Vermilion (red mercuric sulphide, probably synthetic, HgS), used virtually pure for the brilliant red flowers at the left edge; identified microscopically and spectro- graphically (see Plate 3a, p.25). Particles of vermilion are also present in mixture with other pigments, for example in the slate-coloured paint of the roof of the small hut set in the middle distance, and in combination with chrome yellow (see below) for the bright orange strokes of the waterside vegetation. Vermilion is the only brightly coloured 'traditional' pigment to have been used by Monet for ‘Bathers at La Grenouillère’.
Prussian blue (ferric ferrocyanide or a similar compound, invented in Germany by Diesbach, c.1710) for the very dark blue of the bathers' swimming costumes; identified microchemically. Also present as an essential component of the pigment mixture known as ‘chrome green’ (see below). Cobalt blue (cobalt aluminate,CoO.Al2O3, invented in France by Thénard, 1802) (note 4) is the principal mid- blue pigment of the water, used alone and in mixture with lead white, small quantities of Prussian blue and cobalt violet (see below) for the tonal variations (see Plate 3b, p.25). The presence of cobalt blue in a sample was shown by spectrographic analysis (cobalt and aluminium detected by LMA) and by refractive index measurements on extracted pigment particles (RI=c.1.72, by immersion and Becke test). The pigment is also a component in mixed samples of green paint, for example in the dull blue-green of the gunwale of one of the foreground boats (see Plate 3c, p.25).
Chrome yellow (lead chromate, PbCrO4, preparation first described by Vauquelin in France, 1809; introduced as a pigment c.1818) and Lemon yellow (barium chromate, BaCrO4) (note 5), mixed together to form the brightest yellow of the background trees; detected spectrographically, by XRD and microscopically.
Emerald green (copper acetoarsenite, Cu(CH3COO)2. 3Cu(AsO2)2, invented in Germany in 1814) (note 6), present in several pigment mixtures for various shades of green (see for example Plate 3c, p.25). The pigment is recognizable by its characteristic spherical crystalline particle form and strong birefringence; confirmation of its identity was by LMA for copper and arsenic (note 7). Viridian (hydrated chromium (III) oxide, Cr2O3.2H2O, invented in France, 1838; available as an artists' pigment by 1862) (note 8), as one of the pigments in several samples containing heterogenous pigment mixtures. When extracted, viridian particles are microscopically unmistakable as transparent, rich grass-green rounded flakes, showing moderate anomolous birefringence.
Chrome green (not a single pigment, but an homogenous mixture of chrome yellow and Prussian blue (note 9)), for the vivid yellow-green paint of the foliage at the left-hand edge of the picture. LMA of a sample showed it to contain iron, chromium and lead and in addition barium, aluminium and silicon, in accordance with one recorded method of manufacture (note 9).
Cobalt violet (cobalt phosphate or arsenate, preparation as a pigment first described in France, 1859) (note 10), in a number of mixed paint samples, for example the mid- blue tones of the water and the mauve flowers near the left-hand edge. The microscopic appearance of Monet's pigment seems close to that of modern examples being made up of very fine rounded crystalline particles of medium RI and showing strong birefringence. No sample of violet pigment on its own was available, so a chemical identification could not be carried out.