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Ageing behaviour and pyrolytic characterisation of diterpenic resins used as art materials: colophony and Venice turpentine
Journal of Analytical and Applied Pyrolysis 64 (2002) 345–361
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Ageing behaviour and pyrolytic characterisation of diterpenic resins used as art materials: colophony and Venice turpentine Dominique Scalarone, Massimo Lazzari, Oscar Chiantore * Department of Chemistry IPM, Uni6ersity of Torino, Via P. Giuria 7, 10125 Turin, Italy Received 1 August 2001; accepted 4 January 2002
Abstract The ageing behaviour of two diterpenic resins traditionally used as artists’ materials, colophony and Venice turpentine, was investigated with different spectroscopic and chromatographic techniques. In particular, three types of ageing (natural, artificial external conditions with a xenon lamp, artificial indoor conditions with fluorescent tubes) were applied to laboratory samples to study their effects on chemical structures. Thermally-assisted hydrolysis and methylation-gas chromatography-mass spectrometry (THM-GC/MS) was employed for careful characterisation and for identification of markers compounds on the vergin resins and in the course of ageing. The most significant changes were detected in the initial part of ageing, and the principal degradation products coming from oxidation, polymerisation and cleavage reactions identified. The high intensity of xenon lamp irradiation was found to cause, apart from oxidation and polymerisation reactions, further degradation of the chemical structure with molecular fragmentation. From the analytical point of view, the differentiation between colophony and Venice turpentine with THM-GC/MS appears to depend on differences in the lower molecular weight resin components. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Diterpenic resins; Colophony; Venice turpentine; Natural ageing; Artificial ageing; Thermallyassisted hydrolysis and methylation gas chromatography-mass spectrometry; Characterisation
Tribute to Professor Tsuge: Watching the scientific work Professor Shin Tsuge developed over the years has always been to us a pleasant duty, for the quality of results and the sound stimulating achievements. On this occasion it is therefore an honor to acknowledge his scientific accomplishments, sure that the retirement from Nagoya University will not be a retirement from pyrolysis investigation. With our best wishes. Professor Oscar Chiantore and Massimo Lazzari. * Corresponding author. Tel.: +39-011-670-7558; fax: + 39-011-670-7855 E-mail address: [email protected] (O. Chiantore). 0165-2370/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 7 0 ( 0 2 ) 0 0 0 4 6 - 3
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1. Introduction Varnishes based on terpenic resins have been used by old masters to protect paintings from dust, moisture and abrasions. Thanks to their excellent optical, adhesive and protective properties, natural resins are still used by many contemporary artists and restorers and are also frequently added as additives in the formulations of commercial painting media and adhesives [1,2]. Oil varnishes, prepared by mixing natural resins like sandarac, mastic or colophony with drying oils, have been commonly applied by artists from the 11th century. Starting from the 16th century they were gradually replaced by solvent, or ‘spirit’, varnishes, that are solutions of terpenic resins in volatile solvents. Because of their light stability and transparency, solvent varnishes made of dammar are considered the best ones [3]. Varnishes act not simply as a protective layer but also have an aesthetic function. They must be colourless, reversible and their viscosity must be adequate to form homogeneous films [4]. But even if a varnish is supposed to be stable, during ageing it undergoes physical and chemical changes which influence its properties: it becomes brittle and yellow, its solubility in common solvents decreases, and cracks develop on the surface. The understanding of the degradation mechanisms of natural resins is important to prevent the deterioration of old paintings and for the practise of restoration, but is also complicated by a number of factors. First of all, both di- and triterpenic natural resins are a mixture of different substances varying from volatile to polymeric compounds. During ageing, their chemical composition changes qualitatively and quantitatively because of oxidation, polymerisation, isomerisation and cleavage reactions. Finally, interactions with other painting materials, especially pigments, can influence the behaviour of varnishes and their degradation mechanisms. The diterpenic resins commonly used as artists’ materials were colophony, Venice turpentine, copals and sandarac. Diterpenes consist of four isoprene units. The C20 skeleton can have extra functional groups resulting in a great variety of diterpenoids which differ in the degree of unsaturation and in the number of oxygen atoms. Diterpenoids can be classified into two main groups: labdanes which are bicyclic molecules with an highly reactive unsaturated C6 side-chain; pimaranes and abietanes which are tricyclic acids (Fig. 1). The abietane-type molecules, having conjugated double bonds, easily undergo oxidation reactions [5]. Sandarac and copals are fossil resins which, apart from free diterpenoids, consist of a highly polymerised fraction of polycommunic acid or communic acid copolymerised with communol. Fossil resins are hard, tough and almost insoluble in common organic solvents. They can be mixed with oils only after heating and subsequent decarboxylation and depolymerisation of polycommunic acid. Colophony and Venice turpentine exude from trees belonging to the conifer subfamily Pinaceae. Colophony, also known as Greek pitch, is the solid residue obtained after distillation of resins of Pinus species. Venice turpentine comes
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Fig. 1. Chemical structures of some diterpenoid labdanes, pimaranes and abietanes.
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directly from Larix decidua. If spread as a thin layer, colophony darkens rapidly and becomes brittle and dull. For such reasons it was never used on its own, but frequently added to other resins or oils in order to modify some of their properties. At present colophony is often mixed with synthetic resins as an additive in order to increase adhesion, brightness and toughness. The fresh resin consists almost entirely of resinous acids differing in their oxidation behaviour. Sixty percent consists of abietane acids with highly reactive conjugated double bonds, 20– 25% of more stable pimarane acids and 5–10% of acids that are much prone to oxidation (dihydroabietic and tetrahydroabietic acids). When heated, abietane-type molecules isomerise giving a mixture rich in abietic acid, whilst laevopimaric acid almost disappears [6]. These reactions also explain compositional differences between fresh pine resins (Table 1) and colophony. The composition of diterpenic resins can be determined by GC/MS, after methylation of acid groups by diazomethane [6], or by thermally assisted hydrolysis and methylation (THM) with tetramethylammonium hydroxide (TMAH). By means of THM-GC/MS, Pastorova at al. [7] have identified the resinous acids of colophony and also some of their oxidation products. All the oxidised molecules have an abietane-type skeleton, while pimarane acids, lacking conjugated double bonds, are stable towards oxidation and isomerisation. The oxidation products of colophony can be also identified by HPLC/MS [8]. This technique is the most suitable to the analysis of termolabile compounds such as hydroperoxides, whose formation through photo-oxidation has been demonstrated in early studies on oxidation of abietane resinous acids [9–11]. Venice turpentine is often mentioned in ancient recipes for pictorial varnishes even if it darkens quite rapidly. Similar to the other resins exuded by Larix species, Venice turpentine is composed by abietane and pimarane acids, together with smaller amounts of labdane alcohols. The composition of Venice turpentine, determined by gas chromatography and derivatisation with diazomethane [12], is shown in Table 1. Previous studies [13] have demonstrated that also Venice turpentine, like colophony, undergoes oxidation processes, as one could expect on the basis of its chemical composition. For the chemical characterisation of Venice turpentine different types of on-line and off-line derivatisation reactions have been applied before the GC/MS analysis, and the potentialities of each method have been discussed. Methylation with trimethylsilyl (TMS)-diazomethane was successful on most diterpenoids, except for 15-hydroxy-7-oxo-dehydroabietic (DHA) acid, one of the most important markers of diterpenic resins. The best results have been obtained with trimethylsilylation using bis(trimethylsilyl)trifluoroacetamide (BSTFA). This reagent is effective on both acid and alcoholic groups, and it also allows the distinction between free diterpenic acids and diterpenic acid methyl esters resulting from ageing. On-line derivatisation with TMAH is a fast and simple method but is scarcely effective on hydroxyl groups and involves a number of secondary reaction (isomerisation, dehydration and cleavage of hydrolysable bonds) due to the strong alkalinity of the TMAH solution [13].
P. P. P. P. P. L. L. L. L. L.
pinaster halepensis syl6estris ponderosa palustris decidua gmelini russica occidentalis pendula
Dehydroabietic (DHA)
12 39 1.5 15 10 18.5 16.4 7.5 22.8 18.6
39 37 34 40 52 12.5 6.0 2.0 12 7.3
14 10 28 11 9 7.6 5.6 2.7 12.7 5.3
18 1.5 10 11 13 5.6 2.2 tr 5.1 1.6
4 – 4 8 8 1.8 5.1 0.5 7.7 8.6
– – – – – 7.5 13 12.8 4.4 10.7
– – – – – 3.1 23.7 – – 3.5
Larixol
2 10 3 3 1 1.0 1.9 1.5 1.4 1.6
Neoabietic
8.0 1.2 19 7.5 5.1 0.5 – tr 1.6 0.4
Abietic
Epimanool
Laevopimaric/ palustric
Sandaracopimaric
Pimaric
Isopimaric
Other components
Diterpenic acids
Table 1 Chemical composition of resins from Pinus and Larix [5,12]
– – – – – 33 16.6 – – 12.2
Larixyl acetate
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As far as their composition is concerned, Venice turpentine and colophony are very similar. Two characteristic markers of Venice turpentine are larixol and larixyl acetate, but in many cases, especially using on-line thermally assisted methylation with TMAH, they cannot be detected, as larixyl acetate hydrolyses and the methylation of hydroxyl groups is not so effective as for carboxylic acids. This long introduction was necessary to highlight that natural resins are complex chemical systems whose characterisation is difficult not only because of their chemical heterogeneity, but also for the different reactivities and ageing behaviour of the resin components. However their identification is important from the point of view of the knowledge of artists’ techniques and for the practice of conservation and restoration. THM-GC/MS is well suited to this scope because of its response specificity and the capability of obtaining structural informations from very small samples, in the microgram range, that is an absolute requirement in this field. In the present work we have focused our attention on the characterisation of colophony and Venice turpentine and we have applied THM-GC/MS to the study of fresh and aged resins. We have tested different light-exposure conditions in order to evaluate how degradation processes are affected by light ageing, and looked for the possible marker compounds that could be unequivocally used for resin recognition in real pictorial samples.
2. Experimental Colophony and Venice turpentine were purchased from Phase (Firenze, Italy). The samples for ageing were in form of thin films, cast from tetrahydrofuran (THF) or chloroform solutions. Samples for infrared analyses were supported on silicon wafers or KBr discs, while for all other analyses the support was a selected glass. A high-speed exposure unit Suntest CPS (Heraeus, Germany), equipped with a Xenon lamp and a UV filter that absorbs wavelengths lower than 295 nm, was used to simulate outdoor solar exposure. Irradiation was set at 765 W m − 2, and the maximum temperature on the samples was kept at 45 °C by forced air circulation. Under such experimental conditions it has been previously estimated that the rate of photo-ageing is approximately 260 times faster than natural indoor ageing [14]. A set of fluorescent tubes placed behind an ultraviolet filter to cut off radiation below 400 nm were used to reproduce indoor light exposure. The environmental conditions within the box were recorded as about 5 °C above room temperature and approximately 10% below ambient relative humidity (RH). The average light intensity was measured as 22,000 lux. Naturally aged samples, prepared in 1976 according to ancient recipes, were provided by Opificio delle Pietre Dure (Florence). The infrared spectra were collected with a Perkin–Elmer 1710 FTIR instrument with a DTGS detector and 4 cm − 1 resolution. Molecular characterisation of fresh and aged resins was performed with a modular size exclusion chromatography (SEC) system composed of a Waters (USA) M-45 pump, a Rheodyne (USA) 7010 injection valve, a differential refractometer ERC 7510 (Erma, Japan), and PL Gel
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type columns, 10 mm particle diameter. THF solutions were separated from solid impurities by filtration through 0.45 mm PTFE membrane filters. Calibration was obtained with PMMA molecular weight standards and o-dichlororobenzene. Pyrolysis experiments were carried out with an integrated system composed of a CDS Pyroprobe 1000 heated filament pyrolyser (CDS Analytical Inc., USA), a GC 5890A gas chromatograph (Hewlett Packard, USA) equipped with capillary column HP-5MS cross-linked 5% Ph Me Silicone (30 m× 0.25 mm× 0.25 mm), and a Hewlett Packard GC 5970 mass spectrometer. Degradation was carried out with the technique of THM with TMAH, as in this way adequate elutions and separations of the acid compounds in the samples could be obtained. For each analysis a few microliters of 25% aqueous TMAH solution (Aldrich, USA) were added with a microsyringe to the sample in a quartz tube. Sample amounts were on the order of a hundred micrograms. Pyrolyses were performed at 600 °C for 10 s. The pyrolyser interface was set at 300 °C and the injector at 280 °C. The GC column temperature conditions were as follows: initial temperature 40 °C, hold for 2 min, increase at 8 °C min − 1 to 150 °C, increase at 3 °C min − 1 to 280 °C. Helium gas flow was set at 1 ml min − 1. Mass spectra were recorded under electron impact ionisation at 70 eV electron energy, in the range from m/z 40 to 800. Pyrolysis fragments were identified on the basis of their mass spectra and mass library searches (Wiley 138 and NBS).
3. Results and discussion The ageing behaviour of the diterpenic resins has been investigated with infrared spectroscopy and size exclusion chromatography, while the identification of their components on both fresh and aged samples has been carried out by THM-GC/ MS.
3.1. Spectroscopic and size exclusion chromatographic characterisation By means of infrared spectroscopy it is possible to identify easily the different classes of natural compounds (waxes, oils, proteins, resins, etc.), but it is almost impossible to differentiate samples belonging to the same class [15]. The most important components of terpenic resins have similar chemical structures, and the few differences one can notice in the spectra of fresh resins generally disappear in aged ones. Detailed compositional information can be achieved by means of other analytical techniques, in particular gas chromatography coupled with mass spectrometry. In any case infrared spectroscopy is a useful technique to monitor the overall chemical changes occurring during ageing. For instance, comparing the relative intensities of hydroxyl and carbonyl absorptions, it is possible to detect oxidation reactions and molecular cleavages and to evaluate the kinetics of these phenomena. Comparing the IR spectra of fresh colophony and Venice turpentine (Fig. 2) we can notice a few differences: in Venice turpentine the hydroxyl absorption is higher
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Fig. 2. Infrared spectra of unaged and artificially aged colophony (a) and Venice turpentine (b).
than in colophony, whilst the carbonyl peak is lower. Apparently this is the result of the different resin compositions: in colophony the diterpenic fraction consists mostly of resinous acids, whilst in Venice turpentine there is also a remarkable amount of hydroxyl compounds, such as epimalool, larixol and larixyl acetate. The infrared analysis has pointed out that the most relevant molecular changes occur in the initial stage of ageing. Both resins undergo the same oxidation, polymerisation and degradation reactions, and after only a few hours of artificial ageing the infrared spectra of colophony and Venice turpentine are no longer distinguishable. The most important spectral effects of ageing shown in Fig. 2 are: increase of the hydroxyl absorption w(O – H) at about 3400 cm − 1; higher absorption in the region between 900 and 1300 cm − 1, due to w(C–OH) and w(C–O) of esters and acids; moderate increase in double bonds w(CC) at 1610 cm − 1; broadening of the carbonyl peak towards higher wavenumbers, due to new oxidised compounds (ketones, esters, lactones). At longer ageing time a decrease in absorbance within the entire range of frequencies considered has been noticed, attributed to the loss of volatile fragments formed during degradation. This hypothesis is consistent with the weight losses determined on the films during ageing and shown in Fig. 3. At two thousand hours of ageing, loss of volatile fragments results in a total weight loss of about 20%.
Fig. 3. Weight loss percentages of artificially aged colophony () and Venice turpentine ().
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Fig. 4. Infrared spectra of naturally aged colophony (a) and Venice turpentine (b).
The spectra of the naturally aged samples (Fig. 4) are in some ways different from those of artificially aged resins. Hydroxyl absorption is higher, and the component at 1610 cm − 1 denotes an high amount of double bonds. Different origins of the materials and sample preparation may partially explain such differences, but it is also possible that with the different ageing conditions some reactions are predominant in the time scale of the samples we are observing. The ageing of diterpenic resins leads to the oxidation reactions discussed in the previous paragraph, but also to polymerisation reactions. Polymerisation probably proceeds via a radical mechanism involving the double bonds of terpenic substances. Size exclusion chromatography has been used to monitor the molecular weight changes during ageing. SEC curves of Fig. 5 have been normalised so that the peak area represents the same amount of soluble sample differently distributed along the molecular weight scale. The most important effects of ageing which may be observed in the chromatograms are: (i) the fast disappearance of mono- and sesquiterpenic fractions in Venice turpentine (1); (ii) the decrease of the diterpenic component (2); (iii) the simultaneous formation of molecules with molecular weights in the range of diterpenic dimers (3); (iv) the formation of oligomers (4) with maximum molecular weights of about 2200 for colophony and 5000 for Venice turpentine. The changes in the weight average molecular weights of colophony and Venice turpentine are shown in Fig. 6. Obviously, since a PMMA calibration curve was used and the refractometer response is also dependent on molecular composition,
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Fig. 5. SEC curves of colophony (a) and Venice turpentine (b): unaged, artificially aged 25, 100, 500, 1000 h.
the calculated molecular weight averages should be regarded as approximate values. However the trend is correct and it shows that aged Venice turpentine consists of longer oligomeric fragments, probably coming from the polymerisation of labdane type molecules. Even if their shapes are slightly different, the chromatograms of naturally aged samples (Fig. 7) are in good agreement with the artificially aged ones. Average calculated molecular weights are 600 for the colophony sample and 850 for Venice turpentine.
3.2. THM-GC/MS The main terpenic compounds found in colophony and Venice turpentine are listed in Tables 2 and 3. THM-GC/MS has proven to be a well-suited technique for identification of volatile mono- and sesquiterpenoids, abietane and pimarane acids and their oxidation products. Except for the highly volatile components, the pyrograms of the two resins, both fresh and aged, are almost equal (Figs. 8 and 9). All resinous acids have been well separated and identified except for laevopimaric acid. It is known that laevopimaric acid is sensitive to light and heating [5,13] so its absence may be attributed to isomerisation reaction occurring during storage.
Fig. 6. Changes of weight average molecular weights with ageing time of colophony () and Venice turpentine (2).
b
a
20 21
17 18 19
15 16
Artificially aged with fluorescent tubes. Artificially aged under a xenon lamp.
p-Cymene Norchrysantemic acid methyl ester a-Terpineol Longicyclene Longifolene Caryophyllene Methyl pimarate Methyl sandaracopimarate Methyl palustrate Methyl isopimarate 1,2,3,4,4a,10a-hexahydro-1,4a-dimethyl-7-(1-methylethyl)-phenanthrenecarboxylic acid methyl ester Methyl dehydroabietate (DHA) Methyl abietate 1,2,3,4,4a-Hexahydro-1,4a-dimethyl-7-(1-methylethenyl)-phenanthrenecarboxylic acid methyl ester Methyl 7-methoxy-DHA 1,2,3,4,4a,9,10-Octahydro-1,4a-dimethyl-7-(1-methylethenyl)-phenanthrenecarboxylic acid methyl ester Methyl neoabietate Methyl 15-methoxy-DHA 1,2,3,4,4a-Hexahydro-1,4a-dimethyl-7-(1-methylethyl)-9-methoxy-phenanthrenecarboxylic acid methyl ester Methyl 7-oxo-15-hydroxy-DHA Methyl 7-oxo-DHA
1 2 3 4 5 6 7 8 9 10 11
12 13 14
Compound name
No.
Table 2 Terpenoids identified by THM-GC/MS of colophony
344 328
316 344 342
344 312
314 316 310
134 168 154 204 204 204 316 316 316 316 312
Mw
– ã ã – – tr ã
ã ã – ã ã ã ã
– – ã – – – –
ã ã tr
ã ã ã
ã ã –
– ã ã – ã ã ã tr tr tr tr
1000 hb
ã ã ã ã ã
ã ã ã – ã
600 ha
Artificially aged
ã ã ã – ã ã ã ã ã ã –
Unaged
– ã
– ã ã
ã ã
ã tr ã
ã ã – – ã
– ã – ã ã
Naturally aged
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b
a
29 20 30 21
17 18 19
15 16
Artificially aged with fluorescent tubes. Artificially aged under a xenon lamp.
330 344 330 328
316 344 342
344 312
314 316 310
136 136 136 136 135 154 204 204 222 222 316 316 316 316 312
a-Pinene b-Pinene D3-Carene Limonene Allocimene a-Terpineol Longifolene Caryophyllene Elemol b-Eudesmol Methyl pimarate Methyl sandaracopimarate Methyl palustrate Methyl isopimarate 1,2,3,4,4a,10a-Hexahydro-1,4a-dimethyl-7-(1-methylethyl)-phenanthrenecarboxylic acid methyl ester Methyl dehydroabietate (DHA) Methyl abietate 1,2,3,4,4a-Hexahydro-1,4a-dimethyl-7-(1-methylethenyl)-phenanthrenecarboxylic acid methyl ester Methyl 7-methoxy-DHA 1,2,3,4,4a,9,10-Octahydro-1,4a-dimethyl-7-(1-methylethenyl)-phenanthrenecarboxylic acid methyl ester Methyl neoabietate Methyl 15-methoxy-DHA 1,2,3,4,4a-Hexahydro-1,4a-dimethyl-7-(1-methylethyl)-9-methoxy-phenanthrenecarboxylic acid methyl ester Methyl 7-hydroxy-DHA Methyl 7-oxo-15-hydroxy-DHA Methyl 15-hydroxy-DHA Methyl 7-oxo-DHA
22 23 24 25 26 3 5 6 27 28 7 8 9 10 11
12 13 14
Mw
Compound name
No.
Table 3 Terpenoids identified by THM-GC/MS of Venice turpentine
tr ã tr tr ã tr tr tr ã
ã ã – ã ã ã ã tr ã
– ã ã – – – – – ã
ã ã ã
ã tr ã
ã ã –
– – – ã – ã ã – ã ã ã ã ã tr tr
1000 hb
tr
ã – – ã – ã ã – ã ã ã ã ã
600 ha
Artificially aged
ã ã ã ã ã ã ã ã ã ã ã ã ã ã –
Unaged
Tr Tr – ã
– ã ã
ã ã
ã – ã
– – – – – – – – – – ã tr – – ã
Naturally aged
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Fig. 7. SEC curves of naturally aged colophony (a) and Venice turpentine (b).
Some colophony samples have been aged under accelerated solar exposure conditions and under accelerated indoor condition. In both cases chromatographic peaks have lower intensities than in fresh resins, and only few mono- and sesquiterpenoids can be detected. At higher retention times new compounds have been identified. These are all dehydrogenation (peaks 11, 14, 16) [16] and oxidation products of DHA acid, such as 15-hydroxy-DHA acid (18), 7-oxo-15-hydroxy-
Fig. 8. Pyrograms of colophony: (a) unaged, (b) artificially aged under fluorescent tubes for 600 h, (c) artificially aged under a xenon lamp for 1000 h. Peak numbers correspond to those listed in Table 2.
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Fig. 9. Pyrograms of Venice turpentine: (a) unaged, (b) artificially aged under fluorescent tubes for 600 h, (c) artificially aged under a xenon lamp for 1000 h. Peak numbers correspond to those listed in Table 3.
DHA acid (20) and 7-oxo-DHA acid (21). Applying the THM technique all resinous acids are detected as methyl esters, whilst methylation has proven to be less effective on hydroxyl groups, especially in aliphatic compounds [17]. That is why, together with diterpenoids containing methoxy groups (15, 18), also some hydroxy acids (29, 30) have been identified. Pimarane type molecules, having no conjugated bonds, are more stable than abietanes to oxidation. However, during ageing, their abundance decreases and it could be argued that they are involved in other degradation processes. The nature of such reactions has not been clarified yet, but it may be argued that these are polymerisation reactions leading to the high molecular weight fractions detected by SEC. Naturally-aged colophony does not show any significant difference from artificially-aged samples (Fig. 10). The abundance of the diterpenic components is more similar to those of samples aged under indoor exposure conditions. Samples treated under a xenon lamp (u \295 nm) undergo a stronger degradation of both the original components and their oxidation products, causing a further decrease in their abundance. The increasing number and amount of diterpenic fragments eluted between 25 and 35 minutes (Figs. 8 and 9) demonstrate the occurrence of cleavage
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reactions. Comparison of Py-GC/MS and GC/MS data (not reported here) has confirmed that only few degradation products derive from pyrolysis, whilst most of them are the result of photo-induced bond cleavage. Venice turpentine, being a semiliquid resin, is composed of many mono- and sesquiterpenoids. The most abundant ones are a-pinene and elemol (Fig. 9). The diterpenic fraction does not show any significant difference from colophony, except for the presence of traces of oxidised DHA molecules in the fresh resin. It must be noted that larixol, epimanool and larixyl acetate, are among the most important components of Venice turpentine, could not be detected. A possibility is that these compounds were not methylated as it is known that thermally-assisted methylation with TMAH is less effective on aliphatic hydroxyl compounds than on carboxylic acids [13,17]. Secondly labdane type molecules have a highly reactive side chain that is easily subjected to degradation reactions. Compounds identified in fresh Venice turpentine, in the artificially-aged samples and in the naturally-aged ones, are listed in Table 3. The effects of ageing on pyrograms are the same as for colophony: a decrease of the volatile fraction, fragmentation of diterpenic molecules, and formation of oxidation products of DHA acid. THM-GC/MS was also able to establish that the sample supplied by the Opificio delle Pietre Dure as Venice turpentine contains siccative oil, as demonstrated by the presence of mono- and dicarboxylic fatty acids in the pyrolysis products (peaks of the corresponding methyl esters, FAME and DiFAME, respectively, are shown in Fig. 11).
4. Conclusions Natural resins have been extensively used as artistic materials in different ways like, for example, mixed with binding agents for accelerating their drying process, or applied as varnishes for conferring gloss and protecting the surfaces of artworks. Most of these resins are terpenoids and are composed of complex mixtures of
Fig. 10. Pyrogram of naturally aged colophony. Peak numbers correspond to those listed in Table 2.
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Fig. 11. Pyrogram of naturally aged Venice turpentine sample containing drying oil. Peak numbers correspond to those listed in Table 3.
organic molecules having different functional groups and varying in their molecular weights. In this work we have investigated two diterpenic resins, colophony and Venice turpentine, which are often mentioned in ancient recipes for the preparation of varnishes. We have checked three different types of ageing (natural and artificial external conditions with a xenon lamp, and artificial indoor conditions with fluorescent tubes) and monitored the overall structural changes with different analytical techniques. Oxidation processes have been followed with infrared spectroscopy. A number of oxidation products has been identified with THM-GC/MS, and the formation of higher molecular weight compounds has been demonstrated by size exclusion chromatography. The results indicate that the most significant chemical changes occur in the first part of ageing. Measurements on resins submitted to laboratory ageing also indicate that the degradation process is influenced by the type of light exposure conditions. Artificial ageing with a Xenon lamp has proven to be more severe. The high light intensity and UV radiation cause, apart from oxidation and polymerisation reactions, a further degradation of the chemical structure with formation of diterpenic fragments. From the analytical viewpoint, the pyrolytic approach has demostrated that it is well-suited for identifying the presence of diterpenic resins. To distinguish between colophony and Venice turpentine, however, is much more difficult, as the relevant differences are found in the lower molecular weight fractions (mono- and sesquiterpenes). These are also the more volatile or extractable compounds, and for that reason these are likely to be absent in samples from very ancient paintings. Recently we have also focused our attention on two other diterpenic resins, Manila copal and sandarac. Until now only few studies have dealt with these two art materials, so it will be interesting to clarify their composition and, above all, their light-ageing behaviour. Measurements are in progress with this purpose in mind.
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Acknowledgements Thanks are due to Mauro Matteini and Giancarlo Lanterna for the kind supply of the naturally-aged samples from Opificio delle Pietre Dure and to Francesca Cappitelli for the indoor artificial ageing performed with the light box at the Conservation Science Section, Conservation Department, Tate Gallery.
References [1] R.K.T. Oldring, G. Hayward (Eds.), Resins for Surface Coatings, vol. I, Sita Technology, London, 1987. [2] J.M. Challinor, J. Anal. Appl. Pyrolysis 25 (1993) 349 – 360. [3] E.R. de la Rie, Anal. Chem. 61 (1989) 1228A –1240A. [4] E.R. de la Rie, Stud. Conservation 32 (1987) 1 –13. [5] J.S. Mills, R. White, The Organic Chemistry of Museum Objects, Butterworth and Heinemann, Oxford, 1994. [6] J.S. Mills, R. White, Stud. Conservation 22 (1977) 12 – 31. [7] I. Pastorova, K.J. van der Berg, J.J. Boon, J.W. Verhoeven, J. Anal. Appl. Pyrolysis 43 (1997) 41 – 57. [8] K.J. van der Berg, J. van der Horst, J.J. Boon, N. Shibayama, E.R. de la Rie, Adv. Mass Spectrom. 14 (1998) 563 – 573. [9] R.N. Moore, R.V. Lawrence, J. Am. Chem. Soc. 80 (1958) 1438 – 1440. [10] W.H. Schuller, R.N. Moore, R.V. Lawrence, J. Am. Chem. Soc. 82 (1960) 1734 – 1738. [11] W.H. Schuller, R.V. Lawrence, J. Am. Chem. Soc. 83 (1961) 2563 – 2570. [12] J.S. Mills, Phytochemistry 12 (1973) 2407 –2412. [13] K.L. van den Berg, J.J. Boon, I. Pastorova, L.F.M. Spetter, J. Mass. Spectrom. 35 (2000) 512 – 533. [14] M. Lazzari, O. Chiantore, Polym. Deg. Stab. 65 (1999) 303. [15] M.R. Derrick, D. Stulik, J.M. Landry, Infrared Spectroscopy in Conservation Science, The Getty Conservation Institute, Los Angeles, 1999. [16] K.B. Anderson, R.E. Winans, Anal. Chem. 63 (1991) 2901 – 2908. [17] W.C. Kossa, J. MacGee, S. Ramachandran, A.J. Webber, J. Chromatogr. Sci. 17 (1979) 177 – 187.
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Ageing behaviour and pyrolytic characterisation of diterpenic resins used as art materials: colophony and Venice turpentine Dominique Scalarone, Massimo Lazzari, Oscar Chiantore * Department of Chemistry IPM, Uni6ersity of Torino, Via P. Giuria 7, 10125 Turin, Italy Received 1 August 2001; accepted 4 January 2002
Abstract The ageing behaviour of two diterpenic resins traditionally used as artists’ materials, colophony and Venice turpentine, was investigated with different spectroscopic and chromatographic techniques. In particular, three types of ageing (natural, artificial external conditions with a xenon lamp, artificial indoor conditions with fluorescent tubes) were applied to laboratory samples to study their effects on chemical structures. Thermally-assisted hydrolysis and methylation-gas chromatography-mass spectrometry (THM-GC/MS) was employed for careful characterisation and for identification of markers compounds on the vergin resins and in the course of ageing. The most significant changes were detected in the initial part of ageing, and the principal degradation products coming from oxidation, polymerisation and cleavage reactions identified. The high intensity of xenon lamp irradiation was found to cause, apart from oxidation and polymerisation reactions, further degradation of the chemical structure with molecular fragmentation. From the analytical point of view, the differentiation between colophony and Venice turpentine with THM-GC/MS appears to depend on differences in the lower molecular weight resin components. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Diterpenic resins; Colophony; Venice turpentine; Natural ageing; Artificial ageing; Thermallyassisted hydrolysis and methylation gas chromatography-mass spectrometry; Characterisation
Tribute to Professor Tsuge: Watching the scientific work Professor Shin Tsuge developed over the years has always been to us a pleasant duty, for the quality of results and the sound stimulating achievements. On this occasion it is therefore an honor to acknowledge his scientific accomplishments, sure that the retirement from Nagoya University will not be a retirement from pyrolysis investigation. With our best wishes. Professor Oscar Chiantore and Massimo Lazzari. * Corresponding author. Tel.: +39-011-670-7558; fax: + 39-011-670-7855 E-mail address: [email protected] (O. Chiantore). 0165-2370/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 7 0 ( 0 2 ) 0 0 0 4 6 - 3
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1. Introduction Varnishes based on terpenic resins have been used by old masters to protect paintings from dust, moisture and abrasions. Thanks to their excellent optical, adhesive and protective properties, natural resins are still used by many contemporary artists and restorers and are also frequently added as additives in the formulations of commercial painting media and adhesives [1,2]. Oil varnishes, prepared by mixing natural resins like sandarac, mastic or colophony with drying oils, have been commonly applied by artists from the 11th century. Starting from the 16th century they were gradually replaced by solvent, or ‘spirit’, varnishes, that are solutions of terpenic resins in volatile solvents. Because of their light stability and transparency, solvent varnishes made of dammar are considered the best ones [3]. Varnishes act not simply as a protective layer but also have an aesthetic function. They must be colourless, reversible and their viscosity must be adequate to form homogeneous films [4]. But even if a varnish is supposed to be stable, during ageing it undergoes physical and chemical changes which influence its properties: it becomes brittle and yellow, its solubility in common solvents decreases, and cracks develop on the surface. The understanding of the degradation mechanisms of natural resins is important to prevent the deterioration of old paintings and for the practise of restoration, but is also complicated by a number of factors. First of all, both di- and triterpenic natural resins are a mixture of different substances varying from volatile to polymeric compounds. During ageing, their chemical composition changes qualitatively and quantitatively because of oxidation, polymerisation, isomerisation and cleavage reactions. Finally, interactions with other painting materials, especially pigments, can influence the behaviour of varnishes and their degradation mechanisms. The diterpenic resins commonly used as artists’ materials were colophony, Venice turpentine, copals and sandarac. Diterpenes consist of four isoprene units. The C20 skeleton can have extra functional groups resulting in a great variety of diterpenoids which differ in the degree of unsaturation and in the number of oxygen atoms. Diterpenoids can be classified into two main groups: labdanes which are bicyclic molecules with an highly reactive unsaturated C6 side-chain; pimaranes and abietanes which are tricyclic acids (Fig. 1). The abietane-type molecules, having conjugated double bonds, easily undergo oxidation reactions [5]. Sandarac and copals are fossil resins which, apart from free diterpenoids, consist of a highly polymerised fraction of polycommunic acid or communic acid copolymerised with communol. Fossil resins are hard, tough and almost insoluble in common organic solvents. They can be mixed with oils only after heating and subsequent decarboxylation and depolymerisation of polycommunic acid. Colophony and Venice turpentine exude from trees belonging to the conifer subfamily Pinaceae. Colophony, also known as Greek pitch, is the solid residue obtained after distillation of resins of Pinus species. Venice turpentine comes
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Fig. 1. Chemical structures of some diterpenoid labdanes, pimaranes and abietanes.
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directly from Larix decidua. If spread as a thin layer, colophony darkens rapidly and becomes brittle and dull. For such reasons it was never used on its own, but frequently added to other resins or oils in order to modify some of their properties. At present colophony is often mixed with synthetic resins as an additive in order to increase adhesion, brightness and toughness. The fresh resin consists almost entirely of resinous acids differing in their oxidation behaviour. Sixty percent consists of abietane acids with highly reactive conjugated double bonds, 20– 25% of more stable pimarane acids and 5–10% of acids that are much prone to oxidation (dihydroabietic and tetrahydroabietic acids). When heated, abietane-type molecules isomerise giving a mixture rich in abietic acid, whilst laevopimaric acid almost disappears [6]. These reactions also explain compositional differences between fresh pine resins (Table 1) and colophony. The composition of diterpenic resins can be determined by GC/MS, after methylation of acid groups by diazomethane [6], or by thermally assisted hydrolysis and methylation (THM) with tetramethylammonium hydroxide (TMAH). By means of THM-GC/MS, Pastorova at al. [7] have identified the resinous acids of colophony and also some of their oxidation products. All the oxidised molecules have an abietane-type skeleton, while pimarane acids, lacking conjugated double bonds, are stable towards oxidation and isomerisation. The oxidation products of colophony can be also identified by HPLC/MS [8]. This technique is the most suitable to the analysis of termolabile compounds such as hydroperoxides, whose formation through photo-oxidation has been demonstrated in early studies on oxidation of abietane resinous acids [9–11]. Venice turpentine is often mentioned in ancient recipes for pictorial varnishes even if it darkens quite rapidly. Similar to the other resins exuded by Larix species, Venice turpentine is composed by abietane and pimarane acids, together with smaller amounts of labdane alcohols. The composition of Venice turpentine, determined by gas chromatography and derivatisation with diazomethane [12], is shown in Table 1. Previous studies [13] have demonstrated that also Venice turpentine, like colophony, undergoes oxidation processes, as one could expect on the basis of its chemical composition. For the chemical characterisation of Venice turpentine different types of on-line and off-line derivatisation reactions have been applied before the GC/MS analysis, and the potentialities of each method have been discussed. Methylation with trimethylsilyl (TMS)-diazomethane was successful on most diterpenoids, except for 15-hydroxy-7-oxo-dehydroabietic (DHA) acid, one of the most important markers of diterpenic resins. The best results have been obtained with trimethylsilylation using bis(trimethylsilyl)trifluoroacetamide (BSTFA). This reagent is effective on both acid and alcoholic groups, and it also allows the distinction between free diterpenic acids and diterpenic acid methyl esters resulting from ageing. On-line derivatisation with TMAH is a fast and simple method but is scarcely effective on hydroxyl groups and involves a number of secondary reaction (isomerisation, dehydration and cleavage of hydrolysable bonds) due to the strong alkalinity of the TMAH solution [13].
P. P. P. P. P. L. L. L. L. L.
pinaster halepensis syl6estris ponderosa palustris decidua gmelini russica occidentalis pendula
Dehydroabietic (DHA)
12 39 1.5 15 10 18.5 16.4 7.5 22.8 18.6
39 37 34 40 52 12.5 6.0 2.0 12 7.3
14 10 28 11 9 7.6 5.6 2.7 12.7 5.3
18 1.5 10 11 13 5.6 2.2 tr 5.1 1.6
4 – 4 8 8 1.8 5.1 0.5 7.7 8.6
– – – – – 7.5 13 12.8 4.4 10.7
– – – – – 3.1 23.7 – – 3.5
Larixol
2 10 3 3 1 1.0 1.9 1.5 1.4 1.6
Neoabietic
8.0 1.2 19 7.5 5.1 0.5 – tr 1.6 0.4
Abietic
Epimanool
Laevopimaric/ palustric
Sandaracopimaric
Pimaric
Isopimaric
Other components
Diterpenic acids
Table 1 Chemical composition of resins from Pinus and Larix [5,12]
– – – – – 33 16.6 – – 12.2
Larixyl acetate
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As far as their composition is concerned, Venice turpentine and colophony are very similar. Two characteristic markers of Venice turpentine are larixol and larixyl acetate, but in many cases, especially using on-line thermally assisted methylation with TMAH, they cannot be detected, as larixyl acetate hydrolyses and the methylation of hydroxyl groups is not so effective as for carboxylic acids. This long introduction was necessary to highlight that natural resins are complex chemical systems whose characterisation is difficult not only because of their chemical heterogeneity, but also for the different reactivities and ageing behaviour of the resin components. However their identification is important from the point of view of the knowledge of artists’ techniques and for the practice of conservation and restoration. THM-GC/MS is well suited to this scope because of its response specificity and the capability of obtaining structural informations from very small samples, in the microgram range, that is an absolute requirement in this field. In the present work we have focused our attention on the characterisation of colophony and Venice turpentine and we have applied THM-GC/MS to the study of fresh and aged resins. We have tested different light-exposure conditions in order to evaluate how degradation processes are affected by light ageing, and looked for the possible marker compounds that could be unequivocally used for resin recognition in real pictorial samples.
2. Experimental Colophony and Venice turpentine were purchased from Phase (Firenze, Italy). The samples for ageing were in form of thin films, cast from tetrahydrofuran (THF) or chloroform solutions. Samples for infrared analyses were supported on silicon wafers or KBr discs, while for all other analyses the support was a selected glass. A high-speed exposure unit Suntest CPS (Heraeus, Germany), equipped with a Xenon lamp and a UV filter that absorbs wavelengths lower than 295 nm, was used to simulate outdoor solar exposure. Irradiation was set at 765 W m − 2, and the maximum temperature on the samples was kept at 45 °C by forced air circulation. Under such experimental conditions it has been previously estimated that the rate of photo-ageing is approximately 260 times faster than natural indoor ageing [14]. A set of fluorescent tubes placed behind an ultraviolet filter to cut off radiation below 400 nm were used to reproduce indoor light exposure. The environmental conditions within the box were recorded as about 5 °C above room temperature and approximately 10% below ambient relative humidity (RH). The average light intensity was measured as 22,000 lux. Naturally aged samples, prepared in 1976 according to ancient recipes, were provided by Opificio delle Pietre Dure (Florence). The infrared spectra were collected with a Perkin–Elmer 1710 FTIR instrument with a DTGS detector and 4 cm − 1 resolution. Molecular characterisation of fresh and aged resins was performed with a modular size exclusion chromatography (SEC) system composed of a Waters (USA) M-45 pump, a Rheodyne (USA) 7010 injection valve, a differential refractometer ERC 7510 (Erma, Japan), and PL Gel
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type columns, 10 mm particle diameter. THF solutions were separated from solid impurities by filtration through 0.45 mm PTFE membrane filters. Calibration was obtained with PMMA molecular weight standards and o-dichlororobenzene. Pyrolysis experiments were carried out with an integrated system composed of a CDS Pyroprobe 1000 heated filament pyrolyser (CDS Analytical Inc., USA), a GC 5890A gas chromatograph (Hewlett Packard, USA) equipped with capillary column HP-5MS cross-linked 5% Ph Me Silicone (30 m× 0.25 mm× 0.25 mm), and a Hewlett Packard GC 5970 mass spectrometer. Degradation was carried out with the technique of THM with TMAH, as in this way adequate elutions and separations of the acid compounds in the samples could be obtained. For each analysis a few microliters of 25% aqueous TMAH solution (Aldrich, USA) were added with a microsyringe to the sample in a quartz tube. Sample amounts were on the order of a hundred micrograms. Pyrolyses were performed at 600 °C for 10 s. The pyrolyser interface was set at 300 °C and the injector at 280 °C. The GC column temperature conditions were as follows: initial temperature 40 °C, hold for 2 min, increase at 8 °C min − 1 to 150 °C, increase at 3 °C min − 1 to 280 °C. Helium gas flow was set at 1 ml min − 1. Mass spectra were recorded under electron impact ionisation at 70 eV electron energy, in the range from m/z 40 to 800. Pyrolysis fragments were identified on the basis of their mass spectra and mass library searches (Wiley 138 and NBS).
3. Results and discussion The ageing behaviour of the diterpenic resins has been investigated with infrared spectroscopy and size exclusion chromatography, while the identification of their components on both fresh and aged samples has been carried out by THM-GC/ MS.
3.1. Spectroscopic and size exclusion chromatographic characterisation By means of infrared spectroscopy it is possible to identify easily the different classes of natural compounds (waxes, oils, proteins, resins, etc.), but it is almost impossible to differentiate samples belonging to the same class [15]. The most important components of terpenic resins have similar chemical structures, and the few differences one can notice in the spectra of fresh resins generally disappear in aged ones. Detailed compositional information can be achieved by means of other analytical techniques, in particular gas chromatography coupled with mass spectrometry. In any case infrared spectroscopy is a useful technique to monitor the overall chemical changes occurring during ageing. For instance, comparing the relative intensities of hydroxyl and carbonyl absorptions, it is possible to detect oxidation reactions and molecular cleavages and to evaluate the kinetics of these phenomena. Comparing the IR spectra of fresh colophony and Venice turpentine (Fig. 2) we can notice a few differences: in Venice turpentine the hydroxyl absorption is higher
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Fig. 2. Infrared spectra of unaged and artificially aged colophony (a) and Venice turpentine (b).
than in colophony, whilst the carbonyl peak is lower. Apparently this is the result of the different resin compositions: in colophony the diterpenic fraction consists mostly of resinous acids, whilst in Venice turpentine there is also a remarkable amount of hydroxyl compounds, such as epimalool, larixol and larixyl acetate. The infrared analysis has pointed out that the most relevant molecular changes occur in the initial stage of ageing. Both resins undergo the same oxidation, polymerisation and degradation reactions, and after only a few hours of artificial ageing the infrared spectra of colophony and Venice turpentine are no longer distinguishable. The most important spectral effects of ageing shown in Fig. 2 are: increase of the hydroxyl absorption w(O – H) at about 3400 cm − 1; higher absorption in the region between 900 and 1300 cm − 1, due to w(C–OH) and w(C–O) of esters and acids; moderate increase in double bonds w(CC) at 1610 cm − 1; broadening of the carbonyl peak towards higher wavenumbers, due to new oxidised compounds (ketones, esters, lactones). At longer ageing time a decrease in absorbance within the entire range of frequencies considered has been noticed, attributed to the loss of volatile fragments formed during degradation. This hypothesis is consistent with the weight losses determined on the films during ageing and shown in Fig. 3. At two thousand hours of ageing, loss of volatile fragments results in a total weight loss of about 20%.
Fig. 3. Weight loss percentages of artificially aged colophony () and Venice turpentine ().
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Fig. 4. Infrared spectra of naturally aged colophony (a) and Venice turpentine (b).
The spectra of the naturally aged samples (Fig. 4) are in some ways different from those of artificially aged resins. Hydroxyl absorption is higher, and the component at 1610 cm − 1 denotes an high amount of double bonds. Different origins of the materials and sample preparation may partially explain such differences, but it is also possible that with the different ageing conditions some reactions are predominant in the time scale of the samples we are observing. The ageing of diterpenic resins leads to the oxidation reactions discussed in the previous paragraph, but also to polymerisation reactions. Polymerisation probably proceeds via a radical mechanism involving the double bonds of terpenic substances. Size exclusion chromatography has been used to monitor the molecular weight changes during ageing. SEC curves of Fig. 5 have been normalised so that the peak area represents the same amount of soluble sample differently distributed along the molecular weight scale. The most important effects of ageing which may be observed in the chromatograms are: (i) the fast disappearance of mono- and sesquiterpenic fractions in Venice turpentine (1); (ii) the decrease of the diterpenic component (2); (iii) the simultaneous formation of molecules with molecular weights in the range of diterpenic dimers (3); (iv) the formation of oligomers (4) with maximum molecular weights of about 2200 for colophony and 5000 for Venice turpentine. The changes in the weight average molecular weights of colophony and Venice turpentine are shown in Fig. 6. Obviously, since a PMMA calibration curve was used and the refractometer response is also dependent on molecular composition,
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Fig. 5. SEC curves of colophony (a) and Venice turpentine (b): unaged, artificially aged 25, 100, 500, 1000 h.
the calculated molecular weight averages should be regarded as approximate values. However the trend is correct and it shows that aged Venice turpentine consists of longer oligomeric fragments, probably coming from the polymerisation of labdane type molecules. Even if their shapes are slightly different, the chromatograms of naturally aged samples (Fig. 7) are in good agreement with the artificially aged ones. Average calculated molecular weights are 600 for the colophony sample and 850 for Venice turpentine.
3.2. THM-GC/MS The main terpenic compounds found in colophony and Venice turpentine are listed in Tables 2 and 3. THM-GC/MS has proven to be a well-suited technique for identification of volatile mono- and sesquiterpenoids, abietane and pimarane acids and their oxidation products. Except for the highly volatile components, the pyrograms of the two resins, both fresh and aged, are almost equal (Figs. 8 and 9). All resinous acids have been well separated and identified except for laevopimaric acid. It is known that laevopimaric acid is sensitive to light and heating [5,13] so its absence may be attributed to isomerisation reaction occurring during storage.
Fig. 6. Changes of weight average molecular weights with ageing time of colophony () and Venice turpentine (2).
b
a
20 21
17 18 19
15 16
Artificially aged with fluorescent tubes. Artificially aged under a xenon lamp.
p-Cymene Norchrysantemic acid methyl ester a-Terpineol Longicyclene Longifolene Caryophyllene Methyl pimarate Methyl sandaracopimarate Methyl palustrate Methyl isopimarate 1,2,3,4,4a,10a-hexahydro-1,4a-dimethyl-7-(1-methylethyl)-phenanthrenecarboxylic acid methyl ester Methyl dehydroabietate (DHA) Methyl abietate 1,2,3,4,4a-Hexahydro-1,4a-dimethyl-7-(1-methylethenyl)-phenanthrenecarboxylic acid methyl ester Methyl 7-methoxy-DHA 1,2,3,4,4a,9,10-Octahydro-1,4a-dimethyl-7-(1-methylethenyl)-phenanthrenecarboxylic acid methyl ester Methyl neoabietate Methyl 15-methoxy-DHA 1,2,3,4,4a-Hexahydro-1,4a-dimethyl-7-(1-methylethyl)-9-methoxy-phenanthrenecarboxylic acid methyl ester Methyl 7-oxo-15-hydroxy-DHA Methyl 7-oxo-DHA
1 2 3 4 5 6 7 8 9 10 11
12 13 14
Compound name
No.
Table 2 Terpenoids identified by THM-GC/MS of colophony
344 328
316 344 342
344 312
314 316 310
134 168 154 204 204 204 316 316 316 316 312
Mw
– ã ã – – tr ã
ã ã – ã ã ã ã
– – ã – – – –
ã ã tr
ã ã ã
ã ã –
– ã ã – ã ã ã tr tr tr tr
1000 hb
ã ã ã ã ã
ã ã ã – ã
600 ha
Artificially aged
ã ã ã – ã ã ã ã ã ã –
Unaged
– ã
– ã ã
ã ã
ã tr ã
ã ã – – ã
– ã – ã ã
Naturally aged
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b
a
29 20 30 21
17 18 19
15 16
Artificially aged with fluorescent tubes. Artificially aged under a xenon lamp.
330 344 330 328
316 344 342
344 312
314 316 310
136 136 136 136 135 154 204 204 222 222 316 316 316 316 312
a-Pinene b-Pinene D3-Carene Limonene Allocimene a-Terpineol Longifolene Caryophyllene Elemol b-Eudesmol Methyl pimarate Methyl sandaracopimarate Methyl palustrate Methyl isopimarate 1,2,3,4,4a,10a-Hexahydro-1,4a-dimethyl-7-(1-methylethyl)-phenanthrenecarboxylic acid methyl ester Methyl dehydroabietate (DHA) Methyl abietate 1,2,3,4,4a-Hexahydro-1,4a-dimethyl-7-(1-methylethenyl)-phenanthrenecarboxylic acid methyl ester Methyl 7-methoxy-DHA 1,2,3,4,4a,9,10-Octahydro-1,4a-dimethyl-7-(1-methylethenyl)-phenanthrenecarboxylic acid methyl ester Methyl neoabietate Methyl 15-methoxy-DHA 1,2,3,4,4a-Hexahydro-1,4a-dimethyl-7-(1-methylethyl)-9-methoxy-phenanthrenecarboxylic acid methyl ester Methyl 7-hydroxy-DHA Methyl 7-oxo-15-hydroxy-DHA Methyl 15-hydroxy-DHA Methyl 7-oxo-DHA
22 23 24 25 26 3 5 6 27 28 7 8 9 10 11
12 13 14
Mw
Compound name
No.
Table 3 Terpenoids identified by THM-GC/MS of Venice turpentine
tr ã tr tr ã tr tr tr ã
ã ã – ã ã ã ã tr ã
– ã ã – – – – – ã
ã ã ã
ã tr ã
ã ã –
– – – ã – ã ã – ã ã ã ã ã tr tr
1000 hb
tr
ã – – ã – ã ã – ã ã ã ã ã
600 ha
Artificially aged
ã ã ã ã ã ã ã ã ã ã ã ã ã ã –
Unaged
Tr Tr – ã
– ã ã
ã ã
ã – ã
– – – – – – – – – – ã tr – – ã
Naturally aged
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Fig. 7. SEC curves of naturally aged colophony (a) and Venice turpentine (b).
Some colophony samples have been aged under accelerated solar exposure conditions and under accelerated indoor condition. In both cases chromatographic peaks have lower intensities than in fresh resins, and only few mono- and sesquiterpenoids can be detected. At higher retention times new compounds have been identified. These are all dehydrogenation (peaks 11, 14, 16) [16] and oxidation products of DHA acid, such as 15-hydroxy-DHA acid (18), 7-oxo-15-hydroxy-
Fig. 8. Pyrograms of colophony: (a) unaged, (b) artificially aged under fluorescent tubes for 600 h, (c) artificially aged under a xenon lamp for 1000 h. Peak numbers correspond to those listed in Table 2.
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Fig. 9. Pyrograms of Venice turpentine: (a) unaged, (b) artificially aged under fluorescent tubes for 600 h, (c) artificially aged under a xenon lamp for 1000 h. Peak numbers correspond to those listed in Table 3.
DHA acid (20) and 7-oxo-DHA acid (21). Applying the THM technique all resinous acids are detected as methyl esters, whilst methylation has proven to be less effective on hydroxyl groups, especially in aliphatic compounds [17]. That is why, together with diterpenoids containing methoxy groups (15, 18), also some hydroxy acids (29, 30) have been identified. Pimarane type molecules, having no conjugated bonds, are more stable than abietanes to oxidation. However, during ageing, their abundance decreases and it could be argued that they are involved in other degradation processes. The nature of such reactions has not been clarified yet, but it may be argued that these are polymerisation reactions leading to the high molecular weight fractions detected by SEC. Naturally-aged colophony does not show any significant difference from artificially-aged samples (Fig. 10). The abundance of the diterpenic components is more similar to those of samples aged under indoor exposure conditions. Samples treated under a xenon lamp (u \295 nm) undergo a stronger degradation of both the original components and their oxidation products, causing a further decrease in their abundance. The increasing number and amount of diterpenic fragments eluted between 25 and 35 minutes (Figs. 8 and 9) demonstrate the occurrence of cleavage
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reactions. Comparison of Py-GC/MS and GC/MS data (not reported here) has confirmed that only few degradation products derive from pyrolysis, whilst most of them are the result of photo-induced bond cleavage. Venice turpentine, being a semiliquid resin, is composed of many mono- and sesquiterpenoids. The most abundant ones are a-pinene and elemol (Fig. 9). The diterpenic fraction does not show any significant difference from colophony, except for the presence of traces of oxidised DHA molecules in the fresh resin. It must be noted that larixol, epimanool and larixyl acetate, are among the most important components of Venice turpentine, could not be detected. A possibility is that these compounds were not methylated as it is known that thermally-assisted methylation with TMAH is less effective on aliphatic hydroxyl compounds than on carboxylic acids [13,17]. Secondly labdane type molecules have a highly reactive side chain that is easily subjected to degradation reactions. Compounds identified in fresh Venice turpentine, in the artificially-aged samples and in the naturally-aged ones, are listed in Table 3. The effects of ageing on pyrograms are the same as for colophony: a decrease of the volatile fraction, fragmentation of diterpenic molecules, and formation of oxidation products of DHA acid. THM-GC/MS was also able to establish that the sample supplied by the Opificio delle Pietre Dure as Venice turpentine contains siccative oil, as demonstrated by the presence of mono- and dicarboxylic fatty acids in the pyrolysis products (peaks of the corresponding methyl esters, FAME and DiFAME, respectively, are shown in Fig. 11).
4. Conclusions Natural resins have been extensively used as artistic materials in different ways like, for example, mixed with binding agents for accelerating their drying process, or applied as varnishes for conferring gloss and protecting the surfaces of artworks. Most of these resins are terpenoids and are composed of complex mixtures of
Fig. 10. Pyrogram of naturally aged colophony. Peak numbers correspond to those listed in Table 2.
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Fig. 11. Pyrogram of naturally aged Venice turpentine sample containing drying oil. Peak numbers correspond to those listed in Table 3.
organic molecules having different functional groups and varying in their molecular weights. In this work we have investigated two diterpenic resins, colophony and Venice turpentine, which are often mentioned in ancient recipes for the preparation of varnishes. We have checked three different types of ageing (natural and artificial external conditions with a xenon lamp, and artificial indoor conditions with fluorescent tubes) and monitored the overall structural changes with different analytical techniques. Oxidation processes have been followed with infrared spectroscopy. A number of oxidation products has been identified with THM-GC/MS, and the formation of higher molecular weight compounds has been demonstrated by size exclusion chromatography. The results indicate that the most significant chemical changes occur in the first part of ageing. Measurements on resins submitted to laboratory ageing also indicate that the degradation process is influenced by the type of light exposure conditions. Artificial ageing with a Xenon lamp has proven to be more severe. The high light intensity and UV radiation cause, apart from oxidation and polymerisation reactions, a further degradation of the chemical structure with formation of diterpenic fragments. From the analytical viewpoint, the pyrolytic approach has demostrated that it is well-suited for identifying the presence of diterpenic resins. To distinguish between colophony and Venice turpentine, however, is much more difficult, as the relevant differences are found in the lower molecular weight fractions (mono- and sesquiterpenes). These are also the more volatile or extractable compounds, and for that reason these are likely to be absent in samples from very ancient paintings. Recently we have also focused our attention on two other diterpenic resins, Manila copal and sandarac. Until now only few studies have dealt with these two art materials, so it will be interesting to clarify their composition and, above all, their light-ageing behaviour. Measurements are in progress with this purpose in mind.
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Acknowledgements Thanks are due to Mauro Matteini and Giancarlo Lanterna for the kind supply of the naturally-aged samples from Opificio delle Pietre Dure and to Francesca Cappitelli for the indoor artificial ageing performed with the light box at the Conservation Science Section, Conservation Department, Tate Gallery.
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