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Wavelength Selection Using a Modified Camera to Improve Image-Based 3D Reconstruction of Heritage Objects

ABSTRACT Tools for image-based 3D-reconstruction are commonly used for cultural heritage applications; however, wider usage has increased variability in the quality of output 3D models. Geometric variations between 3D models acquired with differing methods make metric conservation applications such as condition monitoring and measuring change over time challenging. This article presents an investigation of wavelength selection using a modified off-the-shelf DSLR camera and bandpass filters to improve input image quality in a 3D-reconstruction study of a wooden sculpture of a coyote and turtle from the Smithsonian American Art Museum. The sculpture has a large crack of concern to conservators, but its curved, dark shiny surface challenges image-based dimensional monitoring. Selecting infrared wavelengths rather than the visible light for 3D reconstruction input images reduced specular surface reflections and improved image contrast resulting in improved recording of the 3D shape. 3D-reconstructions using infrared radiation produce better reconstructions than those using visible light. In this case reconstructed surface discrepancies between visible light are ∼0.6 mm whilst those using infrared are ∼0.3 mm. Results suggest that reflected infrared images are more forgiving and flexible for recording 3D data over time for dark, shiny wooden surfaces and thus improve the reliability and comparability of image-based 3D-reconstruction.

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The molecular characterization of early synthetic dyes in E. Knecht et al's textile sample book “A Manual of Dyeing” (1893) by high performance liquid chromatography - Diode array detector - Mass spectrometry (HPLC-DAD-MS)

This study focuses on the analysis of 61 samples taken from wool fabric specimens present in the appendix of the book “A Manual of Dyeing: For the Use of Practical Dyers, Manufacturers, Students, and All Interested in the Art of Dyeing” by E. Knecht, C. Rawson and R. Loewenthal, published in England in 1893. This manual is a valuable source of information, as it partially describes the chemistry of the molecules used to dye the specimens. The dyes are presented with the common commercial names used at the end of the 19th century and the publication date ensures that they were all synthesized before 1893.The first aim of the study was to explore the correspondence between the dyes described in the manual and the molecules actually present in the samples. High performance liquid chromatography coupled to diode array and mass spectrometric detectors (HPLC-DAD-MS) was used for the molecular characterization of the samples. The results were integrated with a thorough search of historic information on early synthetic dyes, to clarify the dye nomenclature, and ultimately the molecular structures of the detected molecules.The results showed that the labels reported in the manual often do not correspond to the actual dyes present in the samples. In a number of cases, mixtures of dyes of different categories were actually used to obtain a certain color shade. We created a database of more than 160 molecules, including their UV–Vis absorption spectra and MS details, many of which are not reported in the scientific literature. This represents a valuable new comparative dataset useful for the identification of these molecules in historic samples.

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Imaging spectroscopies to characterize a 13th century Japanese handscroll, The Miraculous Interventions of Jiz\u014d 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.

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Integrating liquid chromatography mass spectrometry into an analytical protocol for the identification of organic colorants in Japanese woodblock prints

Three Japanese woodblock prints from the Edo period (1603–1868) underwent a scientific investigation with the aim of understanding the changes in the colorants used in Japanese printing techniques. A multi-analytical approach was adopted, combining non-invasive techniques, such as fiber optic reflectance spectroscopy (FORS), Raman spectroscopy, multispectral imaging (MSI), and macro X-ray fluorescence (MA-XRF) with minimally invasive surface-enhanced Raman spectroscopy (SERS). The results enabled many of the pigments to be identified and their distribution to be studied, apart from two shades of purple of organic composition. Consequently, the potential of high-pressure liquid chromatography tandem mass spectrometry (HPLC–MS/MS) was explored for the first time with application to Japanese woodblock prints. The intrinsic sensitivity of the instrument and an effective extraction protocol allowed us to identify a mixture of dayflower (Commelina communis) blue and safflower (Carthamus tinctorius) red in purple samples constituted of 2–3 single fibers. In addition to the innovative integration of MA-XRF and HPLC–MS/MS to investigate these delicate artworks, the study concluded on the use of traditional sources of colors alongside newly introduced pigments in late Edo-period Japan. This information is extremely important for understanding the printing practices, as well as for making decisions about display, conservation, and preservation of such artworks.

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Open Access
Exploring the transition from natural to synthetic dyes in the production of 19th-century Central Asian ikat textiles

This study focuses on the dye analysis of 26 ikat textiles present in the collection of the Arthur M. Sackler Gallery and originally collected by Dr. Guido Goldman with the aim to gain additional information about their provenance and dating. The investigation exploits the full potential of a multi-analytical approach, starting with a non-invasive survey of all the colors using fiber optic reflectance spectroscopy (FORS), which revealed the presence of indigo and insect-based red dyes. These data were used to select areas from which samples were taken and analyzed by high performance liquid chromatography diode array detector (HPLC–DAD). These results enabled most of the natural sources of dyes to be fully identified, including American cochineal (Dactylopius coccus), madder (probably Rubia tinctorum), lac (probably Kerria lacca), larkspur (Delphinum semibarbatum), pagoda tree flower buds (Sophora japonica), grape vine leaves (Vitis vinifera), indigo and tannins. Complex mixtures of dyes were present in most samples, as a result of both the ikat making process itself and traditional dyeing practices. Synthetic dyes were identified in 9 of the textiles. Samples were re-analyzed using HPLC–DAD coupled to mass spectrometry (HPLC–DAD-MS). Malachite green (basic green 4, C.I. 42000), fuchsine (basic violet 14, C.I. 42510), rhodamine B (basic violet 10, C.I. 45170) and methyl violet (basic violet 1, C.I. 42535) were identified, and a few other tentatively identified synthetic dyes (probably orange I, II and IV, rhodamine 6G, patent blue V and alizarin yellow GG) were detected. As the first synthesis of early synthetic dyes is well documented, their presence was used to refine the dating of these textiles. The contextualization of the results also appeared to support the stylistic assumption that more intricate and colorful designs with a higher level of complexity are dated earlier than simpler, larger and more graphic ones. The overall information acquired reveals a dynamic scenario and an interesting window into the dyers’ experiments and adjustments to the economic and technological changes of the nineteenth century.

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Open Access