Open Access

Characterisation of preparation layers in nine Danish Golden Age canvas paintings by SEM–EDX, FTIR and GC–MS

  • C. K. Andersen1Email author,
  • I. Bonaduce2,
  • A. Andreotti2,
  • J. van Lanschot1 and
  • A. Vila3
Heritage Science20175:34

https://doi.org/10.1186/s40494-017-0147-0

Received: 5 March 2017

Accepted: 5 July 2017

Published: 16 August 2017

Abstract

This study explores the materials used in the preparation layers of nine paintings from the Danish Golden Age as a first approach to understanding the variation in use of materials in the nineteenth century as well as the potential for their degradation. Paintings on canvas have traditionally been suspected to be particularly sensitive to high moisture levels because of the changing quality of materials in the nineteenth century. The explanations have partly included the mechanisation of production methods and partly a more experimental approach to painting. Additionally, collagen-based glue sizing of the canvas is suspected to respond dimensionally to changes in relative humidity. In this study, pigments, fillers and binding media in the preparation layers of nine paintings by different artists were identified using scanning electron microscopy with energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy and gas chromatography–mass spectrometry. The study shows a relatively low degree of variation in materials used in grounds. Surprisingly, no collagen-based binder was found in any of the nine paintings, suggesting that the canvases were not glue sized. All paintings contained calcium, lead, a drying oil and egg, even though only few contemporary recipes in painter’s manuals included egg as an ingredient for preparation layers. These results suggest that the commercial producers of prepared canvas may not have followed the manuals that were written for painters. Egg may have been added in order to increase flexibility and durability of ready primed canvases that were stored and sold in rolls. Moreover, the egg–oil emulsion has the advantage of being more viscous than a pure oil paint and could thus be used without sizing the canvas, rendering the primed canvas less stiff and less responsive to changes in relative humidity. The advantages of using egg in the ground are obvious, and this use, as well as the lack of glue size, has implications for the long-term preservation of the paintings in changing environmental conditions. These results imply that these particular paintings might be less sensitive to relative humidity changes than expected due to the lack of hygroscopic glue.

Keywords

Danish Golden Age paintings Nineteenth century Preparation layers Egg–oil emulsion ground Gas chromatography–mass spectrometry (GC–MS) Fourier transform infrared spectroscopy (FTIR) Energy dispersive X-ray spectroscopy (SEM–EDX) Moisture sensitivity

Introduction

Danish Golden Age paintings were painted in the first half of the nineteenth century and are highly esteemed as an invaluable part of Danish cultural heritage. The painters involved in this study are Christoffer Wilhelm Eckersberg (1783–1853), also called ‘the father of Danish painting’, and some of his most famous pupils at the Royal Danish Academy of Fine Arts: Christen Schillerup Købke (1810–1848), Wilhelm Bendz (1804–1832) and Carl Christian Constantin Hansen (1804–1880), as well as Johan Thomas Lundbye (1818–1848), who was the pupil of professor J.L. Lund, Eckersberg’s colleague at the academy.

This study concentrates on how the canvases used by these painters were prepared before paint application. The purpose of the preparation layers was to produce a uniform background for the painters to work on. In some cases, the procedure for preparing a canvas included sizing, which was the application of a layer of glue from bones, mammal skin, fish swim bladders, or flour paste to the raw canvas [1]. On top of this glue layer, one or more ground layers containing pigment, fillers, and binding media would then be applied. In some cases, a brownish or reddish layer with more inexpensive materials such as ochre or chalk that was bound in oil or suspended in an emulsion was used to fill the voids of the canvas. A thinner layer of more expensive materials was then added to supply the desired background colour. In some cases, painters probably applied their own ground(s) on top of a commercially primed canvas [2, 3].

The colour and texture of the ground layer could influence the tonality and surface appearance of the finished painting; in addition, the materials chosen for the preparation layers can affect the way a painting age. In Denmark as well as abroad, canvas paintings of the nineteenth century have been suspected to be extra sensitive to humidity and water, which cause shrinkage and cracking [46]. Theories regarding variations of preparation layers can be investigated through studies of historic recipes and mock-ups but, to understand the full potential of the preparation layers for influencing degradation, the historic painting materials must be characterised through analyses.

Colour and preparation layers have been studied for some painters from the Danish Golden Age [4, 7] as well as the previous generation. Thus, we know that there was a shift in the use of colours of grounds (from coloured to white) in Denmark from Nicolai Abildgaard [8] through Jens Juel [9] to Christoffer W. Eckersberg [7]. Previous studies furthermore suggest that painters primarily painted on Danish-produced canvases due to the high customs duties on canvases during the first half of the nineteenth century [5]. This behaviour suggests that canvases were locally produced and prepared except for a few specific instances. Eckersberg’s journals show that he bought ready-primed canvases or employed a local craftsman to prepare his canvases [4, 7].

For a canvas that was prepared for commercial sale, we may assume that there were practical issues to consider such as their rolling, storing and shipping [10]. These issues means that flexibility would be an issue of importance, and the binding media plays a significant role in defining these qualities. Certain materials such as collagen-based glues would increase both stiffness and hygroscopic behaviour if used in the preparation of the canvases and might cause failure in the added paint layer [11, 12], whereas we would expect oils to provide more flexibility and less reactivity to moisture.

Recipes suggest that the binding media in grounds in the first half of the nineteenth century were mainly oil or glue or a mixture of both, but a variety of other materials was also introduced. The use of starch is well known [2, 13] and, used less frequently, casein, egg and natural rubber [14]. Grounds were required to have a certain absorbency, which was achieved with glue or an emulsion of oil and water as a binder [15]. However, as mentioned, a certain flexibility that would allow the primed canvases to be rolled for storage or shipping would have become increasingly important with the commercialisation of prepared canvases. It must have been a difficult balance to strike, as more oil provided more flexibility but less absorbency.

This study explores the materials (organic and inorganic components) used in the preparation layers of nine Danish Golden Age paintings in order to improve our understanding of the manufacturing processes and to achieve an impression of the degree of variation in the ground and size layers used at the time. It is furthermore a first approach to understanding the potential for physical degradation by mechanisms such as mechanical stress and to understand the possible sensitivity to changes in relative humidity in order to improve the decision-making platform for conservation and storage of the paintings.

Experimental methods

Golden age painting samples

Statens Museum for Kunst (the National Gallery of Denmark, SMK) possesses a number of samples from the tacking edges of paintings, which include the most prominent Danish Golden Age artworks. These samples were removed by conservators in the first half of the 1960s before conservation treatment, which was usually the application of a wax resin lining. This removal was done in order to have material for later research, and the samples are rather large by modern standards. The size varies from 2 × 50 mm to 20 × 240 mm.

Samples from paintings by influential artists have been chosen for this study (Table 1) and represent different trends and periods of the Danish Golden Age. The paintings furthermore represent different genres (portraits, landscapes and seascapes); two of the nine paintings were executed outside Denmark, namely, in Paris (no. 3) and Rome (no. 4). These canvases were therefore likely to have been prepared abroad.
Table 1

Nine selected paintings

No.

Title artist, year, dimensions no., SMK painting accession no.

Cross section (dark field)

Conservation treatment

1

“The Opera Singer Christoffer Hvid”, Bendz, W. 1825–27, 38.5 × 31 KMS3262

Previous conservation: 1913 glue–paste lined and new stretcher; 1964 wax–resin lined. Glue–paste residue and traces of paper are probably from the glue–paste lining from 1913

2

“The Artist’s Sisters Signe and Henriette Reading a Book”, Constantin Hansen, C.C., 1826, 65.5 × 56.0, KMS3004

Restored (method unknown) 1910, cleaned and “steamed” 1944, wax–resin lined 1963

3

“The Vesta Temple in Rome”, Constantin Hansen, C.C. 1837, 75.5 × 100.0, KMS1072

Wax–resin lined in 1963. This painting was painted in Italy

4

“A View from the Château of Meudon”, Eckersberg, C.W. 1813, 55.5 × 71.0 KMS1623

The painting was glue–paste lined in 1899 and wax–resin lined twice in the 1960s due to severe cracking. The red ground layer has poor adhesion to the canvas

5

“A Russian Fleet at Anchor near Elsinore”, Eckersberg, C.W. 1826, 31.5 × 59.0 KMS1671

1901: new stretcher; 1912 cleaned; 1964 wax–resin lined (no previous lining)

6

“The Russian Ship of the Line “Asow” and a Frigate at Anchor in the Roads of Elsinore”, Eckersberg, C.W. 1828, 63.0 × 51.0 KMS608

1906: yellow varnish possibly removed. Wax–resin lined 1963, no previous lining, good condition

7

“Nude Figure Sitting Boy” Købke, C.S. 1833, 58.3 × 48.7 KMS6177

The painting was wax–resin lined in the 1960s, but the detailed written report disappeared from the conservation archive

8

“Portrait of the Decorative Artist G.C. Hilker”, Købke, C.S. 1837, 65.0 × 54.5 KMS1081

The painting was wax–resin lined 1963/64 and had no previous lining. There is an extensive crack pattern in the painting surface

9

“A Croft at Lodskov near Vognserup Manor, Zealand”, Lundbye, J. Th. 1847, 71.0 × 93.0, KMS1644

1963: wax–resin lined

Cross sections were cut from the tacking edge of samples. Canvas has been included in order to make sure that all preparation layers were included

The following techniques were used for the analytical investigation of the materials in ground samples:

Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM–EDX)

SEM–EDX was performed on cross sections taken from the pieces of tacking edge that were removed during conservation in the 1960s (see Table 1). Samples were embedded in Serifix resin from Struers (methyl ethyl ketone peroxide, butanone and dimethyl phthalate), and the cross sections were polished with sandpaper. Cross sections were analysed in order to obtain elemental information using a Hitachi S-3400N equipped with a Bruker detection system. Spectra and elemental maps were recorded using 20-kV voltage, 50-µA probe current and 10-mm working distance.

Fourier transform infrared spectroscopy (FTIR)

A sample of the ground (approximately 0.5 × 0.5 mm) was taken with a scalpel. The sample was positioned in a Perkin Elmer Spectrum 100 Fourier transform infrared (FTIR) apparatus equipped with an attenuated total reflectance (ATR) diamond/ZnSe crystal. The side that had been closest to the canvas was in direct contact with the ATR crystal. Spectra were acquired in the range from 4000-650 cm−1 with a spectral resolution of 4 cm−1 and at a pressure of 100 N for 4 scans. To corroborate SEM–EDX results from the different layers, cross sections of sample numbers 4 and 9 (KMS1623 and KMS1644) were also measured by micro fourier transform interferometer (µFTIR). In these two specific cases, FTIR analyses were performed with a Bruker Tensor 24® spectrometer coupled to a Hyperion® 3000 microscope that was equipped with a focal plane array (FPA) detector. Measurements were performed in ATR mode with a 20× germanium crystal objective with a refractive index of 4.01 that has an anvil design in an 80-µm tip. An average of 32 scans was used in an accumulation range of 900–3600 cm−1 and at a spectral resolution of 4 cm−1.

Gas chromatography–mass spectrometry (GC–MS)

Samples (between 0.2 and 1.0 mg in weight) were analysed with an analytical procedure based on a wet chemical pretreatment of the sample that includes extraction, desalting and hydrolysis steps. This procedure was done in order to separate the same microsamples into three fractions so that they could be separately analysed by GC–MS. The three fractions were: a saccharide fraction (solution of persilylated dithioacetal derivatives of aldoses and uronic acids that were derived from the hydrolysis of saccharide materials), an amino acid fraction (solution of tert-butyldimethylsilyl derivatives of amino acids that were derived from the hydrolysis of proteins) and a resinous lipid fraction (solution of trimethylsilyl derivatives of acids, alcohols and neutral compounds that were derived from the hydrolysis of lipids, resins and waxes). The procedure is described in detail in the literature [16].

A 6890N GC system gas chromatograph coupled with a 5973 mass selective detector single-quadrupole mass spectrometer equipped with a split-splitless injector and a 6890N GC system gas chromatograph (Agilent Technologies, Palo Alto, CA) coupled with a 5975 mass selective detector single-quadrupole mass spectrometer equipped with PTV injector, also by Agilent Technologies, were used. The MS transfer line temperature was 280 °C; the MS ion source temperature was kept at 230 °C, and the MS quadrupole temperature was at 150 °C. For the gas chromatographic separation, an HP-5MS fused silica capillary column (5% diphenyl/95% dimethyl-polysiloxane, 30 m × 0.25 µm i.d., 0.25 µm film thickness, Agilent Technologies) with a deactivated silica precolumn (2 m × 0.32 µm i.d.) was used. The carrier gas was used in the constant flow mode (He, purity 99.995%) at 1.2 mL min−1. The mass spectrometer was operated in the positive EI mode (70 eV), and MS spectra were recorded both in total ion current (TIC) and single ion monitoring (SIM) modes.

Results

SEM–EDX and FTIR-ATR data are summarised in Tables 2 and 3, and GC–MS data relative to the three analysed fractions are reported in Tables 4, 5 and 6.
Table 2

Elements identified by SEM-EDX on the nine selected paintings

No.

SMK accession no.

Elements

1

KMS3262

Pb, Ca (Al)

2

KMS3004

Ca, Pb (Si, Al, Fe, Na)

3

KMS1072

Pb, Ca (Mg)

4

KMS1623

Whitish layer: Pb (Ca, Al, Si, Fe, K)

Red layer: Si, Fe, Al (Ca, K, Pb/S)

5

KMS1671

Pb, Ca (Al)

6

KMS608

Pb, Ca (Al)

7

KMS6177

Ca, Pb (Si, Mg, Al, K)

8

KMS1081

Whitish upper layer: Pb (Ca)

Lower layer: Ca (Pb, Si)

9

KMS1644

Whitish and yellow upper layer: Pb, Ba, Fe (Si, Al, Na)

Inter layer: particles of Si, Al, Na

Lower layer: Ca

Italics elements are present in relatively high levels. Elements in parentheses are present at trace levels. Pb (lead), Ca (calcium), Al (aluminium), Si (silicon), Fe (iron), Na (sodium), Mg (magnesium), K (potassium), S (sulphur), and Ba (barium)

Table 3

Absorption features in the region between 4000 and 600 cm−1 and their assignments in the FTIR-ATR spectra of the samples collected from the ground samples

No.

SMK accession no.

Lipids

Proteins

Calcium sulphate and/or other compounds

Calcium carbonate

Lead carbonate (basic)

1

KMS3262

2921 s

2852 s

1720 sh

1638 s

1530 s

1327 sh

1240 w

1208 w

1151 w

1078 sh

1030 sh

ca. 1100 br

2515 sh

1795 w

1384 s br

875 vs

715 s

3536 sh

1400 s br

1045 s

756 br

680 s

838 s (neutral)

2

KMS3004

2921 m

2852 m

1708 w

1638 m

1530 vw

1151 w

Silicate ca. 1000 s br

2515 sh

1795 w

1391 s br

875 vs

715 s

1400 s br

756 br

680 s

3

KMS1072

2921 s

2852 s

1738 + 1715 sh

1638 s

1530 s

1327 sh

3530 w

3403 w

1622 w

1110 s br

1393 s br

875 vs

715 s

3536 sh

1400 s br

1045 s

756 br

680 s

4

KMS1623

Top layer

2921 s

2852 s

1730 sh

   

1400 s br

1045 s

(+Pb carboxylates ca. 1515 cm−1)

KMS1623

Bottom layer

2921 s

2852 s

1730 sh

1638 br

1537 br

Silicate ca. 1000 s br

1393 w br

(carbonate)

 

5

KMS1671

2921 s

2852 s

1720 sh

1638 s

1530 s

1327 sh

1240 w

1208 w

1151 w

1078 sh

1030 sh

ca. 1100 br

2515 sh

1795 w

1384 s br

875 vs

715 s

3536 sh

1400 s br

1045 s

756 br

680 s

838 s (neutral)

6

KMS608

2921 s

2852 s

1710 sh

1638 s

1530 s

1151 w

1030 sh

ca. 1100 s br

ca. 673 w

2515 sh

1795 w

1384 s br

875 vs

715 s

3534

1400 s br

680 s

838 s w (neutral)

7

KMS6177

2921 s

2852 s

1738 sh 1715 sh

1638 s

1530 s

1327 sh

1240 w

1078 sh

1030 sh

3530 w

3403 w

1110 s br

673 s

1393 s br

875 vs

715 s

1400 s br

680 s

8

KMS1081

2921 s

2852 s

1720 m

1638 m

1530 m

1327 sh

1240 w

1208 w

1151 w

1078 sh

1030 sh

1620 br (w?)

1110 s (w?)

673 s (w?)

2515 sh

1795 w

1384 s br

875 vs

715 s

3536 sh

1400 s br

1045 s

756 br

680 s

9

KMS1644

Top layer

2921 s

2852 s

1730 sh

 

BaSO4 with bands ca. 1170, 1112, 1075, 981 s

 

1400 s br

1045 s

(+Pb carboxylates ca. 1515 cm−1)

10

KMS1644

Bottom layer

2921 s

2852 s

1738, 1710 sh

1169 w

1094 w

  

2515 sh

1795 w

1389 s br

875 vs

715 s

 

Regarding basic lead carbonate, it is only possible to identify the main components due to the overlap of calcium carbonate, lead carbonate and calcium sulphate peaks. Lead carboxylates could also be present, with bands approximately 1500 and 1400 cm−1; however, because the peaks are located on the same wavelengths as proteinaceous compounds and carbonates, they cannot always be identified

s sharp peak, m medium peak, w weak peak, br broad band, v very, sh shoulder band

Table 4

Amino acid profile (%) of samples with a protein content above the detection limit (DL) or quantitation limit (QL)

No.

SMK accession no.

Ala

Gly

Val

Leu

Ile

Ser

Pro

Phe

Asp

Glu

Hyp

Protein content

1

KMS3262

9.7

9.1

9.5

13.3

8.3

3.8

6.5

7.0

16.7

16.1

0.0

>QL

2

KMS3004

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

No

>DL

<QL

3

KMS1072

12.4

3.5

6.4

10.2

4.9

8.1

6.6

5.7

13.8

28.4

0.0

>QL

4

KMS1623

6.0

15.7

5.1

11.1

4.9

5.9

10.6

7.5

6.5

26.5

0.0

>QL

5

KMS1671

13.8

13.9

13.0

19.2

11.9

7.2

1.8

6.1

10.4

2.7

0.0

>QL

6

KMS608

4.2

2.7

5.4

8.7

5.2

37.5

1.3

1.5

13.1

20.3

0.0

>QL

7

KMS6177

8.5

14.3

8.6

14.3

8.3

5.0

6.0

8.6

14.7

11.6

0.0

>QL

9

KMS1644

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

No

>DL

<QL

Table 5

Characteristic parameters related to the identification of the source of the glycerolipid material in those samples with a lipid content above the QL

No.

SMK accession no.

A/P (azelatic acid/palmitic acid)

P/S (palmitic acid/stearic acid)

O/S (oleic acid/palmitic acid)

Σ Dicarboxylic acids %

1

KMS3262

1.4

1.4

0.0

49

2

KMS3004

2.6

1.3

1.3

63

3

KMS1072

1.1

3.7

36

4

KMS1623

2.6

1.3

0.1

63

5

KMS1671

1.5

1.3

0.0

51

6

KMS608

0.9

3.0

0.4

43

7

KMS6177

1.7

0.9

0.0

51

8

KMS1081

0.8

2.0

2.0

29

9

KMS1644

1.5

1.3

0.0

52

The meaning of the reported parameters are discussed in detail in the literature [20]

Table 6

Relative aldose and uronic acid content (%) of samples with a sugar content above the DL

No.

SMK accession no.

Xylose

Arabinose

Rhamnose

Fucose

Galact uronic acid

Glucuronic acid

Glucose

Mannose

Galactose

Sugar content

1

KMS 3262

0

0

0

0

0

0

36

59

6

>QL

5

KMS 1671

0

0

0

0

0

0

34

58

8

>QL

9

KMS 1644

18

4

2

0

0

0

52

7

17

>QL

For the characterisation of the source of the proteinaceous material, the amino acid profile of each sample whose protein content was above the quantitation limit of the procedure was subjected to a multivariate statistical analysis that included a data set of 121 reference samples of animal glue, casein, and egg (whole egg, albumen and yolk) [17] using principal component analysis (PCA). The resulting score plot is presented in Fig. 1.
Fig. 1

Principal component analysis (PCA) score plot

PCA score plot relative to the ground samples with a protein content above the QL, using a database of reference samples of animal glue, casein and egg.

To determine the source of the saccharide materials, the data were compared to a database of saccharide profiles of reference materials [18], and when polysaccharide gums were hypothesised, they were identified using a decisional scheme that was reported in the literature [19]. The interpretation of the chromatograms relative to the lipid-resinous fractions was based on the recognition of molecular markers for resins and waxes and quantitative evaluation of mono and dicarboxylic acids for lipids [20].

Discussion

The SEM–EDX data allowed us to obtain the elemental composition and stratigraphic information of the ground layers. The EDX information was complemented by the molecular information on organic and some inorganic compounds that were obtained by FTIR. Analyses done with GC–MS on whole samples allowed us to specifically list the organic compounds that were used in grounds and possible size layers without pointing to the specific layers from which these compounds originate. The data are discussed in the following paragraphs and summarised in Table 7.
Table 7

Overview of suggested content in the nine paintings

No.

SMK accession no.

Binders

Pigments and elements

Fillers

1

KMS3262

Drying oil (linseed or walnut) + egg

Lead carbonate

Calcium carbonate, perhaps calcium sulphate

2

KMS3004

Drying oil (perhaps linseed) + egg or casein

Lead carbonate

Silicon and calcium containing filler

3

KMS1072

Drying oil (linseed or walnut) + egg (whole or yolk)

Lead white + (Mg and Si)

Calcium sulphate and calcium carbonate

4

KMS1623

Drying oil and egg (whole or yolk)

Lower layer: iron earth pigment (Fe, Si, Al, Ca, K and Pb/S)

Top layer: lead carbonate + (Ca, Al, Si, Fe, and K)

 

5

KMS1671

Drying oil + egg (whole or yolk)

Lead carbonate

Calcium carbonate, perhaps calcium sulphate

6

KMS608

Drying oil (linseed or walnut) + egg

Lead carbonate, perhaps calcium sulphate

Calcium carbonate, perhaps calcium sulphate

7

KMS6177

Drying oil + egg (whole or yolk)

Lead carbonate, calcium sulphate

Calcium carbonate, calcium sulphate

8

KMS1081

Drying oil + egg (whole or yolk)

Lead carbonate

Silicon and calcium containing filler

9

KMS1644

Drying oil + egg or casein + gum

Top layer: lead carbonate, Fe, Si, Al, Na, Ca

Lower layer: no clear identification

Top layer: barium sulphate

Lower layer: calcium carbonate

Generally, the analysed grounds are rather similar in composition, but minor differences have been noted. The FTIR and SEM–EDX analysis of the examined grounds generally confirmed the presence of lead carbonate and calcium carbonate, and in five cases, calcium sulphate was also observed (KMS3262, KMS1072, KMS1671, KMS608, and KMS6177); however, due to the overlap of peaks in the fingerprint area, it is not always possible to confirm its presence. Silicon was observed in five paintings, and silicate was clearly observed by FTIR in two paintings (KMS1623 and KMS3004). Barium sulphate was only found in sample KMS1644. Calcium carbonate, calcium sulphate and barium sulphate were mainly used as fillers in oil grounds, as they become semi-transparent in oil, whereas lead white provided the white colour to the ground. Furthermore, they acted as dryers to catalyse the oxidation of the oil binder. Apart from a slight tinting of some of the white layers in the grounds, a light-brown bottom layer is the only variation to be found among the pigment composition of the nine grounds [4]. SEM–EDX results from painting number 4 by Eckersberg (KMS1623), which was painted in France, had a red iron earth pigment below the white top layer. The white grounds were also reported in a large survey of Eckersberg paintings [7] and recipes from other European countries [2], confirming a European trend of using white grounds in the period.

GC–MS analyses on the grounds of the nine paintings allowed the identification of a drying oil, most likely linseed or walnut oil, and egg. The oil seemed to have been combined with egg in different amounts in order to form a thick emulsion. It was not possible to determine if yolk or white had been used. The amino acid profiles of egg yolk and egg white proteins are in fact very similar [20], and the P/S ratio could not be used to determine the source of the oil or to verify the presence of the egg lipids. Walnut oil would in fact give rise to similar P/S values as a those of a mixture of egg lipids and linseed oil. The P/S ratios that were obtained were in fact quite low, and according to the literature, this could be interpreted as being due to linseed oil alone [21]. The absence of egg lipids cannot be verified, as egg might be present in relatively minor amounts and consequently not modifying the fatty acid profile of the drying oil. Moreover, it has been demonstrated that fatty acids escape a paint film in time, and considering that palmitic acid evaporates four times quicker than stearic acid [22], the P/S value is expected to significantly decrease over time [23]. The presence of lead white in the preparation layer should make the P/S parameters somewhat more reliable, as lead carboxylates are not volatile. In these samples though, saponification, if it occurred, was not complete, as the νas COO bands of the glycerolipid materials were still very well visible in the spectra, and the νas COO bands of the lead carboxylates, if present, were not distinguishable from the amide II band. The sugar profiles that were observed were in perfect agreement with the presence of egg [18].

Collagen-based compounds, such as animal glue, contain the amino acid hydroxyproline, and the absence of this amino acid in all nine samples ruled out the presence of collagen-based glues in the samples.

Animal glue size was thus not identified in any of the nine samples, although the samples were obtained with great care so that all layers from the canvas on up would be included. Interestingly, this result concurs with discoveries by Stools-Witlox, who concludes that a large proportion of nineteenth century recipes for canvas preparation do not mention a size layer [1].

Furthermore, the analyses evidenced the presence of egg in all cases, a binder that is rarely mentioned in contemporary European sources [1, 2]. However, new materials such as egg did start to occur in recipes in the nineteenth century due to the general trend of experimentation and innovation [1, 24]. As discussed above, the analyses do not clearly show which part of the egg was used because of the simultaneous presence of the drying oil. However, it is well known that egg yolk mixes well with oil due to its emulsifying powers [25, 26].

Conservation-based sources provide an impression of the practical implications of the use of egg–oil grounds. Paints with egg yolk are considered to be strong and durable and have a low tendency to turn yellow, compared to pure oil paints [26, 27]. While egg white is supposed to make brittle paints, yolk-based paints are considered more flexible and tough [28, 29]. A paint based on emulsion of egg and oil would have a higher viscosity than a pure oil ground, and, depending on the particular mixture and the openness of the canvas weave, it could be used directly on a canvas without the use of glue size to prevent it from sinking into the canvas structure.

With an egg–oil ground and no animal glue size, a merchant or artist would probably have been able to roll and store the canvas with ground with a smaller risk of cracks than would have been the case with a glue- or starch-based ground [25]. Furthermore, due to the lack of collagen-based glue, the preparation layer would be less dimensionally responsive at different relative humidity levels, and the risk of mechanical damage could be expected to be lower with this type of preparation. In painting KMS1644, a polysaccharide gum was found. The xylose/arabinose ratio was higher than one, and fucose was absent. Therefore, we can exclude the presence of tragacanth gum and suggest the presence of fruit tree or gum arabic [19], which could very well serve the similar purpose of providing flexibility; alternatively, it could have been used as a sizing agent for the canvas.

KMS3262 and KMS1671 seem quite similar in material composition in the analyses that have been used here, and the two paintings were also painted more or less at the same time but by two different artists. A thread count conducted on these two canvas samples suggest that they are slightly different (KMS3262: 10.0 × 14.5 threads per centimeter and KMS1671: 15.2 × 14.5 threads per centimeter (or 12 × 14.7 when measured with automated thread count on the entire painting [7]), although both are tightly woven [4]. Whether the canvases are in fact different remains uncertain, but the results support the hypothesis that painters used the same supplier(s) of ground, and these suppliers may have had a premixed batch of ground that was applied to more canvases.

Conclusions

SEM–EDX, FTIR and GC–MS analyses of pigments, fillers and binders that were used in grounds of a group of Danish nineteenth century paintings seemed to indicate that the painters of the Danish Golden Age used similar materials. In all the paintings that were analysed, indications of egg–oil mixtures were found as well as lead white and calcium. The latest painting, from 1847, is a little different, as it also contains a gum and barium sulphate. This similarity indicates that certain recipes were favoured amongst the suppliers in Denmark for a period of time or amongst students of the same professor. Furthermore, the results question whether other contemporary countries used similar recipes, since two of these paintings were painted abroad. As animal skin glue seems to be absent in all cases, the findings indicate that these paintings have a relatively low response to relative humidity fluctuations. The presence of egg and gum may indicate a concern for practical matters such as rolling, stretching and storing preprepared canvases.

As all the paintings in this study were wax–resin lined in the 1960s, it does not seem possible to make more general conclusions by comparing the findings reported here and the current condition of the paintings.

Abbreviations

GC–MS: 

gas chromatography–mass spectrometry

QL: 

quantification limit

DL: 

detection limit

PCA: 

principal component analysis

PTV: 

programmed temperature vaporising

EI: 

electron impact

FTIR-ATR: 

Fourier transform infrared spectroscopy

S

sharp peak

m

medium size peak

w

weak peak

br

broad band

v

very

sh

shoulder band

SEM–EDX: 

energy dispersive X-ray spectroscopy

Declarations

Authors’ contributions

CKA developed the concept of this work. AA, JvL, CKA and AV contributed to acquisition and analysis of the data. AA, IB, AV and JvL contributed to data interpretation. CKA provided context and all authors helped drafting/revising the manuscript. All authors read and approved the final manuscript.

Authors’ information

CKA is an assistant professor and a paintings conservator. Her research focuses on mechanical properties of paintings as well as technical examination of paintings technique. IB is an associate professor in analytical chemistry specialising in analysis of organic materials in cultural heritage objects using gas mass spectrometric-based techniques. AA is a graduated technician specialising in analysis of organic materials in cultural heritage objects using gas mass spectrometric-based techniques. JvL is a conservator with extensive experience in Scanning Electron Microscope analyses. AV is a senior researcher in conservation science who is specialised in analyses of artists’ materials using spectroscopic analyses.

Acknowledgements

The Conservation Department at Statens Museum for Kunst (SMK) generously provided the paint samples for this study, which was carried out as a collaboration between the Centre for Art Technological Studies and Conservation (CATS) and the University of Pisa. CATS is a strategic research venture between SMK, the National Museum of Denmark and The Royal Danish Academy of Fine Arts, School of Conservation, in Copenhagen.

Competing interests

The authors declare that they have no competing interests.

Availability of data and materials

Measurements and spectra are shared on ResearchGate.

Funding

The Danish Council for Independent Research, Humanities (FKK).

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

Authors’ Affiliations

(1)
The Royal Danish Academy of Fine Arts, Schools of Architecture, Design and Conservation
(2)
Department of Chemistry and Industrial Chemistry, University of Pisa
(3)
Centre of Art Technological Studies and Conservation

References

  1. Stols-Witlox M. Historical recipes for preparatory layers for oil paintings in manuals, manuscripts and handbooks in North West Europe, 1550–1900: analysis and reconstructions [Ph.D]. University of Amsterdam. 2014. http://dare.uva.nl/search?metis.record.id=430263.
  2. Witlox M, Carlyle L. A perfect ground is the very soul of the art: ground recipes for oil painting 1600–1900. In: Verger I, editor. ICOM committee for conservation 14th triennial meeting, the Hague, 12–16 September 2005 the Hague. London: James & James/Earthscan; 2005. p. 519–28.Google Scholar
  3. Kirby J, Roy A. Paul Delaroche: a case study of academic painting. In: Wallert A, Hermens E, Peek M, editors. Historical painting techniques, materials, and studio practice: preprints of a symposium, University of Leiden, the Netherlands, 26–29 June 1995; Los Angeles, California. Marina del Rey: The Getty Conservation Institute; 1995. p. 166–75.Google Scholar
  4. Andersen CK. Lined canvas paintings. Mechanical properties and structural response to fluctuating relative humidity, exemplified by the collection of Danish Golden Age paintings at Statens Museum for Kunst (SMK) [Ph.D]. Unpublished: KADK Royal Danish Academy of Fine Arts, Schools for Architecture, Design and Conservation, School of Conservation; 2013.Google Scholar
  5. Andersen CK, Filtenborg T, Scharff A, Scharff M. The industrialisation of canvas production in Denmark and its implications for the preservation of Danish nineteenth century paintings. In: Ryhl-Svendsen M, Borchersen K, Odder W, editors. Incredible industry—preserving the evidence of industrial society; May 25–27; Copenhagen. Copenhagen: NKF-dk; 2009. p. 39–49.Google Scholar
  6. Hedley G. The practicalities of the interaction of moisture with oil paintings on canvas. In: Hedley G, Villers C, editors. Measured opinions: collected papers on the conservation of paintings. London: United Kingdom Institute for Conservation; 1993. p. 112–22.Google Scholar
  7. Filtenborg T, Andersen CK. Canvas supports and grounds in paintings by C.W. Eckersberg. In: Townsend JH, Vandivere A, editors. Technology & practice: studying the European visual arts 1800–1850—paintings, sculpture, interiors and art on paper; National Museum of Denmark, Copenhagen, 2016: Archetype; 2017. p. 43–54.Google Scholar
  8. Filtenborg T, Vila Espuna A. Between formula and freestyle, Nicolai Abildgaard and 18th-century painting technique. London: Archetype Publications; 2014.Google Scholar
  9. Slotsgaard TL. An investigation of the painting technique in portraits by Jens Juel. In: Evans H, Muir K, editors. Studying 18th-century paintings and works of art on paper; CATS Proceedings, II, 2014; Copenhagen: Archetype Publication and CATS (Centre for Art Technological Studies and Conservation; 2015. p. 46–57.Google Scholar
  10. Zucker J. From the ground up: the ground in 19th-century American pictures. J Am Inst Conserv. 1999;38(1):3–20.View ArticleGoogle Scholar
  11. Mecklenburg MF. Some aspects of the mechanical behavior of fabric supported paintings. Washington: Report to The Smitsonian Institution; 1982.Google Scholar
  12. Karpowicz A. A Study on development of cracks on paintings. J Am Inst Conserv. 1990;29(2):169–80.View ArticleGoogle Scholar
  13. Carlyle L, Young C, Jardine S. The mechanical response of flour-paste grounds. In: Townsend JH, Doherty T, Heydenreich G, Ridge J, editors. Preparation for painting: the artist’s choice and its consequences. London: Archetype Publications Ltd.; 2008. p. 123–31.Google Scholar
  14. Carlyle LA. The artist’s assistant: oil painting manuals and handbooks in Britain 1800–1900 with reference to selected eighteenth-century sources. London: Archetype Publications; 2001.Google Scholar
  15. Bouviers MBL. Vollständige Anweisung zur Oehlmalerei für Künstler und Kunstfreunde. 2nd ed. Halle: Schwetschke und Sohn; 1838.Google Scholar
  16. Lluveras A, Bonaduce I, Andreotti A, Colombini MP. GC/MS analytical procedure for the characterization of glycerolipids, natural waxes, terpenoid resins, proteinaceous and polysaccharide materials in the same paint microsample avoiding interferences from inorganic media. Anal Chem. 2010;82(1):376–86.View ArticleGoogle Scholar
  17. Bonaduce I, Cito M, Colombini MP. The development of a gas chromatographic–mass spectrometric analytical procedure for the determination of lipids, proteins and resins in the same paint micro-sample avoiding interferences from inorganic media. J Chromatogr A. 2009;1216(32):5931–9.View ArticleGoogle Scholar
  18. Lluveras-Tenorio A, Mazurek J, Restivo A, Colombini MP, Bonaduce I. Analysis of plant gums and saccharide materials in paint samples: comparison of GC–MS analytical procedures and databases. Chem Cent J. 2012;6(1):115.Google Scholar
  19. Lluveras-Tenorio A, Mazurek J, Restivo A, Colombini MP, Bonaduce I. The development of a new analytical model for the identification of saccharide binders in paint samples. PLoS ONE. 2012;7(11):e49383.View ArticleGoogle Scholar
  20. Colombini MP, Andreotti A, Bonaduce I, Modugno F, Ribechini E. Analytical strategies for characterizing organic paint media using gas chromatography/mass spectrometry. Acc Chem Res. 2010;43(6):715–27.View ArticleGoogle Scholar
  21. Mills JS. The gas chromatographic examination of paint media. Part I. Fatty acid composition and identification of dried oil films. Stud Conserv. 1966;11(2):92–107.Google Scholar
  22. Shilling MR, Carson DM, Khanjian HP. Evaporation of fatty acids and the formation of ghost images by framed oil paintings. Western Association for Art Conservation (WAAC) Newsletter. 1998;21(1):4.Google Scholar
  23. Bonaduce I, Carlyle L, Colombini MP, Duce C, Ferrari C, Ribechini E, et al. New insights into the ageing of linseed oil paint binder: a qualitative and quantitative analytical study. PLoS ONE. 2012;7(11):e49333.View ArticleGoogle Scholar
  24. Cove S. Mixing and mingling: John Constable’s oil paint mediums c. 1802–37, including the analysis of the ‘Manton’ paint box. Stud Conserv. 1998;43(supp1):211–6.View ArticleGoogle Scholar
  25. Burley RW, Vadehra DV. Chemistry and biology technological nonfood uses of eggs. In: Burley RW, Vadehra DV, editors. The Avian egg. New York: Wiley; 1989. p. 365–70.Google Scholar
  26. Gettens RJ, Stout GL. Painting materials: a short encyclopedia. Toronto and London: Dover Publications; 1966.Google Scholar
  27. Matteini M, Mazzeo R, Moles A. Chemistry for restoration, painting and restoration materials. Florence: Nardini Editore; 2016.Google Scholar
  28. Khandekar N, Phenix A. Some observations on the effects of pigments on an artificially aged egg tempera paint film. AICCM Bull. 2000;25:1–7.View ArticleGoogle Scholar
  29. Casoli A, Berzioli M, Cremonesi P. The chemistry of egg binding medium and its interactions with organic solvents and water. In: Fuster-Lopez L, Charola AE, Mecklenburg MF, Domenech-Carbo MT, editors. New insights into the cleaning of paintings; Valencia. Washington: Universidad Politécnica de Valencia and Museum Conservation Institute; 2013. p. 39–44.Google Scholar

Copyright

© The Author(s) 2017