John K. Delaney

Use of imaging spectroscopy, fiber optic reflectance spectroscopy, and X-ray fluorescence to map and identify pigments in illuminated manuscripts

Abstract A paradigm using multispectral visible and near-infrared imaging spectroscopy is presented to semi-automatically create unbiased spectral maps that guide the site selection for in situ analytical methods (e.g. fiber optic reflectance spectroscopy and X-ray fluorescence) in order to identify and map pigments in illuminated manuscripts. This approach uses low spectral resolution imaging spectroscopy to create maps of areas having the same spectral characteristics. This paradigm is demonstrated by analysis of the illuminated manuscript leaf Christ in Majesty with Twelve Apostles (workshop of Pacino di Buonaguida, c. 1320). Using this approach the primary pigments are mapped and identified as azurite, lead-tin yellow, red lead, a red lake (likely insect-derived), a copper-containing green, brown iron oxide, and lead white. Moreover, small amounts of natural ultramarine were found to be used to enhance the blue fields around Christ, and a red lake was used to highlight different colors. These results suggest that the proposed paradigm offers an improved approach to the comprehensive study of illuminated manuscripts by comparison with site-specific analytical methods alone. The choice of broad spectral bands proves successful, given the limited palette in illuminated manuscripts, and permits operation at the low light intensity required for examination of manuscripts.

Characterisation of colourants on illuminated manuscripts by portable fibre optic UV-visible-NIR reflectance spectrophotometry

The use of ultraviolet and visible diffuse reflectance spectrophotometry as a preliminary technique in the investigation of illuminated manuscripts is discussed. Because ancient manuscripts are amongst the most fragile and precious artworks, characterisation of the materials used in their decoration should be performed using non-invasive analytical methods. Ultraviolet and visible reflectance spectrophotometry with optical fibres (FORS) allows non-invasive identification of several colourants used by ancient illuminators, causing no damage or mechanical stress to the artworks subjected to analysis. Identification is usually based on the comparison of analytical data with a spectral database built from painted areas on parchment, created by preparing paints according to ancient recipes as described in medieval technical treatises. Such database and the spectral features of the colourants analysed are discussed, along with the benefits of extending the spectral range of analysis into the shortwave infrared (to 2500 nm). FORS can be best appreciated as a rapid preliminary tool that offers an overview on the main colourants employed and guides the selection of painted areas of manuscripts on which more selective techniques, such as X-ray fluorescence or Raman spectroscopy, can be employed for a more complete and accurate identification.

Near Infrared Reflectance Imaging Spectroscopy to Map Paint Binders In Situ on Illuminated Manuscripts

In recent years, visible and near infrared (NIR) reflectance imaging spectroscopy, i.e., the collection of contiguous calibrated spectral images to provide the reflectance spectra for each pixel of the scene, has been applied to the study of old master paintings. Its application to the study of lightsensitive works of art, such as illuminated manuscripts, has been limited to partial pigment identification using visible electronic transitions. Here we show the potential of NIR imaging spectroscopy in the 1000 to 2500 nm (10 000– 4000 cm ) spectral region to map and identify paint binders by utilizing vibrational features associated with methylenic, CH and amide functional groups. This is demonstrated by using a novel hyperspectral NIR imaging spectrometer (1000– 2500 nm, 4.4 nm resolution) to map the use of a fat-containing paint binder (likely egg yolk) for certain compositional elements of a 15th century manuscript leaf. The use of a fatcontaining binder for manuscript illumination is surprising in itself since egg white and gum Arabic (protein and polysaccharides) are historically considered to be the binders preferred by illuminators. This study offers the opportunity to map paint binders in situ on works of art, at a macroscopic scale, for the first time. While analytical techniques using micro-samples (mg) taken from art objects can provide the most accurate identification of artists materials, there is a preference for in situ methods. Among them, site-specific tools such as X-ray fluorescence (XRF), Raman spectroscopy and fiber optics reflectance spectroscopy (FORS) can identify pigments. Production of material maps on the macroscopic scale (entire artwork) has been limited to date, while mapping on the microscopic scale has progressed rapidly and has proven to be useful. Techniques that have been used include scanning electron microscope–energy dispersive spectroscopy (SEMEDS), mid-IR (FT-IR), Raman, XRF, and luminescence spectroscopy using traditional as well as synchrotron sources. Given the 2-D nature of many works of art, the spatial information derived from macroscopic maps can provide important clues about an artist s working methods and help guide conservation choices. Hence, there is interest in the development of macroscopic mapping methods, which utilize existing analytical in situ methods such as XRF and X-ray diffraction, reflectance and luminescence spectroscopy. These methods not only provide the identification and mapping of artist s materials, but also other information such as compositional changes and layering of paint. Unlike XRF mapping, which provides elemental information and is thus limited to being used to infer inorganic pigments, X-ray diffraction and reflectance imaging spectroscopy provide information on the molecular structure of the pigment. Reflectance spectroscopy offers the capability to map also organic materials, such as dyes, and recently a plasticizer in a PVC object. To date most studies using reflectance imaging spectroscopy have relied on electronic transitions in the visible range alone for pigment identification and have been only partially successful. Improved results have been obtained by extending into the NIR range (750–1700 nm) in order to collect vibrational band overtones and combinations associated with hydroxy inorganic pigments. Extending the spectral range to 2500 nm to collect vibrational features associated with carbonate functional groups would be a further improvement. While progress has been made on mapping artists inorganic materials, the mapping of organic materials— paint binders in particular—has succeeded only on the microscopic scale utilizing mid-IR microscopes (650 to ca. 4000 cm ). While remote-sensing hyperspectral imaging cameras operating in the mid-IR exist, such instruments require exotic infrared focal planes and cooling to temper[*] Dr. P. Ricciardi, Dr. J. K. Delaney, M. Facini, Dr. S. Lomax National Gallery of Art, 6th St. and Constitution Ave. NW Washington, D.C. 20001 (USA) E-mail: j-delaney@nga.gov

Near-Infrared Luminescence of Cadmium Pigments: In Situ Identification and Mapping in Paintings

A comprehensive study of the luminescence properties of cadmium pigments was undertaken to determine whether these properties could be used for in situ identification and mapping of the pigments in paintings. Cadmium pigments are semiconductors that show band edge luminescence in the visible range and deep trap luminescence in the red/infrared range. Emission maxima, quantum yields, and excitation spectra from the band edge and deep trap emissions were studied for sixty commercial cadmium pigments that span the color range from yellow to red (reflectance transition 470 to 660 nm). For paints containing cadmium pigments, luminescence from deep traps was more readily observable than that from the band edge, although the yield varied widely from zero to around 4.5%. Optimal excitation for emission is found to be in the visible for both pigments in powder form and mixed with a medium. The maxima of the deep trap emission shift with the band gap energy, providing a potentially useful way to assign pigment type even when used in pigment mixtures. The usefulness of the results of the study on mockups was demonstrated by the mapping of cadmium pigments of different hues with the aid of calibrated luminescence imaging spectroscopy in a painting by Edward Steichen, entitled Study for ‘Le Tournesol’ (1920). Analysis of the luminescence image cube reveals at least six unique spectral components, associated with emission from white pigments, paint binder, and cadmium red and yellow pigments. The results were compared with those from X-ray fluorescence spectrometry (XRF) and fiber-optic reflection spectroscopy (FORS) and the results obtained on paint samples containing cadmium pigments. These results show that, when present, the emission from traps can be used as an analytical tool to identify cadmium pigments, to distinguish among cadmium sulfide, cadmium zinc sulfide, and cadmium sulfoselenide, and to map cadmium pigments, even in mixtures.

Complementary Standoff Chemical Imaging to Map and Identify Artist Materials in an Early Italian Renaissance Panel Painting

Two imaging modalities based on molecular and elemental spectroscopy were used to characterize a painting by Cosimo Tura. Visible-to-near-infrared (400-1680 nm) reflectance imaging spectroscopy (RIS) and X-ray fluorescence (XRF) imaging spectroscopy were employed to identify pigments and determine their spatial distribution with higher confidence than from either technique alone. For example, Marys red robe was modeled through the distribution of an insect-derived red lake (RIS map) and lead white (XRF lead map), rather than a layer of red lake on vermilion. The RIS image cube was also used to isolate the preparatory design by mapping the reflectance spectra associated with it. In conjunction with results from an earlier RIS study (1650-2500 nm) to map and identify the binding media, a more thorough understanding was gained of the materials and techniques used in the painting.

Femtosecond pump-probe microscopy generates virtual cross-sections in historic artwork

Significance We show that a nonlinear microscopy technique (femtosecond pump-probe microscopy) allows for nondestructive 3D imaging of paintings with molecular and structural contrast. Until now, studying the layering structure of a painting has generally required the physical removal of a cross-section sample. Pump-probe imaging has previously been shown on biological tissue, but applications to cultural heritage are more challenging: the variety of pigments in the artist’s palate is enormous compared with the biological pigments present in skin. Nonetheless, we show virtual cross-sectioning capabilities in mockup paintings and nondestructive imaging on an intact 14th century painting. This work represents a comprehensive collaborative effort between laser and biomedical imaging experts and scientists and conservators in national museums. The layering structure of a painting contains a wealth of information about the artists choice of materials and working methods, but currently, no 3D noninvasive method exists to replace the taking of small paint samples in the study of the stratigraphy. Here, we adapt femtosecond pump-probe imaging, previously shown in tissue, to the case of the color palette in paintings, where chromophores have much greater variety. We show that combining the contrasts of multispectral and multidelay pump-probe spectroscopy permits nondestructive 3D imaging of paintings with molecular and structural contrast, even for pigments with linear absorption spectra that are broad and relatively featureless. We show virtual cross-sectioning capabilities in mockup paintings, with pigment separation and nondestructive imaging on an intact 14th century painting (The Crucifixion by Puccio Capanna). Our approach makes it possible to extract microscopic information for a broad range of applications to cultural heritage.

Visible and Infrared Reflectance Imaging Spectroscopy of Paintings: Pigment Mapping and Improved Infrared Reflectography

Reflectance imaging spectroscopy, the collection of images in narrow spectral bands, has been developed for remote sensing of the Earth. In this paper we present findings on the use of imaging spectroscopy to identify and map artist pigments as well as to improve the visualization of preparatory sketches. Two novel hyperspectral cameras, one operating from the visible to near-infrared (VNIR) and the other in the shortwave infrared (SWIR), have been used to collect diffuse reflectance spectral image cubes on a variety of paintings. The resulting image cubes (VNIR 417 to 973 nm, 240 bands, and SWIR 970 to 1650 nm, 85 bands) were calibrated to reflectance and the resulting spectra compared with results from a fiber optics reflectance spectrometer (350 to 2500 nm). The results show good agreement between the spectra acquired with the hyperspectral cameras and those from the fiber reflectance spectrometer. For example, the primary blue pigments and their distribution in Picassos Harlequin Musician (1924) are identified from the reflectance spectra and agree with results from X-ray fluorescence data and dispersed sample analysis. False color infrared reflectograms, obtained from the SWIR hyperspectral images, of extensively reworked paintings such as Picassos The Tragedy (1903) are found to give improved visualization of changes made by the artist. These results show that including the NIR and SWIR spectral regions along with the visible provides for a more robust identification and mapping of artist pigments than using visible imaging spectroscopy alone.

Highly transparent poly(2-ethyl-2-oxazoline)-TiO2 nanocomposite coatings for the conservation of matte painted artworks

A nanocomposite coating based on TiO2 nanoparticles and poly(2-ethyl-2-oxazoline) is used as consolidant of matte painted surfaces (temperas, watercolors, modern paintings). The aim of this work is to provide advances in the conservation of these works of art, while preserving their optical appearance, in terms of colour and gloss. Fiber Optic Reflectance Spectroscopy (FORS) measurements of a painting-model (an acrylic black monochrome) treated with the nanocomposite coatings revealed that it is possible to match the optical appearance of the painted surface by tuning the amount of nanoparticles in the polymeric matrix. The requirement of retreatability of the material has been verified by removing the nanocomposite cast on the painted surface with aqueous solutions. FTIR and SEM/EDX measurements showed that almost no traces of the nanocomposite remained on the painted surface, allowing its use for the treatment of real paintings. Test were performed using a contemporary studio-model on canvas attributed to Agostino Bonalumi (1935–2013).

Use of Imaging Spectroscopy and in situ Analytical Methods for the Characterization of the Materials and Techniques of 15th Century Illuminated Manuscripts

Abstract This study combines site-specific in situ analytical techniques with non-site-specific reflectance imaging spectroscopy and multispectral infrared reflectography to characterize pigments and underdrawing materials in fragile, light-sensitive illuminated manuscripts. In situ methods implemented in this study included fiber optic reflectance spectroscopy and x-ray fluorescence, as well as multispectral visible and near-infrared reflectance imaging spectroscopy and multispectral infrared reflectography. Imaging spectroscopy was used to map areas having similar diffuse reflectance spectra, and the site-specific methods were used to further identify the primary pigments present. This combined approach was used to examine an illumination by Lorenzo Monaco (ca. 1370–1422 or after) and his workshop on the Praying Prophet, in the collection of the National Gallery of Art in Washington, DC (NGA), from an early-15th-century gradual on parchment (codex H74) currently at the Museo Nazionale del Bargello in Florence. The combination of the results along with microscopic examination led to a more complete understanding of the working methods used to create the Praying Prophet, including the primary pigments present and the two forms of underdrawing, ink and metalpoint. These results also allowed for a comparative examination of the primary pigments used in 13 other folios in codex H74. Multispectral infrared reflectography imaging and fiber optics reflectance spectroscopy confirmed that the primary pigments and preparatory drawing used in the NGA folio were consistent with those used in the other folios.

The Role of Varnishes in Modifying Light Reflection from Rough Surfaces - A Study of Changes in Light Scattering Caused by Variations in Varnish Topography and Development of a Drying Model

Abstract To obtain a better understanding of the variations in the appearance of paintings brought about by different varnishes, their ability to form an optically smooth film over rough surfaces was studied using laser scanning confocal microscopy and stylus profi lometry, as well as gloss and distinctness-of-image gloss measurements. Two different varnishes, one made with a low molecular weight resin and having low viscosity, and another, made with a polymer of high molecular weight and having high viscosity, were applied over glass plates that had been given surface roughness in the micrometer range. Measurement of the topography showed that the polymeric resin eliminated mid-to-high spatial frequency roughness of the ground glass but did not eliminate roughness of lower spatial frequencies. The results are discussed in terms of the surface state of the varnish, using concepts such as root-mean-square roughness, total integrated scattering and the power spectral density. These observations indicate that both varnishes reduce the wide-angle light scattering by the rough substrates, but only the varnish made with the low molecular weight resin reduces the small-angle scattering about the specular refl ection. This suggests that the smoothness of the varnished surface depends both on the molecular weight of the resin and on the spatial frequencies of the surface roughness of the surface beneath it. A model for the drying of varnishes is presented that takes the spatial frequency dependence into account.