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NMAI Archaeological Peruvian Textiles:

Imaging & Analysis for Pigment Identification & Elucidation of Manufacture

E. Keats Webb (MCI), Rebecca Summerour (NMAI) and Jennifer Giaccai (FSG)

Four Peruvian Textiles

NMAI Peruvian Textiles (Left to Right: NMAI 220497, NMAI 239073, NMAI 239040, and NMAI 239038)
Images by E. Keats Webb, Smithsonian Museum Conservation Institute

MCI was requested to do imaging and analysis of four painted Peruvian textiles from the collection of the National Museum of the American Indian (NMAI). The four painted textiles are a subset of a technical study of 21 Peruvian textiles as a part of Rebecca Summerour's Mellon Fellow Research Project looking at the materials and manufacturing techniques used to create the textiles. The textiles have only minimal provenance and cultural attribution and the results of the preliminary phases of imaging and analysis will be used to determine some of the pigments used and if imaging techniques will be useful for pigment identification and elucidation of production techniques as well as identification/characterization of pigment and binder.

Imaging techniques included hyperspectral, multiband, reflected infrared digital photography, reflected ultraviolet, ultraviolet-induced visible fluorescence, and reflectance transformation imaging (RTI).

Analytical techniques included X-ray fluorescence spectroscopy (XRF), handheld Fourier transform infrared spectroscopy (FTIR), Fiber Optics Reflectance Spectroscopy (FORS) and imaging; and also destructive techniques including micro X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and high performance liquid chromatography-mass spectrometry. The analytical techniques were carried out by Jennifer Giaccai (FSG), Nicole Little (MCI), and Chika Mori (FSG).

Visible and Infrared Images of NMAI 239040

Visible and Infrared Images of NMAI 239040

Infrared Imaging, both reflected infrared digital photography and infrared reflectography, were completed on all four of the initial group of textiles. Infrared imaging did help with pigment differentiation and characterization and could provide additional information about the manufacture of the textiles. Whether or not the pigment was visible in the near infrared was telling about components of the colorant. For example in NMAI 239040 and NMAI 239076, the black colorant remained visible in the near infrared. Other techniques identified that the black included indigo, but infrared imaging indicated that it also included carbon.

RTI Setup Image

Keats Webb and Rebecca Summerour acquiring images for the RTI of NMAI 239073.

Ultraviolet Imaging, both the UV-induced visible fluorescence and the reflected UV, did not seem to add any additional information to the project.

Reflectance Transformation Imaging (RTI), was requested on one of the textiles to see if the technique would emphasize some of the resinous lines. RTI was completed for the entire view of of one of the texiltes and a detail of another. RTI, similar to just a single raking light image, further emphasizes the texture of the textiles, but did not provide any additional information about the pigment and application.

Multiband and hyperspectral imaging were the most useful techniques for image-based nondestructive characterization and differentiation of pigments and materials in the textiles.

Multiband imaging is similar to multispectral and hyperspectral imaging (reflectance imaging spectroscopy techniques), however for this project it refers to the acquisition of uncalibrated images with bandwidths of 100's nm that are captured using a modified digital SLR camera and bandpass filters. Similar to hyperspectral and multispectral imaging, multiband imaging captures characteristic spectral information about objects, however the uncalibrated image sets cannot produce reflectance spectra. A modified DSLR was used for multiband imaging with a set of bandpass filters with bandwidths in the hundreds of nanometers.

Hyperspectral Imaging of NMAI 220497

Screenshot of the SAO DS9 interface reviewing the hyperspectral imaging data from NMAI 220497.

Multiband imaging proved useful for characterizing select materials, especially when used in combination with hyperspectral imaging or fiber optics reflectance spectroscopy. Multiband imaging was useful for highlighting the consistency of materials throughout each textile because each image can encompass the entire textile, rather than small select areas.

Hyperspectal imaging was carried out using a hyperspectral imaging system with a CCD sensor and spectral sensitivity from 400-1000nm. The imaging system we have acquires 128 images between 400nm to 1000nm creating a data cube that can provide reflectance spectra for pixels or areas of interest, as well as images of the area analyzed at a particular wavelength.

Indigo was successfully characterized with both multiband and hyperspectral imaging. The image subtraction with multiband imaging accurately mapped the location of indigo over an entire textile. The image subtraction alerted researchers that the black colorant in the Chimu Capac or Ancón style textile included indigo, which was supported by the hyperspectral imaging and FORS spectra. This technique using image subtraction with multiband images is likely to be useful for characterizing indigo on other types of objects, such as ceramics or paintings. The imaging did not provide conclusive information for the identification of red or brown colorants.

Visible and Image Subtraction of NMAI 239040

Left: Visible image of NMAI 239040. Right: Image subtraction result from image taken with a 735nm bandpass filter and 660nm bandpass filter. The result highlights the location of indigo in the textile.

Visible and Image Subtraction Detail of NMAI 220497

Visible and Image Subtraction Detail of NMAI 220497.

The multiband and hyperspectral imaging worked well as a means of characterization and identification for the painted Andean textiles because they contained a limited palate that did not include complex mixtures. For example, had ultramarine been present, the image subtraction processing would not have been successful as ultramarine reacts very similarly to indigo. This reminds us how vital our understanding of the historical and cultural context of the object is to our interpretation of image analysis.

The multiband and hyperspectral imaging worked well as a means of characterization and identification for the painted Andean textiles because they contained a limited palate that did not include complex mixtures. Multiband and hyperspectral imaging are important tools, which offer a non-destructive option for material characterization within the context of the entire object.

As of September 2014, this project was presented at two conferences and an abstract was submitted and accepted to a third.

  • "Technical Analysis of Four Archaeological Andean Painted Textiles" poster (text-version) submitted February 28, 2014 for publication in Nuevo Mundo, Mundos Nuevos Presentation was given in November 2013 by Rebecca Summerour. Authors: Rebecca Summerour (NMAI), Jennifer Giaccai (FSG), Keats Webb (MCI), Chika Mori (FSG), Nicole Little (MCI).
  • "A Case Study Using Multiband & Hyperspectral Imaging for the Identification & Characterization of Materials on Archaeological Andean Painted Textiles" presentation at the AIC Annual Meeting, Textiles Specialty Group, in San Francisco, CA, May 29, 2014 by E. Keats Webb. Postprints submitted for publication August 11, 2014. Authors: E. Keats Webb (MCI), Rebecca Summerour (NMAI) and Jennifer Giaccai (FSG).
  • "Technical Analysis of Archaeological Andean Painted Textiles" abstract submitted and accepted for the Dyes in History & Archaeology: 33rd Annual Conference October 29-November 1, 2014 in Glasgow, Scotland. Authors: Rebecca Summerour (NMAI), Jennifer Giaccai (FSG), Keats Webb, Chika Mori (FSG), Nicole Little (MCI).