Fiber Optics Reflectance Spectra (FORS) of Pictorial Materials in the 270-1700 nm range

Introduction

Reflectance spectroscopy in the visible region was applied to the study of paintings for the first time during the decade from 1930 to 1940 [1]. This methodology was subsequently developed at the Conservation Laboratories of the National Gallery, London [2,3]. Starting from the beginning of the Eighties, portable spectrophotometers equipped with optical fibers that operate in the visible and near infrared regions have been extensively used and improved at the Applied Spectroscopy Laboratory of the Institute of Applied Physics "Nello Carrara" (IFAC, Istituto di Fisica Applicata "Nello Carrara" formerly known as IROE, Istituto di Ricerca sulle Onde Elettromagnetiche "Nello Carrara") of the Italian National Research Council (CNR) [4,5].

Fiber optics reflectance spectroscopy (FORS) is a non-invasive technique that makes it possible to perform measurements on totally non-movable objects. It provides scientists and conservators with useful data for the identification of pigments and for the analysis of color and its variation on paintings. Although several variables may affect the reflectance spectrum acquired from a painting, FORS measurements provide, in most cases, information that permits recognition of the spectral features of pigments. The main problem faced in pigment identification, however, is building a suitable database of spectra of artist's materials for comparison. These samples must be prepared as closely as possible following the techniques and pictorial materials used by artists. Therefore, the present database was built with materials selected from those most commonly used both in the past and in the present day [6]. These pictorial materials were painted out onto small wood panels prepared with a traditional ground of gypsum and animal glue. They were applied as pure pigment/dye or as a mixture of different pigments/dyes with different binding media. Some of these pictorial materials were also applied as transparent glazes over opaque paint layers or metal leaf. The main purpose in making this sample collection was to create paint layer structures that follow techniques reported in historical sources.

All the pictorial materials used in the preparation of this sample collection have been previously tested at the Scientific Laboratory of the O.P.D. using IR spectrophotometric analysis. This collection was prepared in 1994, and the samples were then stored in a dust-free place.

The gray paints were obtained from diverse black and white pigments mixed in different amounts and the relative proportions were reported in brackets and expressed as a ratio of weights.

  1. Barnes N.F.: "A Spectrophotometric Study of Artists' Pigments", Technical Studies in the Field of the Fine Arts, 7 (1938), pp. 120-138.
  2. Bullock L.: "Reflectance Spectrophotometry for Measurement of Colour Change", National Gallery Technical Bulletin, 2 (1978), pp. 49-55.
  3. Saunders, D. "The Measurement of Colour Change in Paintings", European Spectroscopy News, 67 (1986), pp. 10-17.
  4. Bacci, M., Baldini, F., Carlà, R., and Linari, R.: "A Color Analysis of the Brancacci Chapel Frescoes", Applied Spectroscopy, 45 (1991), pp. 26-31.
  5. Bacci, M., Baronti, S., Casini, A., Lotti, F., Picollo, M., and Casazza. O.: "Non-destructive spectroscopic investigations on paintings using optical fibers", Materials Research Society Symposium Proceedings, 267 (1992), pp. 265-283.
  6. A. Aldrovandi, M.L. Altamura, M.T. Cianfanelli, P. Riitano: "I materiali pittorici: tavolette campione per la caratterizzazione mediante analisi multispettrale". OPD Restauro, 8 (1996), pp. 191-210.

FORS methodology

The reflectance spectra were acquired using two portable spectroanalyzers: the Zeiss MCS501 model, which operates in the ultraviolet (UV), visible (Vis), and short wavelength infrared (SWIR, 200-1000 nm) range, and the Zeiss MCS511 NIR 1.7 model, operating in the near infrared (NIR, 900-1700 nm) range. The spectral resolution of the two spectroanalyzers was of 0.8 and 6 nm/pixel, respectively.
Calibration was performed by means of a 99% Spectralon diffuse reflectance standard. A xenon lamp (mod. CLX 500) for the 270-820 nm range and a voltage-stabilized 20Watt tungsten-halogen lamp (mod. CLH 500) for the 350-1000 nm and 980-1700 nm ranges were used.
Two geometries of measurements were used: 2x45°/0° with the MCS501 (270-820 nm and 350-1000 nm ranges) and 45°/0° with the MCS511 NIR 1.7 (980-1700 nm range) spectroanalyzers. These two configurations made it possible to work in diffuse reflectance by collecting the light scattered at 45° with respect to the incident light, thus avoiding specular reflected light, from an area of 2 mm in diameter at a working distance of approximately 4.5 mm.
Indeed, since it does not interact with the bulk of the investigated matter, this specular component does not carry out any information on the actual chemical composition of the analyzed compounds. The depth of penetration of the radiation into the painted layers is related in general to several factors, which depend on the specific compounds analyzed: the refractive index, the absorption coefficient, particle size, and so forth as a function of wavelength. Hence, in the UV region the depth of penetration into the paint layers can be of a few microns while in the NIR region it can easily reach one hundred microns. Two bundles of pure fused quartz fibers (one bifurcated - "Y" shaped - consisting of 150 fibers, for sending the incident light that illuminates the sample from two different symmetrical directions, 2x45°; the other, linear consisting of 80 fibers, for receiving the back-scattered light) and two linear bundles consisting of 80 anhydrous quartz fibers were used to convey the light on both the surface under investigation and the detector when measurements were made using the MCS501 and the MCS511 NIR 1.7 spectroanalyzers, respectively.
The probe-head (made at IFAC) is a dark hemisphere, 2.5cm in diameter, terminating with a flat base and having three apertures on the dome. One aperture is at the top of the dome (for receiving the back-scattered light from the sample), and the other two are placed at 45°, symmetrically with respect to the former, for illuminating the investigated area of the sample. Owing to the rapid acquisition time of each single spectrum (a few hundred ms), each measured spectrum is the average of 3 acquisitions.

WARNING:

In the 980-1700 nm spectroscopic range the three typical absorption bands in the 1450 - 1550 nm region, which are evident in almost all the reported spectra, are basically due to the water molecules of the gypsum preparation layer. These bands may somewhat interfere with the absorption bands of other compounds/pigments, for instance with the first overtone of the O-H stretching of lead white pigment.

Materials and Samples preparation

  • Support: 30x200x15 mm wooden panels (plywood)
    • Preparation: rabbit skin glue dissolved in H2O (1:16 ratio) and gilders gypsum added until the glue reached saturation. This preparation was spread by brush on the support in two layers. The second layer was applied while the first was still wet.
    • Imprimatura: rabbit skin glue dissolved in H2O (1:32 ratio).
  • Pigments & dyes: purchased as powders from Zecchi - Colori.
  • Binding media
    • Egg tempera: 50% egg yolk, 25% egg white, 25% vinegar.
    • Linseed oil (stand oil) purchased from Zecchi - Colori.
    • Mastic: pea-like (tears) raw resin from Chios dissolved in turpentine (1:3 ratio).
  • Metal leaf: 80x80 mm 23 Karat gold foils and 100x100 mm 100% pure silver foils both purchased from Giusto Manetti Battiloro s.a.s.
  • Preparation of the ground for gilding with red and yellow bole Assiette à dorer purchased form Lefranc & Bourgeois and fish glue dissolved in H2O (1:20 ratio).
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