Abrasion
Changes to part of the surface coating, paint layer or paint and ground caused by scraping, rubbing or over-cleaning with an abrasive product.
Absorption
1. The taking up of one substance by another through capillary, osmotic or solvent action. 2. The taking up (and consequent weakening) of light by, for example, a pigment layer. The radiation energy is converted into other forms of energy, e.g. heat.
Accretion
An accidental deposit of “foreign” material that was not part of the painting process, f. ex. dried liquid residue, flyspecks, etc.
Adsorption
The taking up by chemical or physical forces of molecules of gases or liquids by the surfaces of liquids or solids with which they are in contact.
Aging -cracks
Age-related cracks, caused over time by environmental, mechanical and other stresses, appear in all older paintings. Starting from the picture support, these cracks run through all the layers of a painting
Auxiliary support
Often used to refer to a stretcher or a strainer, an auxiliary support is the secondary structure of the painting, the framework over which a canvas is stretched.