The viscous mass obtained by long treatment of the black with the binding medium is slowly dried, and again ground when it has become thick. When a completely homogeneous paste has been obtained it is made into sticks which are pressed into moulds, and then very slowly dried so that they do not crack. Small cracks always occur; they are filled in by a brush dipped in the thick paste, finally the sticks are entirely or partially covered with gold leaf. In order to impart to the ink the odour characteristic of the Chinese ink, it is sufficient to use good lamp black, made odourless by extraction with caustic soda, and to add a little solution of camphor in turpentine during the grinding. If it is desired to impart the musk smell which several Chinese varieties possess, a small quantity of a spirit extract of genuine musk may be added. The size solution is made by long boiling isinglass with water; it must be so concentrated that it forms a strong jelly on cooling. A small quantity of acetic acid is added, so that the grinding may not be hindered by the viscous nature of the size, which the acid prevents from gelatinising. When the mixture has been made by long grinding, the mortar is warmed to 40° or 50° C., whereupon the acetic acid is soon volatilised, and the mass rapidly becomes very thick. =Neutral Tint Black.=—A mixture of Indian ink, Chinese blue, and a very small quantity of madder lake forms the colour known as ”neutral tint,” the shade of which is a peculiar greyish violet. By alterations in the proportions of the constituents different shades of ”neutral tint” are produced. APPENDIX.—BLACK MINERAL PIGMENTS. =Chrome Copper Black.=—When copper chromate is strongly ignited in air and then treated with boiling nitric acid a glittering black compound is obtained, which shows up well when printed on fabrics with albumin. This and all similar pigments are distinguished by great durability. =Chrome Black.=—When a mixture of chromium oxide with varying quantities of ferric oxide is strongly heated, pigments are obtained of all shades from dirty yellow and green to the deepest black. This pigment is largely used in porcelain painting to produce a black, which also can be obtained by other but much more costly methods. The best mixture to produce a deep black contains one part of chromium oxide to four parts of ferric oxide. CHAPTER XLIV. ENAMEL COLOURS. An enamel is a glass distinguished from ordinary glazes by a much lower melting point, and generally by opacity. Most enamels are coloured by additions of metallic oxides.