Paintings conservators analyze paintings as layered structures. Identification and characterization of all the component layers of a painting is the essence of the examination process. This analysis informs treatment possibilities and is critical to the work’s long-term preservation.
The primary support refers to the image-bearing surface (e.g. canvas, wood panel) but there may also be an auxiliary support which in turn supports or strengthens the primary support (e.g. stretcher, strainer, wood battens).
Stretcher or Strainer – the wood frame over which a painting on canvas is stretched and fastened through the foldover edge (margins), typically using tacks or staples. A strainer has fixed joins whereas the joins of a stretcher are designed to allow for expansion so that tension in the canvas may be increased.
Canvas – the textile support on which the image is painted. Typically artists utilize textiles designed as painting supports. A variety of weave types, fabric weights, and fibre types (linen, cotton, etc.) are possible.
Ground or Priming – an opaque, preparatory layer may be applied to the primary support to reduce its absorption, provide a good bonding surface for the paint, and alter the visual qualities of the image (contributing surface texture, light reflection, underlying colour, etc.). Size may be applied by the canvas manufacturer (in the case of a textile support) and/or the artist, and is a translucent substance intended to decrease absorption of the textile and increase bonding with the paint (or ground if present). Traditionally the size is a water-soluble, animal-based preparation such as rabbitskin glue. If present, the size is applied first, and may or may not be followed by the application of a priming layer.
Paint – In its simplest form and definition, paint is pigment bound in a medium such as drying oil or acrylic resin. Paint manufacturers modify paints with additives such as fillers and driers, typically with the aim of providing more consistent drying and performance characteristics throughout their entire range of colours. Oil and acrylic are now the most common media utilized by painters, but many other medium types are available, and more than one medium may be employed (“mixed media”). Artists may add other substances to paint such as sand, wax, tar, glue, resins, manufactured mediums, or virtually any substance imaginable…..these additions may or may not be visible but can greatly alter the paint’s characteristics including solubility, durability and ageing properties.
Frequently the image is first drawn by the artist on the prepared support with a medium such as charcoal or graphite – this underdrawing may remain visible where uncovered by paint, or thinly painted/with translucent paint. Of course the design may be worked up in multiple layers of paint, sometimes over a protracted period.
Surface Coatings, Dirt and Accretions
A clear substance such as dammar resin varnish may be applied over the paint to protect the surface and enhance the paint’s optical properties. Natural or synthetic resins may be employed, each having distinct advantages and unfortunately, disadvantages including such as yellowing and increased brittleness with ageing. Other examples of surface coatings include wax, oil, industrial coatings, proteinaceous substances.
Dirt may be trapped below a surface coating as well as accumulating on the topmost surface. Accretions such as accidental paint spatter and fly specks can damage the surface in addition to being unsightly.