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The characterization of commercial artists’ alkyd paints
Available online at
www.sciencedirect.com Journal of Cultural Heritage 9 (2008) 412e419
Original article
The characterization of commercial artists’ alkyd paints Rebecca Ploeger*, Dominique Scalarone, Oscar Chiantore Department of I.P.M. Chemistry and NIS (Nanostructured Surfaces and Interfaces) Center of Excellence, University of Torino, Via Pietro Giuria, 7, 10125 Torino, Italy Received 2 October 2007; accepted 9 January 2008
Abstract There is little information in the conservation literature with respect to artists’ alkyd paints; thus, artists and conservators are somewhat at a loss about how to use and treat alkyds. Recently, analytical methods have been developed to identify the components in these polymers (oil modified polyesters), rates of cross-linking and mechanical properties. Presented in this paper are some of the characterization results of artists’ alkyd paints using THM (thermally assisted hydrolysis and methylation) gas chromatographye mass spectrometry (THMeGC/MS) and Fourier transform infrared spectroscopyeattenuated total reflectance (FTIReATR). Four brands of artists’ alkyd paints containing alkyd resin have been analysed; one containing a phthalic anhydride and pentaerythritol based alkyd resin, two containing isophthalic acid and pentaerythritol based alkyd resins, and the final one containing both phthalic anhydride and isophthalic acid and pentaerythritol based resins among the colours studied. Ó 2008 Elsevier Masson SAS. All rights reserved. Keywords: Alkyd paints; Ageing; Characterization; Artists’ paints; THMeGC/MS; FTIReATR
1. Introduction The name ‘alkyd’ is derived from the alcohol and acid monomers used to make the polyester polymer, and an alkyd resin is an oil modified alkyd polyester. Alkyd resins are made by condensation polymerization of polyols (at least three hydroxyl groups), polybasic acids and a source of fatty acids (either siccative oils or free fatty acids). The final resin is composed of a polyester backbone, on it’s own a highly branched polymer, and dangling fatty acids, which serve to reduce the amount of cross-linking creating a more flexible polymer; Fig. 1 shows a more simple alkyd, composed of phthalic anhydride (difunctional), glycerol (trifunctional) and linoleic acid. The types of fatty acids used are important in determining the final drying characteristics of the material, since they dry through auto-oxidation reactions, much like an oil paint, after solvent evaporation [1]. Out of all the artists’ * Corresponding author. Tel.: þ39 011 670 7554; fax: þ39 011 670 7855. E-mail addresses: [email protected] (R. Ploeger), dominique. [email protected] (D. Scalarone), [email protected] (O. Chiantore). 1296-2074/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2008.01.007
synthetic paints, artists’ alkyd paints have the most similar final visual appearance to traditional oil paint [2]. Their main advantage is their speed of drying, which is 18e24 h (touch dry) due to the high molecular weight alkyd resin and the need for fewer crosslinks for film formation. Alkyds are classified by ‘oil length’, i.e. the weight percent of oil or triglyceride equivalent or fatty acids in the finished resin, and oil type. Varying these two factors has a great impact on the final properties and the ultimate applications of the resins. Alkyd resins used for artists’ paints are classified as ‘long oil’ alkyds, because they contain between 56e70 weight percent oil or fatty acids. ‘Short oil’ (35e45 weight percent), ‘medium oil’ (46e55 weight percent) and ‘very long oil’ (greater than 70 weight percent) alkyd resins are also manufactured and used in various other coatings applications [3]. Drying and semi-drying oils, such as linseed, soya, safflower and castor oil are commonly used in ‘long oil’ applications because of their fatty acid compositions and lower costs [3e 5]. The fatty acid portion gives many important properties to the resin, including, cross-linking potential, flexibility, compatibility with solvents and control of solubility [4].
R. Ploeger et al. / Journal of Cultural Heritage 9 (2008) 412e419
O
O
*
O
O
* n O
O
Fig. 1. An ideal alkyd resin (containing, phthalic anhydride, glycerol and linoleic acid).
Linseed oil and soya oil are the most commonly oils used in alkyd house paint formulations [3]. Linseed oil, a drying oil, contains a large amount of linolenic acid, which tends to yellow considerably as it ages and is avoided for white or light coloured paints. Soya oil, along with safflower oil, castor oil and sunflower oil, contain more linoleic acid that does not yellow as much, but being semi-drying oils, they require more time to dry. Both Lack and Schilling et al. [2,3] claim that Winsor & Newton typically uses a soya based alkyd resin in their Griffin Alkyd paint formulations. Since artists’ alkyd paints contain up to 70 weight percent fatty acids, one can intuitively assume that they undergo similar ageing reactions as oil paints and that similar degradation products will be present. In fact, Staples [6], the technical director of Winsor & Newton in 1982, claimed that ‘‘as many of the constituent parts of alkyd resins come from ingredients found in drying oils, it is not surprising that many of the properties of the alkyd paints are similar to those of natural drying oils’’. However, the assumption that they have similar stability properties has yet to be confirmed and the ageing properties of the polyester portion must also be taken into account. There are very few noted conservation concerns and published information about artists’ alkyd paints, because they are a relatively new artist specific medium. As a result, they have been used less by artists and conservators have not encountered them much in their practices. Winsor & Newton, Middlesex, England, was the first colour maker to introduce an artists’ alkyd line; they introduced their original line of artist alkyds in 1976, Artists’ Alkyds followed by London and Griffin Alkyd Colour in 1980 [2]. In the USA there was PDQ (Paints Dry Quick) with their Artist Oil Color, Quick Dry Oil line; formerly of San Jose, CA, which no longer exists. It has been suggested, in personal communications, that PDQ alkyds appeared on parts of the American market before Winsor & Newton alkyds; however, this is not confirmed. Also available in the USA were Shiva Inc. Alkyd Colors; however, they were also discontinued and Shiva Inc. is now part of Jack Richeson & Co. Inc. in Kimberly, WI. Currently available artists’ alkyd paints are
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Winsor & Newton’s Griffin Alkyd paints, Ferrario Alkyd, Colore Alchidico, Calderara di Reno (BO), Italy and Da Vinci Paint Co.’s Leonardo Oil with Alkyd, Irvine, CA (this is a traditional oil paint with an alkyd media mixed in). Many artists’ materials manufacturers also offer alkyd based media, which can be added to traditional oil paints to alter the rate of drying and film properties. Industrial and household alkyd paints have been used in works of art since their first introduction in the late 1920s/ early 1930s. Some of the more prominent artists that have used industrial alkyds are Jackson Pollock and Pablo Picasso [4,7,8]. The characterization of the main components of the artists’ alkyd resins, additives (fillers) and pigments must be done to provide a foundation for the study and identification of these materials, both artist specific and industrial, in works of art. The literature concerning artists’ alkyd paints is very limited, which forces one to search the literature of oil paints and to make assumptions, although in the past few years several artists’ alkyd studies have been initiated. Characterization studies have been performed by Learner [1] using PyGC/MS and FTIR, by Sonoda [9] using GC/MS with thermally assisted hydrolysis and methylation (THMeGC/MS) and Schilling et al. [3] using GC/MS with an offline methylation procedure for quantitative analysis of the alkyd polymer components. Mechanical properties of artists’ alkyd paints have been studied and compared to the mechanical performance of artists’ acrylic paints by Erlebacher et al. [10], and Civil et al. [11] investigated the cracking in paintings by Antoni Ta`pies, who used commercial household alkyd coatings as his medium. Phenix [12], in his studies of the swelling of oil paint films, mentions the increased sensitivity and plasticization of alkyds in water as a result of the oxidative drying process and the formation of hydrophilic groups. Standeven [4] has studied, in depth, the development of alkyd paints (commercial household) in Britain and the USA and has published her findings for the early years (1910e1960) of the material. Finally Lack [2] investigated some of the performance and working properties of artists’ alkyd paints and Fuesers [13], in an unpublished diploma thesis, characterized artists’ alkyds with an offline derivatization technique and GC/ MS. The additives of these paints were also extracted with various solvents and analysed. To expand the limited conservation literature concerning alkyds, this paper will discuss the characterization of all the current commercially available, to the best of our knowledge, alkyd based paints: Winsor & Newton’s Griffin Alkyd, Ferrario Alkyd’s, Colore Alchidico and Da Vinci Paint Co.’s Leonardo Oil with Alkyd, as well as, the discontinued PDQ alkyd paints. The paints were characterized using THM (thermally assisted hydrolysis and methylation) gas chromatographyemass spectrometry (THMeGC/MS) and Fourier transform infrared spectroscopyeattenuated total reflectance (FTIReATR), both these techniques being regularly used in the cultural heritage field. As mentioned, there is also a variety of alkyd media available to artists to add to traditional oil paints; however, this paper will examine only the alkyd paints.
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It is hoped that the information presented can help clear up confusion or aid in the identification of alkyd materials in works of art. 2. Experimental 2.1. The paints Winsor & Newton alizarin crimson, burnt umber and titanium white were studied; the samples were cast on Mylar in 1980, 1999 and 2006 (3 cm wide strips approximately 0.2 mm in thickness) and aged naturally in a laboratory environment (the 1980 and 1999 samples were provided by the Smithsonian Institution in Washington, DC). A sample, provided by Winsor & Newton, of the alkyd resin used in their current Griffin Alkyd Colours formulations was also analysed. The paints cast in 1980 where from Winsor & Newton’s original Artists’ Alkyd Colour line and the paints cast in 1999 were from their later, and only currently available line, Griffin Alkyd Colour. Ferrario Alkyd, Colore Alchidico, rosso magenta (red magenta) and bianco titanio (titanium white) and Da Vinci Paint Co. Leonardo Oil with Alkyd alizarin crimson and titanium, both purchased and cast in 2006 were studied. The films were cast in a similar fashion and aged naturally in the laboratory. Nine different coloured samples from PDQ (Paints Dry Quick), Artist Oil Color, Quick Dry Oil line, provided by the Art Materials Research and Study Center, National Gallery of Art, Washington, DC were also investigated. The colours were: alizarin crimson, cadmium red medium, cobalt violet, fire red, green gold, hansa yellow (light), indofast orange, Prussian blue and permanent green light. These samples were cast in 2007 on Mylar, undercut and removed from the Mylar substrate with a razor blade and stored in clean glass sample vials. It should be noted that the Da Vinci Paint Co. Leonardo Oil with Alkyd paints are traditional oil based paints with an alkyd medium mixed into them and not alkyd based paints like the Winsor & Newton Griffin Alkyd, Ferrario Alkyd, Colore Alchidico and PDQ paints. The titanium white is listed as a refined safflower oil and alkyd resin, and the alizarin crimson as a refined linseed oil and alkyd resin. These paints were included in the study since they are offered as a commercial oil paint pre-modified with an alkyd resin. 2.2. Characterization studies Fourier transform infrared spectroscopyeattenuated total reflectance (FTIReATR) and thermally assisted hydrolysis and methylationegas chromatographye mass spectrometry (THMe GC/MS) using a 25 wt% aqueous solution of tetramethylammonium hydroxide (TMAH) (from Aldrich), have been used to characterize the binding materials, fillers and pigments used in the paints. FTIReATR was performed with a Thermo Nicolet FTIR NEXUS instrument and Smart Endurance ATR accessory, from 4000 to 500 cm1 and for 32 scans with a resolution of 4 cm1. Data was collected and analysed in OMNIC 6.1a software.
For the Py-GC/MS studies, an Agilent Technologies 6890N Network GC system GC with an Agilent 5973 Network Mass Selective Detector quadrupole mass spectrometer was used along with a CDS Analytical Inc. Pyroprobe 1000 heated filament pyrolyser. Samples were prepared in CDS Analytical quartz tubes with quartz wool. A typical program for painting materials was used, with a 2 min solvent delay (50 C, 2 min; 10 C/min to 300 C; 300 C, 5 min). Data were collected with Agilent Technologies’ Enhanced ChemStation software version D.00.00.38 and analysed with MS Data Analysis Version C.03.00 HP software. Derivatization with TMAH is the recommended method for analysing drying oils [14]. Approximately 7 mL of TMAH solution was injected into the quartz tube and allowed to mix with the sample for at least 1 h to ensure complete contact between the samples and TMAH for full methylation upon pyrolysis. 3. Results and discussion 3.1. FTIReATR FTIReATR results for the Winsor & Newton Artists’ Alkyd Colour (1980) and Griffin Alkyd Colour (1999 and 2006) samples show that they are typical alkyd materials. The peaks observed in this work corresponded well to the peaks (from diamond cell transmission spectra) reported by Learner [1], taking into account the expected small shift of the peaks to lower wavenumbers in the spectra recorded using an ATR accessory [15]. The FTIReATR peaks attributed to OH groups at 3440 cm1 (broad rounded peak) and to (CeH)CH2 asymmetric stretching at 2925 cm1, symmetric stretching at 2855 cm1 and to CH2 bending at 1465 cm1, as well as, to a strong C]O carbonyl stretching around 1726 cm1 are all typical of an alkyd. Also present are the CeH bending, CeO and C-C stretching fingerprint peaks, in specific, at 1258 cm1 (strong and slightly rounded, likely due to the esters) and 1120 cm1. Finally, the peaks at 1600 cm1 and 1580 cm1 (doublet) corresponding to stretching of the aromatic ring, at 1070 cm1 corresponding to unsaturated aromatic in plane deformation, and at 741 cm1 and 705 cm1 corresponding to aromatic out of plane bending were attributed to the polyester portion of an alkyd [16]. Fig. 2 is the FTIReATR spectra of the alkyd resin used by Winsor & Newton to manufacture their current Griffin Alkyd Colour line. Pigments and filler materials were also observed in the various colours and easily distinguished from the alkyd binder. Fig. 3 shows a comparison of the Winsor & Newton 1980, 1999 and 2006 titanium white films. Present in addition to the resin peaks are peaks at 2520 cm1 (small peak), 1820 cm1 (small peak), 1432 cm1 and 878 cm1 corresponding to a dolomite CaMg(CO3)2 filler and a large peak below 700 cm1 corresponding to the pigment, titanium white. The titanium white films all have similar spectra, indicating that the same general type of formulation was used throughout the years; although it is important to note that this technique cannot bring to light minor changes in the formulation or changes in resin sources/providers. The alizarin crimson and burnt umber films both show major changes attributed to changes in the filler
R. Ploeger et al. / Journal of Cultural Heritage 9 (2008) 412e419
Fig. 2. FTIReATR spectrum: Winsor & Newton’s resin used for the modern Griffin Alkyd paints.
material. The spectra of the 1980 burnt umber film contains additional peaks at 1437 cm1 and 878 cm1 corresponding to a dolomite filler (not present in the 1999 and 2006 films) and the 1980 and 1999 alizarin crimson spectra contained a strong peak at 1070 cm1, overlapping the peak of the binder, later identified as a barium sulphate (BaSO4) filler, as well as dolomite. Present in the alizarin crimson spectra were also the peaks associated with the pigment. The burnt umber 1999 and 2006 film spectra did not show evidence of a major filler, only the pigment, and the alizarin crimson 2006 film spectrum contained evidence of a calcium sulphate dihydrate filler (CaSO4 $ 2H2O) with characteristic peaks at 3400 cm1 (rounded), around 1100 cm1 and 668 cm1. The FTIReATR spectra of Ferrario Alkyd, Colore Alchidico samples contained many of the expected alkyd peaks; however, the filler and pigment peaks dominate the spectra. These paints contain a large amount of barium sulphate with distinguishing peaks at 1180 cm1, 1054 cm1, 983 cm1 and 3296 cm1 (due to absorbed water) and calcium carbonate with peaks at 1442 cm1 and 872 cm1. The peak at 1442 cm1 is shifted higher than what is expected for calcium
0,9 0,8
Absorbance
0,7 0,6 0,5
2006
0,4 0,3
1999
0,2 0,1
1980
0,0 4000
3000
2000
1000
Wavenumbers (cm-1) Fig. 3. FTIReATR spectra: Comparison of Winsor & Newton Alkyd Colour and Griffin Alkyd titanium white alkyd paint films. Top: 2006; middle: 1999; bottom: 1980.
415
carbonate, but it is broad, starting at 1415 cm1 and small absorptions at 1795 cm1 and 2510 cm1 support the conclusion of the presence of calcium carbonate. As a consequence of the higher filler content in the Ferrario alkyd paints, when they are removed from the Mylar substrate they are more fragile and tend to crack off rather than peel like the Winsor & Newton alkyds. The alkyd portion of the Da Vinci Paint Co. Leonardo Oil with Alkyd paints was not distinguishable; in fact, the spectra were similar to those of traditional oil paints. This is in accordance with the fact that these paints are oils with alkyd media mixed into them at a low concentration. The titanium white film appeared to contain a small amount of calcium carbonate filler and the pigment; however, the alizarin crimson film spectrum contained a strong peak at 1010 cm1m which is likely a magnesium silicate talc (3MgO $ 4SiO2 $ H2O) filler. The FTIReATR spectra for the PDQ, Artist Oil Color, Quick Dry Oil paints were complicated and difficult to interpret in some cases, because of the complex and overlapping absorbencies from the organic pigments. Spectra interpretation was easier to perform for the paints containing inorganic pigments, which typically have less complicated spectra. For most of the samples it was possible to identify many of the peaks associated with the alkyd binder. No specific filler materials were identified in the PDQ paint samples; however, it is suspected that a barium sulphate filler may be present in some of the samples and it must be confirmed with another scientific technique. Table 1 shows the major peaks for all the films from different manufacturers analysed. It can be seen that there is very little variation between the absorptions of the Winsor & Newton (Griffin Alkyd and Alkyd Colour) and Ferrario Alkyd films; however there is a significant change for the Da Vinci Paint Co. Leonardo Oil and Alkyd, due to the fact it is mainly a traditional oil paint containing very little alkyd resin. Since aged samples of the Winsor & Newton alkyds were available, evidence of ageing was investigated with FTIRe ATR. Inherent to the ATR technique, despite its convenience, there are many factors that make it difficult to perform quantitative analyses, so qualitative comparisons of the samples were performed. In all the Winsor & Newton colours, with age, there was some increase and broadening of the OH groups peak between 3300 cm1 and 3500 cm1 and broadening in the C]O carbonyl stretching peak at approximately 1726 cm1. These are oxidation reactions taking place during natural ageing with the formation of alcohols and carbonyl species, such as carboxylic acids. This can be observed in Fig. 4 which shows details for these regions in the titanium white films. Similar changes are also observed during the ageing of oil paints [17], which indicates alkyds also undergo similar oxidation reactions that occur during the natural ageing of traditional oil paints. By simple observation by eye, the older films appear to be slightly darker and more yellow, and by handling, the older films are more fragile and brittle; all of these observations are characteristic macro-scale property changes with the natural ageing of traditional oil paints.
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Table 1 The characteristic FTIReATR artists’ alkyd resin spectra peaks (in wavenumbers, cm1, with a 4 cm1 resolution) Sample
(CeH)CH2
(CeH)CH2
C]O
CeO
CeO
C]C
str. aro. est.
str.
aro. i-p-d
str., asym
str. sym.
str.
Winsor & Newton 2006 Titanium white Alizarin crimson Burnt umber Griffin resin
2922 2924 2923 2925
2851 2851 2851 2855
1731 1728 1731 1726
1258 1256 1256 1258
1121 1119 1120 1120
1070 1070 1066 1071
Winsor & Newton 1999 Titanium white Alizarin crimson Burnt umber
2922 2920 2923
2851 2850 2851
1731 1731 1729
1257 1266 1257
1121 1118 1119
1068 1068 1066
Winsor & Newton 1980 Titanium white Alizarin crimson Burnt umber
2922 2921 2923
2850 2851 2851
1727 1728 1730
1259 1266 1258
1121 1116 1118
1070 1068 1060
Ferrario 2006 Titanium white Red Magenta
2923 2921
2853 2851
1728 1732
1247a 1248a
1117 1110
1071a 1062a
Da Vinci Paint Co. 2006 Titanium white Alizarin crimson
2921 2925
2852 2854
1736 1737
1226b 1245b
e e
e e
PDQ 2007 9 colours (range)
2923e2924
2852e2853
1728e1731
1254e1265a
1115e1123a
1070e1074a
str. stretching; sym. symmetric; asym. asymmetric; aro. est. aromatic ester; aro. i-p-d, unsaturated in plane deformation. a Small shoulders in pigment/filler peaks. b Associated with triglyceride esters in the oil.
3.2. THMeGC/MS THMeGC/MS revealed that the composition of Winsor & Newton’s Griffin Alkyd Colour alkyd resin, Fig. 5, and all the Winsor & Newton colours (regardless of age), had a polyol composition of pentaerythritol (peak 2, tetramethyl ether of pentaerythritol m/z 45, 75, 85, 101, 115, 128, peak 3, trimethyl
ether of pentaerythritol m/z 45, 71, 75, 85, 101, 114, 128 and peak 4, dimethyl ether of pentaerythritol m/z 45, 61, 71, 75, 85, 101, 114, 128) and a polybasic acid composition of phthalic acid (original monomer used was phthalic anhydride, peak 5, dimethyl ester of phthalic anhydride m/z 77, 92, 133, 163, 194) [3,6,9]. The methylated fatty acid peaks corresponding to azelaic acid (peak 6, result of ageing), palmitic acid (peak 7), oleic acid 0,26
0,18
0,24 0,16
0,22
0,14
0,20
Absorbance
Absorbance
0,18 0,12 0,10 0,08 Age
0,06
0,16 0,14
Age
0,12 0,10 0,08 0,06
0,04
0,04 0,02
0,02 3600
3400
3200
3000
Wavenumbers (cm-1)
2800
1800
1750
1700
1650
1600
Wavenumbers (cm-1)
Fig. 4. FTIReATR spectra: Details of Winsor & Newton Alkyd Colour and Griffin Alkyd titanium white alkyd paint films. On the left are the spectra between 3300 cm1 and 3500 cm1 and on the right the spectra around the carbonyl C]O peak at approximately 1726 cm1. From youngest to oldest in the direction of the arrow.
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Fig. 5. THMeGC/MS chromatogram of Winsor & Newton’s resin used for the modern Griffin alkyd paints (the labels 1e10 are assigned in Table 2).
(peak 8), stearic acid (peak 9) and linoleic acid (peak 10) were all present [17]. Compounds used to control functionality were also observed, such as benzoic acid (most clear in the titanium white samples). The formation of benzoic acid (peak 1, benzoic acid, methyl ester m/z 51, 77, 105, 136) can also occur as a small side reaction involving the elimination of a complete side-group from phthalic acid (or another similar acid) [1] and was likely what was observed in this study, since Schilling et al. [3] reported that the Winsor & Newton Griffin Alkyd paints tested did not contain benzoic acid. Results from the alkyd resin used to manufacture the Griffin Alkyd Colour line are summarized in Table 2. In addition to the expected alkyd peaks, the chromatograms of all the Winsor & Newton colours contain two unidentified peaks towards the end at approximately 21.9 min (m/z 55, 97, 129, 243) and 22.6 min (m/z 55, 74, 87, 97, 143, 197, 229). These were also observed by Fuesers [13] and Sonoda [9]. In the 2006 samples; the second unknown component was present in a smaller quantity compared to the 1999 and 1980 paint films. They were only present in the paints and not in the alkyd resin, so they could be paint manufacturer additives. It is believed that these peaks may be due to 12-hydroxy and 12-methoxy stearic acids [18], although specific molecules have not been identified. It is suspected that the unknown peaks may represent a hydrogenated
Table 2 THMeGC/MS chromatogram peak assignment for Winsor & Newton resin used for the modern Griffin Alkyd paints Molecule detected (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
Retention time (min)
Methylated benzoic acid 8.5 Methylated pentaerythritol 9.0 Tri-methylated pentaerythritol 10.0 Di-methylated pentaerythritol 10.7 Methylated benzenedicarboxylic acid (from phthalic acid) 13.6 Methylated azelaic acid 14.7 Methylated palmitic acid 18.8 Methylated oleic acid 20.5 Methylated stearic acid 20.7 Methylated linoleic acid 20.9
castor oil based rheology modifier, such as castor wax [19]. Other analytical studies are currently being performed to confirm the presence of these fatty acids and the castor wax hypothesis. The THMeGC/MS results of the 1980, 1999 and 2006 all have similar compositions as the alkyd resin used in the Griffin Alkyd Colour paint and peaks due to the organic alizarin crimson pigment were easily identified. THMeGC/MS results for the Ferrario Alkyd, Colore Alchidico paints showed that they had a polyol composition pentaerythritol and a polybasic acid composition of isophthalic acid (dimethyl ester of isophthalic acid m/z 76, 103, 135, 163, 194) and all the expected fatty acid peaks were present (see results for Winsor & Newton samples). The identification of isophthalic acid, with the experimental conditions used, requires derivatization and if this is not done the alkyd resin can easily be mis-identified and confused with an oil paint. It is important to derivatize samples for py-GC/ MS if an alkyd is suspected. The studies reported in the literature using GC/MS techniques (py-GC/MS, THMeGC/ MS and offline derivatization and GC/MS) with respect to artists’ alkyd paints only address Winsor & Newton paints [1,9,13], with a very distinctive phthalic anhydride peak in the underivatized pyrogram and methylated phthalic acid peak in the pyrograms using methylation derivatization techniques; this implies that this phthalic anhydride is the marker for an alkyd, but in fact it is not always the case. In the lack of phthalic anhydride in the underivatized pyrogram, it has been suggested that isophthalic acid based alkyds can be identified by a higher amount of benzoic acid [1]; however, higher amounts of benzoic acid were not observed in the Ferrario Alkyd paint py-GC/MS pyrograms in relation to the Winsor & Newton pyrograms. The two unknown peaks present in the Winsor & Newton paints are not present in the Ferrario samples; however, there is a phthalic acid based plasticizer peak at 24.2 min. The alkyd portion of the Da Vinci Paint Co. Leonardo Oil with Alkyd paints was detected with THMeGC/MS, unlike FTIReATR. Present, in a very low amount, was an isophthalic acid and pentaerythritol based alkyd resin. The dominant
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peaks in the pyrogram were the methylated fatty acid associated with the oil portion of the binder. The PDQ paints yielded interesting THMeGC-MS results. Seven of the nine paint colours showed that they had a polyol composition of pentaerythritol and a main polybasic acid composition of phthalic anhydride, and all the expected fatty acid peaks were present (see results for Winsor & Newton samples), while the other two had a similar polyol composition and fatty acid composition, but contained isophthalic acid as the main polybasic acid. The presence of terephthalic acid (dimethyl ester of terephthalic acid m/z 76, 103, 135, 163, 179, 194) was noted in smaller amounts in some of the samples as well as smaller amounts of isophthalic acid and phthalic acid in relation to the main polybasic acid. It is unknown why there is a change in alkyd resin formulations; there may have been changes in the alkyd supplier, conscience decisions to change the alkyd resin, changes in manufacturing dates or the use of specific resins for certain colours. The detection of terephthalic acid in some samples may also be a result of contamination from the Mylar substrate due to the method used to remove the samples from the substrate. The two unknown peaks present in the Winsor & Newton formulations are also present in all the PDQ paint samples analysed. Phthalic anhydride (phthalic acid) is the most commonly used polybasic acid for alkyd resins. Isophthalic acid is the second most used polybasic acid; these resins have tougher films that are more resistant to hydrolysis compared to phthalic acid based resins. Terephthalic acid is sometimes used in alkyds, because it is less expensive and has a similar hydrolytic stability as isophthalic acid in resins; however, terephthalic diesters tend to partially crystallize in the final resin solution, giving hazy resins and possibly leading to film defects [5]. 3.3. Palmitic to stearic (P/S) peak ratio The palmitic to stearic acid (P/S) ratio is often used to identify oils, because their ratio to each other should not change during the oxidation and film formation processes; they are both saturated and do not participate in the auto-oxidation reactions. The P/S peak area ratio was examined with the hypothesis that it would represent the original oil used in the manufacturing of the alkyd resin. The most widely used oil for alkyds is soya oil [5], and was suspected for these paints, and it has a relatively high P/S ratio of 2.75 [20]. P/S peak area ratio analysis gave mixed results, making it impossible to conclusively evaluate and identify the oil from which the fatty acids were derived. This type of variability amongst oils and difficulty in reproducibility are common issues in THMeGC/MS analyses [21]. Also, a recent study by Tsakalof et al. [22] demonstrated that there was a non-linear GC/MS instrument response when calculating P/S ratios and their dependence on sample dilutions; thus indicating that there is unpredictability in the interpretations of P/S peak ratios, which hinders drying oil identification using this technique. However, it was observed that these paints have different P/S ratios; the P/S ratios for the Winsor & Newton samples (for all the years) ranged between 1.34 0.11 and 1.53 0.12, was 2.06 0.08 for the Ferrario Alkyd, Colore Alchidico paint and was 1.91 0.08 for
the PDQ samples (values given with 95% confidence). There was no way to obtain reasonable P/S peak area ratios for the alkyd resin used in the Da Vinci Paint Co. Leonardo Oil with Alkyd paints. The P/S ratios obtained were an addition of the palmitic and stearic acids present in both the oil portion and the alkyd portion. There appears to be a difference between the fatty acids used for the Ferrario Alkyd, Colore Alchidico and PDQ paints and the Winsor & Newton formulations, but the exact changes could not be determined. An added complication to this is the possibility of mixtures of fatty acids [23] or the substitution of one oil for another based on market availability and prices [5]. These changes can be done, along with modifications in the formulations, without altering the performance of the alkyd [5]. The THMeGC/MS technique cannot distinguish whether the mixture arises from combinations of different natural oils and fatty acids. 4. Conclusions There are no major changes in the polyester monomers in the Winsor & Newton alkyd resins, but it is unknown if there were major changes in the oil used (fatty acids). Also minor additive changes are not detectable, only changes in the fillers used, which has been noted. The evolution of formulations is inevitable, since polymer manufacturers can change their resin formulations without warning and paint manufacturers can modify their formulations in order to provide a better product or change their resin manufacturer. FTIReATR can be used to identify an alkyd resin provided there are no overlapping signals from other additives with the characteristic peaks. Some of the unique peaks are those due to the aromatic portion of the polyester backbone at 1600 cm1 and 1580 cm cm1 (doublet), 1070 cm1, 741 cm1 and 705 cm1. Other unique peaks are those in the fingerprint area around 1258 cm1 and 1121 cm1. The alkyd portion of the Da Vinci Paint Co. Leonardo Oil with Alkyd paints was not visible in the FTIReATR spectra. The major fillers and pigments are also easily identified: dolomite or calcium carbonate and barium sulphate being the predominant fillers in both the Winsor & Newton and Ferrario Alkyd, Colore Alchidico paints and talc in the Da Vinci Paint Co. Leonardo Oil with Alkyd paints. The amount of filler was much higher in the Ferrario Alkyd, Colore Alchidico paints. No fillers were identified in the PDQ paints, although the presence of barium sulphate was suspected in some samples. Some ageing processes were also identified by qualitatively comparing the 1980, 1999 and 2006 Winsor & Newton films. There was broadening of the C]O peak at 1726 cm1 and an increase and broadening of the OH peak between 3300 cm1 and 3500 cm1. These indicate the formation of oxygenated functional groups from oxidative degradation. THMeGC/S with derivatization allows the identification the polyol and polybasic acid monomers used to manufacture the alkyd resin. The fatty acids were also identified, but it was not possible to identify the oil from which they were derived. Winsor & Newton alkyds used a pentaerythritol and phthalic anhydride based alkyd resin and Ferrario Alkyd, Colore Alchidico and Da Vinci Paint Co. Leonardo Oil with Alkyd
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paints used pentaerythritol and isophthalic acid based alkyd resins. The PDQ paints were made with both phthalic anhydride and isophthalic acid based alkyd resins. It is important to always derivatize the sample if a technique similar to that used in this study is used and an alkyd is suspected. Phthalic anhydride will be detected in an underivatized alkyd pyrogram; however, isophthalic acid will not, leading to confusion and the possibility of the paint being incorrectly identified. Other topics to be further investigated using THMeGC/MS are whether this technique can distinguish between a long and short oil alkyd and semi-drying and drying alkyd. The ratio of polyester components (polyols and polybasic acids) to the fatty acids should reveal information about the oil length; the higher the ratio, the shorter the oil length or the percentage of the oil in the alkyd. Schilling et al. [3,24] have suggested a similar technique. If a sample of wet tube paint was available, the identification of a drying or semi-drying oil alkyd could be easily determined by calculating the amount of un-saturated fatty acids present. However, in dry films these acids are consumed in the auto-oxidation reaction and are no longer present [24]. 5. Materials Winsor & Newton Artists’ Alkyd Colour and Griffin Alkyd Colour Whitefriars Avenue, Harrow, Middlesex, HA3 5RH, UK www.winsornewton.com Ferrario Color SRL, Ferrario Alkyd, Colore Alchidico Via Marzocchi, 29, 40012, Calderara Di Reno (BO), Italy http://www.apaferrario.it/ing/index.shtml Da Vinci Paint Co., Leonardo Oil with Alkyd 11 Good Year Street, Irvine, CA 92618, USA http://www.davincipaints.com/ PDQ, Paints Dry Quick, Artist Oil Color, Quick Dry Oilddiscontinued Colours: alizarin crimson, cadmium red light, cobalt violet, fire red, green gold, hansa yellow (light), indofast orange, Prussian blue, permanent green (light) San Jose, CA Acknowledgements We would like to thank Mr. A. Foster at Winsor & Newton for supplying us with the alkyd resin used to make the modern Griffin Alkyd Colour line. Also we would like to thank Dr. M. Mecklenburg at the Smithsonian Institution, Washington, DC for providing us with the aged Winsor & Newton samples from 1980 and 1999. Finally we would like to thank the Art Materials Research and Study Center, National Gallery of Art, Washington, DC for the PDQ paint samples. References [1] T. Learner, Analysis of Modern Paints, Getty Publication, Canada, 2004. [2] C.M. Lack, Performance and Working Properties of Artists’ Alkyd Paints, M.A.C. thesis, Queen’s University, Canada 1988.
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[3] M. Schilling, J. Keeney, T. Learner, Characterization of alkyd paint media by gas chromatographyemass spectrometry, in: A. Roy, P. Smith (Eds.), Modern Art, New Museums, Postprints of the IIC 20th Congress, Bilbao, September 13e17, The International Institute for Conservation of Historic and Artistic Works, London, 2004, pp. 197e201. [4] H. Standeven, The development of decorative gloss paints in Britain and the United States c. 1910e1960, J. Am. Inst. Conserv 45 (2006) 55e65. [5] Z.W. Wicks Jr., F.N. Jones, S.P. Pappas, Organic Coatings: Science and Technology, Vol. 1: Film Formation, Components and Appearance, John Wiley & Sons, Inc., New York, 1992. [6] P. Staples, Master classes in alkyds: the development of alkyd colours, The Artist 97(1) (1982) 11e14. [7] F. Cappitelli, F. Koussiaki, THMeGCMS and FTIR for the investigation of paints in Picasso’s Still Life, Weeping Woman and Nude Woman in a Red Armchair from the Tate Collection, London, J. Anal. Appl. Pyrol 75 (2006) 200e204. [8] J. Crook, T. Learner, The Impact of Modern Paint, Tate Gallery Publishing Ltd., London, 2000. [9] N. Sonoda, Application des me´thodes chromatographiques a` la caracte´risation des peintures alkydes pour artistes, Techne 8 (1998). [10] J.D. Erlebacher, E. Brown, M.F. Mecklenburg, C.S. Tumosa, The effects of temperature and relative humidity on the mechanical properties of modern painting materials, in P.B. Vandiver, J.R. Druzik (Eds.), Materials Issue in Art and Archeology III Proceedings of MRS conference Vol. 267 San Francisco April 27eMay 1, MRS, Warrendale 1992, pp. 359e370. [11] I. Civil, M. Michalski, A. Murray, Cracking the ‘matter paintings’ of Antoni Ta`pies: the role of artistic intent, deterioration and underlying mechanical causes, in: R. Vontobel (Ed.), Preprint of ICOM-CC, 13th Triennial Meeting, Rio de Janeiro, September 22e27, James & James (Science Publishers) Ltd., London, 2002, pp. 407e413. [12] A. Phenix, The swelling of artists’ paints in organic solvents. Part 2, Comparative swelling powers of selected organic solvents and solvent mixtures, J. Am. Inst. Conserv 41 (2002) 61e90. [13] O. Fuesers, Zum Einfluss organischer Lo¨semittel auf die mechanischen ¨ lfarbe, diploma thesis Hochschule Eigenschaften von Alkydharz- und O der Ku¨nste Bern, Switzerland2006. [14] A. Piccirillo, D. Scalarone, O. Chiantore, Comparison between off-line and on-line derivatisation methods in the characterisation of siccative oils in paint media, J. Anal. Appl. Pyrol 74 (2005) 33e38. [15] M.R. Derrick, D. Stulik, J.M. Landry, Infrared Spectroscopy in Conservation Science, J. Paul Getty Trust, U.S.A, 1999. [16] U. Knuutinen, P. Kyllonen, Two case studies of unsaturated polyester composite art objects, e-Preservation Science 3 (2006) 11e19. [17] M. Lazzari, O. Chiantore, Drying and oxidative degradation of linseed oil, Polym. Degrad. Stabil 65 (1999) 303e313. [18] K. Vosmann, E. Schulte, E. Klein, N. Weber, Formation of N- and Omethyl derivatives of lipids containing amino, amide or hydroxy groups by the pyrolytic reaction with trimethylsulfonium hydroxide, Fett. Lipid 100 (1998) 334e342. [19] Cray Valley, Castor wax technical bulletin: Hydrogenated castor oil based rheology modifiers March 2001, http://www.crayvallac.com/ indust.html (December 2007). [20] K.J. Saunders, Organic Polymer Chemistry, Chapman and Hall, London, 1973. [21] F. Cappitelli, T. Learner, O. Chiantore, An initial assessment of thermally assisted hydrolysis and methylation-gas chromatography/mass spectrometry for the identification of oils from dries paint films, J. Anal. Appl. Pyrol 63 (2002) 339e348. [22] A.K. Tsakalof, K.A. Bairachtari, I.D. Chryssoulakis, Pitfalls in drying oils identification in art objects by gas chromatography, J. Sep. Sci. 29 (2006) 1642e1646. [23] T.C. Patton, Alkyd Resin Technology, Formulating Techniques and Allied Calculations, Interscience Publishers, New York, 1962. [24] J.M. Challinor, Structure determination of alkyd resins by simultaneous pyrolysis methylation, J. Anal. Appl. Pyrol 18 (1991) 233e244.
www.sciencedirect.com Journal of Cultural Heritage 9 (2008) 412e419
Original article
The characterization of commercial artists’ alkyd paints Rebecca Ploeger*, Dominique Scalarone, Oscar Chiantore Department of I.P.M. Chemistry and NIS (Nanostructured Surfaces and Interfaces) Center of Excellence, University of Torino, Via Pietro Giuria, 7, 10125 Torino, Italy Received 2 October 2007; accepted 9 January 2008
Abstract There is little information in the conservation literature with respect to artists’ alkyd paints; thus, artists and conservators are somewhat at a loss about how to use and treat alkyds. Recently, analytical methods have been developed to identify the components in these polymers (oil modified polyesters), rates of cross-linking and mechanical properties. Presented in this paper are some of the characterization results of artists’ alkyd paints using THM (thermally assisted hydrolysis and methylation) gas chromatographye mass spectrometry (THMeGC/MS) and Fourier transform infrared spectroscopyeattenuated total reflectance (FTIReATR). Four brands of artists’ alkyd paints containing alkyd resin have been analysed; one containing a phthalic anhydride and pentaerythritol based alkyd resin, two containing isophthalic acid and pentaerythritol based alkyd resins, and the final one containing both phthalic anhydride and isophthalic acid and pentaerythritol based resins among the colours studied. Ó 2008 Elsevier Masson SAS. All rights reserved. Keywords: Alkyd paints; Ageing; Characterization; Artists’ paints; THMeGC/MS; FTIReATR
1. Introduction The name ‘alkyd’ is derived from the alcohol and acid monomers used to make the polyester polymer, and an alkyd resin is an oil modified alkyd polyester. Alkyd resins are made by condensation polymerization of polyols (at least three hydroxyl groups), polybasic acids and a source of fatty acids (either siccative oils or free fatty acids). The final resin is composed of a polyester backbone, on it’s own a highly branched polymer, and dangling fatty acids, which serve to reduce the amount of cross-linking creating a more flexible polymer; Fig. 1 shows a more simple alkyd, composed of phthalic anhydride (difunctional), glycerol (trifunctional) and linoleic acid. The types of fatty acids used are important in determining the final drying characteristics of the material, since they dry through auto-oxidation reactions, much like an oil paint, after solvent evaporation [1]. Out of all the artists’ * Corresponding author. Tel.: þ39 011 670 7554; fax: þ39 011 670 7855. E-mail addresses: [email protected] (R. Ploeger), dominique. [email protected] (D. Scalarone), [email protected] (O. Chiantore). 1296-2074/$ - see front matter Ó 2008 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.culher.2008.01.007
synthetic paints, artists’ alkyd paints have the most similar final visual appearance to traditional oil paint [2]. Their main advantage is their speed of drying, which is 18e24 h (touch dry) due to the high molecular weight alkyd resin and the need for fewer crosslinks for film formation. Alkyds are classified by ‘oil length’, i.e. the weight percent of oil or triglyceride equivalent or fatty acids in the finished resin, and oil type. Varying these two factors has a great impact on the final properties and the ultimate applications of the resins. Alkyd resins used for artists’ paints are classified as ‘long oil’ alkyds, because they contain between 56e70 weight percent oil or fatty acids. ‘Short oil’ (35e45 weight percent), ‘medium oil’ (46e55 weight percent) and ‘very long oil’ (greater than 70 weight percent) alkyd resins are also manufactured and used in various other coatings applications [3]. Drying and semi-drying oils, such as linseed, soya, safflower and castor oil are commonly used in ‘long oil’ applications because of their fatty acid compositions and lower costs [3e 5]. The fatty acid portion gives many important properties to the resin, including, cross-linking potential, flexibility, compatibility with solvents and control of solubility [4].
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O
O
*
O
O
* n O
O
Fig. 1. An ideal alkyd resin (containing, phthalic anhydride, glycerol and linoleic acid).
Linseed oil and soya oil are the most commonly oils used in alkyd house paint formulations [3]. Linseed oil, a drying oil, contains a large amount of linolenic acid, which tends to yellow considerably as it ages and is avoided for white or light coloured paints. Soya oil, along with safflower oil, castor oil and sunflower oil, contain more linoleic acid that does not yellow as much, but being semi-drying oils, they require more time to dry. Both Lack and Schilling et al. [2,3] claim that Winsor & Newton typically uses a soya based alkyd resin in their Griffin Alkyd paint formulations. Since artists’ alkyd paints contain up to 70 weight percent fatty acids, one can intuitively assume that they undergo similar ageing reactions as oil paints and that similar degradation products will be present. In fact, Staples [6], the technical director of Winsor & Newton in 1982, claimed that ‘‘as many of the constituent parts of alkyd resins come from ingredients found in drying oils, it is not surprising that many of the properties of the alkyd paints are similar to those of natural drying oils’’. However, the assumption that they have similar stability properties has yet to be confirmed and the ageing properties of the polyester portion must also be taken into account. There are very few noted conservation concerns and published information about artists’ alkyd paints, because they are a relatively new artist specific medium. As a result, they have been used less by artists and conservators have not encountered them much in their practices. Winsor & Newton, Middlesex, England, was the first colour maker to introduce an artists’ alkyd line; they introduced their original line of artist alkyds in 1976, Artists’ Alkyds followed by London and Griffin Alkyd Colour in 1980 [2]. In the USA there was PDQ (Paints Dry Quick) with their Artist Oil Color, Quick Dry Oil line; formerly of San Jose, CA, which no longer exists. It has been suggested, in personal communications, that PDQ alkyds appeared on parts of the American market before Winsor & Newton alkyds; however, this is not confirmed. Also available in the USA were Shiva Inc. Alkyd Colors; however, they were also discontinued and Shiva Inc. is now part of Jack Richeson & Co. Inc. in Kimberly, WI. Currently available artists’ alkyd paints are
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Winsor & Newton’s Griffin Alkyd paints, Ferrario Alkyd, Colore Alchidico, Calderara di Reno (BO), Italy and Da Vinci Paint Co.’s Leonardo Oil with Alkyd, Irvine, CA (this is a traditional oil paint with an alkyd media mixed in). Many artists’ materials manufacturers also offer alkyd based media, which can be added to traditional oil paints to alter the rate of drying and film properties. Industrial and household alkyd paints have been used in works of art since their first introduction in the late 1920s/ early 1930s. Some of the more prominent artists that have used industrial alkyds are Jackson Pollock and Pablo Picasso [4,7,8]. The characterization of the main components of the artists’ alkyd resins, additives (fillers) and pigments must be done to provide a foundation for the study and identification of these materials, both artist specific and industrial, in works of art. The literature concerning artists’ alkyd paints is very limited, which forces one to search the literature of oil paints and to make assumptions, although in the past few years several artists’ alkyd studies have been initiated. Characterization studies have been performed by Learner [1] using PyGC/MS and FTIR, by Sonoda [9] using GC/MS with thermally assisted hydrolysis and methylation (THMeGC/MS) and Schilling et al. [3] using GC/MS with an offline methylation procedure for quantitative analysis of the alkyd polymer components. Mechanical properties of artists’ alkyd paints have been studied and compared to the mechanical performance of artists’ acrylic paints by Erlebacher et al. [10], and Civil et al. [11] investigated the cracking in paintings by Antoni Ta`pies, who used commercial household alkyd coatings as his medium. Phenix [12], in his studies of the swelling of oil paint films, mentions the increased sensitivity and plasticization of alkyds in water as a result of the oxidative drying process and the formation of hydrophilic groups. Standeven [4] has studied, in depth, the development of alkyd paints (commercial household) in Britain and the USA and has published her findings for the early years (1910e1960) of the material. Finally Lack [2] investigated some of the performance and working properties of artists’ alkyd paints and Fuesers [13], in an unpublished diploma thesis, characterized artists’ alkyds with an offline derivatization technique and GC/ MS. The additives of these paints were also extracted with various solvents and analysed. To expand the limited conservation literature concerning alkyds, this paper will discuss the characterization of all the current commercially available, to the best of our knowledge, alkyd based paints: Winsor & Newton’s Griffin Alkyd, Ferrario Alkyd’s, Colore Alchidico and Da Vinci Paint Co.’s Leonardo Oil with Alkyd, as well as, the discontinued PDQ alkyd paints. The paints were characterized using THM (thermally assisted hydrolysis and methylation) gas chromatographyemass spectrometry (THMeGC/MS) and Fourier transform infrared spectroscopyeattenuated total reflectance (FTIReATR), both these techniques being regularly used in the cultural heritage field. As mentioned, there is also a variety of alkyd media available to artists to add to traditional oil paints; however, this paper will examine only the alkyd paints.
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It is hoped that the information presented can help clear up confusion or aid in the identification of alkyd materials in works of art. 2. Experimental 2.1. The paints Winsor & Newton alizarin crimson, burnt umber and titanium white were studied; the samples were cast on Mylar in 1980, 1999 and 2006 (3 cm wide strips approximately 0.2 mm in thickness) and aged naturally in a laboratory environment (the 1980 and 1999 samples were provided by the Smithsonian Institution in Washington, DC). A sample, provided by Winsor & Newton, of the alkyd resin used in their current Griffin Alkyd Colours formulations was also analysed. The paints cast in 1980 where from Winsor & Newton’s original Artists’ Alkyd Colour line and the paints cast in 1999 were from their later, and only currently available line, Griffin Alkyd Colour. Ferrario Alkyd, Colore Alchidico, rosso magenta (red magenta) and bianco titanio (titanium white) and Da Vinci Paint Co. Leonardo Oil with Alkyd alizarin crimson and titanium, both purchased and cast in 2006 were studied. The films were cast in a similar fashion and aged naturally in the laboratory. Nine different coloured samples from PDQ (Paints Dry Quick), Artist Oil Color, Quick Dry Oil line, provided by the Art Materials Research and Study Center, National Gallery of Art, Washington, DC were also investigated. The colours were: alizarin crimson, cadmium red medium, cobalt violet, fire red, green gold, hansa yellow (light), indofast orange, Prussian blue and permanent green light. These samples were cast in 2007 on Mylar, undercut and removed from the Mylar substrate with a razor blade and stored in clean glass sample vials. It should be noted that the Da Vinci Paint Co. Leonardo Oil with Alkyd paints are traditional oil based paints with an alkyd medium mixed into them and not alkyd based paints like the Winsor & Newton Griffin Alkyd, Ferrario Alkyd, Colore Alchidico and PDQ paints. The titanium white is listed as a refined safflower oil and alkyd resin, and the alizarin crimson as a refined linseed oil and alkyd resin. These paints were included in the study since they are offered as a commercial oil paint pre-modified with an alkyd resin. 2.2. Characterization studies Fourier transform infrared spectroscopyeattenuated total reflectance (FTIReATR) and thermally assisted hydrolysis and methylationegas chromatographye mass spectrometry (THMe GC/MS) using a 25 wt% aqueous solution of tetramethylammonium hydroxide (TMAH) (from Aldrich), have been used to characterize the binding materials, fillers and pigments used in the paints. FTIReATR was performed with a Thermo Nicolet FTIR NEXUS instrument and Smart Endurance ATR accessory, from 4000 to 500 cm1 and for 32 scans with a resolution of 4 cm1. Data was collected and analysed in OMNIC 6.1a software.
For the Py-GC/MS studies, an Agilent Technologies 6890N Network GC system GC with an Agilent 5973 Network Mass Selective Detector quadrupole mass spectrometer was used along with a CDS Analytical Inc. Pyroprobe 1000 heated filament pyrolyser. Samples were prepared in CDS Analytical quartz tubes with quartz wool. A typical program for painting materials was used, with a 2 min solvent delay (50 C, 2 min; 10 C/min to 300 C; 300 C, 5 min). Data were collected with Agilent Technologies’ Enhanced ChemStation software version D.00.00.38 and analysed with MS Data Analysis Version C.03.00 HP software. Derivatization with TMAH is the recommended method for analysing drying oils [14]. Approximately 7 mL of TMAH solution was injected into the quartz tube and allowed to mix with the sample for at least 1 h to ensure complete contact between the samples and TMAH for full methylation upon pyrolysis. 3. Results and discussion 3.1. FTIReATR FTIReATR results for the Winsor & Newton Artists’ Alkyd Colour (1980) and Griffin Alkyd Colour (1999 and 2006) samples show that they are typical alkyd materials. The peaks observed in this work corresponded well to the peaks (from diamond cell transmission spectra) reported by Learner [1], taking into account the expected small shift of the peaks to lower wavenumbers in the spectra recorded using an ATR accessory [15]. The FTIReATR peaks attributed to OH groups at 3440 cm1 (broad rounded peak) and to (CeH)CH2 asymmetric stretching at 2925 cm1, symmetric stretching at 2855 cm1 and to CH2 bending at 1465 cm1, as well as, to a strong C]O carbonyl stretching around 1726 cm1 are all typical of an alkyd. Also present are the CeH bending, CeO and C-C stretching fingerprint peaks, in specific, at 1258 cm1 (strong and slightly rounded, likely due to the esters) and 1120 cm1. Finally, the peaks at 1600 cm1 and 1580 cm1 (doublet) corresponding to stretching of the aromatic ring, at 1070 cm1 corresponding to unsaturated aromatic in plane deformation, and at 741 cm1 and 705 cm1 corresponding to aromatic out of plane bending were attributed to the polyester portion of an alkyd [16]. Fig. 2 is the FTIReATR spectra of the alkyd resin used by Winsor & Newton to manufacture their current Griffin Alkyd Colour line. Pigments and filler materials were also observed in the various colours and easily distinguished from the alkyd binder. Fig. 3 shows a comparison of the Winsor & Newton 1980, 1999 and 2006 titanium white films. Present in addition to the resin peaks are peaks at 2520 cm1 (small peak), 1820 cm1 (small peak), 1432 cm1 and 878 cm1 corresponding to a dolomite CaMg(CO3)2 filler and a large peak below 700 cm1 corresponding to the pigment, titanium white. The titanium white films all have similar spectra, indicating that the same general type of formulation was used throughout the years; although it is important to note that this technique cannot bring to light minor changes in the formulation or changes in resin sources/providers. The alizarin crimson and burnt umber films both show major changes attributed to changes in the filler
R. Ploeger et al. / Journal of Cultural Heritage 9 (2008) 412e419
Fig. 2. FTIReATR spectrum: Winsor & Newton’s resin used for the modern Griffin Alkyd paints.
material. The spectra of the 1980 burnt umber film contains additional peaks at 1437 cm1 and 878 cm1 corresponding to a dolomite filler (not present in the 1999 and 2006 films) and the 1980 and 1999 alizarin crimson spectra contained a strong peak at 1070 cm1, overlapping the peak of the binder, later identified as a barium sulphate (BaSO4) filler, as well as dolomite. Present in the alizarin crimson spectra were also the peaks associated with the pigment. The burnt umber 1999 and 2006 film spectra did not show evidence of a major filler, only the pigment, and the alizarin crimson 2006 film spectrum contained evidence of a calcium sulphate dihydrate filler (CaSO4 $ 2H2O) with characteristic peaks at 3400 cm1 (rounded), around 1100 cm1 and 668 cm1. The FTIReATR spectra of Ferrario Alkyd, Colore Alchidico samples contained many of the expected alkyd peaks; however, the filler and pigment peaks dominate the spectra. These paints contain a large amount of barium sulphate with distinguishing peaks at 1180 cm1, 1054 cm1, 983 cm1 and 3296 cm1 (due to absorbed water) and calcium carbonate with peaks at 1442 cm1 and 872 cm1. The peak at 1442 cm1 is shifted higher than what is expected for calcium
0,9 0,8
Absorbance
0,7 0,6 0,5
2006
0,4 0,3
1999
0,2 0,1
1980
0,0 4000
3000
2000
1000
Wavenumbers (cm-1) Fig. 3. FTIReATR spectra: Comparison of Winsor & Newton Alkyd Colour and Griffin Alkyd titanium white alkyd paint films. Top: 2006; middle: 1999; bottom: 1980.
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carbonate, but it is broad, starting at 1415 cm1 and small absorptions at 1795 cm1 and 2510 cm1 support the conclusion of the presence of calcium carbonate. As a consequence of the higher filler content in the Ferrario alkyd paints, when they are removed from the Mylar substrate they are more fragile and tend to crack off rather than peel like the Winsor & Newton alkyds. The alkyd portion of the Da Vinci Paint Co. Leonardo Oil with Alkyd paints was not distinguishable; in fact, the spectra were similar to those of traditional oil paints. This is in accordance with the fact that these paints are oils with alkyd media mixed into them at a low concentration. The titanium white film appeared to contain a small amount of calcium carbonate filler and the pigment; however, the alizarin crimson film spectrum contained a strong peak at 1010 cm1m which is likely a magnesium silicate talc (3MgO $ 4SiO2 $ H2O) filler. The FTIReATR spectra for the PDQ, Artist Oil Color, Quick Dry Oil paints were complicated and difficult to interpret in some cases, because of the complex and overlapping absorbencies from the organic pigments. Spectra interpretation was easier to perform for the paints containing inorganic pigments, which typically have less complicated spectra. For most of the samples it was possible to identify many of the peaks associated with the alkyd binder. No specific filler materials were identified in the PDQ paint samples; however, it is suspected that a barium sulphate filler may be present in some of the samples and it must be confirmed with another scientific technique. Table 1 shows the major peaks for all the films from different manufacturers analysed. It can be seen that there is very little variation between the absorptions of the Winsor & Newton (Griffin Alkyd and Alkyd Colour) and Ferrario Alkyd films; however there is a significant change for the Da Vinci Paint Co. Leonardo Oil and Alkyd, due to the fact it is mainly a traditional oil paint containing very little alkyd resin. Since aged samples of the Winsor & Newton alkyds were available, evidence of ageing was investigated with FTIRe ATR. Inherent to the ATR technique, despite its convenience, there are many factors that make it difficult to perform quantitative analyses, so qualitative comparisons of the samples were performed. In all the Winsor & Newton colours, with age, there was some increase and broadening of the OH groups peak between 3300 cm1 and 3500 cm1 and broadening in the C]O carbonyl stretching peak at approximately 1726 cm1. These are oxidation reactions taking place during natural ageing with the formation of alcohols and carbonyl species, such as carboxylic acids. This can be observed in Fig. 4 which shows details for these regions in the titanium white films. Similar changes are also observed during the ageing of oil paints [17], which indicates alkyds also undergo similar oxidation reactions that occur during the natural ageing of traditional oil paints. By simple observation by eye, the older films appear to be slightly darker and more yellow, and by handling, the older films are more fragile and brittle; all of these observations are characteristic macro-scale property changes with the natural ageing of traditional oil paints.
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Table 1 The characteristic FTIReATR artists’ alkyd resin spectra peaks (in wavenumbers, cm1, with a 4 cm1 resolution) Sample
(CeH)CH2
(CeH)CH2
C]O
CeO
CeO
C]C
str. aro. est.
str.
aro. i-p-d
str., asym
str. sym.
str.
Winsor & Newton 2006 Titanium white Alizarin crimson Burnt umber Griffin resin
2922 2924 2923 2925
2851 2851 2851 2855
1731 1728 1731 1726
1258 1256 1256 1258
1121 1119 1120 1120
1070 1070 1066 1071
Winsor & Newton 1999 Titanium white Alizarin crimson Burnt umber
2922 2920 2923
2851 2850 2851
1731 1731 1729
1257 1266 1257
1121 1118 1119
1068 1068 1066
Winsor & Newton 1980 Titanium white Alizarin crimson Burnt umber
2922 2921 2923
2850 2851 2851
1727 1728 1730
1259 1266 1258
1121 1116 1118
1070 1068 1060
Ferrario 2006 Titanium white Red Magenta
2923 2921
2853 2851
1728 1732
1247a 1248a
1117 1110
1071a 1062a
Da Vinci Paint Co. 2006 Titanium white Alizarin crimson
2921 2925
2852 2854
1736 1737
1226b 1245b
e e
e e
PDQ 2007 9 colours (range)
2923e2924
2852e2853
1728e1731
1254e1265a
1115e1123a
1070e1074a
str. stretching; sym. symmetric; asym. asymmetric; aro. est. aromatic ester; aro. i-p-d, unsaturated in plane deformation. a Small shoulders in pigment/filler peaks. b Associated with triglyceride esters in the oil.
3.2. THMeGC/MS THMeGC/MS revealed that the composition of Winsor & Newton’s Griffin Alkyd Colour alkyd resin, Fig. 5, and all the Winsor & Newton colours (regardless of age), had a polyol composition of pentaerythritol (peak 2, tetramethyl ether of pentaerythritol m/z 45, 75, 85, 101, 115, 128, peak 3, trimethyl
ether of pentaerythritol m/z 45, 71, 75, 85, 101, 114, 128 and peak 4, dimethyl ether of pentaerythritol m/z 45, 61, 71, 75, 85, 101, 114, 128) and a polybasic acid composition of phthalic acid (original monomer used was phthalic anhydride, peak 5, dimethyl ester of phthalic anhydride m/z 77, 92, 133, 163, 194) [3,6,9]. The methylated fatty acid peaks corresponding to azelaic acid (peak 6, result of ageing), palmitic acid (peak 7), oleic acid 0,26
0,18
0,24 0,16
0,22
0,14
0,20
Absorbance
Absorbance
0,18 0,12 0,10 0,08 Age
0,06
0,16 0,14
Age
0,12 0,10 0,08 0,06
0,04
0,04 0,02
0,02 3600
3400
3200
3000
Wavenumbers (cm-1)
2800
1800
1750
1700
1650
1600
Wavenumbers (cm-1)
Fig. 4. FTIReATR spectra: Details of Winsor & Newton Alkyd Colour and Griffin Alkyd titanium white alkyd paint films. On the left are the spectra between 3300 cm1 and 3500 cm1 and on the right the spectra around the carbonyl C]O peak at approximately 1726 cm1. From youngest to oldest in the direction of the arrow.
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417
Fig. 5. THMeGC/MS chromatogram of Winsor & Newton’s resin used for the modern Griffin alkyd paints (the labels 1e10 are assigned in Table 2).
(peak 8), stearic acid (peak 9) and linoleic acid (peak 10) were all present [17]. Compounds used to control functionality were also observed, such as benzoic acid (most clear in the titanium white samples). The formation of benzoic acid (peak 1, benzoic acid, methyl ester m/z 51, 77, 105, 136) can also occur as a small side reaction involving the elimination of a complete side-group from phthalic acid (or another similar acid) [1] and was likely what was observed in this study, since Schilling et al. [3] reported that the Winsor & Newton Griffin Alkyd paints tested did not contain benzoic acid. Results from the alkyd resin used to manufacture the Griffin Alkyd Colour line are summarized in Table 2. In addition to the expected alkyd peaks, the chromatograms of all the Winsor & Newton colours contain two unidentified peaks towards the end at approximately 21.9 min (m/z 55, 97, 129, 243) and 22.6 min (m/z 55, 74, 87, 97, 143, 197, 229). These were also observed by Fuesers [13] and Sonoda [9]. In the 2006 samples; the second unknown component was present in a smaller quantity compared to the 1999 and 1980 paint films. They were only present in the paints and not in the alkyd resin, so they could be paint manufacturer additives. It is believed that these peaks may be due to 12-hydroxy and 12-methoxy stearic acids [18], although specific molecules have not been identified. It is suspected that the unknown peaks may represent a hydrogenated
Table 2 THMeGC/MS chromatogram peak assignment for Winsor & Newton resin used for the modern Griffin Alkyd paints Molecule detected (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
Retention time (min)
Methylated benzoic acid 8.5 Methylated pentaerythritol 9.0 Tri-methylated pentaerythritol 10.0 Di-methylated pentaerythritol 10.7 Methylated benzenedicarboxylic acid (from phthalic acid) 13.6 Methylated azelaic acid 14.7 Methylated palmitic acid 18.8 Methylated oleic acid 20.5 Methylated stearic acid 20.7 Methylated linoleic acid 20.9
castor oil based rheology modifier, such as castor wax [19]. Other analytical studies are currently being performed to confirm the presence of these fatty acids and the castor wax hypothesis. The THMeGC/MS results of the 1980, 1999 and 2006 all have similar compositions as the alkyd resin used in the Griffin Alkyd Colour paint and peaks due to the organic alizarin crimson pigment were easily identified. THMeGC/MS results for the Ferrario Alkyd, Colore Alchidico paints showed that they had a polyol composition pentaerythritol and a polybasic acid composition of isophthalic acid (dimethyl ester of isophthalic acid m/z 76, 103, 135, 163, 194) and all the expected fatty acid peaks were present (see results for Winsor & Newton samples). The identification of isophthalic acid, with the experimental conditions used, requires derivatization and if this is not done the alkyd resin can easily be mis-identified and confused with an oil paint. It is important to derivatize samples for py-GC/ MS if an alkyd is suspected. The studies reported in the literature using GC/MS techniques (py-GC/MS, THMeGC/ MS and offline derivatization and GC/MS) with respect to artists’ alkyd paints only address Winsor & Newton paints [1,9,13], with a very distinctive phthalic anhydride peak in the underivatized pyrogram and methylated phthalic acid peak in the pyrograms using methylation derivatization techniques; this implies that this phthalic anhydride is the marker for an alkyd, but in fact it is not always the case. In the lack of phthalic anhydride in the underivatized pyrogram, it has been suggested that isophthalic acid based alkyds can be identified by a higher amount of benzoic acid [1]; however, higher amounts of benzoic acid were not observed in the Ferrario Alkyd paint py-GC/MS pyrograms in relation to the Winsor & Newton pyrograms. The two unknown peaks present in the Winsor & Newton paints are not present in the Ferrario samples; however, there is a phthalic acid based plasticizer peak at 24.2 min. The alkyd portion of the Da Vinci Paint Co. Leonardo Oil with Alkyd paints was detected with THMeGC/MS, unlike FTIReATR. Present, in a very low amount, was an isophthalic acid and pentaerythritol based alkyd resin. The dominant
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peaks in the pyrogram were the methylated fatty acid associated with the oil portion of the binder. The PDQ paints yielded interesting THMeGC-MS results. Seven of the nine paint colours showed that they had a polyol composition of pentaerythritol and a main polybasic acid composition of phthalic anhydride, and all the expected fatty acid peaks were present (see results for Winsor & Newton samples), while the other two had a similar polyol composition and fatty acid composition, but contained isophthalic acid as the main polybasic acid. The presence of terephthalic acid (dimethyl ester of terephthalic acid m/z 76, 103, 135, 163, 179, 194) was noted in smaller amounts in some of the samples as well as smaller amounts of isophthalic acid and phthalic acid in relation to the main polybasic acid. It is unknown why there is a change in alkyd resin formulations; there may have been changes in the alkyd supplier, conscience decisions to change the alkyd resin, changes in manufacturing dates or the use of specific resins for certain colours. The detection of terephthalic acid in some samples may also be a result of contamination from the Mylar substrate due to the method used to remove the samples from the substrate. The two unknown peaks present in the Winsor & Newton formulations are also present in all the PDQ paint samples analysed. Phthalic anhydride (phthalic acid) is the most commonly used polybasic acid for alkyd resins. Isophthalic acid is the second most used polybasic acid; these resins have tougher films that are more resistant to hydrolysis compared to phthalic acid based resins. Terephthalic acid is sometimes used in alkyds, because it is less expensive and has a similar hydrolytic stability as isophthalic acid in resins; however, terephthalic diesters tend to partially crystallize in the final resin solution, giving hazy resins and possibly leading to film defects [5]. 3.3. Palmitic to stearic (P/S) peak ratio The palmitic to stearic acid (P/S) ratio is often used to identify oils, because their ratio to each other should not change during the oxidation and film formation processes; they are both saturated and do not participate in the auto-oxidation reactions. The P/S peak area ratio was examined with the hypothesis that it would represent the original oil used in the manufacturing of the alkyd resin. The most widely used oil for alkyds is soya oil [5], and was suspected for these paints, and it has a relatively high P/S ratio of 2.75 [20]. P/S peak area ratio analysis gave mixed results, making it impossible to conclusively evaluate and identify the oil from which the fatty acids were derived. This type of variability amongst oils and difficulty in reproducibility are common issues in THMeGC/MS analyses [21]. Also, a recent study by Tsakalof et al. [22] demonstrated that there was a non-linear GC/MS instrument response when calculating P/S ratios and their dependence on sample dilutions; thus indicating that there is unpredictability in the interpretations of P/S peak ratios, which hinders drying oil identification using this technique. However, it was observed that these paints have different P/S ratios; the P/S ratios for the Winsor & Newton samples (for all the years) ranged between 1.34 0.11 and 1.53 0.12, was 2.06 0.08 for the Ferrario Alkyd, Colore Alchidico paint and was 1.91 0.08 for
the PDQ samples (values given with 95% confidence). There was no way to obtain reasonable P/S peak area ratios for the alkyd resin used in the Da Vinci Paint Co. Leonardo Oil with Alkyd paints. The P/S ratios obtained were an addition of the palmitic and stearic acids present in both the oil portion and the alkyd portion. There appears to be a difference between the fatty acids used for the Ferrario Alkyd, Colore Alchidico and PDQ paints and the Winsor & Newton formulations, but the exact changes could not be determined. An added complication to this is the possibility of mixtures of fatty acids [23] or the substitution of one oil for another based on market availability and prices [5]. These changes can be done, along with modifications in the formulations, without altering the performance of the alkyd [5]. The THMeGC/MS technique cannot distinguish whether the mixture arises from combinations of different natural oils and fatty acids. 4. Conclusions There are no major changes in the polyester monomers in the Winsor & Newton alkyd resins, but it is unknown if there were major changes in the oil used (fatty acids). Also minor additive changes are not detectable, only changes in the fillers used, which has been noted. The evolution of formulations is inevitable, since polymer manufacturers can change their resin formulations without warning and paint manufacturers can modify their formulations in order to provide a better product or change their resin manufacturer. FTIReATR can be used to identify an alkyd resin provided there are no overlapping signals from other additives with the characteristic peaks. Some of the unique peaks are those due to the aromatic portion of the polyester backbone at 1600 cm1 and 1580 cm cm1 (doublet), 1070 cm1, 741 cm1 and 705 cm1. Other unique peaks are those in the fingerprint area around 1258 cm1 and 1121 cm1. The alkyd portion of the Da Vinci Paint Co. Leonardo Oil with Alkyd paints was not visible in the FTIReATR spectra. The major fillers and pigments are also easily identified: dolomite or calcium carbonate and barium sulphate being the predominant fillers in both the Winsor & Newton and Ferrario Alkyd, Colore Alchidico paints and talc in the Da Vinci Paint Co. Leonardo Oil with Alkyd paints. The amount of filler was much higher in the Ferrario Alkyd, Colore Alchidico paints. No fillers were identified in the PDQ paints, although the presence of barium sulphate was suspected in some samples. Some ageing processes were also identified by qualitatively comparing the 1980, 1999 and 2006 Winsor & Newton films. There was broadening of the C]O peak at 1726 cm1 and an increase and broadening of the OH peak between 3300 cm1 and 3500 cm1. These indicate the formation of oxygenated functional groups from oxidative degradation. THMeGC/S with derivatization allows the identification the polyol and polybasic acid monomers used to manufacture the alkyd resin. The fatty acids were also identified, but it was not possible to identify the oil from which they were derived. Winsor & Newton alkyds used a pentaerythritol and phthalic anhydride based alkyd resin and Ferrario Alkyd, Colore Alchidico and Da Vinci Paint Co. Leonardo Oil with Alkyd
R. Ploeger et al. / Journal of Cultural Heritage 9 (2008) 412e419
paints used pentaerythritol and isophthalic acid based alkyd resins. The PDQ paints were made with both phthalic anhydride and isophthalic acid based alkyd resins. It is important to always derivatize the sample if a technique similar to that used in this study is used and an alkyd is suspected. Phthalic anhydride will be detected in an underivatized alkyd pyrogram; however, isophthalic acid will not, leading to confusion and the possibility of the paint being incorrectly identified. Other topics to be further investigated using THMeGC/MS are whether this technique can distinguish between a long and short oil alkyd and semi-drying and drying alkyd. The ratio of polyester components (polyols and polybasic acids) to the fatty acids should reveal information about the oil length; the higher the ratio, the shorter the oil length or the percentage of the oil in the alkyd. Schilling et al. [3,24] have suggested a similar technique. If a sample of wet tube paint was available, the identification of a drying or semi-drying oil alkyd could be easily determined by calculating the amount of un-saturated fatty acids present. However, in dry films these acids are consumed in the auto-oxidation reaction and are no longer present [24]. 5. Materials Winsor & Newton Artists’ Alkyd Colour and Griffin Alkyd Colour Whitefriars Avenue, Harrow, Middlesex, HA3 5RH, UK www.winsornewton.com Ferrario Color SRL, Ferrario Alkyd, Colore Alchidico Via Marzocchi, 29, 40012, Calderara Di Reno (BO), Italy http://www.apaferrario.it/ing/index.shtml Da Vinci Paint Co., Leonardo Oil with Alkyd 11 Good Year Street, Irvine, CA 92618, USA http://www.davincipaints.com/ PDQ, Paints Dry Quick, Artist Oil Color, Quick Dry Oilddiscontinued Colours: alizarin crimson, cadmium red light, cobalt violet, fire red, green gold, hansa yellow (light), indofast orange, Prussian blue, permanent green (light) San Jose, CA Acknowledgements We would like to thank Mr. A. Foster at Winsor & Newton for supplying us with the alkyd resin used to make the modern Griffin Alkyd Colour line. Also we would like to thank Dr. M. Mecklenburg at the Smithsonian Institution, Washington, DC for providing us with the aged Winsor & Newton samples from 1980 and 1999. Finally we would like to thank the Art Materials Research and Study Center, National Gallery of Art, Washington, DC for the PDQ paint samples. References [1] T. Learner, Analysis of Modern Paints, Getty Publication, Canada, 2004. [2] C.M. Lack, Performance and Working Properties of Artists’ Alkyd Paints, M.A.C. thesis, Queen’s University, Canada 1988.
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[3] M. Schilling, J. Keeney, T. Learner, Characterization of alkyd paint media by gas chromatographyemass spectrometry, in: A. Roy, P. Smith (Eds.), Modern Art, New Museums, Postprints of the IIC 20th Congress, Bilbao, September 13e17, The International Institute for Conservation of Historic and Artistic Works, London, 2004, pp. 197e201. [4] H. Standeven, The development of decorative gloss paints in Britain and the United States c. 1910e1960, J. Am. Inst. Conserv 45 (2006) 55e65. [5] Z.W. Wicks Jr., F.N. Jones, S.P. Pappas, Organic Coatings: Science and Technology, Vol. 1: Film Formation, Components and Appearance, John Wiley & Sons, Inc., New York, 1992. [6] P. Staples, Master classes in alkyds: the development of alkyd colours, The Artist 97(1) (1982) 11e14. [7] F. Cappitelli, F. Koussiaki, THMeGCMS and FTIR for the investigation of paints in Picasso’s Still Life, Weeping Woman and Nude Woman in a Red Armchair from the Tate Collection, London, J. Anal. Appl. Pyrol 75 (2006) 200e204. [8] J. Crook, T. Learner, The Impact of Modern Paint, Tate Gallery Publishing Ltd., London, 2000. [9] N. Sonoda, Application des me´thodes chromatographiques a` la caracte´risation des peintures alkydes pour artistes, Techne 8 (1998). [10] J.D. Erlebacher, E. Brown, M.F. Mecklenburg, C.S. Tumosa, The effects of temperature and relative humidity on the mechanical properties of modern painting materials, in P.B. Vandiver, J.R. Druzik (Eds.), Materials Issue in Art and Archeology III Proceedings of MRS conference Vol. 267 San Francisco April 27eMay 1, MRS, Warrendale 1992, pp. 359e370. [11] I. Civil, M. Michalski, A. Murray, Cracking the ‘matter paintings’ of Antoni Ta`pies: the role of artistic intent, deterioration and underlying mechanical causes, in: R. Vontobel (Ed.), Preprint of ICOM-CC, 13th Triennial Meeting, Rio de Janeiro, September 22e27, James & James (Science Publishers) Ltd., London, 2002, pp. 407e413. [12] A. Phenix, The swelling of artists’ paints in organic solvents. Part 2, Comparative swelling powers of selected organic solvents and solvent mixtures, J. Am. Inst. Conserv 41 (2002) 61e90. [13] O. Fuesers, Zum Einfluss organischer Lo¨semittel auf die mechanischen ¨ lfarbe, diploma thesis Hochschule Eigenschaften von Alkydharz- und O der Ku¨nste Bern, Switzerland2006. [14] A. Piccirillo, D. Scalarone, O. Chiantore, Comparison between off-line and on-line derivatisation methods in the characterisation of siccative oils in paint media, J. Anal. Appl. Pyrol 74 (2005) 33e38. [15] M.R. Derrick, D. Stulik, J.M. Landry, Infrared Spectroscopy in Conservation Science, J. Paul Getty Trust, U.S.A, 1999. [16] U. Knuutinen, P. Kyllonen, Two case studies of unsaturated polyester composite art objects, e-Preservation Science 3 (2006) 11e19. [17] M. Lazzari, O. Chiantore, Drying and oxidative degradation of linseed oil, Polym. Degrad. Stabil 65 (1999) 303e313. [18] K. Vosmann, E. Schulte, E. Klein, N. Weber, Formation of N- and Omethyl derivatives of lipids containing amino, amide or hydroxy groups by the pyrolytic reaction with trimethylsulfonium hydroxide, Fett. Lipid 100 (1998) 334e342. [19] Cray Valley, Castor wax technical bulletin: Hydrogenated castor oil based rheology modifiers March 2001, http://www.crayvallac.com/ indust.html (December 2007). [20] K.J. Saunders, Organic Polymer Chemistry, Chapman and Hall, London, 1973. [21] F. Cappitelli, T. Learner, O. Chiantore, An initial assessment of thermally assisted hydrolysis and methylation-gas chromatography/mass spectrometry for the identification of oils from dries paint films, J. Anal. Appl. Pyrol 63 (2002) 339e348. [22] A.K. Tsakalof, K.A. Bairachtari, I.D. Chryssoulakis, Pitfalls in drying oils identification in art objects by gas chromatography, J. Sep. Sci. 29 (2006) 1642e1646. [23] T.C. Patton, Alkyd Resin Technology, Formulating Techniques and Allied Calculations, Interscience Publishers, New York, 1962. [24] J.M. Challinor, Structure determination of alkyd resins by simultaneous pyrolysis methylation, J. Anal. Appl. Pyrol 18 (1991) 233e244.