Formation of zinc oxalate from zinc white in various oil binding media: the influence of atmospheric carbon dioxide by reaction with 13CO2

The formation of metal oxalates in paintings has recently gained a great deal of interest within the field of heritage science as several types of oxalate compounds have been identified in oil paintings. The present work investigates the formation of metal oxalates in linseed oil in the presence of the artists’ pigments zinc white, calcite, lead white, zinc yellow, chrome yellow, cadmium yellow, cobalt violet, and verdigris. The oil paint films were artificially photo-aged by exposure to UVA light at low and high relative humidity, and afterwards analysed by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). The results showed that, compared to the other pigments investigated, zinc white is especially prone to metal oxalate formation and that high humidity is a crucial factor in this process. Consequently, the reactivity and photo-aging of ZnO in various oil binding media was investigated further under simulated solar radiation and at high relative humidity levels. ATR-FTIR showed that zinc oxalate is formed in all oil binding media while X-ray powder diffraction (PXRD) revealed it was mainly present in an amorphous state. To examine whether atmospheric CO2(g) has any influence on the formation of zinc oxalate, experiments with isotopically enriched 13CO2(g) were performed. Based on ATR-FTIR measurements, neither Zn13C2O4 nor Zn13CO3 were formed which suggests that the carbon source for the oxalate formation is most likely the paint itself (and its oil component) and not the surrounding atmosphere.


Introduction
Metal oxalates in oil paintings have recently attracted the attention of heritage scientists as their origin and mechanism of formation has not yet been fully elucidated [1][2][3][4][5][6]. Results published in the scientific literature suggest that different mechanisms might be involved depending on the artworks' constituent materials and the environmental conditions to which they were exposed. For instance, in the case of outdoor wall paintings and monuments constituted of calcareous stone materials, the formation of calcium and copper oxalates has been explained mainly by microbiological activity [7][8][9][10], though formation due to applied conservation treatments and atmospheric pollution has also been proposed [11,12]. For easel (and other weather-protected) paintings the presence of metal oxalates has been attributed to chemical interactions between the pigment and the varnish layer [5,6,13], to reactions between the pigment and oxalic acid present in aerosols [3], or to photodegradation of the binder [4,14], especially given if the latter has a high lipidic content such as in drying oils and egg tempera [15].

Open Access
*Correspondence: francisco.mederos@kikirpa.be 3 Laboratories of the Royal Institute for Cultural Heritage (KIK-IRPA), Parc du Cinquantenaire 1, 1000 Brussels, Belgium Full list of author information is available at the end of the article Indeed, it has been demonstrated that ageing conditions including exposure to ultraviolet (UV) radiation and high humidity, can induce the oxidation of fatty acids and favour the formation of low molecular weight dicarboxylic acids such as oxalic acid [15,16]. In such a process, pigments are considered to be one of the possible sources of the metal cations forming the oxalate salt [17,18]. The catalytic role played by certain copper, lead, zinc and chrome-based pigments has also been pointed out [4,5,16].
In any case, not all pigments are prone to oxalate formation in oil paint films. For instance, copper oxalates have commonly been found in oil paint films containing either copper acetate or basic copper chloride pigments, such as in a series of fifteenth century Gothic Catalan paintings [1] or Hans Memling's Christ with Singing and Music-making Angels altarpiece [17]. Nonetheless, they seem to be absent in paint films containing other copper-based pigments such as azurite [18]. Lead oxalate has been found in red areas containing minium (Pb 3 O 4 ), or in paint layers containing lead-tin yellow [1,2] while cadmium oxalates have been found associated to cadmium yellow (CdS) in paintings such as Flowers in a blue vase by Vincent van Gogh [13] and in Le bonheur de vivre by Henri Matisse [14]. Zinc oxalate has been found in several nineteenth and twentieth-century oil paintings by Munch or Picasso [3] and by van Gogh [19]. In the above-mentioned paintings, it was proven that zinc oxalate originated from the zinc white pigment, possibly due to this pigment's recognized photoactivity [5]. As for calcium oxalates, the most frequently found metal oxalate in easel paintings [1,17,18,20], the origin of the calcium cation is usually attributed to either calcium-containing materials (e.g. fillers and lake substrates) in the paint and ground layers or from particulate matter such as dirt deposited onto the paint surface [17,20]. More recently, the catalytic role of Cr(VI) metal cations in the formation of calcium oxalate from calcite fillers in chrome yellow oil paints has also been described [4].
Considering the above, the present paper investigates the formation of metal oxalates in linseed oil paint films containing pigments such as zinc white (ZnO), calcite (CaCO 3 ), lead white, (Pb 3 (CO 3 ) 2 (OH) 2 ), zinc yellow (KZn 2 (CrO 4 ) 2 (H 2 O)(OH)), chrome yellow (PbCrO 4 ), cadmium yellow (CdS), cobalt violet (Co 3 (PO 4 ) 2 ) and verdigris, (Cu(CH 3 COO) 2 ·H 2 O). Also, given it was found that zinc white-based oil films were particularly prone to metal oxalate formation, further investigations using ZnO dispersed in different oil media were performed. Finally zinc white's capacity to reduce CO 2 (g) into the oxalate anion was tested by exposing ZnO-based oil films to 13 C labelled CO 2(g) . Indeed, semiconductor-mediated reduction of CO 2 into different carbon-containing species such as oxalates are currently investigated as part of environmental chemistry research [21,22]. Thus, it was worth investigating the influence of atmospheric CO 2 as a possible source of zinc oxalate (ZnOx) in oil paint layers.

Syntheses
Reference samples of ZnOx·2H 2 O, PbOx, CaOx·2H 2 O, CdOx·3H 2 O, CuOx, and CoOx·2H 2 O were made by salt metathesis reaction in aqueous solution by adding ammonium oxalate monohydrate (20 mmol in 100 mL, 70 °C) to a stirred solution of the metal salt (nitrate or sulphate, 20 mmol in 100 mL, 70 °C). During addition the desired oxalate salt precipitated. The suspension was allowed to stir for a couple of hours while cooling to ambient temperature. The crystals were then filtered off, washed with 3 × 15 mL of cold water and dried in air. 13 C 2 labelled zinc oxalate was prepared by precipitation from aqueous solutions of zinc nitrate and 13 C 2 labelled sodium oxalate dissolved in deionized water as follows: an aqueous solution prepared by dissolving 0.10 g (0.74 mmol) of Na 2 13 C 2 O 4 in 10 mL of water was dropwise added under stirring to a solution of 0.22 g (0.74 mmol) of Zn(NO 3 ) 2 ·6H 2 O dissolved in 10 mL of water. During addition white crystals precipitated. The mixture was allowed to stir for 2 h after which the crystals were filtered off, washed with 3 × 5 mL of cold water and dried in air. Yield: 0.12 g (86%) of Zn 13 C 2 O 4 ·2H 2 O.

Paint samples for UVA ageing
100 µm-thick paint films for UVA ageing were applied on glass microscope slides using a Zehntner ZUA 2000 Universal Applicator. The paints were produced by mixing a given amount of each pigment, shown in Table 1, with 0.45 g of binding medium (raw linseed oil with 5%(w/w) of Siccative de Haarlem). In order to ensure that all paint films had an equimolar amount of metallic cations (n cation ), the binder to pigment ratios (B/P) were calculated based on that of PbCrO 4 (B/P = 20, e.g. 20 g of oil to 100 g of pigment). The calculated B/P and n cation values are also shown in Table 1. It must be noted that by standardising the molar concentrations of metal ions in the paint samples, the calculated B/P values (B/P exp ) vary to differing degrees from those typically used in reference paint formulations (B/P ref ). The samples were dried in the dark in a fume hood for 1 month before artificial ageing.

UVA light ageing
Experiments with UVA light were performed in a lightcuring oven (Egger EL 1 plus N 2 ) using compact UVA fluorescent lamps (Osram Blue UVA 9W/76) with radiation wavelengths from 350 to 400 nm. Six lamps were used, four at the top and two at the sides. The measured UVA intensity was 160,000 µW/lumen (160 W/m 2 ) while the measured temperature (T = 28 °C) was caused by the fluorescent lamps. The paint samples were then subjected to a repeating cycle of UVA exposure in the Egger box followed by elevated relative humidity exposure in a desiccator containing water. The RH is expected to be around 100% in the desiccator and low in the Egger box due to the fluorescent lamps. The time intervals are shown in Table 2.

ZnO samples in various oil binders
In these experiments, the B/P ratio for ZnO in the various oil binding media was 30 in accordance with its reported value for linseed oil [24]. The different binding media used were: (a) raw linseed oil, (b) raw linseed oil with 5%(w/w) of Siccative de Haarlem, (c) standoil, (d) tall oil modified alkyd, and standoil mixed with an equivalent mass of 10% (w/w) solution of: (e) dammar in mineral turpentine, (f ) shellac in ethanol, and (g) pine colophony in ethanol. 100 µm-thick paint films were applied on melinex polyester films using a Zehntner ZUA 2000 Universal Applicator. All samples were dried in the dark in a fume hood for 1 month before ageing. It is worth noting that, to determine whether the presence of a siccative affects oxalate formation, linseed oil paint films with and without drier were made.

Daylight equivalent ageing of ZnO in various oil binders
Experiments for artificial sunlight ageing were performed in an Atlas Weather-O-Meter Ci3000 fitted with a 4500 W water-cooled xenon arc lamp having combined borosilicate soda-lime glass and coated with infrared absorbing outer filter (CIRA) suppressing radiation below 300 nm. The light intensity was 120,000 lx with a  Light ageing experiments in 13 CO 2(g) enriched atmosphere were performed at concentrations of 4 and 40% (v/v) of 13 CO 2(g) in the Weather-O-Meter and in UVA light (40% (v/v)). The light ageing was performed for 500 h for all trials. The samples were placed in a gas-tight container made from tubular ESCAL ™ Neo gas-barrier film, a heat-sealable polyester film coated with transparent ceramic (Mitsubishi Gas Chemicals). The container was fitted with a 2 mm quartz glass window (10 × 15 cm), which was attached over a hole cut in the gas-barrier film using a double (side by side) bonding with epoxy and silicone glue. The samples of ZnO paint applied on glass microscope slides were placed inside the bag and held in place by double-sided adhesive tape. The volume of the bag was 400 mL and the concentration of 13 CO 2(g) were obtained by injecting 13 CO 2(g) with a glass gas syringe via an inserted needle after which the bag was heat sealed. High RH measured to vary between 70 and 80% was obtained by placing water drops in the plastic bag. The ZnO paint samples had a thickness of 100 µm, B/P = 30, (raw linseed oil with 5% (w/w) Siccative de Haarlem), and were dried in the dark in a fume hood for 1 month before ageing.

ATR-FTIR
ATR-FTIR spectra were recorded on a Perkin Elmer Spectrum One FTIR-spectrometer, fitted with a Universal ATR sampling accessory having a one-bounce composite zinc selenide and diamond crystal. Spectra of the samples were recorded over the range 4000-650 cm −1 with a resolution of 4 cm −1 and 4 accumulations.

PXRD
Powder XRD analyses were performed with a Bruker D8 Advance diffractometer using Ni-filtered Cu-Kα radiation (λ = 1.54056 Å) and Lynxeye position sensitive detector in the range 2θ = 5°-80° (Δ2θ = 0.01°) and total exposure time of 8 h. The oil film material was deposited directly on the Si monocrystal zero-background plate and measured at ambient temperature.

µ-PXRD
µ-PXRD analyses were performed using a low power IµS-Cu HB monochromatic X-ray source (Cu-Kα radiation, Incoatec GmbH) and a PILATUS 200K area detector (Dectris Ltd.) positioned in a reflection geometry. The X-ray source had an incident angle of 12° with respect to the sample resulting in a beam footprint on the sample of around 0.15 × 0.8 mm 2 . Diffraction signals were collected in the range of 2θ = 12°-53°. By moving the X-ray beam over the sample, areas between 0.5-4 cm 2 were analysed in a point-by-point manner with a dwell time of 100 s in each point, for a total period of 4-12 h. Averaged diffraction patterns were calculated from subsets of these areas to improve signal-to-noise ratio. The oil film material was measured either directly on a Si monocrystal zero-background plate or on a melinex polyester film (100 µm). The latter allowed analysing the ZnO oil film from both the top (exposed) and the bottom (unexposed) side. Azimuthal integration was performed using XRDUA [25]. Phase identification was performed using Qualx2 software [26] combined with the crystallography open database (June 2019) [27].

UVA light ageing of various pigments
ATR-FTIR measurement of the surface of the various pigment samples exposed to UVA light showed no sign of oxalate formation after 50 and 224 h. The spectra for all samples were very similar to the spectrum of the dried sample before treatment. The samples were then placed in a desiccator with water for 17 days in order investigate the influence of the relative humidity after which their IR spectra were recorded again. For ZnO the changes in the IR spectrum were remarkable whereas the spectra of the other pigments were only slightly affected. The spectrum of zinc white showed both an increase in the amount of zinc carboxylate (zinc soap) and distinct sign of zinc oxalate (ZnOx) formation (Fig. 1e), which further increased by additional exposure (7 days) to UVA (Fig. 1f ). In Fig. 1 the evolution of the ZnO spectra as a function of UVA-RH treatment (in hours) is shown together with the reference spectrum for ZnOx·2H 2 O.
As indicated by the arrows, the presence of crystal water ν(O-H) = 3359 cm −1 , the symmetrical C-O stretches ν(C-O) = 1362 and 1315 cm −1 , and the C-C stretch ν(C-C) = 818 cm −1 [3] are clearly seen. The strong asymmetrical C-O stretch ν(C-O) = 1621 cm −1 [3] is visible as a shoulder in the absorption region (1700-1500 cm −1 ) for zinc soaps [28,29]. The small peak at 1315 cm −1 observed in the spectrum of dried ZnO (Fig. 1b) may indicate that a minor amount of zinc oxalate is formed together with the zinc soaps while drying of the oil. Likewise, the intensity of characteristic oil binder frequencies (e.g. C-H stretches (3000-2800 cm −1 ), C=O stretches (1750-1730 cm −1 ), C-O stretches (1200-1100 cm −1 ) and the dominant alkyd peak around 1270 cm −1 ) tend to decrease or disappear as the exposure time to the UVA-RH cycles increases.
The IR spectra of the other paint films after the final UVA-RH treatment (726/1080 h) are shown in Fig. 2(top) together with the dried oil film before treatment (middle), and the relevant metal oxalate (bottom). As indicated by the arrows a minor amount of metal oxalate may be observed in the spectra of calcite, zinc yellow, and of cobalt violet.
The minor amount of zinc oxalate observed in the spectrum of zinc yellow (Fig. 2c) may be due to the oxidative nature of the chromate ion. The colour of the aged sample showed a distinct greenish hue indicating the presence of Cr(III). In oil medium, the reduction of Cr(VI) to Cr(III) via the chromate(V) species has been well described [4,30,31], and oxidation of oxalate precursors (CO 2 • , CO 3 −• ) may lower the amount of oxalate ions formed. However, whether chromate ions increase or decrease the formation of oxalate is still unclear. Figure 2c also shows the clear formation of zinc soap [28,29] indicating the reactive nature of zinc yellow with respect to hydrolysis and reaction with the oil medium. This is likely caused by the presence of hydroxide in the pigment, which structure has been recently solved as KZn 2 (CrO 4 ) 2 (H 2 O)(OH) [23]. The reaction between zinc yellow and the oil medium is in contrast to chrome yellow which colour remained almost unchanged after ageing. The IR spectrum showed no sign of lead oxalate and only a slight presence of lead soap (Additional file 1: Fig. S1). The difference between the two chromate pigments is likely due to zinc yellow's higher solubility and to its hydroxide content, making this pigment the most reactive among the chromate pigments [23,30].
Cobalt violet seems also relatively inert with respect to oxalate formation though a minor amount of cobalt oxalate seems to be present. To our knowledge cobalt oxalate has yet to be found in oil paintings.
Neither lead white, chrome yellow, cadmium yellow or verdigris show sign of metal oxalate formation based on FTIR-ATR spectroscopy (vide supra). However, the presence of carbonate in lead white may impair the detection of lead oxalate's symmetrical C-O stretches. Likewise, the amount of metal soap on the surface of the lead white oil film seems comparatively less than for zinc white as indicated by the relative intensity of their characteristic IR vibrations. This result thus suggests a much slower reaction of lead white with the oil binding medium despite its basic nature. Cadmium [13,14] and copper [1,15,17,19] oxalates have been observed in different paintings, but were not found in this experimental setup or may have formed only in concentrations falling below the detection limit of the ATR-FTIR apparatus.

Daylight equivalent ageing of ZnO in various oil binding media
Due to the clear formation of zinc oxalate in the UVA-RH treatment, ageing experiments with zinc white were performed with artificial sunlight (Weather-O-Meter) in various oil binders and in standoil with natural resins (B/P = 30). The IR spectra after 300 h (RH = 80%) are shown in Figs. 3 and 4 together with the reference spectra of the dried oil films before treatment and that of zinc oxalate.
For the four oil media without resin the IR spectrum could be measured both at the surface and at the backside of the oil film. As seen in Fig. 3, zinc oxalate was formed in all samples. Based on the intensity of characteristic ZnOx IR absorption bands, the amount seems higher at the surface compared to the backside. This notable difference also matches the disappearance of the characteristic oil binder frequencies at the exposed surfaces as shown in Fig. 3d (e.g. C-H stretches (3000-2800 cm −1 ), C=O stretches (1750-1730 cm −1 ), C-O stretches (1200-1100 cm −1 ) and the dominant alkyd peak around 1270 cm −1 ). However, in pure linseed oil the amount of ZnOx and the disappearance of frequencies of the binder appears to be similar on both sides.
The paint film with natural resins in standoil became brittle and crumbly after treatment and only surface measurements were possible. As seen in Fig. 4, zinc oxalate was formed in all samples. Based on the intensity of the C-O stretches around 1362 and 1315 cm −1 [3] the relative amount of ZnOx seems highest in the dammar containing paint film and lowest in the sample containing shellac. However, given the resins were dissolved in different solvents, variable solvent evaporation rates might have caused film inhomogeneities [32]. Therefore, it is difficult to draw a conclusion whether the resins have different or any effect on oxalate formation. Nonetheless, our results clearly demonstrate that all the oil binding media, during exposure to light and humidity, react with zinc white to form zinc oxalate.

Ageing in 13 CO 2 atmosphere
In order to investigate if atmospheric CO 2(g) has any influence on the formation of ZnOx, experiments were performed with 13 C labelled CO 2(g) . The IR spectrum after exposure for 500 h in the Weather-O-Meter to a 40% (v/v) 13 CO 2(g) atmosphere is shown in Fig. 5 together with the reference spectra of zinc white in oil, Zn 13 C 2 O 4 ·2H 2 O, and Zn 12 C 2 O 4 ·2H 2 O. Based on the intensity of the ZnOx absorption bands, significantly less zinc oxalate appears to have been formed compared to the spectra shown in Fig. 3b (top), even though the reaction time was considerably longer. Characteristic Fig. 2 FTIR-ATR spectra (asterisk) of six pigments (a-f) in linseed oil with 5% (w/w) Siccative de Haarlem after treatment with UVA for 726 h and placement in desiccator filled with water for 1080 h. Arrows indicate the presence of characteristic metal oxalate IR bands. Reference spectra of the oil film before treatment (black triangle) and of the corresponding metal oxalates (black circle) are also shown for each pigment. The B/P ratios are given in Table 1 vibrations of the binding medium (e.g. C-H and C=O) are also still present. However, the COO − band (1600-1500 cm −1 ), due to zinc soap formation, is dominant showing that reaction between ZnO and the binding medium has taken place. The lower tendency to form zinc oxalate may be attributed to a decrease in effective light intensity due to the quartz window and the plastic bag. Reaction at 4% (v/v) and 40% (v/v) 13 CO 2 in UVA light resulted in similar FTIR-ATR spectra.
As expected from the slightly larger mass of 13 C, the vibrations of the 13 C marked oxalate ion appears at lower wavenumbers than that of the 12 C-containing oxalate [33]. As shown in Fig. 5, the C-O stretches appear at 1364 and 1319 cm −1 , which is similar to the positions observed in Fig. 1 (1364 and 1319 cm −1 ) and Fig. 2b (1364  and 1320 cm −1 ). Comparison to the reference spectra of Zn 12 C 2 O 4 ·2H 2 O and Zn 13 C 2 O 4 ·2H 2 O also confirms the presence of the 12 C-containing oxalate ion while no characteristic absorption bands for 13 C 2 O 4 2− are observed. These results indicate that either no zinc 13 C-oxalate is formed or only at concentrations falling below the technique's limit of detection.
Based on these IR investigations it appears that the formation of zinc oxalates follows the formation of zinc soaps which recently has been shown to be present as ionomeric clusters [34][35][36][37][38]. We believe that these clusters could be the precursors for the formation of zinc oxalates by bond breaking and oxidation of the long chain fatty acids through a radical mechanism, the radicals possibly being caused by light absorption/photoactivity of ZnO. The lack of increased Zn 13 C 2 O 4 ·2H 2 O formation as a function of increased 13 CO 2(g) concentration further suggests that oxalate formation originates from carbon sources in the oil and not from atmospheric ones such as CO 2(g) . Based on our results, we consider it very unlikely that atmospheric carbon dioxide plays any role in the oxalate formation. Hence, the oxalate ions formed are not due to a reductive mechanism that might involve the reduction of CO 2 [21,22] but rather originate from the oxidation of the oil binder.

PXRD characterization
In order to investigate the crystallinity of the formed zinc oxalate, PXRD was performed on the pure linseed oil paint film which showed the largest amount of ZnOx based on the IR spectrum (Fig. 3a). The diffractogram of ZnO in linseed oil after treatment for 300 h is shown in Fig. 6 together with a close-up of the diffractogram. Also shown are reference diffractograms for ZnC 2 O 4 ·2H 2 O, ZnO, and various concentrations of ZnC 2 O 4 ·2H 2 O in linseed oil. The diffractograms of the various zinc oxalate concentrations show that concentrations below 0.5% (w/w) become difficult to observe with PXRD.
As indicated by the arrows in the close-up diffractogram in Fig. 6, weak crystalline zinc oxalate reflections were detected. These results suggest that the formed zinc oxalate is mostly present in an amorphous state, while only a minor fraction is crystalline.

µ-PXRD characterization
As the presence of zinc oxalate was clear on both the surface and backside of the oil film judged by ATR-FTIR (Fig. 3a), and since bulk PXRD indicates that the zinc oxalate formed is mostly present in an amorphous state, the zinc oxalate formation was further investigated by µ-PXRD.
Comparing diffraction data from the exposed top surface (Fig. 7b) and bottom side (Fig. 7c) of the ZnO paint film, showed that the minor amount of crystalline zinc oxalate is present only at the exposed side and is absent on the bottom side, even though IR revealed oxalate vibrations bands with similar intensity on both sides of the paint film. The broad bands at 18.0°, 22.6° and 26.0° 2θ in Fig. 7c originate from the Melinex foil. Regarding the ZnO film aged with the Weather-O-Meter and 40% (v/v) 13 CO 2(g) no indication of crystalline zinc oxalates was found (Fig. 7a).
Next to ZnO and ZnC 2 O 4 ·2H 2 O, the diffraction signals marked ▼ are present in all measurements (Fig. 7a-c). This species is best explained by a hydrated Zn carbonate hydroxide with chemical formula, Zn 4 (CO 3 )(OH) 6 ·H 2 O (PDF-2 entry number 00-011-0287). These diffraction signals are due to an impurity in the ZnO powder (verified with XRPD). Characteristic IR vibrations for this compound [39] have also been detected in the IR spectra of the corresponding samples. It is worth mentioning that the diffraction signal of Zn 4 (CO 3 )(OH) 6 ·H 2 O around 2θ = 24° could hide the presence of anhydrous ZnC 2 O 4 . Nonetheless, the presence of the latter seems unlikely, given the ageing conditions that were employed.

Conclusions
This paper has investigated the formation of metal oxalates by reaction of different pigments in linseed oil paint films under the influence of UVA light and humidity. The pigments investigated were zinc white (ZnO), calcite (CaCO 3 ), lead white, (Pb 3 (CO 3 ) 2 (OH) 2 ), zinc yellow (KZn 2 (CrO 4 ) 2 (H 2 O)(OH)), chrome yellow (PbCrO 4 ), cadmium yellow (CdS), cobalt violet (Co 3 (PO 4 ) 2 ) and verdigris, (Cu(CH 3 COO) 2 ·H 2 O). The UVA light-ageing experiment showed that linseed oil film with zinc white was particularly prone to metal oxalate formation, and that high humidity is a necessary requirement for the formation of metal oxalates. Metal oxalate formation of ZnO was investigated further in different oil binding media under the influence of artificial sunlight and humidity. The different oil binding media were raw linseed with and without Siccative de Haarlem, standoil, and tall oil modified alkyd. Also standoil mixed with dammar, shellac, or pine colophony were investigated. Obtained results show that zinc oxalate was formed in all cases.
The influence of atmospheric carbon dioxide on the formation of zinc oxalate was also investigated by the use of 13 C labelled CO 2(g) . As the vibration bands of the resulting zinc oxalate appear at the frequencies for 12 C-containing oxalate ion, and since the amount of zinc oxalate formed did not increase by increasing CO 2(g) concentration, we consider it unlikely that atmospheric carbon dioxide plays a role in oxalate formation. It therefore supports the hypothesis that oxalate anion formation is due to the oxidation of the oil binder and not to the reduction of CO 2 . Moreover, our results seem to indicate a correlation between the formation mechanism of zinc oxalates and that of zinc soaps. However, further research is needed to clarify the possible link between both mechanisms.
In all cases, the zinc oxalate formed is mostly in an amorphous state; the presence of crystalline ZnC 2 O 4 ·2H 2 O could only be observed in minor amounts in the PXRD patterns, which are dominated by ZnO in the oil paint film. The formation of crystalline ZnC 2 O 4 ·2H 2 O is further limited to the light-exposed (top) side of the paint films.