The moulds in which the cakes are made must be very carefully worked in metal, so that the description and trade mark print clearly upon the cakes. The cakes should be dried at the ordinary temperature or but little higher. They are placed upon smooth boards, and care is taken that the temperature of the drying room remains uniform, which is the condition requisite for the production of the fewest cracks. When it is intended to place on the market only faultless and well-stamped cakes they must be sorted when dry, and the cracked ones rejected; these can be worked up in the next operation.
The dry cakes are given a good appearance by coating them with a weak solution of gum, and then drying. According to the price at which these colours are to be sold the cakes are given a different character. The finest colours are generally made into larger cakes and packed in handsome boxes, whilst ordinary cheap colours are made into small lumps or circular plates, flat on one side and somewhat convex on the other, and packed in boxes of soft wood.
Moist Water Colours.—Instead of grinding water colours with gum Arabic or tragacanth and bringing them on the market in the dry state, they may also be sold in a condition resembling that of oil paints. This may be accomplished by using very viscid glucose syrup instead of gum Arabic, and grinding the pigments in this exactly as in oil. Glucose is very hygroscopic, the colours prepared with it remain moist, and may be spread out upon the palette like oil colours. It is then only necessary to wet the brush in water and mix the mass with it in order to obtain colour of the proper consistency.
Moist water colours have also been known as ”honey colours,” since this mixture of sugars was formerly used for their preparation; it also is hygroscopic. Honey is no longer used; the much cheaper glucose answers the same purpose. On account of their semi-fluid nature, moist water colours are put up in tubes just as oil colours. They are thus rather dear, but are little used.
CHAPTER LXVII.
CRAYONS.
Crayons are coloured pencils by which pictures may be, so to speak, ”dry painted”. At present this method of painting is little used, though it was in vogue in the last century, but coloured pencils, especially blue and red, are much used for writing. Crayons are now used in a very convenient form, being generally made in the same manner as lead pencils. The coloured mixture from which the crayon is made is produced in the form of a paste containing a soft, finely ground mineral, well mixed with the colouring matter and an amount of binding medium just sufficient to hold the powders together.
Gypsum is generally used as the soft white mineral forming the base of the coloured mass. It is much better to use steatite (soap-stone), which is not much dearer, and has many advantages over gypsum. The preference given to steatite is based upon a comparison of the properties of the two minerals; gypsum is crystalline, steatite is non-crystalline. Powdered gypsum has essentially a dry character, whilst powdered soap-stone is of a peculiar greasy nature, and consequently can be readily smeared upon a flat surface; it also imparts a pleasing lustre to the colours with which it is mixed.
The manufacture of crayons consists in preparing and moulding the coloured mass. The process is commenced by mixing the powdered colour and soap-stone in a closed rotating cylinder. The rotation is continued until the mixture is uniform in colour. The quantity of colour to be mixed with the soap-stone depends upon the shade to be produced. Manufacturers who make a speciality of crayons prepare a number of shades of each colour. The artists who use crayons, however, usually require only those of a pure shade, since they themselves can produce the intermediate shades from the essential colours. On this account it is advisable to make crayons of distinct and deep colours. The mineral pigments are the most suitable: for yellow, deep chrome yellow; for red, vermilion or deep madder lake; for green, one of the pure green pigments, such as chrome green; for blue, Chinese blue or ultramarine; for brown, burnt sienna or manganese bistre. For white and black crayons levigated chalk is used, alone or mixed with a sufficient quantity of fine vine black or some other good black. Either gum Arabic or size may be used as binding material. When the former is employed the crayon becomes so brittle when dry that it breaks when pointed with the knife, in spite of the greatest care. Size produces a less brittle crayon, and is also much cheaper, so that it is preferred.
The paste from which the crayons are to be formed is made by mixing thin size with the colouring matter and steatite to a soft pulp, which is then kneaded to secure uniformity. The process then proceeds in a different manner according as the crayon is to be pressed or sawn out.
For producing the crayons by pressing, a simple apparatus is required. It consists of a horizontal metal cylinder with a well-fitting piston. The front of the cylinder is closed by a metal plate in which is an orifice of rather greater diameter than the crayons to be made. In front of the cylinder is an endless band which moves away from it. In using this apparatus to shape the crayons, the paste is made of such consistency that a slight pressure forces it out of the orifice in the front of the cylinder in a coherent rod. When the cylinder is filled with the paste, care must be taken that it includes no air bubbles, for these would cause the rod to break. The piston is then put in position and the mass forced by uniform gentle pressure out of the narrow opening. The endless band must move away at the same rate as the rod proceeds from the cylinder, so that a long stick of the crayon mass rests upon it. This stick is cut up by a blunt knife into uniform lengths, which are dried upon boards covered with blotting paper, and are then enclosed in casings similar to those of the ordinary lead pencil. The rods must be given a rather greater diameter than they are to possess when dry, since they shrink somewhat in drying.