Frescoed walls absorb moisture from the atmosphere. The moisture carries to the wall soluble surface salts that effloresce and injure the fresco pigments. To halt such injury water-permeable fixatives may be applied to help stabilize the pigment and prevent it from flaking off. A more drastic treatment is transfer, by which the mural and upper layer of plaster are cut away from the wall altogether and made fast to a new support. A major instance of successful transfer was carried out on many frescoes unearthed at Pompeii.
Wood-panel paintings undergo much swelling and shrinking with humidity variations. Wood-boring insects and the dry rot of fungus also attack them. The painting may be transferred to a new support, or the old one may be strengthened by impregnation with a consolidating medium (including several plastics) or given auxiliary support. Insecticides and fungicides may suffice to combat woodworms and dry rot; in cases of advanced destruction, reinforcement by impregnation may be necessary.
Canvas supports also absorb and lose moisture, swelling and shrinking, and thereby losing much pigment. In addition, canvases may be weakened or torn with comparative ease. A method of relining (restretching on a second undercanvas) may be effected whereby the old canvas is attached to the new by means of an adhesive. This may be a thermoplastic wax-resin combination or a water-base glue. The painted surface becomes impregnated with the adhesive and is consequently stabilized.
Irregular staining, called foxing, is the bane of print and drawing collectors. In humid conditions foxing attacks the adhesives and mounts of paper-based art, including watercolors, by producing the nutrients favored by molds present in the atmosphere. The work may sometimes be sterilized and remounted on a support chosen for its mold-repellent quality. It may be further treated with a fungicide. Some foxing stains may be removed by careful bleaching and washing, but this is a difficult technique requiring considerable knowledge of materials.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.