Multiband imaging of a sandstone object with blue pigment

1 Multiband imaging of a sandstone object with blue pigment Contents Multiband imaging of a sandstone object with blue pigment ......................................................................... 1 Abbreviations ............................................................................................................................................. 1 Objects description .................................................................................................................................... 1 Purpose ..................................................................................................................................................... 1 Comments on set-up ................................................................................................................................. 2 Comments on standardisation and post processing .................................................................................. 2 Image types ............................................................................................................................................... 2 Equipment ................................................................................................................................................. 3 Light sources ............................................................................................................................................. 3 Raw data ................................................................................................................................................... 5 References ................................................................................................................................................ 5 Images and results .................................................................................................................................... 5 Abbreviations IR Infrared (ca. 700–1100 nm) IRR Infrared reflectance IRRFC Infrared-reflected false-colour MBI Multiband imaging UV Ultraviolet (ca. 200–400 nm) UVL Ultraviolet luminescence UVR Ultraviolet reflectance UVRFC Ultraviolet-reflected false-colour VIL Visible-induced luminescence VIS Visible (ca. 400–700 nm) Objects description Sandstone with blue pigment Purpose Multiband imaging (VIS, IRR, UVL, UVR, IRRFC, VIL) was performed for documentation and to visualize the blue pigment particles. Date 2024-03-26 Dnr RAÄ-2024-0156 Author Sara Norrehed Date of analysis 27th of March 2024 Place of analysis Heritage Laboratory, Swedish National Heritage Board Analyst Sara Norrehed 2 Comments on set-up All images were captured with the object placed horizontally with the camera mounted above. The object was placed on a black paper background and supported to keep the area with blue pigment flat. The camera was positioned c. 50cm above the object and two light sources were placed on opposite sides with approximately a 45-degree angle towards the object. Comments on standardisation and post processing The Heritage Laboratory follow the guidelines for multiband imaging as presented in the user manual that was developed by the British Museum during the EU-funded CHARISMA project1. The manual details practices to improve the reproducibility of the method between users and institutions, including the use of specific standards. For post processing Nip2 is a graphical interface for the image processing system VIPS that is open-source and can be downloaded free of charge2. Nip2 has many features; for example, it can utilize standards to colour calibrate and correct for spatial inhomogeneities of the light source. The processing and use of Nip2 is described in detail in the manual. Every object and on-site photography setup provide different circumstances and challenges, therefore some variation in set-up between sessions may occur. Image types Image Radiation source Filter General purpose VIS VIS (flash) VIS Standard image. IRR IR (flash) IR IR reflection upon illumination by IR radiation. Useful to study underdrawings in paintings. Many materials such as organic dyes and binders become transparent under these conditions. UVL UV VIS Luminescence in the VIS region upon illumination by UV radiation. Useful to visualize organic binders and dyes. Some inorganic materials also have luminescent properties, e.g. zinc oxide. UVR UV UV UV reflection upon illumination by UV radiation. Useful to visualize surface structure, coatings and varnishes. IRRFC In IRRFC the red channel from IRR is combined with the red and green channel from the VIS image. Some pigments have characteristic colour changes. UVRFC In UVRFC the blue channel from UVR is combined with the green and blue channel from the VIS image. Some pigments have characteristic colour changes. VIL VIS (LED) IR For visualisation of Egyptian blue, Han blue and Cd-pigment 3 Equipment Camera Nikon D850 (modified to full spectrum, UV-IR blocking filter removed) Lens Nikon AF NIKKOR 105 mm Filters CHSOS Robertina filters for VIS, UV and IR, B+W 093 for IR Light sources Godox speedlight V860ii for VIS and IRR CHOS Fabrizio UV-light for UVL and UVR Godox LED1000 II for VIL Software Adobe Photoshop, Adobe Camera Raw, Adobe Bridge, Nip2 with Bm-workspace Standards Calibrite Classic Mini McBeth color chart Spectralon© reflectance standards; 99 % Radiation sources Irradiance profile of Nikon Speedlight V860ii measured at 100 cm distance with an Ocean Optics Flame spectrometer. 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 Irradiance Wavelenght (nm) Speedlight V860ii 4 Godox LED1000 II measured at 100 cm distance with an Ocean Optics Flame spectrometer. CHSOS Fabrizio UV-light measured at 100 cm distance with an Ocean Optics Flame spectrometer. 0 2 4 6 8 10 12 14 16 18 20 250 300 350 400 450 500 550 600 650 700 750 800 850 900 μW/cm2/nm Wavelength (nm) Godox LED1000 II 5 Raw data Tif and jpg images are stored on the Swedish National Heritage Boards raw database Lida (L:) and available upon request. References 1. Dyer, J., Verri, G. & Cupitt, J., ‘Multispectral Imaging in Reflectance and Photo-induced Luminescence modes: a User Manual’, 2013, Web publication/site, European CHARISMA Project, Online. 2. GitHub - BritishMuseum/bm-charisma: A nip2 workspace for technical imaging https://github.com/BritishMuseum/bm-charisma (2023-03-24) Images and results Results from MBI are indicative and should always be accompanied by analytical methods. MBI was performed on the part of the object that contain blue pigment. The area with pigment particles measures around 2x2cm. The expected results from blue pigments containing copper are summarized in Table 1. The main differences between azurite and Egyptian blue should be seen in IRRFC and VIL. Interference from the substrate (sandstone) is evident in these images, a heterogenous background makes conclusions difficult. The imaged blue pigment did not show any visible-induced luminescence (VIL), hence azurite is the most likely conclusion that can be drawn by MBI. MBI also indicate that the blue pigment is the same material and not a mixture as no variations can be seen in the image response from the pigment. In the UVL image glue is visible. Table 1. Expected MBI results from azurite and Egyptian blue (Cu-containing blue pigments) Pigment VIS IRR IRRFC VIL UVR UVL Azurite Blue Dark Purple - Dark - Egyptian blue Blue Dark/Transp Red Bright Dark - 6 VIS IRR 7 IRR False colour UVL 8 UVR VIS (for VIL) 9 VIL Azurite references A set of azurite references demonstrate the expected response in MBI. Azurite ground to different particle size in two different binders (rabbit glue and linseed oil), on a white (chalk) and a black (ivory black) background. A mineral sample (USA) is also included. All references were available from the Heritage Laboratory reference collection.