Paintings are complex, multilayered objects, of which we can usually only see the surface layer. These objects result from a series of processes involving the selection and application of painting materials and techniques. The painter chooses a support for the painting (often wood or canvas), a ground material on which to apply the paint, and pigments that are bound in a medium of the artist’s choosing. Before paint layers are applied, an underdrawing, or an initial plan for the overall composition, might be set down to help guide the subsequent stages. The paint can be applied thinly or thickly, roughly or smoothly, and in one or multiple layers. Once the painting is finished, varnish can be applied to protect the paint below and saturate the colours, but the artist might also desire a matte finish and decide not to use varnish. There are many variations to this process, which are usually influenced by the place and period in which the painting was created. Technical art historians study these aspects of a painting to help answer questions on issues such as the attribution and the dating of an artwork. By combining an array of art historical and scientific tools, we can approach questions about artists’ working methods. Technical art history is, by definition, a highly interdisciplinary effort, since it draws on instruments and expertise from fields outside the humanities. Through such exchanges, scientists can better understand their own equipment: its limitations, extensions, and possible improvements. Technical art history also brings together curators and conservators whose experience and knowledge deepen our understanding of the artworks and their state of preservation.[i] The McMaster Museum of Art’s exhibition, The Unvarnished Truth: Exploring the Material History of Paintings, demonstrates that science can provide essential data to further enhance our understanding of the creative processes that produced the fascinating and beautiful objects on display.
The Electromagnetic Spectrum
The human eye is the first tool used to examine paintings. Using only a very small range of the electromagnetic spectrum (Fig. X), our eyes are able to detect a wide range of colours, hues, and tones, through which we experience the world. The colours that we can see are a function of wavelength, all arranged within this visible range of the spectrum, between approximately 400 to 700 nanometres (nm), or billionths of a metre. From short to long wavelengths, the visible spectrum runs from violet, blue, green, yellow, and orange to red.
After this initial visual analysis, a decision can then be made to proceed with examinations using forms of radiation that cannot be detected by the human eye. Beyond the red end of the visible spectrum lies the range of infrared (IR), with wavelengths that are longer than 700 nm. The wavelengths of ultraviolet (UV), on the other hand, are too short to be seen by humans; they are beyond the purple end of the visible spectrum. X-rays have even shorter wavelengths and, thus, more penetrating energy. All these different types of radiation provide complementary information about applied materials and techniques and, typically, about layers that lie below the surface of a painting. However, before using these other parts of the spectrum to analyze a painting, it is possible to learn a great deal about the painting’s condition simply by closely examining its surface.[ii] As paintings age they are transformed: varnishes yellow, some paint mixtures become transparent, and the surface can crack. Human intervention is another factor that often affects the way a painting ages. Over the course of history, many paintings have been subject to alteration by people including amateur painters and professional conservators. These attempts may be an effort to address the effects of age. Such interventions can often be detected with the human eye, sometimes simply by manipulating a light source. For example, shining a light from behind a painting produces light that is transmitted through the painting’s surface. When a painting that is executed on canvas is illuminated using transmitted light, it is possible to see variations in the thickness of paint. Raking light, or light that is shone from one side of a painting, emphasizes surface texture. The surface texture is very informative about the state of preservation of a painting. Using our eyes, we may also see more than the uppermost layer of a painting. For instance, certain paint layers will become increasingly transparent as the work ages, making it possible to see the underdrawing, if present, through these layers.
A great deal of information can be gathered while still using the visible part of the spectrum through magnification of the surface of a painting. This is achieved with the help of lenses. Such lenses can be handheld, or can be attached to a headband and worn as a visor to allow both hands to be free (Fig. X). The close examination of the surface of a painting can tell us about the condition of the work, the presence of additions, and the painting technique. Cameras are also capable of producing magnified images of a painting’s surface; macro photography uses a special lens to create detailed images of a small area of a painting (Fig. X).
Photography is an important tool for creating more permanent records of this type of analysis by making it possible to print hard copies of an image. Like analog photography, digital photography requires direct access to the paintings. Once created, digital image files are more accessible because they can be shared with relative ease over the Internet. This enables researchers without firsthand access to artworks to conduct research from afar. Digital photographs can also be effectively compared with digitized X-radiographs, IR reflectograms, and other photographs, facilitating comparisons of the different layers of a painting. For The Unvarnished Truth, digital photographs of all nine paintings were taken and uploaded to a website for sharing among experts. Digital copies of the UV images, IR reflectograms and X-radiographs were also placed on this website. This allowed specialists to examine the paintings in detail before participating in discussions at the museum.
Stereomicroscopes can help direct research because they produce extreme close-ups of the surface of a painting, which cannot be achieved by a camera lens of the type discussed in this essay. Magnification of this strength can allow researchers to see into cracks in the paint surface. This type of microscope generally uses the visible range of the electromagnetic spectrum (about 400 to 700 nm). Stereomicroscopes have two objective lenses (lenses that are closest to the object being examined) that create a three-dimensional image. By making it possible to get a detailed sense of the landscape of a painting, the stereomicroscope yields valuable information about the material composition and the way these materials were applied. Another type of microscope that is often employed in technical analysis of paintings is the research microscope. It is used to examine cross sections, which are tiny samples that are removed from a painting. Research microscopes can also use UV light (approximately 30 to 400 nm) to analyze the structure and materials of the cross section.
IR radiation is composed of longer wavelengths than visible light (between about 700 to 1 000 000 nm or 1 mm). This range is subdivided into the following groups: near infrared (NIR) extends from approximately 750 nm to 2500 nm, mid infrared (MIR) from about 2500 nm to 30 000 nm, and far infrared (FIR) from approximately 30 000 nm to 1 000 000 nm. NIR is used for the analysis of paintings and is a part of the IR spectrum that is not involved with heat detection.
Initially, conservators used IR photography to assess the condition of paintings. Later, the idea developed that this technique might be able to reveal the painting’s underdrawing. Once the support had been prepared for painting, artists were able to apply their compositional designs to it; this is what is called the underdrawing.[iii] Examining underdrawings reveals a great deal of information about the creative process and can assist in clarifying issues of attribution, for example. While IR photography was successful at producing images of the underdrawing beneath most paint mixtures, it is limited as a tool because it is not capable of penetrating some blues and greens. Infrared reflectography (IRR), which uses longer wavelengths than IR photography, was developed by J. R. J. van Asperen de Boer in the 1960s to address this limitation.
Early Netherlandish paintings are particularly well suited to examination with IRR because these works typically include an underdrawing made of a carbon-containing medium applied on top of a white chalk ground. Carbon absorbs IR, whereas white chalk reflects this radiation, creating a high-contrast image of the underdrawing layer. IR is also absorbed by carbon-containing materials used in other layers of a painting besides that of the underdrawing. A careful comparison of the painting as it appears in regular light and in the IR image is therefore necessary to avoid confusing these layers. The entry in this volume on the painting attributed to Jan Gossaert provides a good example of IRR analysis.
Ultra Violet Examination
UV light lies just beyond the purple end of the visible light spectrum (between approximately 30 and 400 nm). UV lamps are useful tools for examining the surface condition of a painting. UV excite fluorescence in materials contained in the uppermost layer of an artwork.[iv] Photographs of paintings under UV light (also known as UV fluorescence photography) can be taken to create a more permanent document for analysis.[v]
It is possible to identify substances based on an understanding of the way particular materials appear when they fluoresce. Different varnishes exhibit characteristic fluorescence, making it possible to classify them under UV illumination. For example, an aged natural resin varnish appears bright greenish-blue under UV. One of the most common uses of the UV lamp is to identify recent areas of retouching, applied within approximately the last several decades (although this range may vary). These retouches will typically appear dark under the UV lamp. UV illumination was used in the analysis of the painting by Aert van der Neer; for more details, see the entry on this particular work in this volume.
X-radiography uses radiation of the electromagnetic spectrum that falls below about 0.01 to 10 nm. The technique was invented in 1895 and was soon applied to the study of paintings.[vi] X-rays are potentially able to penetrate the entire layered structure of a painting, all the way through the support to anything that is attached to the back of the artwork. X-radiography allows us to visualize the materials of a painting and its support because different elements absorb X-rays in distinctive ways. Areas of paint that contain elements of high atomic number, such as lead white and vermilion, appear as white areas in X-radiographs, and are thus easily identified. In contrast, elements of low atomic number, which are usually part of earth pigments—including umbers, earths, and siennas—appear dark. The negatives of X-radiograph films are used for interpretation because, under regular illumination, the lighter colours on the painting’s surface often appear more like the X-radiograph negatives rather than the developed X-radiograph films.
X-radiographs provide useful information about the construction of a painting as well as the artist’s technique. Compositional changes made in a dense material like lead white, which took place at an early stage of the painting process and are now hidden by subsequent layers of paint, can be revealed by X-radiography. Patches of paint loss that have been filled with a material of high atomic number can also be detected in X-radiographs. As with other methods, it is critical to compare X-radiographs with results from other methods, like IRR. These techniques complement each other, enriching our understanding of artists’ working methods. A practical application of X-radiography can be found in the entry on Adriaen Brouwer’s The Drinker /The Bitter Draught in this volume.
X-Ray Fluorescence Analysis
With X-ray fluorescence (XRF) analysis, a small area of the painting’s surface is exposed to X-rays (between approximately 0.2 to 0.1 nm). These X-rays excite atoms contained in a small spot on the sample and cause X-ray radiation or photons to be emitted. This radiation can then be detected and analyzed to identify the elements that make up the object. Each element reacts differently when exposed to this type of radiation, meaning that qualitative XRF analysis can help identify pigments present in an area of a painting. XRF provides semi-quantitative information about the relative concentrations of these elements within a sample spot. Whereas earlier XRF equipment required samples be removed from a painting for analysis, modern equipment for XRF is nondestructive. For some time, to use nondestructive XRF required that the painting be brought to the instrument. More recently, portable, handheld XRF devices have been developed that can be taken into a museum or conservation studio.
The interpretation of XRF data is often complex. The high-energy X-rays used for XRF analysis are capable of penetrating the entire layered structure of a painting and, therefore, can detect elements located below the surface. XRF can also provide important insights about the physical history of paintings because it can also detect non-original materials including fills that have been added in areas of loss. A preliminary examination with a microscope can help establish the condition of the sample area and detect whether or not non-original materials may be present. It is also useful to identify, with as much certainty as possible, what pigments may be present in the sample spot before conducting XRF as this will facilitate the interpretation of the results. Here, the interdisciplinary nature of technical research is of primary importance, since art historians and conservators collaborate with scientists to interpret the spectra. XRF analysis was used to examine Alexej von Jawlensky’s Murnauer Landscape with Three Haystacks, discussed elsewhere in this volume.
Harnessing the power of the electromagnetic spectrum and combining scientific analysis with art historical research reveals new information that is hidden within a painting. The Unvarnished Truth exhibition demonstrates the way in which the study of paintings is deepened by technical art history’s application of scientific methods. By conducting this valuable interdisciplinary research, new insights have been generated that shed light on artistic processes and issues such as dating and attribution.
[i] Both conservators and curators tend to the protection and preservation of artworks. Curators are the caretakers of artworks within a collection and have a wide range of responsibilities. These duties include, but are not limited to, conducting research, organizing exhibitions, and performing administrative tasks related to objects in their collection. Conservators examine artworks to determine their condition and perform treatments to ensure that these works are preserved in the most stable state possible. To this end, the conservator can make adjustments to the storage environment or artwork itself.
[ii] Stephanie S. Dickey’s essay “The Art of Connoisseurship” in this volume discusses an important method of analyzing paintings using the visible range of the electromagnetic spectrum and the keen visual memories of this type of specialist.
[iii] Some artists did not include this underdrawing stage in their creative process. Furthermore, in some parts of the world, the material used for the underdrawings did not contain carbon, meaning that these underdrawings are invisible to IRR.
[iv] Fluorescence is the emission of longer wavelength radiation from atoms stimulated or irradiated with shorter wavelengths of electromagnetic radiation. These longer wavelength emissions can be in the visible spectrum and show up as colours.
[v] Although not used as part of this exhibition, another technique involving this wavelength group is UV reflection photography. This involves taking photographs with a UV-sensitive camera of the UV light that is reflected by a work. See R. de la Rie “Fluorescence of Paint and Varnish Layers (Part I)” and “Fluorescence of Paint and Varnish Layers (Part II)” in Studies in Conservation 27, no. 1 (1982): 1–7 and 65–9.
[vi] Ron Spronk, “More Than Meets the Eye: An Introduction to Technical Examination of Early Netherlandish Paintings at the Fogg Art Museum,” Harvard University Art Bulletin 5, no.1 (1996): 5.