In green areas which have become bluish a final glaze, either yellow or green, may have been lost, either by action of cleaning agents and/or abrasion or flaking. At the same time the surface of the blue or bluish opaque underpaint may have been revealed as roughish and matt, or become so as a result of cleaning. Blue leaves are sometimes conspicuous in seventeenth century Dutch landscapes and flowerpieces otherwise remarkable for botanical accuracy of form and colour. The blue colour of such foliage is usually explained as the fading of a light-fugitive component of the paint, such as a yellow lake pigment. Such may indeed sometimes be the case (see 2. below), but painters of flowers and landscapes in order to achieve an adequate range of shades of green with the limited number of pigments at their disposal had to resort to many different techniques, and one of these was the, by that time well-established, art of glazing. There is, for example, in the Metropolitan Museum of Art, New York, a flowerpiece by Margareta Haverman (a pupil of Jan van Huysum) in which the majority of the foliage is completely blue with only here and there a patch of rich deep green. In the green areas, there survives a green glaze, which was identified from samples as being of copper 'resinate' type (note 1). It is cracked and crazed almost like shattered glass in the few places where it has not fallen away to expose the matt, opaque blue underpaint. Loss of a yellow glaze over blue or green-blue underpaint would produce a similar effect.
In the affected greenish areas fading of a yellow lake pigment or of a yellow organic pigment may have occurred, present either in a final glaze or as a component of a mixture of pigments in a single paint layer. Such fading has been detected in some seventeenth century Dutch landscapes. A notable example occurs in the foliage of a ‘Landscape with Cattle’ by Adriaen van de Velde, signed and dated 1664, in the Fitzwilliam Museum, Cambridge, which presented much the same appearance, both on the picture surface and in paint samples under the microscope as foliage in badly blanched Claudes (note 2). The top surface of the paint layer of some samples of foliage from the van de Velde was seen under the microscope as a matrix of lead white with scattered blue verditer (basic copper carbonate) particles, but aluminium was detected chemically in the layer suggesting the presence of aluminium hydroxide which could be accounted for as the substrate of a yellow lake pigment now faded to the point of disappearance. Elsewhere in the picture there were one or two examples of unchanged yellow lakes, mixed with blue pigment to give green, sometimes surviving unchanged in underlayers. It is conceivable that if the dyestuff of a yellow lake pigment faded with complete loss of colour, the residual substrate plus medium might have a degree of turbidity which could give the visual effect of blanching. More extensive use seems to have been made of yellow lake pigments in the seventeenth century than at earlier periods. Lakes were made from dyestuffs extracted from a number of the many different plants which give a yellow colouring matter and there are recipes not only for lakes on the usual substrates of aluminium hydroxide and chalk, but also for yellow pigments in which the dyestuff was adsorbed onto lead white. Yellow lakes seem early on to have gained notoriety for their tendency to fade; a Dutch term for them is ‘Schietgeel’, an abbreviation for ‘Verschietgeel’, i.e. 'disappearing yellow', and indeed such samples as have been prepared or painted out in the laboratory seem to deserve this reputation, fading rapidly in a matter of months when exposed to strong light. It is not proposed here to deal in any detail with this class of pigments for it is expected that at some future date they will be the subject of research. Another possibility which cannot be excluded is the use of yellow organic pigments (as distinct from lakes) particularly saffron, used as a pigment from medieval times at least, and gamboge which was probably introduced into Europe from the Far East at the close of the sixteenth century. Even less seems to be known about these materials or their permanency, whether as colorants or as film-forming substances and, like the yellow lake pigments, they are very difficult to identify in small paint samples even when-not faded. In the sixteenth century the most frequently-occurring green pigment was verdigris (note 3), used by itself, in combination with other pigments, particularly with lead white or lead-tin yellow, or dissolved in a medium to form a copper 'resinate' type material. In the seventeenth century, particularly in the Netherlands, but also elsewhere, there seems to have been a tendency to make greens from a combination of blues and yellows, either by physical mixture of blue and yellow pigments or by means of an optical mixture produced by a yellow glaze over a blue underpaint or a blue glaze over yellow. Some of the green areas in the pictures by Claude reveal under the microscope both complex pigment mixtures and multilayer structures not always easy to interpret (see Plates 2a and 2b, p.21).