The frame is important for the preservation of a miniature as it acts as a protective and stable environment. All the Prado's miniatures are framed although many of the frames are not original and a large number have been subsequently altered. It is common to find pieces of paper, card or cloth that fill in the space at the back and fix the miniature into the frame. In the case of works from the Perera collection the pieces of paper and card found during restoration were prescriptions or from boxes of medicines. Arturo Perera was a doctor and thus used materials from his everyday surroundings for his miniatures fig. 15. An interesting case is that of the miniature of the Duchess of Alba (O-694) by Guillermo Ducker. When the back of the frame was removed a carefully folded sheet of paper was found, which was a page from a book with a detailed description of an aphrodisiac called satirión or 'dog's testicle' (orchis mascula). Examples of this type raise the question of whether the piece of paper simply acted as filling or whether it was placed there for another purpose. fig. 16
Some frames take the form of lockets for hair. In commemorative pieces of this type the hair is arranged decoratively in curls or is woven or plaited. In both cases the hair was prepared in the same way: it was soaked in an aqueous adhesive 2 and then woven into the desired pattern while still damp. When the adhesive dried the woven hair was placed in the locket. fig. 39
The sheets of glass in old frames were blown by hand and are unique to each frame. They were known as vidrios lupa [magnifying glass] although this effect is only evident at the edges as the centre is flat. This subtle effect functions to enlarge the miniature. Many of these sheets of glass have deteriorated, resulting in the hydration of the glass and the appearance of tiny surface drops that crystallize in relation to atmospheric conditions fig. 25. A raking light study revealed that their internal structure was micro-fractured, which is an associated type of damage that occurs when hydrated glass dries out fig. 24 - fig. 24.1. An analysis of the “drops” revealed that they were deliquescent salts of sodium acetate, 3 for which reason the compound that destabilised and started to corrode the glass was acetic acid. This type of deterioration was frequently triggered off by inappropriate wrapping material and damp combined with gas emissions from unstable materials such as wood, resulting in the onset of the chemical process of crizzling.
In some cases these 'drops' only appear on the inside face of the glass and seem to have spread out as if wiped with a cloth. This suggests that a product containing acetic acid such as vinegar was used when handling the miniatures and that this substance had not been dried or completely removed before the frames were sealed up, leading to the deterioration of the glass.
Given that the sheets of glass are concave the drops have only been in contact with the miniatures at the edges, although in some cases we encountered small surface marks only visible with a binocular lens. However, the acidification of the atmosphere can affect the paint layer. The most damaged work in this respect was Portrait of a Girl (O-675) by Dechateaubourg in which some white brushstrokes in the area of the dress had blackened on the surface and acquired an iridescent effect. When analysed, the pigment was found to be lead white, which oxidises in an acidic environment and takes on the dark grey tone found in the miniature fig. 26. In addition, the pictorial layer was fragile and brittle, for which reason various micro-fragments had become detached. Their analysis revealed that the above-mentioned agglutinant, gum arabic, was damaged, possibly due to hydrolysis characteristic of an acidic incident.