The first half of the nineteenth century saw some striking discoveries in chemistry which were to be of particular relevance to the introduction of new artists' materials. In terms of the number of new pigments which were to flow from the discovery, Vauquelin's 'Memoir' of 1809 on chromium and its compounds is especially noteworthy(‘Annales de Chimie’, LXX, pp.90–1). The isolation of another previously unknown metal, cadmium, by Stromeyer in 1817 also contributed significantly to the palette, providing by the mid-nineteenth century a range of yellow to orange shades of cadmium sulphide (the cadmium yellow pigments). Subsequently, early in this century, cadmium yielded another series of pigments based on the metal's sulphoselenide compounds; in this case ranging from red to maroon.
Developments in organic chemistry in the last century can be considered as of no less importance, with the preparation by William Perkin of mauve in 1856, the first ever organic dyestuff to be made synthetically. In 1868 the first natural dyestuff to be duplicated artificially, alizarin, was synthesized by the German chemists Graebe and Lieberman.
Three samples from ‘Bathers at La Grenouillère’ have been analysed for their paint medium by gas-chromatography, and the results reported in the last issue of this ‘Bulletin’. Since then a fourth sample has been examined, and in all cases the P/S ratio was found to lie between the expected figures for poppyseed and walnut oils. It can be concluded that one or another of the oils, or possibly a mixture of the two had been employed. See ‘National Gallery Technical Bulletin,4’(1980), p.67.
See note by Elizabeth H. Jones on Monet's materials in ‘Monet Unveiled: A New Look at Boston's Paintings’, Museum of Fine Arts, Boston (Boston 1977), p.6.
In a sample from the white ground of the National Gallery picture, XRD has shown the priming to be composed of lead white incorporating a little barium sulphate as an extender. Monet presumably used a commercially primed canvas.
Cobalt blue represented a very considerable addition to the palette, being chemically completely stable and possessing a pure blue colour. It has a fair covering power and tinting strength, with good drying properties in oil. Like the traditional blue glass pigment smalt, Thénards blue owes its colour to the cobalt (II) ion; unlike smalt the chemical stability and hiding power are much greater. The structure of cobalt aluminate is that of a normal spinel, with a cubic close-packed array of oxygen atoms in which cobalt (II) ions occupy the tetrahedral holes and aluminium occupies the remaining octahedral sites. Binary oxide structures of this type are likely to make pigments which are permanent.
The significance of the discovery of chromium for the artist has been referred to in Note 1 above. Chrome yellow whilst a moderately stable pigment is subject to blackening by sulphides; lemon yellow (barium chromate) is not so affected. However the latter tends to lack brightness and covering power. To an extent the yellow pigments based on chromium tended to be replaced by the more reliable cadmium yellow pigments in the later part of the nineteenth century.
Emerald green apparently enjoyed a certain popularity in the earlier part of the nineteenth century, providing a brilliant green substitute for the traditional pigment verdigris. It was however unsatisfactory from two points of view. It was both exceedingly toxic as a result of its relatively soluble arsenic content, and reputedly liable to react with sulphur-containing pigments, although this does not seem to be a particular defect when the pigment is protected in a dried oil film.
Emerald green was eventually entirely displaced from the artists' palette by the introduction of transparent chromium oxide (viridian, see Note 8 below), which suffers none of the disadvantages of the earlier green.
Although emerald green is microscopically characteristic, the possibility exists for it to be taken for the spherulitic particles of green verditer (artificial malachite). See p.55 and Note 10, p.57 of this ‘Bulletin’. Confirmation of the nineteenth century pigment requires either detection of arsenic as well as copper in the sample, or X-ray diffraction analysis.
The invention of viridian provided the artist with a pigment of highly desirable properties. It is chemically very stable and suitable for all media both in tint and as a glazing pigment.
It is interesting to find emerald green and viridian present together in Monet's painting of 1869, but this may have been because of some general confusion over the identity of the two pigments soon after the introduction of the newer pigment in 1862. In France viridian is called 'vert emeraude' (emerald green), whereas copper acetoarsenite (called in England emerald green) is known in France as Veronese green.
The two component pigments of chrome green cannot be distinguished under the microscope at normal magnifications since the Prussian blue tends to form a thin intimate coating over the particles of chrome yellow. The pigment mixture will react with alkalies discharging the blue colour of the Prussian blue, leaving a yellowish brown residue of lead chromate. With acids the chrome pigment passes into solution leaving the Prussian blue unaffected. These two microchemical tests are indicative of chrome green.