The paint layer
Miniaturists began their portraits with a preparatory drawing. Various examples in pencil have been encountered but it is not easy to access them as on some occasions they are concealed beneath the paint while in other cases they were executed with a brush in the same technique and using the same pigments as the miniature itself. Dechateaubourg's Portrait of a Girl (O-675) reveals a particularly precise under-drawing of the dress and the bow of the edging of the dress. fig. 3. Another interesting example is Juan Pérez de Villamayor's Portrait of Isabel II (O-698), fig. 4 which reveals an area of squaring-up behind the dress and various lines that establish the position of the mouth. This was an institutional portrait that may have been made on the basis of a larger preparatory sketch that was reduced using the squaring-up technique.
For the paint layer artists used pigments with water-soluble agglutinants. It is difficult to specify exactly which agglutinants were used due to the small scale of these works and the thinness of the pictorial layer, which has prevented us from taking representative micro-samples for analysis. In general, therefore, we have referred to old treatises that discuss the use of tempera and gouache techniques in general terms. However, during the restoration of Portrait of a Girl (O-675), a fragment of the paint layer became detached and could therefore be analysed using gas chromatography and mass spectrometry. These tests revealed that the agglutinant was gum arabic, which is typically used with watercolour. When used, the amount of pigment could be varied or other elements added, allowing the artist to deploy both translucent brushstrokes that exploited the transparent nature of the ivory, and dense, opaque ones that concealed the support.
Three principal pictorial techniques have been identified in the Prado's miniatures: transparencies in the manner of watercolour, stippling, and hatching over superimposed, opaque strokes. There are miniatures in which only one of the three is used, but the most common practice was to combine them. In general, the faces and flesh areas were painted using stippling and hatching, fig. 5 as in Portrait of an elderly Lady by Antonio Tomasich, which reveals a large number of tiny dots, or in the above-mentioned Portrait of a Girl by Dechateaubourg, which uses a lesser density of dots. In some cases stippling was painted over glazes applied with the brush and which functioned as the initial lines of the composition. Only in a very few miniatures in the collection are the faces painted with transparencies characteristic of watercolour technique, of which the most spectacular example is Portrait of a Boy in which the anonymous artist used a very limited number of brushstrokes over the translucent tone of the ivory to create an impressionistic effect of volume fig. 6. In the grisaille of Profile Portrait of two young Women attributed to Bourgeois de la Richardière the brushstrokes are executed in black with a few white highlights fig. 7.
To depict the hair artists generally applied long, fine brushstrokes that overlapped the stippled area of the face, while for the clothes a wide range of techniques were deployed: the interplay of transparencies was exploited for areas of thin, light-coloured material while for the other colours a thicker, more opaque layer of pigment was applied, starting with the darkest under-layers, which were then graduated with increasingly pale toned strokes. For areas of embroidery, mantillas and above all jewels such as brooches and necklaces, a sense of volume was achieved through the use of heavily charged spots of white pigment that stood out in relief. In some cases we also encountered the use of gold dust to complete this effect fig. 8 - fig. 8.1 - fig. 8.2. Scraping was also employed, both to define an outline and to lighten the effect of an area of colour, while another technique occasionally encountered, particularly in the backgrounds, is that of the application of drops of water to create a pale spot with a dark outline fig. 14. Very few pentimenti or errors were encountered in these miniatures due to the fact that they were based on carefully conceived preparatory studies, and the few corrections that were found were carried out by scraping out rather than painting over fig. 9. The backgrounds tend to be thick and opaque and in miniatures that used a sheet of silver leaf the edges of the sheet had to be concealed from the front given it would otherwise have showed through. This is evident in the unfinished miniature Portrait of a Lady in a Black Dress (O-715) fig. 38, in which the sheet of silver shows through around the head.