Second and third harmonic generation measurements of glues used for lining textile supports of painted artworks
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- Filippidis, G., Melessanaki, K. & Fotakis, C. Anal Bioanal Chem (2009) 395: 2161. doi:10.1007/s00216-009-3060-x
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In this study, we present the implementation of non-linear spot measurements for obtaining specific and novel information related to various types of natural and synthetic glues used for lining of painted artworks. Third harmonic generation measurements were employed, in transmission mode, for the accurate and non-destructive thickness detection of lining glues. Furthermore, second harmonic generation signals were collected, in reflection mode, providing complementary information for the discrimination between different types of lining glues.
KeywordsSHGTHGLining gluesPainted artworks
An essential problem in conservation of painted artworks is the removal of old linings. In this procedure, a piece of fabric, that has been glued for stabilizing reasons on the verso of the original “textile support” of the painting, needs to be replaced due to deterioration effects [1, 2]. The original “textile support” of an artwork is a fragile and complex structure mainly consisted of cellulose (textile, animal glue and filler like gypsum). “Textile support”, due to certain properties of cellulose, aged and deteriorates . Cellulose absorbs atmospheric oxygen and radiation, and is under attack by acid sources (such as CO2) and microorganisms. Those processes initiate chemical reactions and with the catalytic presence of humidity, “textile support” oxidizes and decays. These effects accelerated with the time, forcing “textile support” to lose its elasticity, firmness and become brittle. Tensions and deformations are created to the painted layer, and the “textile support” is no longer able to carry and stabilize the delicate painted surface . This fact raises the need to take actions and protect paintings against damage by enforcing the old “textile support”. To achieve that, an additional textile is glued on the verso of the original one using a very delicate and complex technique called lining, which is traced back to the seventeenth century .
It is well known that initially natural adhesives were applied in lining procedures. Natural adhesives are organic materials extracted from animals such as glutoline, bone, casein glues and from plants such as waxes, starch flours, gums and natural resins . These materials are under attack by microorganisms, a procedure that leads to discolor effects and looseness of their mechanical properties. Thus, natural adhesives are not currently in use in conservation.
In the early 1900s, the discovery of synthetic resins, polyvinyl acetates, acrylic solutions and microcrystalline waxes open the road for the replacement of natural adhesives . Aging resistance, controllable setting, hardening procedure, improved adhesion and flexibility are some of the main advantages of synthetic materials .
However, the lining glues and the lining textiles also deteriorate and usually suffer from fresh deformations, after a certain period of time, rising up the need to be replaced with new ones, an operation called relining . This conservation stage take place in two steps: a relatively easy operation, the separation of the lining textile from the old “textile support” by employing mechanical methods and a more elaborate one, the removal of the remaining lining glue via chemical and mechanical procedures. In order to assist and guideline, the latter step is crucial to understand and categorize the adhesives used and also be able to measure their thicknesses. In this respect, non-linear (second and third harmonic generation) spot measurements were applied to explore the feasibility of precise thickness detection and composition identification of different natural and synthetic lining glues.
Second and third harmonic generation (SHG–THG) processes are scattering phenomena. Two or three photons of angular frequency ω are destroyed and a photon of angular frequency 2ω (for SHG) or 3ω (for THG) is simultaneously created in a single quantum-mechanical process. SHG and THG modalities present the capability of intrinsic three-dimensionality, high axial resolution, the ability to section deep within the sample and the reduction of “out of focal plane” photobleaching in the specimens. Furthermore, optical higher harmonic generation does not deposit energy to specimens due to its energy-conservation characteristics, providing minimum sample disturbance (e.g. thermal, mechanical side-effects) which is desirable for art conservation studies.
SHG and THG microscopy measurements are mainly used as tools for the in vivo imaging and mapping of sub-cellular biological structures and processes. SHG modality provides information related to stacked membranes and arranged proteins with organized structures, such as collagen [4–6]. In addition, SHG is a useful technique for probing membrane-potential-induced alignment of dipolar molecules . THG is proven to be generated from regions with optical inhomogeneity  and is used for probing structural and anatomical changes of biological samples at cellular or sub-cellular level [9, 10].
More recently, non-linear imaging techniques were used as diagnostic tools for the precise detection of multilayer structures of painted artworks . Specifically, via the detection of THG and multi-photon fluorescence (MPEF) signals from model painted artworks, the thickness determination of varnish-protective layers and the identification of the chemical composition of painting materials were achieved . Detection took place in both transmission and reflection mode, demonstrating the ability of this non-destructive technique to be applied on the evaluation of original artworks.
In this study, higher harmonic generation (SHG–THG) spot measurements are employed for the extraction of novel information related to thickness determination and composition discrimination of various types of lining glues used in painted artworks.
The samples were placed to a round glass slide of 35 mm diameter and ~45 μm thickness (Marienfeld) that fit into a motorized xyz translation stage (8MT167-100-Standa). The minimum step of the stage in each direction was 1 μm. The focal distance between the objective lens and the sample was controlled by the motorized stage employing a specially designed software (National Instruments, Labview 7.1). The recording time was 10 ms in every step. A CCD camera (PLA662-PixeLINK) was used for the observation of the samples. The average laser power on the specimen was 30 mW (0.6 nJ per pulse). No damage in the sample was observed at this power.
THG signals were collected and collimated in transmission mode by a condenser lens (Plan-Apochromat ×100 NA 1.4 Carl Zeiss). After passing through a 340-nm colored glass filter (U 340-Hoya), the signals were sent to a photomultiplier tube (Hamamatsu H9305-04) connected to a lock-in amplifier (SR810-Stanford Research Systems).
SHG signals were collected in the backward direction using another photomultiplier tube (PMT Hamamatsu R4220) connected to a lock-in amplifier. The photomultiplier tube was attached at the position of the microscope eye-piece. A 514-nm interference filter (F03-514.5-CVI) was placed at the photomultiplier input.
Eight different samples representing natural and synthetic lining glues, casted on thin cover-slips, were analyzed. All the samples were fresh.
Sample 1 is flour paste, the most common used adhesive. Flour consists of starch and gluten, and when combined with boiling water forms a thick and viscous paste. It is stable and remains water-soluble after drying .
Sample 2 is rabbit skin glue. Rabbit skin is impure gelatine with some lower molecular weight residues of collagen, keratin and elastin. It forms top-quality and high viscous glue when diluted in warm water, and remains water-soluble after drying .
Sample 3 is starch paste. It is an adhesive prepared by mixing one part of flour paste, one part of rabbit skin glue and some drops of linseed extracts and garlic juice . Those materials, once added to boiling water forms a thick, viscous paste. It forms a strong and water-insoluble paste, which can be removed only with the aid of enzymes, after drying.
Sample 4 is Beva 371 a synthetic wax. It is a thermoplastic elastomeric polymer mixture, containing ethylene vinyl acetate (EVA) resin with a variety of waxes and ketone resins .
Sample 5 is Lascaux 498 HV acrylic adhesive. Lascaux 498 HV is a water-based emulsion of thermoplastic acrylic resin containing butyl acrylate thickened with methacrylic acid .
Sample 7 (Fluor-Lascaux) is a combination of synthetic and natural materials and is prepared according to a modern recipe of flour paste . A relatively small amount of Lascaux 498 HV is added to the flour paste (15 grams to 500 cm3 of paste) forming a water-soluble and a very stable adhesive.
Finally, sample 8 is Vinavil, a synthetic adhesive. Vinavil is polyvinyl acetate water dispersion. It is a water-soluble material (in contrast to sample 6) and upon drying remains soluble in ethanol/water mixture .
Complementary information related to the composition of the glues is acquired via the realization of SHG measurements. High SHG signals are collected from the flour paste of sample 1 (Fig. 2). The main ingredient of the flour paste is starch. Starch has been shown to have a naturally high χ2 coefficient [15, 16]. The starch granule, a highly birefringent structure, consists of crystalline amylopectin lamellae organized into effectively spherical blocklets and large concentric growth rings . It is feasible that these optically active structural features are responsible for the strong SHG signals arising from the starch-based samples (like flour paste). Similarly, high SHG signals are obtained from starch glues (data not shown).
Consequently, THG represents a new qualitative way for the precise detection of the thicknesses of various types of natural and synthetic glues. Furthermore, the non-destructive modality of SHG allows the discrimination, rather than the determination, of the composition between different lining glues.
In the current study, THG signals were detected in transmission mode while SHG signals were collected in reflection mode. However, since THG measurements can be realized in reflection mode , the non-destructive technique of optical higher harmonic generation (SHG–THG) can be integrated in a compact, novel instrument with the capabilities of diagnosis and laser cleaning for art restoration purposes. The great degree of axial resolution, the increased penetration depth and the minimal sample disturbance that second and third harmonic generation diagnostic modalities present, are essential for the accurate online control of any cleaning procedure and the elimination of the damage effects on the exposed painted surfaces.
Non-linear modalities (such as SHG–THG) comprise powerful, non-destructive, diagnostic tools that provide detailed information regarding lining glues thicknesses and composition. Eight different types of glues have been investigated. THG modality is an appropriate technique for the precise determination of the thicknesses of the adhesives. SHG measurements attribute complementary information related to the composition of glues (existence or absence of collagen and starch granules). Consequently, these diagnostic non-linear modalities provide essential, unique information for the assessment of the appropriate conservation method that has to be followed on the lining “textile support” of a painted artwork.
Furthermore, we have to note that the compact size of the employed excitation source and the reduced time of data acquisition (each measurement lasts less than a minute) make this innovative technique ideal for in situ laser diagnosis of painted artworks.
This work was supported by the UV Laser Facility operating at IESL-FORTH under the European Commission “Improving Human Research Potential” program (RII3-CT-2003-506350) and by the Marie Curie Transfer of Knowledge project “NOLIMBA” (MTKD-CT-2005-029194).