Original articleDiscrimination of painting binders subjected to photo-ageing by using microspectrofluorometry coupled with deconvolution analysis
Introduction
Paint layers typically consist of inorganic and organic pigments dispersed in an oil- or protein-based binding medium and applied over a preparation layer. Other organic substances are usually present in contiguous layers, and comprise animal glues in the preparation layers and natural resins in the varnish layers [1]. The difficulty in the mechanical separation of the various contiguous layers, as well as the reduced amount of the sample to be collected for the analysis, make identification of the binding media one of the most challenging steps in the diagnostic investigation of paintings.
Oil is a natural product that can be pressed out of a variety of plant seeds. Some types of oils, known as drying oils (e.g. linseed oil), form a solid film when exposed to air due to oxidation and cross-linking reactions, thus they assume good working properties as binding media. The protein-based materials mainly employed include: egg, used whole or after separation of the yolk from the glair; animal glue, extracted from skin or bones; milk, which is usually treated in order to isolate the casein fraction.
Binding media are subjected to ageing processes that are typically represented by physical, chemical and biological phenomena [2]. Photo-oxidation is one of the degradation mechanisms most involved in the ageing of painting media [3]. Some proteinic binders having an heterogeneous composition, such as egg, can also experience a variety of reactions among the various components of the medium [4]. Lastly, humidity, pH gradients and chemical interactions with the pigments can play important roles during ageing processes [2]. All these factors can lead to dramatic changes in the chemical composition of binding media within the paint layer, thus making their identification quite difficult. Hence, a further analytical problem adds to the previously mentioned ones for studying the original techniques of old paintings, as well as for planning their correct restoration.
Fluorescence spectroscopy is a well-developed and powerful technique that was proposed in the past as a practical tool for investigating the chemical composition of painting materials [5], [6]. Indeed, when exposed to ultraviolet excitation, many compounds found in oil- and protein-based paints can exhibit different colours and intensities of fluorescence, which are closely correlated to their chemical nature.
Ultraviolet sources have been applied to the examination of old pictures ever since they became commercially available around the 1920s. Nevertheless, no studies of interest on the fluorescence of paintings have been published since the pioneer works of Eibner [7] and Wolf and Toeldte [8] appeared around 1930. The most significant progress was made in the 1980s by de la Rie [9], [10], [11]. He studied the fluorescence properties of linseed oil and natural resins submitted to different periods of artificial ageing using a fluorescence spectrometer equipped with a mercury lamp. In further studies, several laser sources were employed to excite the endogenous fluorophores of the paints [12], [13], [14]. An excimer and a Nd:YAG laser coupled with a spectrometer to collect emission spectra was recently proposed as a non-destructive and potentially portable system [15], [16].
A different approach is represented by microspectrofluorometric analysis. This technique has been applied to investigations of organic binders as a promising tool for coupling the sensitivity of fluorescence measurements with the spatial resolution of a microscope [17], [18]. By using this technique, it is possible to carry out stratigraphic analyses of paint samples in order to obtain important information on the nature of the components, in addition to their distribution within the layers.
However, it has been observed that the fluorescence properties of organic binders vary depending on their physical state and ageing time. Protein-based binding media in wet form are poorly fluorescent when excited in the UV A (320–400 nm range) (P. Matteini, personal communication). The same behaviour has been observed for linseed oil, the most popular drying oil [10]. After drying, several alterations in the components of the medium take place, leading to an increase in the fluorescence signal that can be easily acquired by a microspectrofluorometric apparatus [17].
Fluorescent products found in aged proteins may derive from the oxidation of amino acids [19]. Nevertheless, in the presence of oxidative lipids (e.g. in egg yolk), other reactions associated with fluorescence products, can take place [20].
Under UV excitation, dried linseed oil shows a bright signal, which grows and shifts to longer wavelengths during ageing [11]. This behaviour has been attributed to the formation of soaps between free fatty acids and the metal ions of the pigments [7]. Furthermore, lipid oxidation can also lead to the formation of several conjugated Shiff bases [10], some of which have been suggested to have fluorescence properties [21].
The aim of this work was to assess the capability of microspectrofluorometry to identify and distinguish between different binding media used in works of art, by investigating their endogenous fluorescence properties in the presence or in the absence of mineral pigments. Particular attention has been devoted to the effects produced on fluorescence by accelerated photo-ageing process. This was done in order to evaluate the response of this technique on old paintings and “fill in” the lack of knowledge in this field. To achieve this objective, the emission responses of various organic binders, both on thin film cast onto microscope glass slides and on cross-section of paint layers, were analysed. A standard epifluorescence microscope equipped with a spectrum analyser was used to detect the emission spectra. Moreover, a curve-fitting deconvolution of the fluorescence spectra was performed in order to better discriminate the responses obtained from the different binders investigated.
Section snippets
Samples preparation and photo ageing procedure
Binding media were prepared following artist's accounts and historical recipes [1] as much as possible. Animal glue was swollen in water overnight and then warmed at 40 °C to give a 20% solution (w/w). Whole egg and commercial partially defatted milk were diluted in water (1:1). Linseed oil (boiled with catalytic quantities of cobalt salts) was purchased from Zecchi (Florence, Italy) and used without dilution.
Pure pigments (azurite, malachite, red ochre and white lead) were purchased from Zecchi
Assessment of fluorescence and re-absorption effect of the pigments
Preliminary tests were performed to verify the fluorescence properties of the inorganic pigments considered in the study, namely azurite, malachite, red ochre and white lead. All the pigments lacked fluorescence or showed a very weak signal when excited at 365 nm wavelength, even at the longest acquisition time used for the measurements (5 seconds). Hence, their contribution to the fluorescence of paint samples was considered to be negligible. These observations are in agreement with an earlier
Conclusions
This work points out that microspectrofluorometry coupled with deconvolution analysis can be a useful tool for distinguishing between oil- and protein-based media found in art works. In addition, accurate curve fitting can provide suggestions with regard to the different proteinaceous materials by highlighting the small differences in their fluorescence properties upon ageing. Due to its easy use, short times required for the analysis, and low cost, the proposed technique could be employed as a
Acknowledgements
We wish to thank Cinzia Mancuso and Maria Antonietta Gallone for helpful discussions.
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