·Table of Contents
·Conservation and Restoration in Art and Architecture
Identification and Classification of Natural Organic Binding Media and Varnishes by Micro-Raman Spectroscopy
Peter Vandenabeele, Luc Moens,
Ghent University Laboratory of Analytical Chemistry Proeftuinstraat 86, B9000 Ghent (Belgium)
University of Bradford Department of Chemical and Forensic Sciences Bradford BD7 1DP West Yorkshire(United Kingdom)
Martine De Reu and Guido Van Hooydonk,
Ghent University Central Library Rozier 9, B9000 Ghent (Belgium)
Many of the problems in the conservation and restoration of ancient masterpieces are caused by the degradation of the binding medium or the varnish layer. Numerous different natural organic binding materials have been used; therefore, the identification of these materials is of high importance.
For the characterisation of binding media with separation methods, such as gas chromatography (GC), liquid chromatography (HPLC) and capilary electrophoresis (CE) generally a large sample is required (typically more than 100 µg). Moreover, the extensive sample preparation and the multi-parameter operating conditions make alternative methods desirable. Raman spectroscopy is a nondestructive spectroscopic method, which can be used for the identification of small samples (typically less than 1 µg) of the materials used in objects of art, such as inorganic and organic pigments and natural binders and varnishes. The Raman-spectra are a fingerprint of the molecular structure of these materials, which can be used for their identification and for the study of degradation processes.
A classification of natural organic binding media and varnishes is proposed which is based on their characteristic chemical functionalities, as revealed by their molecular Raman-spectra . Four major groups can be distinguished, namely the proteinaceous binding media, the resins, the fatty acid containing materials and the polysaccharide binding media.
Ramanspectra are recorded using a Renishaw System-1000 spectrometer, which is equiped with a 780 nm diode laser, a 1200 lines mm -1 grating and a Peltier-cooled CCD detector, enabling the recording of Raman spectra with a spectral resolution of ca. 1 datapoint/cm -1. An Olympus BH-2 research grade microscope is used to focus the laser beam on the sample and to collect the backscattered Raman light. The system makes it possible to record spectra of inorganic and organic particles down to 1 µm.
Classification of Natural Organic Binding Media and Varnishes
There are many different ways to classify organic binding media and varnishes. In this work we use a classification based on the chemical composition and try to retrieve this classification by the spectroscopic examination of several samples. As natural media consist of a complex mixture of components, the classification is not always straightforward. Moreover, for some applications mixtures of different media are used. Despite this, the unmixed natural products can be classified as belonging to:
- proteinaceous media,
- resinous materials,
- fatty acid containing binding media or
- polysaccharide media.
Many animal materials are applied as a binding medium or glue and many proteinaceous binding media have been applied in tempera paintings. Examples are egg white, casein and gelatin. Casein is a phosphoprotein that is a major constituent of milk. Ramanspectra of these natural products are presented in Fig. 1. Characteristic Raman bands for this type of binding media are the amide I (v(CONH), ~1666 cm -1) and amide III bands (~1245 cm -1). Between these Raman bands an intense feature is observed at ca. 1450 cm -1, which is attributed to the CH2 scissoring mode. All these proteins have a sharp Raman band around 1000 cm -1, which has been assigned to the aromatic ring breathing of phenylalanine.
Fig 1: Raman spectra of some proteinaceous binding media (baseline-corrected): a. Egg white (after pre-treatment with UV radiation), b. Casein, c. Gelatin. Marked Raman bands are Amide I and III vibrations and the aromatic ring breathing of the phenylalanine amino acid.|
Although these materials clearly belong to the same group of materials and these similar binding media have several common features in their spectra, by comparing the spectral properties it is possible to differentiate between the individual samples.
All the members of this heterogeneous group are of terpenoid origin. These media, which are mainly applied as a varnish, consist of a complex mixture of alcohols, carboxylic acids, ketones, polycyclic hydrocarbons and aromates. In Fig. 4, four Raman spectra of resins are shown. These materials all contain mainly terpenoid components with multiple C=C double bonds. A characteristic Raman band of these components is the v(C=C) stretching band of cis alkenes (~1650 cm -1). A weak band at ca. 1240 cm -1 can be attributed to the in plane C-H bending of cis alkenes. The broad band at ca. 1450 cm -1 can be assigned to the d(CH2) scissoring mode. As the resins consist of complex mixtures of terpenoid material, their spectra depend on this composition and it is straightforward to discriminate between the different types.
Fig 2: Raman spectra of four resinous binding media (baseline-corrected): a. Shellac, b. Dammar, c. Colophony, d. Congo copal. Marked Raman bands are the cis C=C stretch and the in plane d(C-H) bending vibration of cis alkenes.|
Fatty acid media
The most important binding media are the fatty acid containing binders. Linseed oil is a drying oil, while poppyseed oil is a semi-drying oil. The first type contains relatively more C=C double bonds (linolenic acid has 3 double bonds) and as curing occurs at these reactive centers, the latter dries slower. Beeswax is a complex mixture of saturated long chain aliphatic compounds (acids, alcohols, esters and hydrocarbons). This trend is easily seen in the Raman spectra (Fig. 3), by observing the intensities of the in-plane C-H bending of cis alkenes (~1240 cm -1) relative to the d(CH2)2 deformation (~1300 cm -1). The bands at ca. 1745, 1655 and 1450 cm -1 are assigned to d(C=O), v(C=C) and d(CH2) respectively.
Fig 3: Raman spectra of some fatty acid media (baseline-corrected):|
a. Linseed oil (fresh sample, 1st cold expression), b. Poppyseed oil (fresh sample, 1st cold expression), c. Beeswax. Marked Raman bands are v(C=O), v(C=C)cis , d (CH2), d (CH2)2 and the in plane d (C-H) bending vibration of cis alkenes.
Pollysaccharide media contain gums and starch. Gums are secretions from plants, but while the resins, they swell or dissolve in water. Raman spectra of potato starch, gum Arabic and cherry gum are presented in Fig. 4. This type of binding media is characterised by COC sugar ring vibrations. In spectra of unknown components, it is difficult to clearly identify these bands, as CC deformations overlap in this region. An obvious way for the characterisation of polysaccharides in natural binding media can be seen in the region between 1500 and 1800 cm -1: polysaccharides don't have (strong) Raman bands in this region, whereas the other natural organic binding media have strong and characteristic bands. By comparing the spectral features of the different polysaccharides, the different gums can be distinguished.
Fig 4: Ramanspectra of some polysaccharide binding media (baseline-corrected): a. Potato starch, b. gum Arabic, c. Cherry gum. Marked Raman bands are COC sugar ring vibrations.|
In this work, we have demonstrated the possibility of micro-Raman spectroscopy for the distinction between the different types of natural organic binding media. We can differentiate and by comparing the spectra within each group, the identification of the material itself is possible:
The proteinaceous media, such as egg white, casein and gelatin have distinct bands in their Raman-spectrum, that correspond to vibrations of the amide-functions.
- Resinous materials (like shellac, dammar, colophony and copal) are characterised by distinct C=C bands in their spectrum..
- The fatty acid containing materials have long-chain carboxyl groups, which correspond to bands at typical wavenumbers. This group consists of drying oils, semi-drying oils and waxes.
- The absence of C=C or C=O moieties in the spectrum, gives an indication for polysaccharide media, like starch or gums.
- P. Vandenabeele, B. Wehling, L. Moens, H. Edwards, M. De Reu and G. Van Hooydonk, Analytica Chimica Acta 407 (2000) 261-274.