Some pastel colors—such as carmine, which was used to evoke the hue of the sitter's complexion—are prone to fading, while many others have greater durability. Regardless of color, all pastels must be protected from exposure to high levels of light or prolonged periods of illumination. Unlike oils, pastels' vulnerability to fading is increased because they are not protected by a varnish, nor are the powdery components surrounded by a resin.
In most eighteenth-century pastel portraits, color extends across the entire support, but those with exposed areas of paper, such as the préparations by Maurice Quentin de la Tour, are at the greatest risk of color alteration from exposure to light. Often they have discolored and turned brown in the reserve areas. Museums generally limit the display of pastels to no more than three months per year at five foot-candles; if they are displayed for longer periods of time a light level of approximately three foot-candles should be maintained.
Pastels must always be framed and glazed to best preserve them. Most acrylic sheeting is not satisfactory to glaze pastels because its static charge will attract pastel particles, a problem that is exacerbated when the plastic is rubbed (during cleaning, for example). To protect the surface of these fragile works, shatterproof glass with an Ultraviolet (UV) barrier is recommended. Pastels are occasionally glazed with AR acrylic sheeting, but we do not yet know this material’s long-range properties (such as deleterious off-gassing, which is common to many plastics as they age and deteriorate).
If a pastel is glazed with non-static acrylic sheeting, it is recommended that it be used only for the purposes of transport or loan rather than for permanent framing. A pastel that is framed with the original eighteenth-century glass, which does not have a UV barrier, can be protected from fading and color alteration by maintaining low light levels, keeping curtains drawn when the work is not being viewed, and coating nearby windows with a UV film. To prevent the entry of dust, a seal made of strips of paper should be applied to the back of a framed pastel.
Pastels must also be protected from mold and other forms of biodeterioration, problems to which they are vulnerable because of the organic binders in the crayons, the adhesives used in the mounting structure, and their paper supports. Environmental levels must be kept in a range of 68 to 72 degrees Fahrenheit and 48 to 52 percent relative humidity. High humidity can provoke staining; low levels can lead to desiccation of the support.
Understanding the particular properties of pastel enhances our appreciation for the distinctive light and beautiful richness of color of the medium, qualities underlying its great acclaim in the eighteenth century. It also enables us to take the steps needed to preserve these fragile portraits so that we may continue to enjoy them and ensure their future in good condition.
For travel and transport, pastels should be framed and kept in a horizontal, face-up position. One of the greatest hazards to a pastel is vibration. This can be reduced by cushioning crates with ethafoam, or, for short distances, wrapping the composition in bubble wrap with the bubbles facing outward.