Mauro Bacci

Fibre optics applications to works of art

Abstract In recent years physical and chemical investigations on works of art have significantly grown both for obtaining information on the materials and techniques used by the artist, and for developing useful guidelines on restoration. The main difficulty, which is encountered when precious and unique objects are examined, involves performaing measurements in the safest way possible for the object. Therefore, we proposed, as the principal objective of our research, the development of totally non-destructive methodologies of investigation. Fibre optics reflectance spectroscopy (FORS), where fibre optics is simply used as an intrinsic optrode, is particularly suitable for our purposes, because it allows recording of reflectance spectra up to more than 2.0 μm without any damage to the art work, and therefore offering the possibility of a large sampling. This last point is particularly interesting in comparison to the common microanalytic procedures, which, on the contrary, can only work on a few samples at a time. We used FORS as a powerful tool for: (a) pigment identification by means of a correlation analysis with a ‘palette’ appropriately prepared in a laboratory; (b) analysis of colour changes induced in paintings by the environment; and (c) documentation of the colour before and after restoration. As an example, we report the results of our investigations on paintings of the Middle Ages and the early Renaissance (Giotto, Masaccio, Luca Signorelli).

A COLOR ANALYSIS OF THE BRANCACCI CHAPEL FRESCOES

About one hundred significant spots (area = 0.5 cm2) that were homogeneously colored were selected on the frescoes “Resurrection of the son of Teofilo,” “Saint Peter in pulpit,” and “Saint Peter heals invalids by his shadow” in the Brancacci Chapel (Chiesa del Carmine, Florence). The reflectance spectra of these spots were recorded before and after restoration with the use of an external integrating sphere, which was connected through optical fibers to a UV/visible spectrophotometer, and the data were stored in a personal computer. Spectrophotometric investigations were also performed on small samples of frescoes, which were appropriately prepared, in a laboratory, with pure pigments. The results we obtained are discussed in the light of our main purposes: (1) to achieve nondestructive identification of pigments; (2) to help create time-unalterable color archives; (3) to establish a method for monitoring eventual color changes; and finally (4) to obtain useful information for the fields of art restoration and art history.

IDENTIFICATION OF THE PRE-COLUMBIAN PIGMENT MAYA BLUE ON WORKS OF ART BY NONINVASIVE UV-VIS AND RAMAN SPECTROSCOPIC TECHNIQUES

Abstract UV-visible reflectance spectroscopy and Raman microspectroscopy provide a rapid way to unequivocally identify the pre-Columbian pigment Maya blue. Spectra of modern synthetic materials are compared with data from a contextualized archaeological sample and from an object in the collection of the Los Angeles County Museum of Art. UV-visible spectroscopy and Raman microspectroscopy, together with complementary techniques such as Fourier transform infrared microspectroscopy, reveal significant differences between spectra of indigo as pure crystalline solid or as complexed by palygorskite. These techniques are thus extremely specific, being able to identify Maya blue as a prepared pigment rather than detecting only its ingredients, indigo and palygorskite. Fiber optics UV-visible reflectance spectroscopy and Raman microspectroscopy present the additional advantage of being completely noninvasive and therefore suitable for the study of works of art. Lightweight portable fiber optics UV-visible reflectance spectroscopy devices make it possible to perform measurements in situ on wall paintings and other immovable objects. The spectral differences between pure indigo and the indigo-palygorskite complex can be interpreted in terms of different hydrogen bonding configurations for the indigo molecule.

Mid-Infrared Fiber-Optics Reflectance Spectroscopy: A Noninvasive Technique for Remote Analysis of Painted Layers. Part I: Technical Setup

Mid-infrared chalcogenide fiber optics coupled with Fourier transform infrared (FT-IR) benches has made it possible to perform noninvasive reflectance measurements of layers of paint. The technique has potential applications for noninvasive analysis of works of art. These measurements can present large distortions in the spectrum, both in band shape and absorption frequency, which may depend on the band strength, on the concentration of the sample, or on the optical layout of the measuring system. Therefore, it is difficult to compare reflectance spectra with those collected in the transmission mode and, consequently, with the available databases. The work deals with an overall survey of the limits and problems involved in the utilization of this analytical technique, an estimate of the reproducibility of the measurements, and the development of a correct measurement procedure.

Non-Invasive Identification of White Pigments on 20Th-Century Oil Paintings by Using Fiber Optic Reflectance Spectroscopy

Abstract Fiber optic reflectance spectroscopy in the 270–1700 nm range was used to identify white pigments typically used in 20th-century paintings and as restoration materials, such as lead white, zinc white, titanium white in the two crystalline forms anatase and rutile, lithopone, and kaolin. Use of the non-invasive, in situ technique was undertaken in accordance with the principles of minimum intervention, which guide the restoration and conservation of artworks. With this technique, a suitable spectroscopic database of the pigments was first created, using various instrumental set-ups. Subsequently, fiber optic reflectance spectroscopy was applied to the study of two paintings from the Galleria di arte moderna, Palazzo Pitti, Florence: Mercato by Elisabeth Chaplin, dating probably from the 1920s, and Renaioli, painted in 1950 by Bruno Rosai. The purpose was to better understand the evolution of the use of such white pigments during the past century, and also to provide useful information concerning questions of chronology.

Spectroscopic behavior of acid-base indicators after immobilization on glass supports

This paper describes an extensive spectrophotometric study on several acid-base indicators immobilized, by means of a silylation process, on quartz powder and on controlled pore glass. The spectrophotometric properties of the immobilized indicators are at variance with those of the free indicators with respect to pH dependence and ranges. The different behavior of both types of indicators in the liquid and immobilized phases may be explained by both a modification in the electronic structure of the indicators and a distribution of the molecules on different sites. The differences among the spectrophotometric properties of the indicators, when these are immobilized on different solid supports, can be justified by taking into account the steric hindrance and the reactivity of the indicators. Particular attention was paid to the pH-sensitive bands and their position, keeping in mind that the investigated material could possibly be used as a transducer for pH fiber-optic sensors.

Spectrophotometric investigations on immobilized acid-base indicators

Abstract Diffuse reflectance spectra of four acid-base indicators immobilized on Amberlite XAD-2 resin are reported as a function of pH. Adsorption modifies the acid-base properties of the indicator by broadening the pH intervals over which the dyes change colour and by shifting these intervals to higher pH values. The different behaviour in the liquid and immobilized phases may be explained by multilayered adsorption of some indicators. The occurrence of hysteresis during a pH cycle is reported.

Evaluation of the museum environmental risk by means of tempera-painted dosimeters

The monitoring and control of environmental parameters provide useful information in order to reduce the decay of works of art. Dosimeters that reproduce the pictorial structure of paintings as closely as possible were prepared and tested to evaluate the effects induced on works of art by the ‘‘global’’ indoor environment. These dosimeters were aged under different conditions in ageing chambers, and successively were placed in the Uffizi Gallery for several months. The data obtained from the naturally aged dosimeters were compared with those found for the artificially aged, thus providing information on the alteration process induced by the environmental factors. # 2000 Elsevier Science B.V. All rights reserved.

Library of UV-Vis-NIR reflectance spectra of modern organic dyes from historic pattern-card coloured papers.

An accurate characterisation of the organic dyes used in artworks, especially those made of paper, is an important factor in designing safe conservation treatments. In the case of synthetic organic dyes used in modern works of art, for example, one frequently encountered difficulty is that some of these dyes are not still commercially available. Recognizing this problem, the authors of this paper present the results of an analysis of UV-Vis-NIR fibre optic reflectance spectra of 82 samples of dyed paper prepared with 41 dyes. The samples come from a historic book, The Dyeing of Paper in the Pulp, which was published by Interessen-Gemeinschaft (I.G.) Farbenindustrie in 1925. The dyes used in the paper pulp belong to the azo compounds, acridine, anthraquinone, azine, diphenylmethane, indigoid, methine, nitro, quinoline, thiazine, triphenylmethane, sulphur and xanthene classes.

A new kind of oxygen-sensitive transducer based on an immobilized metallo-organic compound

Abstract A new kind of oxygen-sensitive transducer has been realized based on the reflectance change of bis(histidinato)cobalt(II) [Co(His)2] immobilized on a thin-layer chromatographic plate (TLC plate). This plate is further coated with a two-component silicone rubber layer in order to maintain a wet environment for the cobalt complex and to provide mechanical protection of the oxygen-sensitive layer. An experimental characterization of the sensor material shows it to be capable of detecting oxygen at levels as low as 0.2% (v/v) in an argon stream with rate constants of 12 s and 65 s for the oxygenation and deoxygenation process, respectively.