Art is important because of its aesthetic appeal, perceived monetary value, or because of the historical or emotional significance attached to either the work itself or to the artist who created it. It is the physical vestige of mankind’s cultural heritage. Yet when the ravages of time, less than perfect environmental conditions, and other circumstance cause damage and deterioration, it falls on those in the field of art conservation to perform a scientific analysis on the materials from which the objects were made and determine the best method for their preservation or restoration.

Paintings come to the conservation laboratory in various states of disrepair; the materials used in their creation widely diverse. Media range from oil-based paint, egg tempera, wax, protein glue, and a variety of synthetic material; the range of supports includes canvas, wood, stone, masonry, and a variety of man-made composites. Over time, the innate properties of these materials can change in their structure and appearance. For example, a 400-year-old oil-on-wood panel may have begun to warp, split, or may have incurred insect damage. Another oil painting, done later in time, may have paint losses or tears in its canvas support. Contemporary paintings, in which the artist may have used experimental substances, may have begun to deteriorate in unexpected ways that can only be understood by an in-depth technical analysis.

Generally, there are three techniques conservators use to determine the nature of the painting’s material composition: a surface examination under a microscope or by using ultraviolet and/ or raking light; X-radiology and infrared reflectography, which penetrates through the surface of the painting; and sampling, in which trace specimens are removed from the work and analyzed with a variety of complex instruments.

Raking light (an illumination method that utilizes rays of light almost parallel to the surface) records the topographical surface and textural irregularities and can detect cupping, flaking, lifting of paint, minute cracks, or planar distortion. The extreme angle of raking light can also reveal subtle changes in the shape of a surface, which may result from uneven stretching of a canvas, warping of a support, or cracks in paint or emulsion. It can also indicate how an artist applied or changed paint.

X-radiology makes visible surface features of the painting that are impossible to see with the naked eye. This technique picks up on the density of the materials, creating an image based both on the pigment’s elemental properties and the thickness of the paint layers. This can reveal any repairs that have previously been made to the painting. (In modern conservation, a repair in an area of paint loss is called “in-painting”). It is important that the conservator never make a repair that might not be compatible with the original painting, or one that might not be reversible by other conservators in the future.

These analysis techniques are non-intrusive and are not harmful to the works of art. However, sampling does require a specimen extraction, therefore, extreme caution is used in its employment. Usually, the small sample will be taken from the painting’s edge or from an area adjacent to the pre-existing damage.