The deterioration of cadmium sulphide yellow artists’ pigments

Bronwyn Leone and Aviva Burnstock*Courtauld Institute of ArtSomerset HouseStrandLondon WC2R 0RNUnited KingdomE-mail: aviva.burnstock@courtauld.ac.uk

Chris JonesThe Natural History MuseumCromwell RoadLondon SW7 5BDUnited Kingdom

Peter HallebeekInstituut Collectie Nederland8 Gabriel Metsustraat1071 AmsterdamThe Netherlands

Jaap Boon and Katrien KeuneAMOLF Kruislaan 4071098 SK AmsterdamThe Netherlands

*Author to whom correspondence should be addressed

Abstract

This paper investigates the deterioration of cadmium sulphide yellow pigments, through analysis of artificially aged pigments and samples from 12 paintings. The deterioration occurs mainly in the lighter zinccontaining yellows and leads to discolouration and loss of adhesion within the paint, resulting in a chalking and crumbly surface. Samples were characterized using light/ultraviolet and electron microscopy/energy dispersive X-ray (EDX) spectroscopy and X-raydiffraction. Darkening of yellow paint was characterized by the formation of brown cadmium oxide at the surface of some samples, while the formation of colourless/white products includingcadmium sulphate and zinc hydroxide, and the degradation of the oil bindingmedium, is associated with chalking. A hypothesis is suggested involving the oxidation of cadmium zinc sulphide. The cause of instability may be linked with early manufacturing methods, where the product is largely amorphous.

Keywordscadmium yellow, pigment deterioration, colour change, X-raydiffraction, artificial ageing, paintings

Introduction

As relatively modern pigments, the cadmium sulphides are generally regarded as stable. However, various cases of specific deterioration noted on paintings dating 1887–1923 have indicated that not all varieties are stable. The deterioration appears to occur mainly in the lighter yellows, and involves discolouration (both a fading or whitening and a darkening to a dull patchy ochre-like colour) and a loss of adhesion within the paint itself, producing a chalking and crumbly surface. Deterioration is most pronounced in areas of thick impasto, while loss of the discoloured uppermost layers reveals the bright yellow below. Cadmium sulphide, first observed in 1817 and recorded the following year after artificial synthesis, was early on recommended as an artist’s pigment, and appears in colourman’s catalogues from the 1840s. Early commentators recommended the pigment highly, although tests on its stability and permanence showed varying results, with the paler cadmiums being considered the least permanent: mainly discolouration and fading are reported. According to some commentators, the pale cadmiums were fugitive or permanent according to their method of manufacture, with pigments considered permanent being manufactured by well-guarded and secret processes. Cadmium sulphide is made as a pigment by the wet process whereby the product is precipitated from the reaction of a soluble cadmium salt (sulphate, chloride, nitrate, or iodide) with a soluble sulphide (hydrogen-, sodium- or various thiosulphates). The time and temperature of the reaction, concentration of the reactants, presence of impurities, and pH of the solution all affect the colour and properties of the resulting pigment. The inclusion of zinc, in solid solution within the crystal lattice results in the lighter yellows. Thorough filtering and washing is essential to eliminate all soluble reaction products and ensurestability. Calcination, carried out at about 600 °C in an inert atmosphere, results in reproducibility and increased permanence by establishing the desired crystal structure and surface (Curtis and Wright 1954, Fiedler and Bayard 1986). Previous studies on the deterioration of cadmium yellow artists’ pigments have described the effects on paintings by Piet Mondriaan dating 1917–20 (Van Asperen De Boer 1994), Aflred Munnings dating 1911–19, and Arthur Briscoe (1914) (Whitehouse and Eastaugh 2001), where friability was noted throughout the paint layers, whereas Huigen and Phenix (1997) provide background information on cadmium pigments and their behaviour in paints. The aims of the present project were to compare the effects of artificially aged samples to examples of the deterioration seen on paintings, in an attempt to replicate and analytically investigate the deterioration.

Experimental

Sample preparation

Eight powdered pigments from the Courtauld Institute archival collection and two tube paints from Claus & Fritz were selected for study. Two contemporary cadmium sulphides from Cornelissen and six modifications thereof were included in an attempt to replicate the possible defects of earlier cadmium sulphide pigments (Table 1). The dry pigments were bound in cold pressed linseed oil (Kremer), pre-polymerized linseed oil (Winsor & Newton), and Paraloid B72.These were painted out onto eight polytetrafluoroethene (PTFE) sample boards and allowed to dry for 10 days. Three sample boards were light-exposed in the Courtauld Institute lightageing chamber, equipped with six OSRAM L58W/12-950 Lumlux de Luxe Daylight lamps giving an illluminance of 12,000–15,000 lux. Using reciprocity, the light received by the samples was equivalent to 45 years in museum conditions (based on 200 lux 8 h per day). For each sample

board exposed to light a corresponding board was kept in the same conditions in the dark, as a control. The temperature in the chamber was 45 ± 1 °C. Local conditions of relative humidity (RH) were controlled by placing two sets of samples in sealed plastic containers with saturated salt solutions (magnesium nitrate for about 45 per cent RH, and potassium nitrate for about 85 per cent RH), following the design described by Saunders et al. (2002). The third set of samples was placed in the chamber, which measured about 10 per cent RH. Two sample boards were kept separately in ambient conditions as controls, one in room light, the other in the dark. Figure 1 shows the layout of sample boards and the sample numbers, which are referred to throughout the text. Samples and surface characteristics of degraded cadmium sulphide yellow were examined from 12 paintings, listed in Table 2.

Table 1. Pigments and media used in sample preparation, and surface effects observed after artificial ageing

Three sample boards were light-exposed in the Courtauld Institute lightageing chamber, equipped with six OSRAM L58W/12-950 Lumlux de Luxe Daylight lamps giving an illluminance of 12,000–15,000 lux. Using reciprocity, the light received by the samples was equivalent to 45 years in museum conditions (based on 200 lux 8 h per day). For each sample board exposed to light a corresponding board was kept in the same conditions in the dark, as a control. The temperature in the chamber was 45 ± 1 °C. Local conditions of relative humidity (RH) were controlled by placing two sets of samples in sealed plastic containers with saturated salt solutions (magnesium nitrate for about 45 per cent RH, and potassium nitrate for about 85 per cent RH), following the design described by Saunders et al. (2002). The third set of samples was placed in the chamber, which measured about 10 per cent RH. Two sample boards were kept separately in ambient conditions as controls, one in room light, the other in the dark. Figure 1 shows the layout of sample boards and the sample numbers, which are referred to throughout the text. Samples and

surface characteristics of degraded cadmium sulphide yellow were examined from 12 paintings, listed in Table 2.

Figure 1. Layout of sample boards showing sample numbers

Table 2. Analysis of samples from paintings using XRD and EDX

Methods of analysis

Energy dispersive X-ray (EDX) spectroscopy was used to characterize the elemental composition of the pigments and samples from paintings, using a Link AN10 85 EDX attached to a JEOL S100 scanning electron microscope (SEM). Samples were coated with carbon using a standard evaporator. High-resolution topographic images of dry pigments and degraded sample surfaces after ageing were produced using a Philips XL30 FEG SEM (Natural History Museum, London), which employs a field emission electron source, low acceleration voltage and high vacuum (10–9

mBar). Samples were coated with 20 nm of gold palladium (Au (80 per cent)/Pd (20 per cent)) using a Cressington sputter coater 208HR. X-ray-diffraction (XRD) was performed on pigment samples before and after ageing using a D8TM Discover GADDS microdiffractometer (Siemens-Bruker) with a HI-STAR Area Detector and phase identification by integration over a selected range of chi (ICN, Amsterdam). Samples of solid material sized 0.5 mm2 were placed on the tip of a glass fibre of diameter 0.1 mm, thinly coated with cedar oil. This was attached to a small

piece of wax over a copper tube that was fixed in the centre of the back plate of the microdiffractometer. Exposure

Table 3. Analysis of pigment samples using XRD and EDX

parameters were 40 kV and 30 mA for 600 s. In addition, a NONIUS PDS120 X-ray diffractometer (Natural History Museum, London) was used. Where possible the degraded surface of aged samples was characterized separately from the bulk sample by careful separation of the sample material. The static secondary ion mass spectrometry (SIMS) experiments were performed on a Physical Electronics (Eden Prairie, MN, USA) TRIFT-II time- of-flight SIMS (TOF-SIMS). The surface of the sample was scanned with a 15 keV primary ion beam from an 115In+ liquid metal ion gun. The beam was rastered over a 100 μm × 100 μm sample area, divided into 256 pixels × 256 pixels, the pulse length was 20 ns, non-bunched, the current was 600 pA and the spot size about 120 nm. The surface of the sample was charge-compensated with electrons pulsed between the primary ion beam pulses. The measurements were performed in both positive and negative mode.

Results

Elemental analysis of the dry pigments was done using EDX (Table 3). All the materials contained cadmium and sulphur. The lighter yellows contained zinc, whereas the darker pigments contained a lower proportion of zinc. XRD analysis showed that all samples contained solid solutions of cadmium zinc sulphide in a hexagonal crystal lattice. Cadmium zinc sulphide was present in low proportions even for those samples appearing not to contain zinc by EDX analysis. Three samples (13, 15, 16) also contained barium sulphate, which may have been added separately, or co-precipitated as a lithopone.

SEM study of pigment morphology

Significant variations in particle size and morphology of the dry pigments were revealed using SEM/SED imaging. Most samples contained irregularly shaped particles between 1.0–0.1 μm. Some samples appeared fused (Figure 2a), whereas others contained additional aggregates of larger, semi-spherulitic or cubic particles (0.5–1 μm) (Figure 2b). Both darker shades from Field (samples 10, 11) exhibited spherulitic morphology (Figure 2c). Sample 14 contained rod-shaped entities of varying length (0.1–5.0 μm) and thickness (Figure 2d).

(a) (b)

(c) (d)

Figure 2. SEM images showing dry pigment morphology for (a) sample 6 (20,000×), (b) sample 16 (25,000×), (c) sample 10 (25,000×), (d) sample 14 (20,000×)

Analysis of samples from paintings

EDX analysis of the samples from paintings showed that all contained cadmium and sulphur (Table 2). The presence of amorphous cadmium sulphide in seven paintings was implied where yellow samples containing elemental cadmium and sulphur showed no clear XRD patterns. An amorphous component may also have been present in other samples, combined with the crystalline material characterized using XRD.

Surface examination of artificially aged samples

Some of the paint samples artificially aged showed similar degradation phenomena to the deteriorated cadmium-containing yellow paint in the paintings examined. The surface changes observed using light microscopy are listed in Table 1. In general, the deterioration of the lighter-coloured paintsbound in cold-pressed linseed oil was most pronounced, resulting in matt, etched and sometimes chalked surfaces after light-ageing at high RH. Chalking corresponded to whitening of the surface, which revealed the brighter yellow pigment below when scraped away. Darkening to a dull ocherous colour occurred predominantly at higher RH while some samples obtained dark grey specks on the surface after ageing. SEM imaging of the aged surface of samples showed contraction of the medium away from the pigment in the more degraded samples, with underbound particles exposed at the surface (Figure 3a). This effect was exaggerated in sample 14 where the binding medium is barely evident at the surface (Figure 3b). In general, light exposure and/or high RH produced surfaces where particles were

(a) (b)

(c) (d)

Figure 3. SEM images showing sample surfaces after ageing for (a) sample 15 in cold pressed linseed oil after light ageing at 85% RH (6500×), (b) sample 14 in cold pressed linseed oil after light ageing at 85% RH (3500×), (c) sample 14 in pre-polymerized linseed oil after light ageing at 85% RH (2000×), (d) sample 14 in pre-polymerized linseed oil after light ageing at 85% RH (5000×)

exposed or more thinly coated with binding medium than in the controls. Samples aged at high RH in the dark exhibited similar though less extremely roughened surfaces, and it is likely that the surface changes are to some extent related to the effects of moisture on the oil medium. Sample 14 in all three media light exposed at 85 per cent RH exhibited crystallike entities protruding from the surface (Figure 3b, c). Elemental analysis of these entities showed the presence of sulphur, sodium and oxygen, which corresponds to the elements found in the rod-like structures present in the dry pigment (Figure 2d). SEM images of samples 9 and 14 bound in pre-polymerized linseed oil and light-aged at 85 per cent RH exhibited feather-like structures characterized in previous studies as fatty acids evaporating from the oil film (Figure 3d) (Burnstock et al. 1993).

Examination of cross sections

Almost all cross-sections prepared from the most deteriorated samples revealed a paler, discoloured upper layer, which in ultraviolet light fluoresced differently from the rest of the layer, usually a white to grey-greenish hue (Figure 4a, b). This was also evident in cross-sections prepared from Mother and Child by Frank Heath (Figure 4c, d). Analysis of cross-sections was carried out using EDX and SIMS to characterize the difference between the bulk sample and the degraded surface. Compared with the bulk sample, cadmium was generally depleted in the uppermost-discoloured layer. SIMS mapping showed a lower concentration of fatty acids on the surfaces of samples that exhibited chalking, whereas the bulk pigment was associated with a higher concentration of fatty acids, and is therefore better bound in samples where darkening occurred. All pigment samples examined in cross-section fluoresced orange-pink in ultraviolet light. Samples 9, 13 and 15 exhibited brighter specks of luminous pink,

(a) (b)

(c) (d)

Figure 4. Cross-sections from sample 13 in pre-polymerized linseed oil after ageing at 45% RH in the dark in (a) ordinary light and (b) ultraviolet light, and from Mother and Child by Frank Heath in (c) ordinary light and (d) ultraviolet light

whereas sample 14 fluoresced bright red. A similarly bright fluorescence had been observed for The Daffodil Fields by Jessica Heath. Such strong fluorescence suggests the presence of impurities which can act as luminescence centres within the pigment structure (De la Rie 1982).

Characterization of the degradation products

Several possible degradation products in aged pigment samples and samples from paintings were characterized by XRD, listed in Tables 2 and 3. Cadmium oxide sulphate was found after light ageing at 85 per cent RH in sample 14. Cadmium sulphate and zinc hydroxide were detected at the degraded surface of sample 1. This sample also showed changes in pigment morphology at the surface after light ageing at 85 per cent RH (Figure 5a) with the emergence of sharp-edged, flat particles (greater than 1 μm), characteristically different from those observed in the dry pigment (Figure 5b). Cadmium carbonate as well as barium cadmium chloride was detected in sample 17 after light ageing at 85 per cent RH. This sample also revealed small particles emerging from the surface of the paint film that corresponded to a heavy grey metallic-like surface discolouration.

(a) (b)

Figure 5. SEM images showing (a) sample 1 in pre-polymerized linseed oil after light ageing at 85% RH (15,000×) and (b) sample 1 dry pigment (25,000×)

A sample from the discoloured paint of the Jessica Heath contained cadmium carbonate otavite and the chalking yellow from Picasso’s Flowers contained cadmium carbonate oxide. Leger’s Mechanical Element contained cadmium oxide hydroxide in the pale upper layer, while cadmium sulphate and zincite were found in the discoloured upper layer of Bart van der Leck’s The Storm. In some cases, such as for the Seurat, it was not possible to characterize the darkened degradation product evident on the surface as the degraded layer was too thin to separate from the bulk sample. The same was true for the sample of darkened yellow from the Bonnard where the brown product was not detected, however a secondary degradation product, white zinc oxide, was characterized. Another noteworthy observation was that the addition of excess sulphur in the artificially aged samples tended to hinder deterioration of the medium, although these samples all appeared darker after ageing.

Discussion

The presence of amorphous cadmium sulphide in seven paintings was implied where yellow samples containing elemental cadmium, zinc and sulphur showed no clear XRD patterns. The presence of amorphous cadmium sulphide in batches of the pigment is related to its method of manufacture, where calcination, which results in the formation of a more stable crystalline

product, does not always seem to have occurred historically. Early accounts on pigment manufacture report how heating must be avoided as it darkens the pigment and ruins the colour (Bersch 1901, Ward and Dudley 1927). This is most likely the case in the presence of oxygen, but calcination should take place in an inert or reducing atmosphere to avoid oxidation. The process must be perfectly controlled to ensure even colour, and seems only later to have been refined (Curtis and Wright 1954, Huckle et al. 1966). XRD characterized degradation products associated with surface darkening and chalking in pigments and samples from paintings, including brown cadmium oxide and the colourless and white products cadmium sulphate, zinc hydroxide and zinc oxide. Cadmium carbonate was also detected, but it is not clear to what extent this is formed as a degradation product, as it is known to have been added to the pigment in an attempt to produce a lighter colour (Fiedler and Bayard 1986). An hypothesis about the degradation reaction and degradation products is proposed: this involves the oxidation of reactive amorphous cadmium zinc sulphide yielding cadmium oxide and sulphate, zinc oxide and sulphur dioxide gas. Dimitrov et al. (2002) also suggest oxidation as the most likely reaction pathway. Further reactions of the degradation products, for example in oil media, may include the formation of zinc carboxylate salts, which would explain the absence in the analysis of zinc oxide expected as a degradation product via this reaction. The predominance of zinc-containing compounds in the degraded upper layers of samples can be explained by the mobility of zinc ions in the presence of moisture, whereas damp conditions might promote degradation of the drying oil, whereby sulphur dioxide reacts to form sulphuric acid leading to acid hydrolysis of the binding medium. This could explain the etched surfaces observed in several degraded samples in this study. Acid reactions (involving SO4/H2SO4) may be mopped up or inhibited through mixing with lead white and chalk (alkaline pigments to form salts) resulting in more pronounced degradation only when the pigment is more or less used pure. There may also be links to the pigment volume concentration and whether the paint is medium rich or lean. The deterioration was observed to be more pronounced in areas of impasto, whereas thinly or evenly applied areas tend to be less degraded. A second degradation phenomenon related to the manufacture of the pigment was characterized in sample 14, Laurie’s cadmium yellow. After ageing this sample exhibited erosion of the binding medium, exposed particles on the surface and crystal-like entities containing sulphur oxygen and sodium (Figure 3b). In one of his recipes for cadmium sulphide, Laurie (1895) recommends precipitation from cadmium sulphate and sodium thiosulphate. It is likely that the presence of these impurities, also implicated in the bright fluorescence of this sample, are the result of improper washing during the manufacturing process. The whitened upper layer of The Daffodil Fields by Jessica Heath was very similar in appearance to sample 14 after ageing, but contained a significant amount of chlorine. Although this may have been introduced by airborne deposit or surface dirt, it may result from the manufacture of the pigment from cadmium chloride, as it is a well-known tendency for cadmium sulphide to occlude salts from the solution in which it is precipitated. The deterioration of Cornelissen’s cadmium yellow was surprising, as this sample was included as a control because modern cadmium yellow is considered stable and used for retouching. Although deterioration was observed in both the linseed oils, the sample in Paraloid B72 appeared unchanged, which suggests that this medium affords the pigment protection from reactive species.

Conclusion

Evidence from the present study suggests that the deterioration of cadmium yellow paints includes deterioration of the binding medium and alteration of the pigment, resulting in the

formation of brown, white and colourless compounds. A hypothesis about the oxidation of amorphous cadmium zinc sulphide is proposed, yielding cadmium oxide and sulphate, zinc oxide and sulphur dioxide gas. Further reactions of the degradation products may include acid hydrolysis of the binding medium and the formation of zinc salts. The presence of amorphous cadmium sulphide in artists’ pigments is related to the early manufacture of the pigment where calcination, which results in the formation of a crystalline and thus more stable product, does not always seem to have occurred. Samples containing a high proportion of amorphous cadmium yellow pigment may be most reactive, and pigments made using manufacturing methods where the amorphous intermediate stage is not properly converted to the crystalline product may explain why some pigments are more prone to deteriorate. The implications for conservation include an awareness of the physical and chemical changes in degraded paint films containing cadmium yellow pigments, and the consequent risks involved in cleaning and the consolidation of loose pigment at the surface. Change in paint solubility due to the formation of soluble degradation products is an issue in the choice of solvents used for cleaning, consolidation or coatings. This study points towards the possibility that moisture treatments or even environments of high humidity may trigger the deterioration.

Acknowledgements

We thank Gordon Cressy, Natural History Museum, London, for X-raydiffraction of selected samples.

References

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Materials

Cadmium sulphidesCornellisen and Son Ltd105 Great Russell StreetLondon WC1B 3RYUnited KingdomTel.: +44 (0)207 636 1045Web site: www.cornellisen.com

Cold pressed linseed oilDr Georg F KremerFarbmühleD-88317 AichstettenGermanyTel.: +49 756591120Fax: +49 75651606E-mail: kremer-pigmente@t-online.de

Pre-polymerized linseed oilWinsor & NewtonWhitefriars AvenueHarrowMiddlesex HA3 5RHUnited KingdomTel.: 0208 4274343Web site: www.winsornewton.com

PTFE SheetRS Components Ltd

PO Box 99CorbyNorthants NN17 9RSUnited KingdomTel.: +44 01536 201201Web site: www.rswww.com