Giovanni Verri

The exceptional near-infrared luminescence properties of cuprorivaite (Egyptian blue)

Cuprorivaite (CaCuSi(4)O(10), also known as Egyptian blue) exhibits an exceptionally high emission quantum efficiency in the near-infrared region (lambda(max) = 910 nm, Phi(EM) = 10.5%) and a long excited state lifetime (107 mus); these properties make it appealing for several applications in the fields of biomedical analysis, telecommunications and lasers.

The spatially resolved characterisation of Egyptian blue, Han blue and Han purple by photo-induced luminescence digital imaging

The photo-induced luminescence properties of Egyptian blue, Han blue and Han purple were investigated by means of near-infrared digital imaging. These pigments emit infrared radiation when excited in the visible range. The emission can be recorded by means of a modified commercial digital camera equipped with suitable glass filters. A variety of visible light sources were investigated to test their ability to excite luminescence in the pigments. Light-emitting diodes, which do not emit stray infrared radiation, proved an excellent source for the excitation of luminescence in all three compounds. In general, the use of visible radiation emitters with low emission in the infrared range allowed the presence of the pigments to be determined and their distribution to be spatially resolved. This qualitative imaging technique can be easily applied in situ for a rapid characterisation of materials. The results were compared to those for Egyptian green and for historical and modern blue pigments. Examples of the application of the technique on polychrome works of art are presented.

Non-invasive identification of organic materials in wall paintings by fiber optic reflectance infrared spectroscopy: a statistical multivariate approach

The aim of this study is to develop a method for the non-invasive and in situ identification of organic binders in wall paintings by fiber optic mid-FTIR reflectance spectroscopy. The non-invasive point analysis methodology was set-up working on a wide set of wall painting replicas of known composition and using statistical multivariate methods, in particular principal component analysis (PCA), for the interpretation, understanding, and management of data acquired with reflectance mid-FTIR spectroscopy. Results show that PCA can be helpful in managing and preliminary sorting of the large amount of spectra typically collected during non-invasive measurement campaigns and highlight further avenues for research. The developed PCA model was finally applied to the case of a Renaissance wall painting by Perugino assessing it predictability as compared to the interpretation of the single spectrum.

Application of the Kubelka–Munk Correction for Self-Absorption of Fluorescence Emission in Carmine Lake Paint Layers

The variations of the fluorescence emission of carmine lake travelling through an absorbing and scattering medium, such as a paint layer, were investigated by ultraviolet (UV)–visible absorption, fluorescence spectroscopy, and imaging techniques. Samples of the lake were studied in dilute and saturated solutions, on a reference test panel and a real case study. Relevant spectral modifications have been observed as a function of the lake concentration mainly consisting of a fluorescence quenching, red shift of emission maxima, and deformation of emission band. The application of a correction factor based on the Kubelka–Munk model allowed fluorescence spectra obtained in solution and on painted samples of known composition to be compared and correlated, highlighting that the fluorescence of the lake within paint layers is affected by both self-absorption and aggregation phenomena. This approach has been successfully applied on a painting by G. Vasari for the noninvasive identification of carmine lake. The results reported here emphasize the necessity of taking physical phenomena into account in the interpretation of the fluorescence spectra for a proper and reliable characterization and identification of painting materials in works of art.

Correction of Ultraviolet-Induced Fluorescence Spectra for the Examination of Polychromy

Ultraviolet-induced fluorescence spectroscopy is a commonly used technique for the characterization and identification of painting materials, such as organic binders and colorants. Its interpretation is strictly connected to both the experimental setup and an understanding of the physical and chemical interactions among materials in paint layers, which are commonly composed of a fluorescent organic binder and a pigment. When irradiated with ultraviolet radiation, the light emitted by fluorophores present in the organic binder undergoes several types of interactions, in particular scattering and absorption by neighboring pigmented particles and auto-absorption. As a result of scattering and absorption phenomena, the emission spectrum is deformed according to the physical properties of the surrounding pigmented particles. This can lead to shifts of the emission maxima and/or to the formation of apparent new emission bands. The extent of the modifications to the emission spectra, caused by auto-absorption and selective absorption phenomena, may lead to the erroneous characterization or identification of the fluorescent materials. As a consequence, the interpretation of the emission signal can be greatly compromised. A correction based on the Kubelka–Munk theory is proposed to evaluate the extent of the spectral distortion and is assessed on modern replicas of wall paintings of known composition. Although the model cannot be applied to all cases, qualitative distinctions between real and apparent emissions are achieved.

The application of visible-induced luminescence imaging to the examination of museum objects

Visible-induced luminescence imaging in the near infrared range (800-1700 nm) can play a key role in the spatial characterization of certain historical blue pigments (principally Egyptian blue, Han blue and Han purple). All three of these pigments show a very strong infrared emission when excited across the visible range. The setup required for this type of measurement comprises a recording device with some sensitivity to near infrared radiation and an excitation source in the visible range. Inexpensive and widely available excitation radiation sources that can be used for this application include fluorescent lamps and LEDs. While visible LEDs do not usually emit infrared radiation, commercially available fluorescent tubes may emit some stray infrared radiation. Although the presence of such stray infrared radiation may in some cases be considered beneficial, allowing the user easily to locate the presence of the pigments under investigation within the composition of the piece, it can be easily removed using a simple subtraction method. This method, based on the measurement of the reflective properties of the surface under investigation in the emission range of the luminescent pigments, is described. The emission results obtained for Egyptian blue, Han blue and Han purple are compared to those of a set of representative historical and modern blue pigments, including manganese blue, which was the only other pigment found to show detectable luminescence properties. Examples of the application of visible-induced luminescence imaging to archaeological objects of the Antonine period (AD 100-200) in the collections of the British Museum are also presented.

DIGITAL MAPING OF EGYPTIAN BLUE : CONSERVATION IMPLICATIONS

Abstract Egyptian blue was extensively used throughout the areas surrounding the Mediterranean from early dynastic Egypt until after the Roman Empire ended. A new, easily applied, technique to detect and map this pigment has been developed that takes advantage of its very strong photo-induced infrared luminescence. Using this method, single particles of pigment can be detected, even when concealed by layers of discoloured organic binding media, varnishes or inorganic patinas. A survey of sculptures, friezes, mosaics and wall paintings from the ancient cultures of the Mediterranean and the Middle East has revealed expected and unexpected occurrences of Egyptian blue and rediscovered lost decorative patterns. This technique not only offers information on the manufacture, decoration and condition of the object, but also provides an opportunity to map the use of Egyptian blue both geographically and temporally. These new perspectives on ancient polychromy can help to inform future preservation strategies and conservation treatments that take this new information into account.

Infrared Reflectography of the Mughal Painting Princes of the House of Timur (British Museum, 1913,0208,0.1)

Infrared reflectography of the Mughal painting known as Princes of the House of Timur (British Museum, 1913,0208,0.1) suggests at least four phases of overpainting, reveals previously unseen inscriptions, clarifies issues of iconography, and provides parallels with workshop practice—in particular, with bookbinding and book illustration.

A New Application of Fiber-Optic Reflection Spectroscopy (FORS): Characterization of Low-Temperature Alteration of Chlorite Schist and Implications for Understanding Ancient Stone Cooking Vessels

Bulk thermal alterations to chlorite schist occurring at temperatures above 450 °C are traditionally studied using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, or thermogravimetric analysis (TGA), but lower temperature changes that lead to structural and chemical alteration, including changes in oxidation state that are not followed by a crystalline modification, are not detectable using these techniques. In this paper we present the results of the first study to examine the application of fiber-optic reflection spectroscopy (FORS) for the detection and quantification of low-temperature thermal alterations in chlorite schist. Such changes have been observed during research on the thermal behavior of medieval (12th–13th centuries AD) chlorite cooking pots from the archaeological site of Merv, Turkmenistan. FORS was used to investigate these changes, testing the archaeological samples against a model data set of experimental reference specimens. The results demonstrate the potential of FORS for tracking low-temperature thermal alterations and offer the opportunity to examine temperatures attained by ancient chlorite vessels during their past use in cooking activities.

A Note on the Examination of Silverpoint Drawings by Near-Infrared Reflectography

Abstract A group of 17 Italian fifteenth-century metalpoint drawings were examined using near-infrared reflectography and analysed non-invasively by X-ray fluorescence and Raman spectrometry. Metals, including silver and lead, reflect infrared (IR) radiation specularly and selectively. As expected, all of the lead-based metalpoint in this group of drawings proved to be entirely opaque when examined by IR reflectography. However, the silver-based metalpoint found on these drawings was observed to be wholly or partially transparent to infrared radiation in the 800–1700 nm range. The results obtained on drawings were compared with those from reference material of known composition. The response of silver-based metalpoint to infrared radiation is here attributed to the presence of silver corrosion products, particularly silver sulphide (Ag2S), a commonly encountered degradation product of silver. This compound shows high transmission in the spectral range under investigation.