The Chemistry of Pigments

THE CHEMISTRY OF PIGMENTS BY EENEST J. PAEEY, B.Sc. (LoND.), E.I.C., F.C.S. " AUTHOR OF THE CHEMISTRY OF ESSENTIAL OILS AND ARTIFICIAL PERFUMES," ETC. JOHN H. COSTE, F.I.C., F.C.S. WITH FIVE ILLUSTRATIONS LONDON SCOTT, GEEENWOOD & CO. 19 LUDGATE HILL, E.G. 1902 [All rights reserved] t> .; -^C^ GENERM- PREFACE. THE authors have in the following pages endeavoured to indicate the chemical relationships, composition and properties of most of the better -known pigments. During recent years they have given a great deal of attention to the examination of painters' colours, and have from time to time found some difficulty in obtaining reliable information on the subject. This led to a considerable amount of work in obtaining and examining specimens of pigments, and it was felt that the information thus gained might usefully be published. Speaking generally, the plan of the authors has been to treat the various pigments in groups, allied chemically rather than chromatically. Motives of convenience have in some cases led to a modifica- tion of this scheme. The methods of manufacture of colours have been considered rather from the chemical than the technical of view it is point ; not suggested by the authors that the present work is in any sense a manual of colour-making. Analytical processes which, in most cases, the authors have by experience found suitable are described, and the nature of probable impurities, 104628 VI PREFACE. adulterations and other causes of inferiority pointed out, and numerous analyses of genuine and sophisti- cated pigments, for the most part by the authors, are given as illustrating the composition of these bodies. Preliminary chapters on colour and on the application of pigments have been added. It is the hope of the authors that this work may be found of use by those who are called upon to use or examine pigments as a guide to the selection of those which are suitable, and the rejection of those which as a class or through individual inferiority are unsuitable for the class of work to be undertaken. E. J. P. J. H. C. LONDON, October, 1901. CONTENTS. CHAPTER I. INTRODUCTORY. Light White Light The Spectrum The Invisible Spectrum- Normal Spectrum Simple Nature of Pure Spectral Colour The Kecomposition of White Light Primary and Comple- mentary Colours Coloured Bodies Absorption Spectra pages 1 to 17 CHAPTER II. THE APPLICATION OF PIGMENTS. Uses of Pigments: Artistic, Decorative, Protective Methods of Application of Pigments : Pastels and Crayons, Water Colour, Tempera Painting, Fresco, Encaustic Painting, Oil-Colour Painting, Keramic Art, Enamel, Stained and Painted Glass, Mosaic pages 18 to 76 CHAPTER III. INORGANIC PIGMENTS. T White Lead Zinc W hite Enamel White Whitening Red Lead Litharge Vermilion- -Royal Scarlet The Chromium Greens Chromates of Lead, Zinc, Silver and Mercury Brunswick Green The Ochres Indian Red Venetian Red Siennas and Umbers Light Red Cappagh Brown Red Oxides Mars Colours Terre Verte Prussian Brown Cobalt Colours Cceruleum Smalt Copper Pigments Malachite Bremen Green Scheele's Green Emerald Green Verdigris Bruns- wick Green Non-arsenical Greens Copper Blues Ultramarine Carbon Pigments Ivory Black Lamp Black Bistre -Naples Yellow Arsenic Sulphides: Orpiment, Realgar Cadmium Yellow Vandyck Brown pages 77 to 181 Vlll CONTENTS. CHAPTER IV. ORGANIC PIGMENTS. Prussian Blue Natural Lakes Cochineal Carmine Crimson Lac Dye Scarlet Madder Alizarin Campeachy Quercitron Bliamnus Brazil Wood Alkanet Santal Wood Archil Coal-tar Lakes Red Lakes Alizarin Compounds Orange and Yellow Lakes Green and Blue Lakes Indigo Dragon's Blood Gamboge Sepia Indian Yellow, Purree Bitumen, Asphaltum, Mummy pages 182 to 275 INDEX pages 277 to 280 THE CHEMISTRY OF PIGMENTS. CHAPTEE I. INTRODUCTORY. COLOUR phenomena may, broadly, be divided into two classes. In the larger and more important group they are due to the coloured bodies selective action of what we v know as upon light, whilst in the smaller group they are dependent upon quite other causes, which, from the point of view of the present work, need not be touched upon. The colour effects produced by pigments belong entirely to the former class, and to intelligently understand the principles of colouring materials, an elementary but exact knowledge of the theories of light is necessary. In the present chapter the theoretical aspect of the subject is briefly referred to from this point of view, but no mathematical details are introduced, except where absolutely necessary for the comprehension of the point at issue. Light. Light is due to a series of intensely rapid vibrations of a medium which is assumed to be imponderable, and to pervade all space and matter. Such a medium is, of course, physically quite incomprehensible, but its existence must be assumed to satisfactorily explain the ordinary phenomena of light. These vibrations are transverse, that is, in planes at right angles to the direction of propagation of the beam. The vibrations in sound waves are, on the other hand, 1 2 CHEMISTEY OF PIGMENTS. of the of longitudinal, that is, in the direction propagation the wave. This hypothesis is known as the wave theory, or the undulatory theory of light, the vibrations evidently par- taking of the nature of waves. Indeed they may be well illustrated by ordinary waves, and the distance between two successive crests is termed the ivave length, whilst the distance from crest to trough is the amplitude. The intensity of the wave depends on the latter, whilst its specific character, as will be seen later, depends essentially on the wave length. White Light. The ordinary light of the sun, which we recognise as white light, is a mixture of innumerable vibrations of different wave lengths, or, what amounts to the same thing, of different colours. This is best illustrated by the classic experiment of Newton, which was undertaken by him to prove the pro- " position that the light of the sun consists of rays differently refrangible" (Newton, Opticks, book i., prop. ii.. theorem 2). This experiment may be briefly described as follows : Into a perfectly dark room (see Fig. 1) a beam of light is ad- mitted through a small round hole, 0, and in its path is placed a triangular glass prism P. The light falling on the prism is refracted or bent (as is always the case when light passes from any medium to any other of different density), and on being received on the screen Q it is seen that instead of a white spot we have an elongated band of different " " colours. This is the spectrum S. The Spectrum, This coloured band, which is called the prismatic spectrum, in order to distinguish it from that produced in another man- ner (vide infra), is composed of a series of colours, situated in the same order as are those in the rainbow, which owes its INTRODUCTORY. 3 existence to an action of this kind. As a matter of fact, every vibration of different wave length is refracted to a different degree, the rays of shorter being more refracted than those of longer wave length. Hence the fact that the colours in the spectrum merge gradually into one another, and although each separate position as we advance from one end to the other is of a different colour absolutely, we cannot, with the eye, enjoy such fine distinctions, so that we are compelled to separate the spectrum into arbitrary divisions in which the FIG. i. main colour is represented in its various shades. For convenience Newton divided the spectrum into seven parts, starting from the least refracted rays, as follows : Ked, orange, yellow, green, blue, indigo and violet. Any other similar division is quite legitimate such, for example, as the more extended red, orange red, orange, orange yellow, yellow, etc., etc. To exactly define the spectral colours, as they are called, however, we need something more exact. We have this in the Frauenhofer lines (so named from their discoverer), a series of fine dark lines which break the con- 4 CHEMISTEY OF PIGMENTS. tinuity of the spectrum of sunlight. As these dark lines depend on the existence of definite elements in the sun's atmosphere their position is absolutely fixed, and their positions having been determined with accuracy, they can be used as points of reference. The most prominent lines are those named from A to H, starting from, the red end of the spectrum. These lines are reproduced as bright lines in the same position, when, for example, the element responsible for the given line is volatilised in a colourless flame. The D line (which is in reality two lines extremely close together), for example, is characteristic of the element sodium. If the visible portion of the prismatic spectrum, that is, from the red to the violet, be divided into 1,000 parts, the positions of these lines are as follows : A . E 363-11 B . 74-02 F . 493-22 C . 112-71 G . 753-58 D . 220-31 H . 1,000 The Invisible Spectrum. Hitherto we have referred only to the visible spectrum, that is, the coloured bands which are produced when white light is decomposed by, for example, a glass prism. But it has been shown beyond doubt that heat waves are subject to many of the same laws as light waves, and it appears that the same class of vibration is responsible for the phenomena of heat and light. If a delicate thermometer be placed in the various parts of the visible spectrum, it will be found that, if we use a prism of, for example, rock salt, which absorbs practically none of the vibrations, the temperature varies with the portion of the spectrum in which the thermometer is placed.

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