Natural Varnishes Research Articles

Evaluation of Deep Eutectic Systems as an Alternative to Solvents in Painting Conservation.

Natural deep eutectic solvents (NADESs) are considered as green solvents, and due to their promising sustainability, they have been applied in many research fields. In this study, the main goal is to use various NADES systems to replace the traditional solvents used in conservation and restoration to remove varnish layers in a painting. The toxicity of traditional solvents, such as toluene or acetone, is well known in the chemistry field. To replace them, it is important to understand the intrinsic physicochemical properties of a solvent that may act as a substitute. Polarity and solubility are proposed as the best parameters required for this study. The Nile red probe was used to confirm the similarity between the polarity of deep eutectic systems (DESs) and traditional solvents. According to their polarities and Hansen solubility parameters, it is possible to predict the best solvents to solubilize the natural resin varnishes. Besides this, some arithmetic models can also be applied to estimate the critical or thermodynamic properties, which are useful tools to predict the behavior of these solvents. We have further proven the possibility of dissolving natural varnishes such as dammar or mastic in hydrophobic DESs, such as menthol + lauric acid, menthol + decanoic acid, or menthol + thymol.

Drying Oil and Natural Varnishes in Paintings: A Competition in the Metal Soap Formation

Metal soaps formation is a well-known issue in oil paintings. Along the lifetime of the painting, carboxylic acids coming from drying oil (free fatty acids, acids from hydrolysis of triglycerides and from oxidation processes) can react with cations of some pigments (in particular, smalt, lead white and zinc white) forming the related carboxylic salts. As observed by many authors, the formation of these carboxylates, with the tendency to migrate and to aggregate, not only modifies the behavior and the aspect of the paint film but also complicates the cleaning approach. In previous works we have demonstrated that a similar pigment reactivity is possible even in presence of natural resins (such as colophony, dammar, mastic, etc) historically used as final varnishes on paintings. In this case, in the reactions the terpenic acids, among the main components of the resins, are involved. In this work, the carboxylates formation kinetics has been studied starting from two representative acids (palmitic and abietic) of painting oils and natural varnishes. Successively, the reactivity of the palmitic acid with the potassium abietate and of the abietic acid with the potassium palmitate has been verified. This investigation aims at clarifying in which way terpenic acids can be involved in the metal soaps reactivity confirming that also surface varnishes may play a significant role in the carboxylates formation and reactivity. It is important to keep in mind that a finishing varnish can be removed and reapplied many times during the lifetime of a painting, thus renewing the provision of reactive terpenic acids at the interface of the painted layers.

Varnishes with Biocidal Activity: A New Approach to Protecting Artworks

Keeping agents that cause biodeterioration under control is a common task in restoring and conserving artworks. However, there are very few studies, specifically those concentrating on pictorial works, that provide solutions compatible with the complex mix of organic and inorganic materials to be found in such cultural assets. This study aims to use biocide products that are commonly used in this field (benzalkonium chloride, o-phenylphenol, and tributyltin naphthenate) and which are associated with natural varnishes with a long tradition in artistic practices, in order to protect paintings and polychrome sculptures from environmental fungi and bacteria without altering the original materials or the visual appearance of the artworks. The effectiveness or innocuousness of the treatments was tested chemically and visually via optical microscopy, gas chromatography–mass spectrometry (GC-MS), and a multidimensional scaling analysis (MDS); the treatments produced good results as regards the inhibition of microbial growth and scarce interaction with the artistic materials being studied.

Preliminary study into the reduction and removal of naturally aged varnishes from painted surfaces using an Er:YAG laser in a two-step cleaning process with solvents

A preliminary study into the reduction and removal of naturally aged varnishes, aimed at collating observations to encourage further study of the cleaning of paintings with intractable, difficult to remove coatings, is presented in this article. Er:YAG laser cleaning trials with dry laser exposures followed by solvent removal of natural resin varnishes were conducted on a naturally aged test panel, and an expendable nineteenth-century oil painting. Surfaces were examined with normal and ultraviolet light, a headband magnifier, and low-power digital microscopy, videography and colorimetry. Standard solvent cleaning tests were first carried out, followed by a two-step cleaning protocol where Er:YAG irradiation of the coating was followed by solvent cleaning, the laser being used to promote a weakening or disaggregation of the topmost portion of the layer or layers of varnish. The aim was to assess the laser cleaning outcomes conservators might expect on ‘real’ as well as mocked-up surfaces; results are presented from one of each. Cleaned areas of paint were also directly exposed to Er:YAG irradiation to determine a safe fluence with observations limited to relatively low magnifications. Overall these efforts inform the risks and contribute to the development and the safe use of this novel cleaning method.

Laser cleaning of paintings: in situ optimization of operative parameters through non-invasive assessment by optical coherence tomography (OCT), reflection FT-IR spectroscopy and laser induced fluorescence spectroscopy (LIF)

Nowadays the use of laser technology as a highly precise tool for the cleaning of paintings is gaining ground. The development of a non-invasive analytical protocol aimed at thorough assessment of the treated surfaces and real time monitoring of the laser cleaning action is thus becoming imperative. This ensures that no side effects (e.g. discoloration, darkening, blackening) will occur on the painting surfaces due to laser ablation. In the present study the potential of the combined use of optical coherence tomography (OCT) and reflection FT-IR spectroscopy for in situ non-invasive assessment of laser cleaning procedures has been investigated on a historical easel painting donated to science. Specifically, OCT and FT-IR analyses were carried out before and after each cleaning test in order to carefully assess the condition of the painting surfaces upon their irradiation with a KrF excimer laser and evaluate the removal of weathered and/or non-original materials (i.e. waxes, aged natural varnishes, oxalates, deposition/migration compounds). This specific laser radiation, operating in the ultraviolet region (λ = 248 nm), is highly absorbed by coating materials (i.e. oxidized natural varnishes) and thus high selectivity and control of the cleaning process can be pursued. A systematic integration of the information on the chemical changes of the molecular composition of the painting surface provided by reflection FT-IR spectroscopy with the visualization of the stratigraphical changes by OCT allowed the evaluation of the sensitivity of the painting materials to different operative parameters of the laser treatment, such as laser fluence and number of pulses. In parallel, laser induced fluorescence (LIF) spectroscopic measurements, recorded with the same laser beam at significantly attenuated energy density values, were exploited to provide information on the fluorescence properties of the irradiated varnish surfaces. The combination of the results obtained by OCT and FT-IR complemented with those from LIF measurements are discussed with the aim of developing an efficient methodology for assessing in situ the irradiated painting surfaces after the laser cleaning treatments and, thus, setting the basis for a future monitoring scheme that would ensure optimum cleaning interventions.

Derivatization Technique To Identify Specifically Carbonyl Groups by Infrared Spectroscopy: Characterization of Photooxidative Aging Products in Terpenes and Terpeneous Resins

Analysis of bioorganic materials by infrared spectroscopy (FT-IR) is frequently limited due to overlapping of diagnostic bands from the various components, which poses a fundamental problem to this analytical technique. The distinction of oxidized di- and triterpenes, for example, is hindered by the superposition of similar absorption bands of carbonyl functional groups summing up to a broad, nondistinctive signal. This study presents a technique for selective fluorination of various carboxylic acids by exposure to gaseous sulfur tetrafluoride. The derivatization treatment leads to characteristic band shifts, allowing the separation of otherwise overlapping bands. Accordingly, the IR bands of primary acids, α,β-unsaturated acids, tertiary acids, peroxy acids, esters, ketones, and α,β-unsaturated ketones are split into distinct absorption bands. The capability of this method is demonstrated on the example of natural resins and their ingredients, which are commonly known to be susceptible to oxidation at ambient conditions. The derivatization method enables one to identify various carbonyl containing functional groups by infrared spectroscopy, even in complex mixtures of terpenes. It unveils previously hidden degradation reactions running in terpenes and natural resins exposed to artificial aging by irradiation with light. New insight is presented on the individual reaction pathways of the terpenes hydroxydammarenone and abietic acid as well as of natural resin varnishes made from dammar and colophony.

Photo-oxidative kinetics of solvent and oil-based terpenoid varnishes

Here, the photo-oxidative degradation of several terpenoid varnishes usually encountered in conservation of cultural heritage has been investigated. Samples were prepared by dissolving dammar, mastic, colophony, sandarac and bleached shellac in suitable solvents or by heating mastic, colophony, sandarac and Manila copal with linseed oil. The alteration effects of this broad set of samples induced by light exposure in an ageing chamber, were thoroughly characterized using transmission FTIR spectroscopy, colorimetry, and gravimetric measurements. The various varnishes exhibited a similar saturating photo-oxidative kinetics upon the exposure time but significant quantitative differences were pointed out. A decreasing of the methyl and methylene stretching bands was observed for both solvent and oil based varnishes, which suggests the alteration effects begin with hydrogen atom abstractions. The consequent reaction with oxygen, catalyzed by irradiation and temperature rise lead to the increase of carbonyl and hydroxyl functionalities, thus suggesting the formation of new compounds containing carboxylic, ketone and ester groups and lactonised structures. At the same time, cross-linking and extensive polymerization were clearly observed for sandarac and shellac solvent varnishes. Measurement and comparison of the photo-oxidative and discoloration effects of the present set of samples, provide a general picture of the ageing phenomenology of the natural varnishes, which can be usefully exploited in characterization and conservation of easel paintings and other artworks.

Assessment of in-depth degradation of artificially aged triterpenoid paint varnishes using nonlinear microscopy techniques.

The present work investigates the applicability of nonlinear imaging microscopy for the precise assessment of degradation of the outer protective layers of painted artworks as a function of depth due to aging. Two fresh and artificially aged triterpenoid varnishes, dammar and mastic, were tested. Nonlinear imaging techniques have been employed as a new diagnostic tool for determination of the exact thickness of the affected region due to artificial aging of the natural varnishes. The measured thicknesses differ from the calculated mean penetration depths of the samples. These nondestructive, high resolution modalities are valuable analytical tools for aging studies and they have the potential to provide unique in-depth information. Single photon laser induced fluorescence measurements and Raman spectroscopy were used for the integrated investigation and analysis of aging effects in varnishes.

Effect of photodegradation on the identification of natural varnishes by FT-IR spectroscopy

In this study, natural varnishes used in ancient painting techniques were recreated following ancient recipes. They were irradiated by artificial sunlight and analysed by FT-IR all along the accelerated ageing. The main natural resins used were: sandarac, copal, colophony, mastic and dammar in alcohol, spirit turpentine and oil media. This study aims to understand and follow the evolution of characteristic absorption bands for the differentiation of varnish types by FT-IR analyses. Kinetics and evolutions of the compositions of the varnishes during photo-ageing were also studied. Photodegradation reactions lead to the loss of many characteristic absorption bands and the apparition of new ones. To assess the specific fingerprints of different resins which do not depend on the degradation state, results were analysed by statistical treatment (Principal Component Analyses). This multi-disciplinary approach leads to the identification of new criterions including the decrease of specificity caused by the photochemical reactions.

An approach to the study of the fungal deterioration of a classical art material: Mastic varnish

Mastic, one of the best natural varnishes, is frequently used as protective and finishing layer or as component of oleo-resinous media in paintings, both in the past and currently. However, this resin is affected by complex deterioration processes which can change its characteristics and thus the visual aspect of works of art. The alteration processes caused by radiation have been widely studied, but there is a lack of information on the biodeterioration of this natural product. In this paper, fungi from collections as well as from oil paintings of the Fine Arts Museum of Granada (Spain) were inoculated onto slides covered with mastic. The samples, after an incubation period of 15 days, were analysed by gas chromatography-mass spectrometry (GC-MS) to identify the chemical changes undergone, and a visual monitoring of the samples was performed to determine the formation of mycelia onto solidified resins. Major changes were detected in Chrysonilia sitophila, Phoma herbarum, and P. chrysogenum, showing evidence of alteration processes caused or favoured by these microorgamisms.

Optical Characterization of Varnish Films by Spectroscopic Ellipsometry for Application in Artwork Conservation

The specific aim of this paper is to measure the optical constants of fresh varnish layers up to a thickness of 10 mum by spectroscopic ellipsometry. It is the first time that this technique has been used in artwork conservation and it may prove very promising due to its nondestructive character. Samples of fresh dammar varnish (natural resin) and Paraloid B72 (synthetic resin) applied on glass and carbon black acrylic paint were analyzed. Both varnishes were considered as perfect dielectrics, and the real part of their refractive index was described by the Cauchy model: n (lambda) = A + B/lambda(2) + C/lambda(4). The Cauchy coefficients for dammar varnish and Paraloid B72 were then determined for layers of known thickness. The ellipsometric data were fitted to a model, which includes a mixed varnish-air layer. The optical properties of this layer were calculated by the Bruggeman effective medium approximation. In the case of carbon black acrylic paint, another mixed layer (paint-varnish) was added to the model. The results are very close to the values given in the literature. Given the measurement reproducibility, the results show that ellipsometry can discriminate between dammar varnish and Paraloid B72. This is very important in artwork conservation studies, because it has been done by time-consuming, destructive techniques up to now. Future work includes measurements of other types of natural and synthetic varnishes, in an attempt to introduce a nondestructive method for picture varnish identification and aging studies.

Fine art painting characterization by spectroscopic ellipsometry: preliminary measurements on varnish layers

The long-term conservation of the original state of paintings requires high quality multi-spectral digital image technologies. CRISATEL Program addresses fine art professionals as primary end-users base. The analysis by spectroscopic ellipsometry of the varnish on fine art paintings will allow the curator to correct the effect of aged or glossy varnish on the digital images and to simulate the original state of the masterpiece through this superficial layer. Sets of varnish samples have been measured first at the University of Cyprus using a SOPRA GES5 Spectroscopic Ellipsometer and then at SOPRA facilities with a Fourier transform infrared spectroscopic ellipsometer. The capability to measure a varnish layer (thickness and optical index) on glass and to distinguish synthetic from natural varnishes has been proved. The goal of this paper is to review the first results obtained in the characterization of varnishes for fine art paintings, and to present some problems that are still open and can represent a challenging research field for the future.

Industrial hygiene characterization of automotive wood model shops.

A suspicion of an excess cancer risk in automotive model shops prompted the Industrywide Studies Branch, NIOSH, to conduct a proportionate mortality study and an industrial hygiene characterization of operations in these shops. The mortality study showed a statistically significant excess proportion of deaths due to colon cancer and leukemia (for woodshops only). The materials used in the model shops include various natural woods, laminated woods, plastics, resins, varnishes, putties and paints. Personal breathing zone samples were collected for total and respirable dust, amines, various hydrocarbons (including styrene, and toluene), formaldehyde, and nitrosamines. Particle size distribution studies were conducted on the wood dust and bulk airborne samples of dusts were subjected to various mutagenicity test systems. Work practices, ventilation and general housekeeping were checked. Total wood dust samples ranged from 0.03 to 25 mg/m3 with an average around 1.0 mg/m3. The percent respirable dust ranged from 19 to 38% as measured with Andersen impactors. Solvent exposure samples ranged from non-detectable to about 10% of the OSHA Permissible Exposure Levels. Relevant recommendations for improvement of contaminant control were made.

Some Picture Varnishes

A review of recent research at the National Gallery, London. Experiments have shown insolubility to occur through cross-linking of certain polymethacrylate varnishes. Accelerated tests have produced highly insoluble films, but the process has been found to have begun after 6 years of normal museum conditions. The natural resin varnishes and AW-2 are compared for tendencies to bloom, wrinkle, and yellow. Lastly there are some notes on optical properties.