High-resolution MA-XRF images of the entire painting
In Fig. 2, a series of MA-XRF images of the entire painting are shown, as well as two false color composites of the area around the left quince. While the Ca–K, Pb–L and Pb–M images mostly clearly show the painted composition as a whole, they are less informative in a chemical sense. The light green of the foliage can be associated with high levels of Sn and Cu, suggesting that a mixture of lead tin yellow and either a Cu-green (e.g. malachite and/or verdigris) or a Cu-blue (e.g. azurite) was used to obtain this tint of green. The darker greens are also associated with these two elements but show a lower Sn–L intensity, possibly because of the presence of a lake layer (rich in K and Ca) that covers the lighter green paint below, resulting in a darker appearance. The association of the darker parts of the foliage with higher K signals can clearly be seen in the As, K, Pb composite MA-XRF image of Fig. 2. The yellow parts of the painting (the yellow quince and the pomegranate fragment) have been painted with an As-containing pigment, possibly orpiment (As2S3) or realgar (As4S4) or a mixture of both. Next to a high K–K signal, the left brown medlar is associated with a high Ca–K intensity, but this signal may originate in part from the ground layer beneath (see further). In the bottom composite image of Fig. 2, it can clearly be seen that slightly shaded edges and ridges of the quince and the crown leaves correspond to higher Ca signals, possibly due to the local presence of bone black. Another source of Ca may be the substrate of the red lakes. None of the depicted fruits, with the exception of the details of the walnut, show elevated Fe–K signals, generally excluding the use of earth pigments. An unclear structure is present in the left upper part of the background for which an Fe-containing paint was used; since this causes the Pb–M signals to be absorbed, this material may be present in a superficial retouching layer. In the area of the quince (and of the walnut), extensive retouching with a Ti-containing pigment (probably TiO2, see further) has been done. In the K–K, Ca–K, Pb–M, Sn–L and Cu–K images, parts of a two-toned leaf (now hardly visible with the naked eye), positioned just above the quince, can be discerned.
Macroscopic X-ray powder diffraction mapping of a degraded yellow paint area
In Fig. 3, macroscopic distributions of specific chemical elemental and crystalline phases of a small subarea of the painting are shown. This subarea is indicated in Fig. 1 and corresponds to the upper left corner of the left quince that exhibits a greyish-pale yellow appearance. This quince is suspected to have had a much more saturated light-yellow color, typical of quinces. Since the X-ray beam impinges the painting under a shallow angle (of around 12°), only the most superficial paint layers (approximately the top 5–10 micrometers) are sampled [11, 25]. By combining information of the large scale (Fig. 2) and small scale MA-XRF maps (Fig. 3, upper panel) with the visual color of the investigated area and with background knowledge on common painters’ pigments available in 17th C Amsterdam, already an approximate identification of the pigments employed for painting the quince can be made.
It is clear from the diffraction maps that the resolution of reflection mode MA-XRPD is insufficient to map out fine details such as the insect visiting the quince. The grey/black pigment in this feature is also not detected by XRPD. Some of the lower resolution/small scale MA-XRF maps of Fig. 3 were recorded with an 8.04 keV primary beam during the combined MA-XRF/MA-XRPD scans; these include elemental distributions such as S, P and Cl that could not be detected by the commercial MA-XRF instrument in the conditions used here (polychromatic primary beam of higher energy, 5–22 keV range; and a much shorter dwell time). Several XRF maps (such as those of Ca, K, P, Mn and Fe) allow to visualize the aforementioned two-toned leaf, situated in the upper left corner of the imaged area. The green shade of this leaf has been realized with a pigment that contains Sn, likely lead tin yellow. Some of the darker parts of the scanned area (such as the crown leaves of the quince) are associated with higher P–K levels, suggestive of the use of (phosphate-rich) bone black in these areas. Other dark areas (background, crown leaves) are associated with Mn and Fe (not included in Fig. 3), suggestive of the use of earth pigments (and more in particular umber or Siena earths) to obtain the dark color. The S–K and As–K distributions clearly correspond to the yellow areas and thus very likely with an As-sulfide pigment such as orpiment (the presence of realgar is also consistent with the S and As distributions but the yellow colour makes this less likely). Clearly no lead tin yellow was employed to paint the yellow fruit. The bright areas of the Pb–M map seem to resemble the map of Cl–K, suggesting that a (Pb, Cl) containing compound may be present. In some areas, the Pb–M distribution is complementary to that of Ca–K, suggesting that a Pb-containing material is present on top of a Ca-containing paint layer. The presence of Ca in the yellow area is not illogical since gypsum is known to have been added to orpiment-based paints [26]. There are, however, significant differences among the S–K and Ca–K distributions. The K–K and Cl–K distributions appear complementary to each other and are not so easily associated with a specific pigment. Together with Ca–K, the K–K map does reflect to some extent the areas on the surface of the fruit where shadows were painted and thus the K–K distribution may reflect (in part) the presence of (red) lakes applied on top of the yellow paint. The same could be true for Ca–K in case a lake was employed precipitated onto a Ca-containing inorganic substrate. Generally speaking, however, the Pb–M, Cl–K, K–K, Ca–K are not easily interpretable in terms of either pigments or other materials present based on their XRF data alone.
A more specific pigment identification is possible when the corresponding MA-XRPD maps are considered (Fig. 3, middle and lower panel) in comparison to the corresponding elemental maps (also shown in Fig. 3, upper panel). The As2S3 map clearly confirms the use of only orpiment (i.e. no realgar encountered above the detection limit of MA-XRPD) for painting the yellow fruit. Lead tin yellow (another yellow pigment) is only used in the green of the leaf. Both in the leaf and in the crown leaves of the quince some hydroxyapatite (bone black) is present. The gypsum distribution resembles that of orpiment while that of calcite is largely confined to the (slightly shaded) edges of the quince and ridges around the crown leaves. In this particular area, the hydrocerussite distribution is somewhat similar to that of calcite. Cerussite and hydrocerussite appear to be co-localized in the white highlights that were applied on the yellow surface of the quince.
Next to orpiment, of which the presence in a 17th century painting can be expected/is plausible, also the rare As-containing minerals schultenite (PbHAsO4) and mimetite (Pb5(AsO4)3Cl) are found to be present in the general yellow area, but with distributions that are quite different from each other. Consistent with the recent literature on this topic [9, 11], the rare nature of these minerals strongly suggests that they are secondary products. These were not part of the paint employed by Nellius during the creation of the painting and were formed in the course of the past 350 years on or near the surface of the paint as a result of spontaneous chemical transformation processes. Indeed, both orpiment and realgar are known to be light-sensitive, causing their color to fade [6]. For orpiment, a direct photo-oxidation to arsenolite (As2O3) takes place, while in the case of realgar an intermediate, pararealgar (As4S4), is initially formed [27,28,29]. Recently it was found that in a subsequent oxidation step arsenolite can be further transformed into soluble arsenates [9].
Other secondary products that are found on this flower still life are palmierite (K2Pb(SO4)2) and syngenite (K2Ca(SO4)2.H2O), two mixed K-based sulfates with either Ca2+ or Pb2+ as divalent ions. Both these secondary products are abundantly present in the leaf above the quince where they have the same distribution. In the yellow areas, they are also present but only with a partially similar distribution. In the MA-XRPD data, no indication of the presence of the alum substrate of the lakes (assumed to be a primary material) is visible in this case, suggestion that most or all of it became converted into other sulphates. Both secondary sulfates are less abundantly present in the radial areas around the crown leaves of the quince where there is also less schultenite and less K (see XRF image) present. In turn, somewhat more Ca, corresponding to calcite and gypsum, as well as more hydrocerussite are found in this region. While the cerussite image largely resembles the hydrocerussite distribution, it does not show a higher abundance in the aforementioned radial areas. The comparison of the Ca MA-XRF map with the MA-XRPD map of calcite, bone black (hydroxyapatite), gypsum and syngenite clearly shows the different spatial distribution of these four calcium compounds. The MA-XRPD map of lead tin yellow is quasi identical to the Sn–L MA-XRF map, identifying this traditional yellow pigment as the single Sn-containing compound present. In the brighter green of the leaf, and to a lesser extent in the crown leaves and in the background area, malachite is shown to be present.
Syngenite is a frequently encountered secondary salt, either as a weathering product in black crusts or as efflorescence layers on stone monuments, mural paintings and medieval (K-rich) glass [30,31,32,33,34,35,36]. In some cases it has been found below the surface of artworks, such as in a red–orange Baroque bole ground or as a raw material in the plaster of a Chinese wall painting [37, 38]. On the other hand, palmierite is less commonly reported as a sulfate salt on stone sculptures, medieval glass windows and wall paintings [39,40,41]. Palmierite has been encountered in multiple paintings from 17th century Old Masters such as Vermeer, Jordaens and Rembrandt [42]. After migration of Pb2+ from lead white to upper paint layers, it can react with K+ (present in e.g. smalt, lake substrates and earth pigments) and SO42− (present in lake substrates, such as potassium alum (KAl(SO4)2∙12H2O), or from environmental SO2) to precipitate as palmierite [42,43,44].
It is noteworthy that the schultenite distribution is quite different from that of the parent pigment orpiment, and similar to that of K and palmierite, suggesting that the formation of schultenite is facilitated when more K is locally present at or just below the surface, e.g. in lake brush strokes. In the very K-rich parts of the scanned areas (especially the leaf above the quince), K also appears to have become the preferred cationic partner for capturing sulfate ions; it would appear that in the presence of both Ca2+ and Pb2+ ions (next to K+), the sulfates syngenite and palmierite coprecipitated.
Although, based on these maps, certain hypotheses on the formation of these various salts can be made, it remains very difficult/impossible to infer information on the depth ordering of the various primary and secondary products that can be identified. Do the secondary arsenate compounds form (i) on top of orpiment, (ii) in the same stratum as their parent compound or (iii) below it? Have the secondary sulfates coprecipitated with schultenite on the surface of the painting or are they located at different depths below the surface. To obtain more information about the sequence of these layers, a paint cross section taken from the central part of the quince was examined.
Microscopic X-ray powder diffraction mapping of a minute paint cross section
In Fig. 4, two composite false color maps of relevant primary and secondary crystalline compounds obtained by μ-XRPD from a minute paint cross section sampled in the central yellow part of the quince are shown. The sampling position is indicated in Fig. 1. With μ-XRPD, information on the composition of each individual paint layer can be obtained. From these data, and consistent with the MA-XRPD data discussed above, the presence of a number of white/colorless compounds such as calcite and hydrocerussite in well-defined paint strata can be clearly visualized as well of that of yellow orpiment. These compounds can be readily assumed to have been original components of Nellius’ paint(s). Again consistent with the MA-XRPD data, the presence of compounds that are likely to be secondary compounds such as palmierite, mimetite and schultenite can now be unambiguously linked to specific layers in the paint stratigraphy. The second lead-arsenate based rare mineral, mimetite (Pb5(AsO4)3Cl)) features a substantially higher Pb:As atomic ratio (5:3) than schultenite (1:1).
Previously, mimetite has been reported on three Hellenistic steles from Alexandria [45, 46], in several murals [47,48,49] and in one case was considered to be a degradation product formed from the interaction between orpiment and red lead (Pb3O4) [50]. Schultenite was recently reported as a degradation product of orpiment, together with arsenolite (As2O3), on a colonial American polychromed chest on stand [9]. Fully consistent with the observation of Fig. 4b, Vanmeert et al. have recently encountered both arsenates in degraded areas on several paintings by De Heem [11].
From the depth distributions of a number of primary compounds (Fig. 4), a relatively simple, four-layered structure of the original paint can be inferred, consisting of a ground layer ① containing calcite (CaCO3), covered by a second layer ② containing coarse particles of cerussite (PbCO3) and hydrocerussite (2PbCO3∙Pb(OH)2), which in turn is covered by a yellow layer ③ containing finely ground particles of orpiment (As2S3). Inside layers ② and ③, also a few large grains of gypsum (CaSO4∙2H2O) can be observed in the μ-XRPD maps. On top of layer ③, although not discernable by XRPD, a (yellow) lake layer ④ is present e.g. to create the effect of shading on the quince surface. This layer is visible in UV image of this sample (not shown).
It is highly relevant to establish where exactly in the original stratigraphy, the highest local concentration of the secondary products has formed. For the arsenates, unsurprisingly, this is at the interface between the As-rich orpiment layer ③ and the Pb-rich lead white layer ② (labelled ❷′ and ❸′ in Fig. 4). Next to that, inside/near the top layer ④ also some arsenate minerals are present. On the basis of Fig. 4, it is possible to formulate a relatively simple mechanism of formation for the secondary products, consisting of essentially three steps: (i) release of specific ions as a result of (photo)degradation in their parent paint layers, (ii) migration of these ions towards other layers and (iii) local precipitation of one or more crystalline secondary products. One can imagine that in layers ② and ③, light-induced and other degradation processes in first instance give rise to respectively mobile AsO43−, SO42− and Pb2+ ions while from layer ① Ca2+ ions can be released. The chemical transformation to produce the arsenate and sulfate ions from orpiment clearly is an oxidation process since the As(III)-species that are originally present in orpiment become As(V)-species while the sulfidic counter ions become sulfate species, as already described more in detail by Keune et al. [9] and Vermeulen et al. [10]. This oxidation process can only take place in the presence of moisture and oxygen inside the porous paint, according to the following reactions:
$${ 2 {\text{ As}}^{ 3+}}_{{({\text{s}})}} + 6 {\text{ H}}_{ 2} {\text{O}} + {\text{O}}_{{ 2({\text{g}})}} \to {{{ 2 {\text{ HAsO}}_{ 4}}}^{ 2- }}_{{({\text{aq}})}} + {{ 1}0{\text{ H}}^{ + }}_{{({\text{aq}})}}$$
$${ 3 {\text{ S}}^{{ 2} - }}_{{({\text{s}})}} + 1 2 {\text{ O}}_{{ 2 { }({\text{g}})}} \to {{ 3 {\text{ SO}}_{ 4}}^{ 2- }}_{{({\text{aq}})}}$$
…….
$${\text{As}}_{2} {\text{S}}_{{3({\text{s}})}} + 6{\text{ H}}_{2} {\text{O}} + 13{\text{ O}}_{{2({\text{g}})}} \to 2{{{\text{ HAsO}}_{4}}^{2 - }}_{{({\text{aq}})}} + 3{{{\text{ SO}}_{4}}^{2 - }}_{{({\text{aq}})}} + 10{{\text{ H}}^{ + }}_{{({\text{aq}})}}$$
(In the above equations, “HAsO42−” should be understood to denote all protonated forms of arsenate ions, ranging from the fully protonated H3AsO4 to the completely deprotonated AsO43−; the average degree of protonation is pH dependent.) The above redox reaction leads to the formation of acidic species and a lowering of the pH of the aqueous phase present in the paint. Either the latter is present as microdroplets of solution inside the porous paint layer (pore water) or as a absorbed layer of water molecules at a (polar) paint surface. In the latter case, the high polarity of the surface of the pigment grains may enhance the reaction rates. Through diffusion or a process of cyclic moisture evaporation/drying and water condensation inside/wetting of the paint layers, the acidic solution that originates in the orpiment layer ③ can reach the lead white layer ② where the dissolution of the lead white can be enhanced above its ‘normal’ level. The release of Pb2+-ions from lead carbonates, in some cases leading to e.g. the in situ formation of lead soaps and other compounds, is a well-known and elaborately studied phenomenon (see e.g. [42]); the acidic groups of the oil binding medium are already sufficient to promote the dissolution of Pb2+ ions. This process consumes free protons and involves the release of CO2:
$${\text{PbCO}}_{{3({\text{s}})}} + {2{\text{H}}^{+}}_{{({\text{aq}})}} \to {{\text{Pb}}^{2 +}}_{{({\text{aq}})}} + {\text{CO}}_{{2({\text{g}})}} + {\text{H}}_{2} {\text{O}}$$
$$2\,{\text{PbCO}}_{3} \cdot {\text{Pb}}\left({\text{OH}} \right)_{{2\,({\text{s}})}} + {6\,{\text{H}}^{+}}_{{({\text{aq}})}} \to {3\,{\text{Pb}}^{2 +}}_{{({\text{aq}})}} + 2\,{\text{CO}}_{{2 ({\text{g}})}} + 4\,{\text{H}}_{2} {\text{O}}$$
Since the concentration of H3O+ ions in the aqueous phase will be the largest at the interface between the orpiment layer ③ and the lead white layer ②, it will be in this contact area (labelled with ❷′ in Fig. 4) that the highest concentration of Pb2+ ions in solution will be encountered. The further into the lead white layer the acidic solution diffuses/penetrates, the more it will become neutralized as H3O+ ions are replaced by (solvated) Pb2+ ions.
At the orpiment–lead white interface area, we hypothesize that two streams of ions may encounter one another: on the one hand solvated arsenate ions that are formed in the orpiment layer and that are migrating towards the lead white layer, while on the other hand solvated Pb2+ ions move in the other direction. In this [already partially (?) neutralized] acidic solution, all necessary ions are available to allow precipitation of the arsenate mineral schultenite (PbHAsO4). This precipitation reaction also consumes protons:
$${{\text{Pb}}^{2 +}}_{{({\text{aq}})}} + {{{\text{HAsO}}_{4}}^{2 -}}_{{({\text{aq}})}} \to {\text{PbHAsO}}_{{4({\text{s}})}}$$
It is interesting to note that slightly below the schultenite layer, i.e. slightly closer to the source of Pb2+ ions and slightly more distant from the source of HAsO42− ions, possibly at a location where the pH is slightly higher and the acid–base equilibrium between HAsO42− and AsO43− favours the presence of AsO43−, a second rare arsenate mineral, mimetite (Pb5(AsO4)3Cl) has precipitated (labelled ❸′ in Fig. 4):
$${5\,{\text{Pb}}^{2 +}}_{{({\text{aq}})}} + {{\text{Cl}}^{-}}_{{({\text{aq}})}} + {3\,{{\text{AsO}}_{4}}^{3 -}}_{{({\text{aq}})}} \to {\text{Pb}}_{5} \left({{\text{AsO}}_{4}} \right)_{3} {\text{Cl}}_{{({\text{s}})}}$$
In addition, when we compare the value of the precipitation products of these two minerals (Ks,25 °C ≈ 10−23–10−24 for schultenite and Ks,25 °C ≈ 10−76–10−83 for mimetite) and finally consider that the effect of [Pb2+]aq, i.e. the equilibrium concentration of solvated Pb2+-ions in the aqueous phase, on the precipitation behavior is much larger in the case of mimetite than for schultenite, it becomes understandable why mimetite has precipitated closer to lead white layer ② than schultenite. (The source of Cl− ions required for the formation of mimetite is not easily identified; however, many paintings and painters’ materials generally contain an abundance of chlorides [2,3,4, 11, 26, 41] and the MA-XRF data of Fig. 3 (Cl–K map) also points out that it is present (at least at the surface). In this case, the lead white, likely produced by means of the Dutch stack process, is considered to be a primary source of Cl.
In Fig. 4 we further see that the largest abundance of palmierite ((Na,K)2Pb(SO4)2), a secondary compound formed by precipitation of solvated Pb2+ with SO42− and K+ (and/or Na+) ions,
$${{\text{K}}^{+}}_{{({\text{aq}})}} + {{\text{Pb}}^{2 +}}_{{({\text{aq}})}} + {{{\text{SO}}_{4}}^{{2} -}}_{{({\text{aq}})}} \to {\text{K}}_{2} {\text{PbSO}}_{{4({\text{s}})}}$$
is situated at the surface (near/in layer ④). Also this phenomenon and in particular the fact that this secondary lead sulfate mineral has not precipitated in the same location as the lead arsenate minerals becomes understandable when the higher solubility (and thus the larger Ks value) of palmierite is taken into consideration (Ks,25 °C ≈ 10−10–10−8). At [Pb2+]aq levels that force the arsenates to precipitate locally, sulfate ions can still remain in solution and thus have the possibility to migrate over a larger distance. Thus, e.g. as a result of drying/condensation cycles, many (but not all) sulfate ions that were formed (or were originally present) inside orpiment layer ③ may have become transported to the original paint surface (top of layer ③ and/or lake layer, see Fig. 4) where they may have been forced to precipitate because of water evaporation (layer ④ in Fig. 4b). Depending on the available cations, several sulfates may have formed, including palmierite and syngenite. It follows that, if conditions are such that e.g. palmierite is precipitating, then also the less soluble minerals such as schultenite and mimetite will behave similarly, provided all required ions are present in the thin layer of evaporating solution present at the surface. Another relevant difference between the arsenate and sulfate salts is that a possible source of the sulfates is atmospheric SO2, making formation of the sulfates closer to the paint surface more probable that at greater depths.
Note: The observation that the above-mentioned arsenate minerals are not homogeneously distributed within the As-containing paint layer strengthens our initial assumption that they were formed in situ and were not already present at the time of painting.
