Use of artificial blue and green copper carbonate pigments: The very small size and regular rounded shape of copper carbonate pigments in many areas and paint layers of Claude's pictures indicate that they may be manufactured rather than the natural minerals azurite and malachite, although the artificial and mineral forms seem indistinguishable on the basis of their X-ray diffraction powder patterns. Although the artificial forms were known and produced from late medieval times their more extensive use, particularly in combination with yellow lake and organic pigments to form greens, seems to be associated with seventeenth century painting and has frequently been observed in samples from Dutch and Flemish pictures of the period. The very fine particle size results in poor colouring strength (whether the copper carbonate is of artificial production or the finely-ground mineral) and this means that the blue or green colour of the paint will be worse affected by any change in colour or optical properties of the medium. In addition, the fine particles, because of higher ratio of surface area to volume, are likely to be more vulnerable to chemical attack from action of chemical reagents like acids or alkalis, or to effect of moisture. Also it has only recently been realized that not only verdigris but copper carbonate pigments have a tendency to react chemically with the organic materials of which paint media are composed (note 3), and this tendency is likely to increase with decreasing particle size. For the blue pigments, at least, there survive a large number of recipes, not all chemically sound, and early manufacturers may not have been too scrupulous about washing out excess reagents such as alkalis from the precipitated pigment, thereby introducing factors which could lead to instability of the resulting paint.
Effect of alkaline cleaning reagents: In the past alkaline materials, of which the most common is ley (an aqueous solution of potash) have been used in picture cleaning and may well have been found necessary for the removal of some early varnishes of the boiled hard resin type or even of later copal/oil varnishes. As well as being disruptive to the medium of the paint film caustic alkali would be capable of attacking copper carbonate pigments dissolving them, or partially dissolving them and re-precipitating the copper as whitish copper hydroxide. If the copper carbonate is totally converted to hydroxide, the latter may on exposure or dehydration become blackish or brownish copper oxide, but slight attack might just result in a turbidity or greying of the paint film. Again pigment of a small particle size would be most vulnerable to change. Ultramarine, unlike the blue and green copper pigments, is unaffected by alkali. Caustic alkali could alter the colour of yellow lake pigments.
In the blue areas, true 'ultramarine sickness', i.e. the discolouration of ultramarine to a yellowish grey or white by action of acids (note 4). The sulphur in the ultramarine molecule is displaced with evolution of hydrogen sulphide gas and simultaneous loss of the blue colour of the material, leaving only a yellowish grey or whitish silicaceous mass. Mineral acids in dilute solution have an almost instantaneous effect, but weak organic acids such as maleic acid will gradually produce the same result and even the fatty acids in oil media have been suspected of being capable of causing the disorder, as is sulphur dioxide or other acid fumes present in polluted atmospheres. Fortunately true ultramarine sickness seems a rather rare phenomenon in easel paintings compared with the number of cases of discolouration which seem to be associated with another blue pigment, smalt (see 6. below). It is not, however, unknown in the National Gallery; an instance is reported on p.27 of this issue of the ‘Technical Bulletin’ and an extreme case can be seen in Sassoferrato's ‘The Virgin in Prayer’ (No.200) in which the Virgin's blue cloak has a very blanched and greyish appearance. Ultramarine, which in the seventeenth century was still the genuine lapis lazuli variety, was used quite extensively by Claude, both in skies, where it is mixed with a high proportion of lead white, and in the drapery of figures, where it is often used alone or mixed with only a very little lead white to give an intense deep blue. Lead white, chemically a basic material, may serve to protect ultramarine particles with which it is mixed from effect of acids. Blanching is certainly prominent on some of the deep blue ultramarine drapery of foreground figures in Claude's paintings, for example the blue jacket and hose of the man in the bottom left corner of No.30, ‘Seaport: The Embarkation of S. Ursula’. (See also Plate 8, p.48.) In others the deep blues seem in perfect condition. Samples of paint from pictures by Claude so far examined reveal that ultramarine present, like the copper carbonate pigments, tends to be of rather small particle size which, again, would render it more vulnerable to chemical attack.