Under the Microscope: The Turin Shroud is an ancient linen kept in Turin Cathedral. It bears the shadowy outline of a naked man, claimed to be Christ, writes Prof William Reville.
It shows what appear to be bloodstains at the wrist, feet and head, consistent with crucifixion and a crown of thorns. In the late 1970s Walter C McCrone, a chemical expert on art forgeries, examined samples from the shroud under a microscope. The stains did not contain blood, but contained two red pigments, red ochre and vermilion, readily available in the middle ages. He concluded that the shroud was painted around 1355, and radiocarbon dating later confirmed the tests. McCrone describes the scientific investigation of art forgery in the January/February 2001 edition of The Sciences.
The art of forgery is very old. The ancient Egyptians made false gems out of glass and workshops in ancient Rome trundled out copies of Greek sculptures and jewellery. By the 17th and 18th centuries, forgers were copying the great European painters including Rubens, Rembrandt and Vermeer. They covered their tracks with great ingenuity.
For example, no forger would paint an Old Master on a fresh canvas. They would purchase old paintings of no value and scrub the canvases clean. To fake age, some forgers would alternately expose their paintings to heat and cold, thereby creating a random pattern of cracks on the surface of the painting.
The book The Turin Shroud: In whose Image? by Lynn Picknett and Clive Prince (Harper Collins, New York, 1994), proposes that the shroud was an elaborate hoax played by Leonardo da Vinci (1452-1519), in which he used a primitive photographic technique to place an image onto the shroud. For various reasons this seems unlikely, but Leonardo was a known hoaxer. He subtly built his own image into many of his paintings. He is one of the disciples in The Last Supper and he himself was the model for The Mona Lisa (right).
The Dutch painter Han van Meegeren was a famous modern forger. Several of his works appeared in the 1930s as "lost Vermeers". Some of them were bought by major museums. Van Meegeren was meticulous - he even ground his own pigments of known historical authenticity. He was tried in 1945 as a Nazi collaborator because he had sold a "Vermeer" to Hermann Goring. He pleaded guilty to the lesser crime of art forgery and in 1947 he received a sentence of only 12 months for this offence.
There are powerful methods other than scientific ones for detecting art forgery. Art connoisseurs familiar with the technique, style and subject matter of an artist may quickly detect a forgery by spotting a detail from the wrong period or even inappropriate paint colours or brush-stroke patterns.
Scientific analysis of a potential forgery relies on techniques such as microscopy, radiocarbon dating, and examination with ultraviolet light or X-rays. If a scientist can show that any material used in a painting was unavailable during the lifetime of the attributed artist, this will prove the painting to be a forgery.
Radiocarbon dating is very useful for determining age, but it can only be used for materials derived from plants and animals, for example fabric. Carbon in the atmosphere exists in two forms - stable carbon-12 and radioactive carbon-14.
Living plants and animals exchange carbon in their bodies with carbon in the atmosphere, and the ratio of carbon-14 to carbon-12 in living tissue is the same as it is in the atmosphere. When an organism dies it stops exchanging carbon with the atmosphere and the subsequent radioactive decay of carbon-14 in the dead tissue causes the ratio of carbon-14 to carbon-12 to decline below the atmospheric ratio. Carbon-14 decays at a precise rate and by measuring the ratio of carbon-14 to carbon-12 in an object, one can calculate how long ago the organic material in that object died.
The technique of dendrochronology can be used to date a painting made on wood. Trees grow in girth in annual cycles and this is recorded in the form of tree rings. In a good year the tree will grow more than in a lean year, and consequently the ring for that year will be fatter than for the lean year. In any particular geographical area all the tree rings in a species will record the same pattern as the years pass by.
Examination of a painting under ultraviolet light reveals if it has been cleaned or retouched. Examination with X-rays can reveal hidden layers of paint. But the most useful scientific instrument is the polarised-light microscope. This can identify all paint pigments. A few pigments made by grinding up naturally occurring minerals have been in use since prehistoric times.
Art forgery is very still much alive and well. Queries are still raised about the authenticity of many works attributed to Van Gough, including Sunflowers, bought in 1987 by a Japanese insurance company for $39.9 million. Authentication is a perennial problem for museums and galleries.
William Reville is associate professor of biochemistry and director of microscopy at UCC.