Near-IR Hyperspectral Imaging: A New Tool in Art Conservation

Jim Mansfield
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Spectral Imaging: A New Tool in Art Conservation

Wide-band infrared photography has been used for decades in art analysis because some pigments are nearly transparent in the near infrared and infrared spectral regions. This means that art conservationists and art historians can sometimes look at the lead underdrawings that are were sketched before the painting was made and get an idea about the thought process or intent of the artist.

However, digital near-IR imaging has not been as widely applied as other spectroscopic techniques. Near infrared (near-IR) spectral imaging of works of art offers both the power and sensitivity of near infrared photography and the specificity of spectroscopy in one technique. Combining spectral imaging with multivariate data analysis techniques makes this very powerful method of analyzing works of art.

One example of using spectral imaging in the analysis of works of art was of work I did while back at the Institute for Biodiagnostics. I imaged Untitled (The Holy Trinity), a 15^th Century drawing attributed to Viet Hirschvogel the Elder, which is in the collection of the Winnipeg Art Gallery. Full details of the imaging and results have already been published.[1][2] This work of art was imaged using a 650 to 1100 nm spectral imaging system.


Figure 1: Colour photograph of Untitled (The Holy Trinity), a 15th Century drawing attributed to Viet Hirschvogel the Elder and in the collection of the Winnipeg Art Gallery	Figure 2: Pseudo-colour RGB image of the first three principal component images from a PCA calculation of the imagecube taken from Untitled (The Holy Trinity).

Figure 1 shows a colour photograph of the drawing, which is composed of three or four materials sketched onto paper. Figure two shows the results of principal components analysis (PCA) on the dataset. The first three PCA images were put into the red, green and blue channels of a colour image, the results of which can be seen in Figure 2. The red lines in Figure 2 represent the lead underdrawing, some parts of which can be seen on close visual inspection of the drawing. However, much of the lead underdrawing is not visible, as it has been inked over. The ink shown in pale blue corresponds to the iron gall or iron tannate ink that was used to fill in the sketch. The yellow regions correspond either to a darker ink, which is either a more concentrated version of the iron gall or iron tannate ink shown in pale blue, or an ink made from different components.

Figure 3: Absorbance spectra of representative art materials. These materials are among those that are thought to possibly to have been used in Untitled (The Holy Trinity). These material spectrally fall into three main classes: blackbody absorbers (ivory black, carbon black, lead, graphite and charcoal); organic based (peat, iron tannate); and iron-based (iron oxide, red chalk).

It is also possible to draw representative art materials onto a substrate and image them under the same conditions as the works of art are being imaged. Untitled (The Holy Trinity) appeared to have been made up for four materials. We took modern versions of these types of materials and drew or painted them onto a paper similar to that used in the original drawing. These were then imaged under the same conditions, giving the spectra shown in Figure 3. It is interesting that these fall into three categories (blackbody absorbers, organic-based, and iron-based. It is also interesting that “iron tannate” contains no iron! The iron is used in the preparation of the organic dye but turns to metallic iron and is not dissolved in solution.

Longer wavelength spectral imaging systems will likely prove even more useful in the analysis of works of art, as they image in the same spectral region as the near infrared imaging systems that have proven so useful already in art conservation.

Still under construction!

[1] Michael Attas, Edward Cloutis, Douglas Goltz, Claudine Majzels, James R. Mansfield, Henry H. Mantsch “Spectroscopic Imaging in Art Conservation: A New Tool for Materials Investigation” Leonardo: Journal of the International Society for the Arts, Sciences and Technology Volume 36, Number 4, 2003, 304-307.

[2] J.R. Mansfield, M. Attas, C. Majzels, E. Cloutis, C. Collins, and H.H. Mantsch, "Near Infrared Spectroscopic Reflectance Imaging: A New Tool in Art Conservation," Vibrational Spectroscopy, 28, 59-66, 2002.