Reflectance spectroscopy in the visible region was applied
to the study of paintings for the first time during the decade
from 1930 to 1940 . 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 . 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
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.
- Barnes N.F.: "A Spectrophotometric Study of Artists'
Pigments", Technical Studies in the Field of the Fine
Arts, 7 (1938), pp. 120-138.
- Bullock L.: "Reflectance Spectrophotometry for Measurement
of Colour Change", National Gallery Technical Bulletin,
2 (1978), pp. 49-55.
- Saunders, D. "The Measurement of Colour Change in
Paintings", European Spectroscopy News, 67 (1986),
- Bacci, M., Baldini, F., Carlà, R., and Linari, R.:
"A Color Analysis of the Brancacci Chapel Frescoes",
Applied Spectroscopy, 45 (1991), pp. 26-31.
- 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.
- 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.
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,
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.
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
- 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).