Crayon is a general name given to various mineral and vegetable substances used in designing or painting in pastil, whether they have been beaten and reduced to a paste, or are used in their primitive consistence, after sawing or cutting them into long narrow slips. In this last manner red crayons are made from red chalk; white, from white chalk; black, from charcoal and black lead. Crayons of all other colours are compositions of earths reduced to paste. The tempering of crayons is found to be an operation of great nicety, to avoid their being so hard as to impart an insufficient supply of colour; or, on the contrary, so soft as to crumble away, and to be little better than a powder upon the paper. A variety of slightly glutinous fluids have been proposed to give them the due degree of coherence; but the strength and the kind of fluid used requires to be varied according to the nature of the colour to be employed. The English manufacturers employ a variety of substances for this purpose; among which may be mentioned ale-wort, rendered glutinous by boiling, and gum tragacanth. It is obvious that the marks made upon paper by such compositions as we have mentioned, can be but slightly attached to the paper, and that they are extremely liable to be injured or defaced.
Various means have been resorted to for fixing them in such a manner, that, without having their tints injured, they may be enabled to bear rubbing. When the picture is made on unsized paper, Cathery recommends the back to be brushed over with a size made of half an ounce of isinglass, and two drachms of powdered alum, boiled for a quarter of an hour in a quart of water, and strained. This size, used milk-warm, penetrates the paper, and effectually fixes the picture. He also recommends another way, which is applicable to large drawings done on sized paper; it consists in sponging with the glutinous fluid a piece of unsized or blotting paper, of the same size as the picture. This wetted paper being laid flat upon a table, the face of the picture is pressed upon it in every part. The chalk thus becoming wet with size adheres to the original surface, and, by taking care wholly to avoid the smallest sidewise motion whilst the two surfaces are in contact, the colours are not in the least daubed, nor is the minute quantity of colour transferred to the blotting paper any injury to the piece.
Sebastian Grandi, an Italian artist, communicated to the Society of Arts a process for preparing crayons, which are stated to be of a quality greatly superior to those commonly or previously in use, being fixed so as to prevent their being rubbed off the paper when used, and are applicable alike to water or oil paintings. These crayons are made of bone-ash powder, mixed with spermaceti, adding thereto the colouring matters. The proper proportion is three ounces of spermaceti to one pound of the powder. The spermaceti to be first dissolved in a pint of boiling water, then the white bone-ash added, and the whole to be ground well together with as much of the colouring matter as may be necessary for the shade of colour wanted. They are then to be rolled up in their proper form, and gradually dried upon a board.