Initial findings from the survey of the NM’s conservation dossiers revealed that 48 paintings from the total of 57 Munch paintings had a varnish coating and that 30 of these 48 varnished paintings were documented as having been varnished by the NM [27]. The reports also revealed a gradual shift from using traditional natural resin varnishes (post-1950) towards the dominant use of two synthetic types (Laropal K 80 and MS2A) by the mid-1980s. Information from the NM’s historic varnish recipes confirmed the use of common additives in small percentages to various mixtures [27]. Microcrystalline wax was included in some recipes to reduce gloss and in some cases a light stabilizer, bis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate and methyl-1,2,2,6,6-pentamethyl-4-piperidyl sebacate (Tinuvin 292, Ciba-Geigy Marienberg GmbH, Germany), had been incorporated as a preventive method against light ageing [28]. In some cases, the anomalous addition of a heat-bodied linseed oil (stand oil) had also been introduced, possibly to control the application process, saturation and gloss. Notwithstanding the significance of the documentary information, inconsistencies and variations in recording conservation data between 1949 and 1993 were noted in the dossiers, emphasising the need for a chemically specific form of identification of the varnish coatings.
Depth of penetration tests undertaken in DR on varnish applied to glass slides recorded interference signals from the Si–O–Si stretching from the glass between thicknesses of 250 µm and 500 µm. This helped to establish an approximate depth of penetration range for the detection of Laropal K 80 grade 9 varnish, applied to the paintings.
Flower Meadow Field (Woll 148)
Munch’s small plein air painting, Flower Meadow Field was painted whilst he resided with his family on the island of Veierland, South of Oslo in the summer of 1887 [8]. The composition (66.5 cm × 44 cm) donated to the museum in 1921 has been executed rapidly with thick brush strokes and raised impasto on a relatively thin white ground layer. Evidence of early framing marks in the wet oil paint can be seen around all four edges of the composition and the thinly woven canvas support has been later marouflaged onto a cardboard support. Conservation dossiers only record the application of Laropal K 80 grade 9 varnish in 1983 [24]. The purple coloured fluorescence emitted under UV and the stereomicroscopic investigations confirmed the presence of a thick, discoloured (yellowing) and evenly distributed top varnish layer (Fig. 3).
pFTIR spectral readings were taken from four different coloured areas with varnish (Green Gr.02, Blue Bl.01, Bl.02, White W.01 and Red R.01) and one green spot without (Gr.01, old cleaning test). Spectra obtained from all the varnished areas showed intense peaks related to the aliphatic C–H stretching band around 2857 cm−1 and 2945 cm−1. These, together with the strong peaks noted at 1451 cm−1 and 1712 cm−1, in the green (Gr.02) and red (R.01) pigmented areas, appear to be in line with spectral features of the synthetic resin Laropal K 80 [29] (Fig. 4). For quantifying the reproducibility of the measurements, the coefficient of variation (CV) of the wavenumbers of three independent readings carried out at different times was calculated on different bands (stretching bands of OH, CH2–CH3, C≡N and C=O). This ranged from 0.027% for the CH2–CH3 peaks centred at around 2930 cm−1 to 0.695% for the same bands. This shows a very good reproducibility of the measurements. When relating gloss measurements with the quality of the spectra obtained, a slightly lower value, 8–10 GU, was observed in the varnished green (Gr.02) and red (R.01) areas compared to those noted in the varnished white and blue areas (12–14 GU). In raking light, the surface topography of the paint in the blue sky and white meadow regions revealed a slightly smoother (glossier) surface compared to the more irregular topography from the impasto in the green and red passages of paint. Significantly lower surface gloss measurements (1–2 GU) were recorded in the unvarnished and matt area (Gr.01) and its pFTIR spectrum, plotted with the adjacent varnished area (Gr.02), recorded a definite reduction of the bands around 2854 cm−1 and 2940 cm−1 (Fig. 5). This confirms that the synthetic Laropal K 80 varnish layer had been removed or reduced to undetectable in the Gr.01 area, after cleaning. Supplementary micro-invasive analysis by THM-GC/MS confirmed the presence of Laropal K 80 in the Gr.02 area. Synthetic resin, and additionally detected components of dammar resin, probably from a previous unrecorded varnish application, as well as a substantial amount of siccative oil, which could have migrated from the paint to the varnish layer [30] (Fig. 6).
The prominent peak at 2088 cm−1 noted in many of the spectra could be attributed to the C≡N stretching associated with the pigment Prussian blue from the paint layer [31]. The presence of a stretching band at 3545 cm−1 (OH) and a stronger one around 1450 cm−1 (carbonate stretching) in the red area (R.01) indicate also the presence of lead white (2PbCO3·Pb(OH)2) in that coloured paint.
The identification of the various pigments was difficult to ascertain by their colouration under UVA examination alone, given the overriding and dominant fluorescence of the varnish coatings present. The diffuse presence of lead in all the pXRF readings helped to confirm the use of lead white throughout. Mercury was identified in passages with red paint and implied a palette containing vermillion whilst the iron peaks noted in the XRF spectra of the blue sky helped to further confirm the pFTIR finding for Prussian blue. The presence of chromium recorded in the green areas combined with the dark character (UVA-absorbing) of the unvarnished green area (Gr.01) in UVA light, suggested the use of a chromium green oxide based pigment [32]. Peaks for cadmium and iron were detected in the XRF spectra from various passages of yellow paint and indicated the use of both cadmium yellow and yellow earth, respectively.
Portrait of Hans Jæger (Woll 174)
Munch painted the portrait of his friend, the bohemian author Hans Jæger (1854–1910), in March/April of 1889 and the painting (109 cm × 84 cm) was owned by Munch until it was purchased by the NM eight years later. Conservation dossiers only mention one major restoration carried out in 1954 when it was cleaned, glue-paste lined and varnished with mastic [22]. Like Flower Meadow Field, the paint application is thick with little exposed ground.
UV photography confirmed the presence of a varnish coating with a dull yellowish, green fluorescence evenly applied throughout (Fig. 7). pFTIR spot readings were taken from different areas, the blue coat (Bl.01 and Bl.02), white background (W.01 and W.02), red/pink flesh tints (R.10), the dark red/brown wood colour from sofa (R.01–R.09) and the yellow paint along the top edge (Y.01). Marked spectral differences were observed between the different coloured areas. The spectra for the reddish/brown spots (R.01) showed intense peaks at 2849 cm−1 and 2918 cm−1 (Fig. 8). The shapes of the aliphatic CH stretching bands suggested the presence of an oil-natural resin rather than a synthetic one. Moreover, the spectra contained similarities to the mastic reference sample with the association of the carbonyl band at 1740 cm−1 with an oil-resin. In contrast, spectral readings taken from the white painted areas (W.01 and W.02) had strong signals typically associated with presence of lead white in the paint layers with an OH band at 3540 cm−1 and the broad carbonate band at about 1450 cm−1 [33]. This made it difficult to assign spectral characteristics associated with a specific resin varnish. As with Flower Meadow Field, for quantifying the reproducibility of the measurements, the CV of three independent readings carried out at different times was calculated on different bands (stretching bands of OH, CH2–CH3, C≡N, and CO32−). This ranged from 0.010% for the CO32− peaks centred at around 1450 cm−1 to 0.653% for the CH2-CH3 stretching bands centred at around 2950 cm−1. Again, this showed a very good reproducibility of the pFTIR measurements. Surface gloss measurements revealed higher values recorded in the smoother surface topography of the white regions (17–22 GU) compared to those from the irregular surface topography of the textured brush strokes in the reddish/brown areas (4–8 GU). As noted in the painting, Flower Meadow Field, the clarity of the spectra for varnish resin identification appeared to be influenced by its thickness, surface texture, and pigment composition. A similar low 1–2 GU range was also recorded in unvarnished regions, along the painted tacking edges.
X-radiographs taken in 1993 revealed that Munch had extensively re-worked the bottom right-hand corner of the composition, the table, and had also undertaken some minor alterations to the sitter’s seated position by lowering the right-shoulder, arm and hand [34]. In contrast to Flower Meadow Field, the distinctive colouration of certain pigments, when illuminated by UVA radiation, could be detected. Two different types of white pigment, lead white and zinc white, appear to have been used by Munch, possibly in relation to the compositional process and later alterations. In the passages of white paint and pigment mixtures, a difference between the characteristic reddish-purple UVA-fluorescence colouration of lead white and the opaquer greenish-yellow colouration associated with zinc white was noted [32]. The presence of lead and zinc was further supported by pXRF. A typical dark blue UVA-induced fluorescence witnessed in the blue region (Bl.02), combined with traces of iron detected by pXRF, helped to confirm the presence of Prussian blue. Vermillion was also probably identified using pXRF through the presence of mercury.
Night in Nice (Woll 224)
The NM’s first Munch acquisition is the small painting, Night in Nice (40 cm × 54 cm), which was purchased from the autumn state art exhibition (Høstutstillingen—Statens Kunstutstilling) in the same year as it was painted, 1891 [1, 2, 8]. As an early work, Munch’s painting technique is relatively traditional in terms of paint application. The build-up is simple compared to the two other works examined, thinly executed and with no distinct raised impasto or surface textural effects. Part of the motif is present on the lower tacking edge and a canvas stamp with no. 15 on the reverse indicates a French commercial standard format of 65 cm × 54 cm. The canvas has been later reduced (cut) in height from 65 to 48 cm by the artist and stretched onto a smaller stretcher. According to the conservation records, a coat of Laropal K 80 grade 9 varnish was sprayed over the paint surface in 1983 after a light surface cleaning [23]. The presence of varnish was confirmed by the strong yellow/green coloured fluorescence under UVA examination (Fig. 9).
Closer microscopic examination further confirmed a varnish coating that does not extend to all four edges of the canvas. Moreover, drip marks in the varnish along the lower canvas edge appear to correspond precisely with a fluorescence pattern detected under UVA light along the lower inner sight-edge of the original frame. These findings confirmed the presence of an earlier and previously undocumented varnish layer underneath the aforementioned thinly applied synthetic varnish (Laropal K 80 grade 9) recorded from 1983. It is possible that this varnish was directly applied whilst the canvas was still in its frame. In situ pFTIR measurements were recorded over different coloured areas of blue (Bl), violet (V), white (W) and red (R), and along the lower unvarnished tacking margin including areas with exposed ground (Gd) (Fig. 9). Preliminary results captured characteristic spectral features for lead white with typical absorptions for OH bond at 3540 cm−1 and for carbonate around 1470 cm−1, predictably more prominent in the unvarnished areas [33] (Fig. 10).
As in Flower Meadow Field, the reoccurring prominent peak at 2088 cm−1 present in many of the spectra could be associated with the presence of Prussian blue in the underlying paint. However, no bands typical of a synthetic Laropal K 80 resin, allegedly applied to the surface in 1983, were detected in any of the spectra. As noted by Miliani et al. [20] caution is required when extrapolating data from the FTIR spectra due to the often-complex stratigraphic character and heterogeneous nature of paint layers and surface coatings, which need to be understood. Comparisons between readings taken from two similarly pigmented areas, one varnished (V.06) and one unvarnished (V.09), revealed some minor spectral differences. In the unvarnished area, sharp derivative inverted bands at 2862 cm−1 and 2942 cm−1, typical of the CH2 and CH3 stretch for an oil, were detected, as was the carbonyl stretch at around 1752 cm−1. In the varnished area, the absorptions for the CH2 and CH3 stretch are relatively weak. Supplementary micro-invasive analysis was thus undertaken for clearer identification of the varnish coatings. The presence of Prussian blue was confirmed with ATR-FTIR and Raman analyses (V.06) and further tests using THM-GC/MS confirmed the presence of a drying oil and components of dammar. No Laropal K 80 resin was detected suggesting that either it had been removed post-1983 or that the painting was never actually varnished with it, contrary to the conservation report. Gloss measurements taken from all the varnished pFTIR spot locations recorded values of 12–14 GU and low values (1–2 GU) in the unvarnished regions. The similar high gloss surface readings recorded on the various varnished parts of the paint surface correspond to the painting’s relatively flatter surface topography and minimal surface texture. Like with Flower Meadow Field, the identification of the various pigments proved difficult to be ascertained by their colouration under UVA radiation. pXRF helped to confirm the use of lead white, vermillion and cadmium yellow. Traces of iron, associated with Prussian blue, were also detected.