Authentication of Paintings

Paintings can be accurately authenticated through both forensic and stylistic analysis that renders subjective connoisseurship obsolete. Under the circumstances, this essay is only about a few of the many scientific tools available and how they can be used to authenticate paintings.

Forensic science studies anomalies in the chemical and physical composition of paintings. This includes the paint's ingredients, the chemical makeup of the canvas or panel, and markings that lie below the paint surface. Analysing such anomalies is critical to gaining understanding of the painting's composition, origin and age. For example, scientists may uncover a forged 16th century Titian painting it contains zinc-white paint.

Science studies art opposite the way that a connoisseur would. While the connoisseur generally tries to expand the opus of artwork, forensics aims to exclude forgeries. This sort of "guilty until innocent" approach that forensic science takes to art research is one whereby a work is not considered authentic until its attributes conform to set standards.

Forensic analysis also offers critical insights into a painting's style and physical composition. Even if a forged painting is made using authentic materials, forensics can reveal anomalies in its content or other features. For example, a forged Titian can be eliminated if the paint's copper content does not conform to established parameters, which is possible even if the forger had used the correct types of paints.

Basically, there are two types of forensic analysis. The first one involves photographic techniques that use infrared, X-ray and ultraviolet light. This is the most common form of scientific test, but its major weakness is that it does not study actual samples.

Infrared Reflectography identifies markings or drawings underneath the painted surface. In the old masters, under-drawings were often drawn directly on the canvas as a sketch for the painting. Examining under-drawings can help to establish the painting's authenticity and can be compared against the artist's style. This radiation can also detect authentic signatures indistinguishable to the naked eye, or reveal fake signatures that were added after the completion of a forgery.

Infra-red light lies just outside the visible spectrum. It overlaps with the red area of the spectrum and the microwave region. Conservators use wavelengths of radiation from the near infra-red part of the spectrum Ð'- mostly in the range of 750Ð'-2000 nanometres. This relatively long, low frequency wavelength is able to penetrate through the upper layers of a painting or work on paper, such as oil paint, to the drawing underneath. An infra-red reflectogram is created by capturing an image of the infra-red wavelengths that are reflected into a camera lens.

Infra-red examination is commonly used to look at an artist's working technique. It provides clues as to how a work of art has been constructed, and often gives an indication of the materials that the artist used. As some paints and varnishes can appear transparent in infra-red light, details hidden beneath the surface may be revealed. Graphite pencil, charcoal lines and other carbon-based drawing media used during the early stages of developing a work of art are enhanced using infra-red reflectography.

X-ray photography uses short-wave radiation to detect alterations in a painting, areas of a painting that have been repaired or changed and also identify certain types of X-ray absorbing pigments, like lead white and led-tin yellow. Since dates when these paints were introduced have been determined, their presence can shed light on the painting's time of execution. Coupled with UV light analysis to reveal areas of in-painting, these techniques can aid in the identification of pigments.

However, though X-ray photography is able to detect lead-based paints, it cannot quantify the paint's precise lead content. In addition these technologies are incapable of analyzing organic material such as the binding ingredients in paints. This means investigating a painting through photographic examination alone can produce highly deceptive results, and they must be employed in tangent with additional tests to properly assess a painting.

The second (and more effective) category of scientific testing involves the extraction and analysis of samples from a painting. The most advanced method is Reflection X-ray Fluorescence Spectrometry Analysis (TXRF), which examines pigments taken from a painting using X-rays. The extraction process involves obtaining a micro-sample by brushing a cotton swab over the surface of a painting. The sample is then subjected to high-intensity X-ray radiation, stimulating the chemicals in the sample and causing them to release secondary X-ray signals. Each element in the sample emits a unique signature, revealing the precise elemental contents of the sample. TXRF examination differs from other photography techniques because it involves the analysis of actual paint samples rather than non-intrusive photography. Thus it can produce far more detailed forensic chemical reports compared to non-intrusive photography.

According to scientists R. Klockenkamper et al, "a characterisation of the pigments may help in assigning a probable date...to the painting." This is because each type of paint has a unique chemical composition which is traceable to the time it was introduced. For example, Prussian Blue was introduced in 1710. Therefore, if a painting attributed to Rembrandt contains Prussian Blue, it can be immediately dismissed as a forgery.

However, there are also more intrusive forensic analysis techniques that require paint samples. These methods are very effective, but are considered unacceptable to many galleries because they cause a small amount of damage to a work. One such test is the Atomic Absorption Spectrophotometry (AAS) test.

Lab samples for AAS can be carefully cut from the back side of a painting, but although it is a destructive technique, the sample size needed is very small (typically about 10 milligrams). Solid samples are then dissolved, often using strong acids.

A minute quantity of the liquid sample solution, about 0.01Ð'-0.02 the size of a raindrop (20Ð'-50 μL), is sprayed into a nitrous oxide-acetylene