush and white of egg after the first blocking and then again pressing under less heat. Calf leather, and undressed calf in particular, receives its splendid brown colour by being previously damped with boiled or distilled water to which a little spirit has been added. The impression on a damped ground is first made somewhat warmer and afterwards repeated when the press has cooled a little, occasionally wiping the blocks with a waxed cloth. The impression must in all cases be even in colour and have a high finish; if this is not so, the lighter places must be once more damped with a small brush and again impressed.
Articles to be blocked in gold must be prepared with some substance to which the gold will adhere. The finisher uses a fluid or dry adhesive according to the material he is working upon. Of liquid sizes, diluted white of egg or, in some cases, dissolved gelatine or blood serum is used almost exclusively. These liquids, which are applied with a sponge or, for fine work, with a brush, are called the "ground" or "sizing." Of dry adhesives, we know only the gilding powder, which consists of resin with now and then an addition of dried white of egg. The other powders used by the finisher are rice flour or potato flour; these are not used as adhesives but to prevent the gold sticking at places where it is not wanted.
Leather and cloth are sized with white of egg; silk and other woven materials are finished without special sizing, and also surfaced papers and cardboard goods, as they are treated with finishers' powder, a wet preparation being unsuitable.
Size for blocking is thinner than for hand finishing, and when it has to be washed over the whole of a cloth cover it is still further diluted. Glair for brushing over an impression consists of one part white of egg and one part vinegar; to every 1/2 litre 1 g. of powdered borax is added, the whole beaten to a froth and filtered. For coating-over, take one part white of egg, two parts water, and to every 1/2 litre add 10 drops of glycerine and 1 g. borax. If a larger quantity of glair is required for coating over, it may be thinned with water or vinegar; but size made with vinegar must not be applied to fancy coloured cloths.
Gilding powder may be yellow or white, the latter kind is rougher and is suitable for silk stuffs and also where blocking has to be done over colour on cloth. Yellow powder is softer and is apt to clog the finer cut tools; it is more suitable for the preparation of paper de luxe, for blocking surface and chromo papers, and for photo cases.
Gilding powder is sprinkled by means of a box which has a double layer of fine gauze instead of a bottom. By tapping upon the box a fine shower of powder is dusted on to the surface to be gilded.
Glair is made in the workshop; the powder is bought ready prepared—to make it would take too much time.
For blocking in alloyed metals, gelatine size is used, as it is a powerful adhesive. A tablet of gelatine is soaked in 1/4 litre of water overnight and next morning is dissolved in the bath. Gelatine is used to cover the whole surfaces and is laid on when lukewarm. Blood serum serves the same purpose. Ox blood is allowed to coagulate, and the hardened mass thus produced is placed over a cloth strainer, or hair sieve, and the liquid albumen allowed to drain from it; the process is hastened by cutting the caked blood into small pieces. Only the clear liquid which is strained off is to be used—a pinch of borax will make it keep longer.
For finishing bindings, real gold-leaf—orange tint—is used almost exclusively, the cheaper green gold being used only for sample cases and school books. Alloyed metal and aluminium (this is now commonly used instead of the quickly oxydizing silver) are used almost solely in the manufacture of papers de luxe and for placards.