The paint layer of a painting consists of a composition of materials: a binder, a charge, pigments and varnish
Pigments constitute the coloured base of the paint layer. These are powders extracted from natural materials, principally vegetable and mineral. The oldest known pigments date from Prehistoric times (rock paintings) where calcium carbonate (chalk), soot residue (composed of carbon and obtained from the calcinations of organic matter) and ocres (yellow coloured sedimentary rock) are found. Since Antiquity, the opening of commercial routes between Orient and Occident encouraged new techniques and the appearance of new materials. For example, vermillion (red extracted from mercury ore) was used by the Chinese for their seals. The ultramarine blue Lapis Lazuli is a rare rock found in Afghanistan, Russia and Tibet. Sculpted and polished in Ancient Egypt, it was used for illuminated manuscripts during the Middle Ages. Extremely beautiful examples of the brilliance of this colour can be seen in L’Allégorie de la Prudence by Simon Vouet (room 11) or La Vierge au Lys by Carlo Dolci (room 13).
The Binder enables the amalgamation of pigments and their transformation into a homogenous material. Until the Renaissance the binders used were animal glues (glue from skin, fish, deer antlers) wax or egg. Subsequently oil was mostly used. It had the advantage of drying more quickly in reaction to contact with the oxygen in air. Linseed oil was the most widespread. In the 20th century synthetic binders appeared which replaced oil, such as vinyl or acrylic resins.
The Charges are material added to the preparation to achieve a gloss or matt effect (talc, kaolin, marble powder…). Their use varies depending upon the artist.
The Varnish is traditionally obtained by dissolving natural resins in a solvent, most commonly turpentine essence or ethyl alcohol. It serves to protect the paint layer and to give an overall glossy finish to the painting.
Behind the composition and the colours of an artwork the artist has posed a ground layer which gives the coloured tonality of the painting. It is chosen by the painter in keeping with his aesthetic objectives. Thanks to invaluable research we know that the painters from the 17th century Dutch school (Rubens, Van Dyck…) mixed 25% red ochre (obtained from heating the natural yellow ochre) with 75% minium (bright orange containing lead). At the Musée Fabre Vénus et Adonis by Nicolas Poussin (room 11) contains a red preparation which gives a particular tonality to the painting. We know that Rembrandt superimposed this preparation with a grey second layer composed of 80% ceruse (white lead), 5% soot black and 15% yellow ochre whose qualities gave his paintings their sombre and brilliant tonality. In the 19th century Gustave Courbet, in his first canvases still marked by Romanticism, used dark preparations. However, his work realised in the Languedoc was painted upon a white preparation, in keeping with the clear light of the south of France (see La Rencontre or Le bord de mer à Palavas in room 37).
Therefore a painting constitutes different stratum composed of heterogeneous materials which must stay in cohesion. This equilibrium is fragile, sensitised by the painting’s travels and subject to the variations of temperature and humidity. When the painting is degraded a specialist has to be consulted to study and restore it.