Laboratory Diagnostic and Forensic Science
Unlock the mysteries to your artwork’s past
If you are buying, appraising or interested to know more about your artwork’s history, conservation laboratory techniques and tools, coupled with historical research can help to detect and identify mysteries. Every restorative intervention also starts with diagnostic testing. This information allows conservators to develop strategies unique to the artwork to proceed with caution. Further more, some of these techniques are applied at intervals during an artworks lifespan to monitor its ageing as a preventative testing measure ensuring the stable health of the art.
Ultraviolet lighting is a method used by conservators to reveal surface anomalies pertaining to materials used and the originality of the artwork.
Located just beyond the violet band of visible light, ultraviolet (UV) radiation causes materials to fluoresce and phosphoresce depending on their properties. Invisible to the naked eye, ultraviolet radiation has a shorter wavelength (365 nanometer range) than visible radiation.
For a few examples of how UV helps us conservators, original paint layers fluoresce, while retouchings and over-paintings appear as non-fluorescing dark spots. Organic materials frequently fluoresce while it is a rare occurrence for inorganic materials. When examining varnishes with UV light, the age and composition of these layers are revealed by the colour of fluorescence. For example, aged natural resin varnishes appear greenish yellow, while newer synthetic varnishes appear milky white to purple. UV light also aids restorers in fine-tuning cleaning strategies. For example, UV light reveals if an artwork’s surface is evenly cleaned of varnish and other contaminants such as fungal growth. Finally, UV fluorescence photography is particularly useful in examining inscriptions and signatures, which allows for authentication of the artwork.
Ultraviolet examination does have some limits though. For instance, although we previously described retouchings and over-painting appearing as non-fluorescing dark spots, restorations executed many years ago tend to fluoresce after eighty to a hundred years. This means that when UV light is used to examine the painting, these areas may be difficult to distinguish from the original painted layers. In these instances, examination using further diagnostic tools can provide additional information.
X-Rays are used by conservators to identify material composition, artist techniques and composition changes, preparatory sketches, structural supports and more.
It is a common practice for artists to recycle their canvases, usually by painting over a rejected image. X-radiography is a useful analytical technique, which can reveal information regarding an artist’s working process. It is able to detect different materials, preparatory sketches, changes to the composition of the image and structural supports. For these reasons, the use of x-radiography has become particularly useful and common among art authenticators, who can compare the information gained from analysis with the known methods and materials used by the artist in question.
X-rays are located beyond ultraviolet in the electromagnetic spectrum. Similar to ultraviolet, x-rays are invisible but have even shorter wavelengths and greater energy than visible light. X-rays penetrate through paint layers and supports to varying degrees depending on the atomic weight of the material being x-rayed. Materials of low atomic weight allow x-rays to pass through easily and therefore create dark areas on x-ray film, while those of high atomic weight block x-rays, leaving the film transparent. For example, lead white paint contains lead atoms of a higher atomic weight. This causes the white paint to block the x-rays and therefore appear transparent on the x-ray film. In contrast, carbon-based paints, such as some blacks, allow x-rays to pass through more easily, creating darker areas on the film.
Fourier-Transform Infrared Spectroscopy
Cross sections viewed under a microscope with natural and UV light can provide important information relating to paint stratigraphy, pigments composition, and the presence of unoriginal restoration layers.
Cross-sections are microscopic paint samples that are carefully collected from an artwork. Ethical factors are always considered when choosing a sampling location and occasionally it may be decided for a given artwork that sampling is inappropriate. However, paint cross-sections have proved valuable in many instances when questions remain regarding an artwork’s condition, authenticity, or provenance.
Usually samples (typically no larger than the head of a pin) are taken with a scalpel from the cracks, damaged parts, or the edges of the artwork. Often samples are collected with the aid of a microscope, a head loupe, or other types of magnifiers. The microscopic paint sample is then cast in a clear resin, typically polyester or resin-based, that hardens over a period of several hours. The resin is polished smooth using fine abrasives or with the aid of a micro-tome until the paint sample is reached within the center of the resin block. Once finished the sample can then be placed under a microscope of varying magnitude. These samples provide important information relating to paint stratigraphy (preparatory layers, paint layers, patinas, gilding and varnish layers), pigments composition (colour, quantity and dimension of pigment grains) and the presence of unoriginal restoration layers.
Cross-sections are typically viewed through a microscope that is additionally equipped with an ultraviolet light source as well. Both visible light and ultraviolet light can provide complementary information. For example, certain organic materials will auto-fluoresce when exposed to ultraviolet light (e.g. natural resins or organic red lakes). Additional information can also be obtained by subjecting the sample to biological stains on the surface of the cross-section. For example, when staining a sample using Amido Black (an azoic dye) protein based materials turn a blue color. This could be helpful to identify a substance such as rabbit glue (a traditional protein binder that consists of hydrolysed collagen) or egg (used in authentic tempera paintings).
FTIR is used to precisely characterize natural and synthetic materials in works of art.
FTIR (fourier transform infrared spectroscopy) is a scientific technique valued for its great accuracy in identifying materials. FTIR requires samples of only miniscule size—weighing just several micrograms—taken from an inconspicuous location on the work of art. The sample is put on an infrared window and analyzed under infrared light. In a matter of seconds the data taken produces a computer-generated graph. The peaks on the graph’s spectrum represent chemical bonds between two atoms or groups of atoms in a molecule. Since each material has a unique combination of atoms, no two compounds produce identical spectra – like a fingerprint, each one has a unique profile. Comparing your sample’s spectrum to spectra of known materials is how conclusions are met. Conservators around the world document their samples in order to share data, drawing similarities between materials. Unlike many other diagnostic techniques, which require the sample to be broken down by heat or chemicals in order to be analyzed, FTIR is a nondestructive technique. The sample can be recovered to carry out further analyses. FTIR can also be performed on a cross-section with several layers of paint, adhesives and varnish.
Knowing exactly what components make up a given artwork allows conservators to decide the best treatment procedures and materials to use for conservation. Moreover, the information also gives art historians and curators a better understanding of what, where, why and how artist’s materials were used over the decades. Pinpointing materials used during artistic periods in particular regions can assist in deciphering if an artwork is an original. For example if a modern pigment, which was not invented before a specific time is identified in an artwork that is said to be ancient, it can prove that that artwork is not original. A more positive outcome would identify a combination of material used uniquely by one artist.