Reflectance Imaging Spectroscopy (RIS) for Operation Night Watch: Challenges and Achievements of Imaging Rembrandt’s Masterpiece in the Glass Chamber at the Rijksmuseum

Visible and infrared reflectance imaging spectroscopy is one of the several non-invasive techniques used during Operation Night Watch for the study of Rembrandt’s iconic masterpiece The Night Watch (1642). The goals of this project include the identification and mapping of the artists’ materials, providing information about the painting technique used as well as documenting the painting’s current state and ultimately determining the possible conservation plan. The large size of the painting (3.78 m by 4.53 m) and the diversity of the technical investigations being performed make Operation Night Watch the largest research project ever undertaken at the Rijksmuseum. To construct a complete reflectance image cube at a high spatial resolution (168 µm2) and spectral resolution (2.54 to 6 nm), the painting was imaged with two high-sensitivity line scanning hyperspectral cameras (VNIR 400 to 1000 nm, 2.54 nm, and SWIR 900 to 2500 nm, 6 nm). Given the large size of the painting, a custom computer-controlled 3-D imaging frame was constructed to move each camera, along with lights, across the painting surface. A third axis, normal to the painting, was added along with a distance-sensing system which kept the cameras in focus during the scanning. A total of 200 hyperspectral image swaths were collected, mosaicked and registered to a high-resolution color image to sub-pixel accuracy using a novel registration algorithm. The preliminary analysis of the VNIR and SWIR reflectance images has identified many of the pigments used and their distribution across the painting. The SWIR, in particular, has provided an improved visualization of the preparatory sketches and changes in the painted composition. These data sets, when combined with the results from the other spectral imaging modalities and paint sample analyses, will provide the most complete understanding of the materials and painting techniques used by Rembrandt in The Night Watch.

Imaging spectroscopies to characterize a 13th century Japanese handscroll, The Miraculous Interventions of Jizō Bosatsu

Scientific imaging of a large fragile work of art can be especially challenging, but especially rewarding to better grasp the complexity and changes that have occurred during its creation and lifetime. Here, noninvasive imaging, macro X-ray fluorescence (MAXRF) imaging spectroscopy and reflectance imaging spectroscopy, from the visible to the near infrared spectral range, are utilized to document a 14-m-long Japanese narrative handscroll, The Miraculous Interventions of Jizō Bosatsu. Due to the scroll’s age and its handling during past use as a teaching tool, it has a number of conservation needs and shows evidence of past repairs. The scroll has extensive and severe creasing, breaks and tears, as well as unstable and powdering pigments. Microscopic observation and scientific analyses were performed both to document the current condition of the scroll and to better understand its long history. Combining RIS and MAXRF allowed for pigment characterization through elemental and molecular information. While RIS and MAXRF previously have been applied to the study of other painted materials, their application to East Asian paintings is rare. The obstacles of the scroll’s length and fragile uneven surface were overcome by optimizing the setups of the two imaging systems. The MAXRF and RIS analyses, here focused on a select scene of the scroll, found certain original pigments common in early Japanese scroll paintings were used frequently, such as vermilion, iron-based compounds (yellow and red ochres), and copper-containing greens, while others occurred sparingly, such as azurite and red lead. A chloride-containing lead-based white pigment was employed. Faded organic colorants, notably indigo as well as an organic yellow/brown, could be detected but their vibrancy has been muted over the centuries. In the case of indigo, it may be visibly observed in some areas; however, analysis revealed its previously unknown presence mixed with a copper green in a select area. This focused study sets a foundation for further studies on both this object and other Asian works of art.

Synchronized Hardware-Registered VIS-NIR Imaging Spectroscopy and 3D Sensing on a Fresco by Botticelli

We discuss a synchronised sensing technique for the analysis of painted surfaces of frescos. Specifically, the performance of Visible-Near Infrared (VIS-NIR) Reflectance Imaging Spectroscopy (RIS) synchronized with three-dimensional (3D) acquisition is demonstrated in the study of a detached mural painting by Alessandro Botticelli. Synchronized sensing generates georeferenced data for simplified data treatment and interpretation. We show how such output data can provide key information to interpret important fresco surface and subsurface features (e.g., painting technique, material composition, pentimenti).

A Preliminary Study on the Differentiation of Linseed and Poppy Oil Using Principal Component Analysis Methods Applied to Fiber Optics Reflectance Spectroscopy and Diffuse Reflectance Imaging Spectroscopy

This paper presents results from the examination of a set of experimental samples using fibre optic reflectance spectroscopy (FORS) and diffuse reflectance imaging spectroscopy in the short-wave infrared (SWIR) range, combined with statistical analysis of the data for the discrimination and mapping of poppy and linseed oil. The aim was to evaluate the efficacy of this non-invasive approach for the study of paint samples with a view to the application of the method for characterisation of the two drying oils in painted art. The approach allowed discrimination between the two drying oils based on FORS spectra and the hyperspectral cube, indicating the influence of the spectral region around 1700 nm on the statistical discrimination based on the anti-symmetric and symmetric first overtone stretching of methylenic CH2 groups. This method is being studied as a potential non-invasive method of organic analysis of oil types that have formerly been studied using gas chromatography-mass spectrometry (GC-MS), which requires micro-samples.

Beauty is skin deep: the skin tones of Vermeer’s Girl with a Pearl Earring

The soft modelling of the skin tones in Vermeer’s Girl with a Pearl Earring (Mauritshuis) has been remarked upon by art historians, and is their main argument to date this painting to c. 1665. This paper describes the materials and techniques Vermeer used to accomplish the smooth flesh tones and facial features of the Girl, which were investigated as part of the 2018 Girl in the Spotlight research project. It combines macroscopic X-ray fluorescence imaging (MA-XRF), reflectance imaging spectroscopy (RIS), and 3D digital microscopy. Vermeer built up the face, beginning with distinct areas of light and dark. He then smoothly blended the final layers to create almost seamless transitions. The combination of advanced imaging techniques highlighted that Vermeer built the soft contour around her face by leaving a ‘gap’ between the background and the skin. It also revealed details that were otherwise not visible with the naked eye, such as the eyelashes. Macroscopic imaging was complemented by the study of paint cross-sections using: light microscopy, SEM–EDX, FIB-STEM, synchrotron radiation µ-XRPD and FTIR–ATR. Vermeer intentionally used different qualities or grades of lead white in the flesh paints, showing different hydrocerussite/cerussite ratios and particle sizes. Lead isotope analysis showed that the geographic source of lead, from which the different types of lead white were manufactured, was the same: the region of Peak District of Derbyshire, UK. Finally, cross-section analysis identified the formation of new lead species in the paints: lead soaps and palmierite (K2Pb(SO4)2), associated with the red lake.

Near-UV to mid-IR reflectance imaging spectroscopy of paintings on the macroscale

Broad spectral range reflectance imaging spectroscopy (BR-RIS) from the near UV through the mid–infrared (IR) (350 to 25,000 nm or 28,571 to 400 cm−1) was investigated as an imaging modality to provide maps of organic and inorganic artists’ materials in paintings. While visible–to–near-IR (NIR) reflectance and elemental x-ray fluorescence (XRF) imaging spectroscopies have been used for in situ mapping, each method alone is insufficient for robust identification. Combining the two improves results but requires complex data processing. To test BR-RIS, image cubes from early Italian Renaissance illuminated manuscripts were acquired using two spectrometers. Maps of pigments, including trace minerals associated with mined azurite, and their associated binding media were made. BR-RIS has a more straightforward analysis approach as implemented here than visible-to-NIR, mid-IR, or XRF imaging spectroscopy alone and offers the largest amount of macroscale information for mapping artists’ materials by comparison.