Similar to x-radiography, infrared reflectography (IRR) is an imaging method used to reveal details about a work not visible to the eye. IRR imaging is a technique by which a specialized infrared camera uses long wavelengths of light to penetrate layers of paint, producing a grey-scale image that illustrates the various degrees of absorption and penetration. Some paint layers will allow more penetration than others, depending on the type and thickness of the paint. Thinner paint layers are easier to penetrate and often appear partially or completely transparent in the infrared reflectogram. Black paint, for example, easily absorbs infrared radiation and will appear opaque. The resulting infrared reflectogram captures the stages of making, from under-drawing to final paint layer. Researchers and connoisseurs observe the evolution of a painting’s composition, allowing for a more complete visual analysis of a work.
An infrared reflectogram of the Palmer’s Virgin and Child with Donors triptych (below) provided critical physical evidence to reject the previous attribution of the piece to Memling.
The image is characterized by flatness. A distinct lack of under-drawing is underscored by areas of transparent paint. Contours of forms are absent in the infrared reflectogram. Color modeling, which gives the figures mass, depth, and believable physical form, is replaced by flat ghostly figures with piercing black eyes.
Jan Van Eyck, Ghent Altarpiece, detail of infrared reflectogram of Virgin Enthroned panel. Image: Royal Institute for Cultural Heritage, Closer to Van Eyck: Rediscovering the Ghent Altarpiece.
An infrared reflectogram of the Virgin Enthroned panel of the Ghent Altarpiece (above) illustrates the complexity of the paint layer structure of a true early Netherlandish painted panel. Color modeling that defines the folds of drapery remains visible under infrared imaging, lending dimensionality to the image despite absence of the full color spectrum.
The complete transparency of the paint layers of the Palmer Triptych suggests that pigments used on these panels were not typical fifteenth-century formulas, such as those used on the Ghent Altarpiece. The peculiar nature of the paint layer structure of the Palmer Triptych led to further material analysis, specifically of the pigments using x-ray spectrometry.
X-Ray Spectrometry (EDS)
Energy-dispersive x-ray spectrometry (SEM/EDS) is a quantitative elemental analysis that determines the chemical composition of an unknown sample. The process is an elaboration of microscopic study of a sample, which generally involves three parts: the subject, glass lenses which magnify the subject, and a light source positioned below the subject. A scanning electron microscope (SEM) replaces the light beam with a focused beam of electrons and the glass lenses with a series of electromagnets. The beam of electrons interacts with the unique atomic structure of the sample, generating x-rays that are specific to the elements present in the sample. Energy-dispersive x-ray spectrometry (EDS) software detects and measures x-ray energy. The resulting EDS spectrum plots data referring to the x-ray energy given off by the individual elements in the sample which analysts can study to determine the sample’s chemical composition.
EDS analyses may be employed to examine paint samples in order to determine the composition of the pigments used on a work of art. In the case of the Palmer Triptych, specialists at The Metropolitan Museum of Art Sherman Fairchild Paintings Conservation Center extracted a small cross-section of paint from the area of blue sky at the edge of the central panel. Tests revealed that the pigment is a mixture of lead white and barium sulfate. The presence of lead white is unsurprising as it has been in use as a pigment since antiquity. The combination of lead white and barium sulfate, however, is remarkable in that this particular mixture is known as “Venice White,” a formulation that was used extensively beginning in the twentieth century. Barium sulfate, also known as barium white, is a synthetic pigment developed in the late eighteenth century and manufactured since the early nineteenth century. The presence of barium sulfate is therefore anachronistic to early Netherlandish painted panels and suggests that the Palmer Triptych was either heavily restored or produced entirely in the twentieth century using Venice White, containing barium sulfate.
For more information about pigment analysis, visit The National Gallery YouTube channel where curators and scientists take viewers behind the scenes at the Gallery to learn how the Scientific Department employs technical analyses of pigment, materiality, and more.
Dendrochronology is a method of technical analysis used to approximate the date of a wood artifact. To determine the earliest possible felling date for a wood specimen, scientists measure growth rings to calculate growth rate and compare this data to an established database. By identifying this chronological boundary, dendrochronology allows art historians to affirm or refute an attribution of a painted work on wood supports.
The growth rings of a tree vary in width depending on climate conditions throughout the growing season, including temperature and rainfall, resulting in a unique pattern. Ring width is indicative of growth rate, or the average amount of wood produced per year; narrow rings indicate slow growth while wide rings indicate rapid growth. The average growth rate of an individual tree corresponds to predictable patterns in tree growth for a specific species of a known location and time. Additionally, ring density is a variable that can determine at what point in a tree’s lifespan it was felled, or cut, by noting the pattern of sapwood rings (younger, softer, less dense wood) and heartwood rings (older, denser wood). This biological regularity – by which the rings form in predictable ways in response to certain geographical and climactic conditions – allows scientists to compare the ring sequences of an unknown wood to known data.
Comparative studies to make chronological determinations requires a large sample size of known data for tree species. For dendrochronologists, the most valuable tool is a collaborative database that assembles data from researchers in various institutions in Europe and guarantees a steady exchange of data among laboratories. Scientists contribute data on growth rings for tree species in order to establish the diagnostic against which the unknown sample is compared (similar to a control group in experimental science). Experts can situate the felling of the tree that produced the unknown sample within a range of dates that has been determined by statistical analysis of this large sample size. For example, if study of an unknown sample of oak panel produces data points that correspond to a known sample of oak in terms of ring width and density, then experts can propose a chronological window within which the unknown sample was probably felled – the earliest year from which the tree could have been cut to the date after which it could not have been cut down.
Dating wood artifacts using dendrochronology must also take into account the time interval between cutting down the tree and the completion of a painting. Several months or years may elapse, during which time the wood is shipped over land and by sea, shaped, dried, and potentially stored in a warehouse or workshop for an extended period. The systematic estimation of the interval between felling and use is unrealizable given this variability. Therefore, the interpretation of dendrochronological data is an estimation and is generally used in combination with other forms of technical analyses when it comes to attributing or dating wood artifacts.
For more information about dendrochronology, visit The Getty Foundation and the Royal Institute for Cultural Heritage (KIK-IRPA, Brussels) web application Closer to Van Eyck: Rediscovering the Ghent Altarpiece. Specifically, investigators document their dendrochronological study of the altarpiece here.