Mapping of egg yolk and animal skin glue paint binders in Early Renaissance paintings using near infrared reflectance imaging spectroscopy

Abstract

In situ chemical imaging techniques are being developed to provide information on the spatial distribution of artists' pigments used in polychrome works of art such as paintings. The new methods include reflectance imaging spectroscopy and X-ray fluorescence mapping. Results from these new methods have extended the knowledge obtained from site-specific chemical analyses widely in use. While these mapping methods have aided in determining the distribution of pigments, there is a growing interest to develop methods capable of identifying and mapping organic paint binders as well. Near infrared (NIR) reflectance spectroscopy has been extensively used in the remote sensing field as well as in the chemical industry to detect organic compounds. NIR spectroscopy provides a rapid method to assay organics by utilizing vibrational overtones and combination bands of fundamental absorptions that occur in the mid-IR. Here we explore the utility of NIR reflectance imaging spectroscopy to map organic binders in situ by examining a series of panel paintings known to have been painted using distemper (animal skin glue) and tempera (egg yolk) binders as determined by amino acid analysis of samples taken from multiple sites on the panels. In this report we demonstrate the success in identifying and mapping these binders by NIR reflectance imaging spectroscopy in situ. Three of the four panel paintings from Cosimo Tura's The Annunciation with Saint Francis and Saint Louis of Toulouse (ca. 1475) are imaged using a highly sensitive, line-scanning hyperspectral imaging camera. The results show an animal skin glue binder was used for the blue skies and blue robe of the Virgin Mary, and egg yolk tempera was used for the red robes and brown landscape. The mapping results show evidence for the use of both egg yolk and animal skin glue in the faces of the figures. The strongest absorption associated with lipidic egg yolk features visually correlates with areas that appear to have white highlights. The results are in agreement with prior site-specific amino acid analysis, underscoring the synergy of both methods. The work here demonstrates that NIR reflectance imaging spectroscopy is a useful technique that can identify and map paint binding media based on differences in chemical composition.

Visible and infrared imaging spectroscopy of paintings and improved reflectography

“…endmembers), the image cube was processed using hyperspectral algorithms developed for the analysis of remote sensing data [20] and found useful for analyzing image cubes of paintings and manuscripts [4,15,16,18]. Because of the large number of reflectance spectra in the image cube, multivariate statistics were used to reduce the dimensionality of the data.…”

“…These features are often related to functional groups like hydroxyl (−OH) and carbonate (−CO 3 ) [7]. Absorption features from organic materials like the paint binding media, including the amide groups of proteins and lipid bands of oils and waxes, can also be observed [14] and even used to map their spatial distribution [15,16].…”

“…However, to discriminate between pigments having sharp transition edges, such as red and yellow semiconductor pigments, a resolution of <5 nm may be necessary. In the SWIR region, the required resolution should be <10 nm in order to resolve the narrow vibrational bands associated with some binding media [14][15][16]. X-ray fluorescence, in contrast, provides information on the elemental composition of materials and allows one to infer the possible colorants present [17].…”

“…Using highly sensitive focal plane arrays, the images can be acquired under lighting conditions suitable for examination of paintings and even illuminated manuscripts [15,18]. Collection rates are typically on the order of a few minutes to scan an area of 640 by 640 spatial pixels in 150-300 spectral bands [2,4,15,16]. In contrast, XRF imaging spectroscopy has been implemented using a raster scanning method, which provides detailed information about the elements used and in some cases the way they are layered within a work of art.…”

Qualitative analysis of pigments and binders in paint colors used in ‘Gvozden’ by Mića Popović

“…Non-destructive analysis by traditional macroscopic methods (X-radiography, near-infrared reflectance imaging, UV-visible fluorescence photography) can give the information on the support of the painting, compositional paint changes, under drawings, paint and varnish applications and restorations [3]. On the microscopic scale, materials can be identified by using Raman spectroscopy [4]; in macroscopic scale by using reflectance imaging spectroscopy [5], and X-ray fluorescence intensity mapping [6]. The methods offering 3D information are under investigation [7][8][9].…”