The identification of works of art follows the same general procedures as detecting a forgery or determining the treatment needed to restore a damaged item. The technical specialist works with the art historian or curator to discover who was the actual executor of a particular painting, drawing, or sculpture. This kind of work is particularly significant when large sums of public money are to be committed to a major purchase for an art gallery. The question is often whether a newly discovered work, typically a painting, is really that of a well-known artist. It is not uncommon for such finds to be made in an antique shop or someone's attic. Determining their authenticity is a significant challenge for the investigating team. Often the claim proves to be unsubstantiated, but there is always a chance of discovering a lost or previously unknown Grand Master.
It is important to determine the history of the artwork under question in as much detail as possible. Sometimes this is difficult. Families may pass art collections down through generations and accompanying documentation can be lost. Paintings are stolen or lost and can disappear for many years. For instance, the upheavals of World War I and World War II led to the dispersal of many valuable artworks by looting. Some may have since been acquired by art dealers, museums, and private collectors, but questions remain as to their authenticity.
An important task in art identification is to compare the questioned item with examples from the artist's main body of work. This falls to the art historian, who has specialist knowledge of the artist's approach, themes, favorite materials, and techniques. The extent of technical examination needed depends very much on the context and circumstances—it can be relatively easy to disprove or prove authenticity in some cases, but other cases may be more challenging. The investigation begins with a visual investigation of the painting, accompanied by photography in normal, infrared, and ultraviolet light. Photographs are taken in front of the painting with both normal illumination and tangential illumination, the latter revealing surface irregularities.
A painting is generally made up of four distinct layers: the support; the ground, which serves as a foundation for the paint; the paint itself; and varnish. The materials used in each layer are often characteristic of the time of creation of the painting. Most old materials can be re-created, so their presence alone cannot confirm a painting's age. However, the presence of acrylic paint means a work that has been made in the twentieth or twenty-first century.
The support is usually made of wood or canvas. The age of wood such as oak can readily be determined by dendrochronology , which is the examination of tree rings. Canvas age can be solved by carbon-14 dating. When it comes to pigments, the presence of lead-tin yellow can always exclude the nineteenth century. This pigment was widely used in the fifteenth, sixteenth, and seventeenth century, but disappeared completely around 1850. It was rediscovered in 1941. Another yellow pigment that has been used in identification is cadmium yellow, which was introduced about 1800. In 1942, the St. Louis Art Museum bought a still life said to be by the Spanish master Francisco de Zurbaran (1598–1664). However, the discovery of cadmium yellow in the painting meant it could not date back to the seventeenth century.
The presence of underdrawings, as revealed by infrared reflectography, is an interesting aid to identification. The paint layer is transparent to infrared light, but it is absorbed by lines drawn on the ground layer. The rest of the infrared is reflected back by the ground. Thus an infrared photograph reveals any underdrawing that the artist may have done. The absence of underdrawings may be indicative of a forgery, because they may not be necessary if someone is just making a copy.
X-radiography can also reveal what is beneath the surface of a painting. The technique is not unlike the x rays that are used to examine the human body. Most elements of the paint layer are transparent to x-radiation, save for white pigments containing lead and vermilion, a red that contains mercury. X-radiography can show revisions and additions to the painting if these have been done with lead or mercury-containing pigments. It can also reveal the structure of the support, showing any marks that have been made in its construction.
In one well-known case, a painting long assumed to be a copy was identified as being authentic, thanks to technical testing. A small panel of the Virgin and Child first attributed to Jan Gossaert (1478–1532) had been consigned to the storerooms of the National Gallery in London because it had been decided it was only a copy, probably made in the eighteenth or nineteenth century. Further testing in 1994 involved studying the surface and cross-section samples in ultraviolet light. This showed re-painting and addition of a varnish layer had been done in the nineteenth century. However, there were older paint layers containing a red pigment from the sixteenth century. Dendrochronology showed the support to be equally old. Removal of the later paint and varnish restored the original Gossaert, which matched an engraved copy from 1589. The painting was the original in a group of six closely related paintings which survive.
see also Art forgery; Holocaust, property identification; Paint analysis.