CHAMBERS'S INFORMATION FOR THE PEOPLE. 



COLOURING-MATTERS. 



The object of dyeing is to give a fast colour to 

 animal or vegetable fibres or fabrics made from 

 them. It differs from painting or mere staining 

 in that the colour is fast that is, so fixed to or 

 in the fibre that it cannot be washed off or out 

 by water or soap. In painting, on the contrary, 

 the colouring-matter is mechanically spread upon 

 the object to be painted. As, however, many 

 colouring-matters are used both in dyeing and in 

 painting, we shall first give a description of the 

 most important colouring-matters, without refer- 

 ence to their special use. 



Colouring-matters, among which, for convenience, 

 we have included white paints, may be most con- 

 veniently divided into Inorganic and Organic. 



Of the Inorganic, the most important are : 



White-lead, a basic carbonate of lead, or com- 

 pound of carbonate of lead and hydrated oxide 

 of lead. Red-lead, an oxide of lead prepared by 

 heating massicot (plumbic oxide) in a current of air. 

 Chromate of lead (PbO,CrO 3 ), known as chrome 

 yellow, is prepared by precipitating a soluble salt 

 of lead, usually the acetate, by means of chromate 

 of potash, or by acting upon solid carbonate, 

 chloride, or sulphate of lead with chromate of 

 potash. Basic chromate of lead (2PbO,CrO 3 ), 

 chrome red, is prepared by the action of caustic 

 alkalies upon chrome yellow, by treating chrome 

 yellow, suspended in water with finely divided oxide 

 of lead, or by fusing chrome yellow with nitre. 

 Oxide of zinc (ZnO), zinc white, is used as a 

 paint instead of white-lead in places exposed to 

 the action of sulphuretted hydrogen. Sulphuretted 

 hydrogen converts white-lead into the black sul- 

 phide of lead, but does not affect the colour of 

 zinc white. Sulphide of cadmium (CdS), cadmium 

 yellow, jaune brilliant; sesquioxide of chromium, 

 or chromic oxide (Cr 2 O 3 ), chrome green ;* sesqui- 

 oxide of iron, or ferric oxide (Fe 2 O 3 ), Venetian red. 

 Ferric oxide is also deposited in cloth for the 

 purpose of dyeing it buff; hydrated ferric oxide is 

 the colouring-matter of ochre and umber. Per- 

 oxide of manganese is not used as a pigment, but 

 is deposited in cloth as a black or brown dye (see 

 page 346). Smalt is a blue powder consisting 

 of finely divided glass coloured blue by means 

 of silicate of cobalt. Cobalt ultramarine, or 

 Thdnard's blue, is a compound of oxide of cobalt 

 with alumina, in which the alumina plays the part 

 of an acid. Stannate of cobalt (CoO,SnO 2 ) is also 

 used as a blue pigment. Cobalt green is a com- 

 pound of oxide of cobalt with oxide of zinc. 

 Cobalt yellow is a double nitrite of cobalt and 

 potash. Brunswick green is a basic carbonate 

 of copper (CuO,CO 2 ,CuO,H 2 O). Bremen blue is 

 hydrated oxide of copper (CuO,H 2 O); when mixed 

 with linseed-oil it becomes green, from the for- 

 mation of linoleate of copper. Scheele's green 

 is arsenite of copper. Schweinfurt (or emerald) 

 green is a compound of acetate of copper and 

 arsenite of copper. Normal acetate and various 

 basic acetates of copper occur in commerce under 

 the name of verdigris. Vermilion is mercuric 

 sulphide (HgS). When mercury and sulphur are 

 mixed together, a black compound is formed, 

 which, when sublimed, is converted, without change 



* The name chrome green is sometimes given to a mixture of 

 chrome yellow and Prussian blue. 

 310 



of composition, into a beautiful scarlet substance,. 

 vermilion. The same change is produced when. 

 the black mercuric sulphide is digested with a 

 solution of an alkaline sulphide, containing excess- 

 of sulphur. Orpiment is arsenious sulphide 

 (As 2 S 3 ), and is prepared by heating together 

 arsenious acid (As 2 O 3 ) and sulphur, or realgar 

 (AsS) and sulphur, or by precipitating a solution 

 of arsenious acid in hydrochloric acid by means 

 of sulphuretted hydrogen. Ultramarine is, prop- 

 erly speaking, a blue pigment, prepared from the 

 blue mineral 'lapis lazuli.' A blue substance is 

 now prepared artificially of a similar composition. 

 It is prepared by heating a mixture of China-clay, 

 soda-ash, sulphate of soda, sulphur and coal. The 

 mass thus obtained is mixed with sulphur and 

 heated in a current of air. 



The organic colouring-matters may be con- 

 veniently divided into Artificial and Natural, al- 

 though it is not always easy to adhere strictly to- 

 this classification. 



i. Artificial Organic Colouring-matters. Lamp- 

 black consists chiefly of carbon mixed with oily 

 and tarry matters ; it is prepared by collecting 

 the soot of resin or pitch burned with a supply of 

 air insufficient to oxidise completely the carbon 

 and hydrogen. Prussian blue is ferric ferrocya- 

 nide (see CHEMISTRY). It can be prepared 

 by precipitating a solution of ferrocyanide of po- 

 tassium (yellow prussiate of potash) by means of 

 a solution of a ferric salt (ferric chloride, or 

 muriate of iron, for instance) this method gives 

 ' neutral ' Prussian blue. It can also be prepared 

 by precipitating the solution of yellow prussiate by 

 means of a ferrous salt (such as ferrous sulphate, 

 green vitriol), and oxidising the white precipitate 

 of ferrous ferrocyanide ; by this method, what is 

 called 'basic' Prussian blue is formed. Turn- 

 bull's blue is ferrous ferricyanide, and is formed 

 by the action of a ferrous salt (green vitriol) on 

 ferricyanide of potassium (red prussiate of potash). 



Many very valuable and interesting colouring- 

 matters have been obtained within the last twenty 

 years from coal-tar. We shall shortly describe 

 the preparation of the most important of these. 

 The constituents of coal-tar from which colouring- 

 matters are obtained are : i. Benzol ; 2. Phenol or 

 carbolic acid ; 3. Naphthaline ; and 4. Anthracene. 

 As these substances boil at different temperatures, 

 they are partially separated from one another 

 when the tar is distilled, and by repeated distilla- 

 tions they can be still further purified. Benzol, as- 

 it occurs in commerce, is a mixture of several 

 hydrocarbons, chiefly benzol (C 6 H 6 , boiling-point 

 176 F.) and toluol (C ? H 8 , boiling-point 226 F.). 

 By the action of nitric acid, benzol and toluol 

 are converted into nitrobenzol and nitrotoluol,. 

 thus: 



C 6 H 8 

 Benzol. 

 C 7 H 8 



Toluol. 



HNO, = 



Nitric Acid. 



HNO 3 = 

 Nitric Acid. 



NitrobenzoL 



NitrotoluoL 



H 2 O. 



Water. 

 H 2 O. 

 Water. 



As commercial benzol contains both benzol and' 

 toluol, of course commercial nitrobenzol contains 

 both nitrobenzol and nitrotoluol. The odour of 

 nitrobenzol resembles very closely that of oil of 

 bitter almonds, and on this account the sub- 

 stance is used in perfumery under the name of 

 artificial oil of bitter almonds, or Essence de 

 Mirbane. 



