# Characterisation of rouaite, an unusual copper-containing pigment in early modern English wall paintings, by synchrotron micro X-Ray diffraction and micro X-Ray absorption spectroscopy

Ellen H. Purdy, Sarah Critchley, Clément Holé, Marine Cotte, Andrea Kirkham, Michael Casford

► To cite this version:

Ellen H. Purdy, Sarah Critchley, Clément Holé, Marine Cotte, Andrea Kirkham, et al.. Characteri-sation of rouaite, an unusual copper-containing pigment in early modern English wall paintings, by synchrotron micro X-Ray diffraction and micro X-Ray absorption spectroscopy. Applied physics. A, Materials science & processing, 2024, 130 (11), pp.817. 10.1007/s00339-024-07954-1 . hal-05105620

HAL Id: hal-05105620

https://cnrs.hal.science/hal-05105620v1

Submitted on 10 Jun 2025

**HAL** is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

# Characterisation of rouaite, an unusual copper-containing pigment in early modern English wall paintings, by synchrotron micro X-Ray diffraction and micro X-Ray absorption spectroscopy

Ellen H. Purdy<fnref n="1" /> · Sarah Critchley<fnref n="2" /> · Clément Holé<fnref n="3" /> · Marine Cotte<fnref n="3" />,<fnref n="4" /> · Andrea Kirkham<fnref n="5" /> · Michael Casford<fnref n="1" />

Received: 30 April 2024 / Accepted: 30 September 2024 / Published online: 21 October 2024
© The Author(s) 2024

## Abstract

Although the use of verditer, a synthetically produced copper carbonate pigment chemically analogous to azurite (Cu₃(CO₃)₂(OH)₂), in early modern wall paintings is regularly reported in grey literature, its detection in wall paintings is typically unavailable in a public forum. This article focusses on two particular samples (M9 and T1) initially suspected to contain copper blue pigments from decorative wall painting schemes in domestic houses across a range of status groups, in eastern and south-eastern England, dated to 1550-1650. Interest in verditer synthesis procedures is found in textual sources including both historic manuals and current scientific literature. The unreliability of the synthesis, particularly for blue verditer, however, is widely documented. Here, we report a multimodal study employing SEM-EDX, SR-μPXRD mapping, and SR-μXANES spectroscopy to characterise the phases present in two wall painting samples, one c. 1600/early 1600s (M9) and the other c. 1600 (T1). In addition to assessment of the presence of verditer, this study presents the first identification of rouaite, a synthetic copper hydroxynitrate mineral (Cu₂(NO₃)(OH)₃), in wall painting samples. Rouaite, shown in a previous publication to be a common byproduct of replications of the historical verditer synthetic recipes, appears in combination with verditer and alone. Its infrequent identification in paintings and murals is considered in light of analytical challenges. The presence of copper hydroxychlorides is also discussed in the context of rouaite stability and degradation. These results represent a significant development in our knowledge of the early modern palette in England and historical verditer synthesis and advances our understanding of how these painting schemes may be best preserved in the future.

**Keywords** Verditer · Mural · Rouaite · Synchrotron

<figure><img src="image_3.png" /></figure>

<footnote>Michael Casford
mtlc2@cam.ac.uk</footnote>

<footnote>Ellen H. Purdy
ep575@cam.ac.uk</footnote>

<footnote>Sarah Critchley
sarah_ann.critchley@th-koeln.de</footnote>

<footnote>Clément Holé
clement.hole@esrf.fr</footnote>

<footnote>Marine Cotte
marine.cotte@esrf.fr</footnote>

<footnote>Andrea Kirkham
andreakirkham4@gmail.com</footnote>

<footnote n="1">¹ Department of Chemistry, University of Cambridge,
Lensfield Road, Cambridge CB21EW, UK</footnote>

<footnote n="2">² Cologne Institute of Conservation Sciences, Cologne
University of Applied Sciences, Ubierring 40, Köln 50678,
Germany</footnote>

<footnote n="3">³ European Synchrotron Radiation Facility (ESRF), 71 Av. des Martyrs, Grenoble 38000, France</footnote>

<footnote n="4">⁴ Laboratoire d'Archéologie Moléculaire et Structural (LAMS CNRS UMR 8220), Sorbonne Université, 4 Pl. Jussieu, Paris 75005, France</footnote>

<footnote n="5">⁵ Andrea Kirkham Conservation Ltd, 31 Silver Street,
Norwich NR3 4TT, UK</footnote>
# 1 Introduction

The use of a good quality blue verditer, a synthetic basic copper carbonate chemically analogous to the naturally occurring mineral azurite ($Cu_3(CO_3)_2(OH)_2$), is identified in a number of seventeenth-century domestic English wall paintings, especially in Suffolk and Essex. Kirkham suggests that blue verditer was used in houses of the 'middling sorts' as an affordable alternative to natural azurite [1]. According to Delamare, the first possible documentation of blue verditer is in 1537–38 [2] at Falkland Palace, Scotland as 'verditer bis', though it is important to mention that 'bis' may refer to a darker green pigment rather than a true blue verditer [3, 4]. However, the earliest known references in English treatises by Thomas de Mayerne and Norgate are from the early seventeenth century, and the earliest documents describing the synthesis process are from the 1660s [1, 5–9].

Although organic blues (commonly derived from woad) were extensively used over a long period in a wide range of decorative schemes (sometimes used as blue but often in pigment mixtures to make green), reasonably priced vivid blues were not readily available prior to the introduction of verditer. The use of natural azurite in the Palaces and elite houses in sixteenth and seventeenth century England is well documented, for example at the Charterhouse, London [10], but its expense limited its wider use in wall paintings in middling households during this period [1, 11]. Probate inventories and tax documents suggest that, once available, blue verditer was sold at a substantially lower cost compared to natural azurite [1, 11, 12]. The supply of good blue verditer could clearly meet demand in the seventeenth century in decorative schemes commissioned by middling status clients. The increased desirability/demand of blue pigments may, in part, result from the Parisian trend of the 'blue room' which influenced interior design choices in England from the second quarter of the seventeenth century [1, 13].

The 22 samples in this corpus are from the eastern and south-eastern counties in England. The regional bias is partly because 1) domestic wall paintings in these areas are well-documented by Dr. Andrea Kirkham who supplied the samples; and 2) the material evidence for blue and green verditers survives well and paint samples are available for some (but not all) of the surviving schemes. Furthermore, early documented examples of blue verditer are found in a Norwich grocers' probate inventory where two grades of blue verditer are listed in 1617 [1]. However, it is worth noting that research by Dr. Davies in the Welsh Marches has

not identified any schemes with blue verditer, to date which may indicate a regional bias in the use of the blue.<fnref n="1" />

Previous work on medieval and early modern wall painting has also identified blue pigments in mixture. Hughes discusses 'washing [verditer] with smalt' to improve the blue hue at Little Castle Bolsover [15], a procedure also described in Van Dyck's early seventeenth century Antwerp Sketchbook.<fnref n="2" />.

Although the date of the first use of blue verditer is debated, its use in domestic wall paintings increases appreciably in the early seventeenth century. The unpredictability and frequent failure of verditer synthesis was widely documented, and it is possible that poor quality verditers appeared first as the production process was refined and perfected [1, 6, 12]. If unsuccessful, a green product rather than a blue was produced-a significantly less valuable pigment [12, 17, 18].

The documented verditer synthesis was often carried out by precious metal refiners as a way to generate additional income from a byproduct of the metal parting procedure [19]. Silver and gold were purified by reaction with nitric acid and recovered by reaction with copper metal, producing a blue copper nitrate solution. This 'copper-water' was then mixed with chalk and allowed to react, producing an increasingly pure blue product over several reaction cycles:

> They put into a Tub a Hundred Pound weight of Whiting, and thereon pour the Copper-Water, and stir them together every Day, for some Hours ... when the Water grows pale, they take it out, and set [it] by for farther Use, and pour on more of the Green Water; and so continue till the Verditer be made...

[7]

The verditer synthesis and particularly the causes of its unpredictability have been studied previously by Mactaggart and Mactaggart [17, 20–22] and are the subject of a recent publication [23]. In their original (and widely cited) publication, the identity of the green byproduct was assumed

<figure><img src="image_4.png" /></figure>

<footnote n="1">¹ Kirkham pers comm., observation based on data in K. Davies, *Artisan art: vernacular wall paintings in the Welsh Marches, 1550-1650* [14]</footnote>

<footnote n="2">² Michael Jaffe's translation (Vol. 2, page 213) provides the following translation: 'To make a common blue: take nitric acid and dissolve it in copper filings with fine white lead or chalk, but you have to wash it a number of times to remove the nitric acid. This green, washed again with smalt, gives common blue.' This recipe may therefore refer to green, rather than blue, verditer. Jaffe's notes support a date of approximately 1615-1620 for the sketchbook and his discussion of the artist's materials mention that they may be based on as yet unidentified earlier treatises or passed down orally by other artists [16]</footnote>
to be synthetic malachite.<fnref n="3" /> Later work by Mactaggart and Mactaggart mentions the possibility of gerhardtite, the naturally occurring polymorph of rouaite, occurring as the green product [26]. Previous work by the author, however, found that malachite was very rarely produced in replications of the synthesis process. The common undesirable byproduct of blue verditer production is indeed a basic copper nitrate, rouaite ($Cu_2(NO_3)(OH)_3$) [27]. Rouaite and gerhardtite are polymorphs, sharing their chemical structure but not their crystal structure. Rouaite is monoclinic, while gerhardtite is orthorhombic. Notably, attempts at gerhardtite synthesis by Yoder et al. produced only rouaite. The crystal structure, therefore, indicates the origin of the mineral: synthetic if monoclinic, and natural if orthorhombic.

Although verditer has been identified in wall paintings and easel paintings in England and continental Europe[2, 28–32], rouaite has not been frequently reported. We find one reference to it in a miniature painting attributed to a follower of Nicholas Hilliard and dated to 1613 (Portrait of an unknown man, called Sir Arnold Breams, Evans.2, Victoria & Albert Museum, London), used in combination with massicot and indigo [33, 34]. To our knowledge, this phase has not been previously identified in wall paintings. Its polymorph, naturally occurring gerhardtite, has recently been identified for the first time alongside natural atacamite in an Andean colonial painting [35]. Both remain elusive in published literature, and rouaite is likely under- or mis-identified in studies where characterisation is carried out using elemental analysis only and not combined with techniques giving structural information such as X-Ray Diffraction (XRD) or Raman spectroscopy.

Rouaite, although rarely identified as a pigment, is a common copper corrosion product in both modern patinas and historical archaeological finds [36–41]. It is also significant as a catalyst precursor due to the ability to easily substitute its nitrate ions for other anions including organic molecules [42–45]. Its stability, therefore, is of interest within conservation science and more broadly.

While evaluating the presence of verditer in a set of historical wall painting samples, we report the first identification of rouaite in two early modern English wall painting samples by synchrotron micro powder X-Ray diffraction (SR-μPXRD, sometimes abbreviated as SR-μXRPD) and synchrotron micro X-Ray absorption spectroscopy (SR-μXANES). We highlight the value of multimodal and chemically specific analytic methods for detection of unusual pigment phases. The significance of

the results is contextualised within the wider use of verditer and development of its synthesis. Finally, we briefly address the challenges of detecting rouaite as well as its potential degradation to copper chlorides. Initial work consisted of a comprehensive study of 22 samples from early modern English wall paintings previously suspected to contain azurite, verditer, or unidentified blue/green pigments, obtained by Dr. Andrea Kirkham.

Copper-containing blue and green paint layers were first evaluated using optical microscopy and Scanning Electron Microscopy coupled to Energy Dispersive X-Ray spectroscopy (SEM-EDX) to characterise samples as likely natural azurite or synthetic verditer on the basis of many qualitative and quantitative factors including pigment particle morphology and size and presence or absence of associate minerals (publication of comprehensive results from this study is forthcoming). This analysis brought the composition of several samples into question. A subset of several samples containing morphologically unusual or ambiguous copper-containing particles and, in some cases, significant colocalisation of copper and chlorine in elemental mapping data, were selected for further analysis as their chemical composition and morphology precluded identification as either azurite or verditer.

Unfortunately, intense fluorescence from the embedding resin prevented analysis of these samples by Raman spectroscopy. In order to study this subset with greater chemical specificity, two experiments were therefore planned at the European Synchrotron Research Facility (ESRF).

First, SR-μPXRD was used to study the mineral phases present in the subset of as yet unidentifiable samples. The possibility that pigment particles were poorly crystalline or amorphous necessitated a second experiment using SR-μXANES at the Cu and Cl K-edges to further study the local atomic environments of these two elements. The combination of nondestructive analysis capability, high spatial resolution, and chemical specificity of these techniques made them appropriate for such study.

As detailed below, two samples were found to contain rouaite based on the SR-μPXRD data: M9 and T1. Figure 1 presents photographs of these schemes. The results of analysis of these two samples and the significance of rouaite in their composition is presented in this work.

Sample M9 is from a first floor chamber in a great house in Hertfordshire.<fnref n="4" /> Dating of this scheme is difficult due to its fragmentary condition, although context suggests c. 1600 - early seventeenth century. Sample T1 is from a painted attic chamber in a gentry house in Norfolk. The sample comes from a border detail, part of an imitation textile scheme in a

<footnote n="3">³ Spherulitic semi-synthetic malachite has been identified in other work and its production in Germany has been documented [24, 25]. It appears, however, to have been *more* desirable than the natural mineral, possibly due to superior workability or colour, the opposite of the relationship between blue verditer and azurite).</footnote>

<footnote n="4">⁴ The samples were part of an investigation of the wall paintings carried out by Andrea Kirkham (2012) [46]</footnote>
<figure><img src="image_5.png" /><figcaption>**Fig. 1 a-b)** Photographs of the fragmentary wall painting from which sample M9 was taken, with sample location marked with red arrow. Additional blue areas (not sampled) which are difficult to see in the photographs due to the fragmentary condition of this scheme are marked by black arrows. **c-d)** Photographs of the wall painting from which sample T1 was taken, with sample location marked with red arrow. Detailed areas **b** and **d** are marked in **a** and **c** with red boxes. All images © Dr. Andrea Kirkham and reproduced with permission</figcaption></figure>

small and richly decorated chamber.<fnref n="5" /> The two samples are discussed separately as they come from different sites and display different characteristics apart from the presence of rouaite.

# 2 Materials and Methods

## 2.1 Materials

Historical wall painting samples were supplied as previously prepared cross sections by embedding in polyester resin and polishing to reveal sample surface. For SEM-EDX analysis, resin blocks were coated with 25-50 nm amorphous carbon (Quorum Technologies Q150T ES) and wrapped in conductive copper tape to minimise surface charging. For synchrotron analyses at the ID13 (μPXRD) and ID21 (μXRF/μ

<table><caption>**Table 1** XANES reference compounds</caption><thead><tr><th>Sample</th><th>Origin</th></tr></thead><tbody><tr><td>Atacamite, Cu<sub>2</sub>(OH)<sub>3</sub>Cl</td><td>Lab synthesis[47]</td></tr><tr><td>Azurite pigment, Cu<sub>3</sub>(CO<sub>3</sub>)<sub>2</sub>(OH)<sub>2</sub></td><td>Natural</td></tr><tr><td>Copper(II) chloride, CuCl<sub>2</sub></td><td>Sigma CAS 7447-39-4</td></tr><tr><td>Copper(I) chloride, CuCl</td><td>Sigma CAS 7758-89-6</td></tr><tr><td>Copper(II) sulfate, CuSO<sub>4</sub> * 5 H<sub>2</sub>O (chalcanthite)</td><td>Lab synthesis</td></tr><tr><td>Copper(II) acetate hydrate, Cu(CO<sub>2</sub>CH<sub>3</sub>)<sub>2</sub> * xH<sub>2</sub>O</td><td>Sigma, CAS 66923-66-8</td></tr><tr><td>Copper(I) Oxide, Cu<sub>2</sub>O</td><td>ESRF database</td></tr><tr><td>Copper(II) Oxide, CuO</td><td>ESRF database</td></tr><tr><td>Mixture of rouaite and verditers</td><td>Lab synthesis</td></tr><tr><td>Malachite, Cu<sub>2</sub>CO<sub>3</sub>(OH)<sub>2</sub></td><td>Natural</td></tr><tr><td>Rouaite, Cu<sub>2</sub>(NO<sub>3</sub>)(OH)<sub>3</sub></td><td>Lab synthesis</td></tr><tr><td>Verdigris pigment</td><td>Purchased, L. Cornellisen</td></tr><tr><td>Verditer pigment, Cu<sub>3</sub>(CO<sub>3</sub>)<sub>2</sub>(OH)<sub>2</sub></td><td>Purchased, L. Cornellisen</td></tr><tr><td>Verditer pigment, Cu<sub>3</sub>(CO<sub>3</sub>)<sub>2</sub>(OH)<sub>2</sub></td><td>Supplied by collaborator</td></tr><tr><td>Verditer pigment, Cu<sub>3</sub>(CO<sub>3</sub>)<sub>2</sub>(OH)<sub>2</sub></td><td>Lab synthesis</td></tr><tr><td>Rabbit skin glue</td><td>Purchased, L. Cornellisen<br>prepared per Cardell et. al. (2017) [48]</td></tr></tbody></table>

XANES) beamlines, the resin blocks were thinned down to 100 µm thick cross samples in 1 mm thick resin blocks using a fine saw and dremel tool.

Reference powder samples were purchased or synthesized in the laboratory as detailed in Table 1. For ID21 μXANES analysis, thin layers of bulk powder reference samples were spread on sulfur free tape (FluXana TF500, 47 µm polypropylene), enclosed by a layer of ultralene film (Cole Palmer, thickness 4 µm).

It was not possible to obtain pure reference samples for some compounds of interest, including gerhardtite, clinoatacamite, and paratacamite. The development of robust reference libraries for pure species is critical for researchers in heritage science, and it is important to acknowledge this limitation in the current study.

## 2.2 Methods

### 2.2.1 SEM-EDX

SEM images and EDX maps were collected using a TESCAN MIRA3 FEG-SEM coupled to an Oxford Instruments Aztec Energy X-maxN 80 EDX system and software. An accelerating voltage of 15 keV was chosen so that high Z elements could be effectively resolved, and

<footnote n="5">⁵ Information provided by Andrea Kirkham. See forthcoming publication, *Decorative painting in domestic houses, 1560-1660: Colour, Design and Society* by Dr. Andrea Kirkham for further information.</footnote>
the beam intensity was attenuated by decreasing the beam
current in order to reduce surface charging under high
vacuum conditions. Secondary electron (SE) and back-
scattered electron (BSE) detectors were used to image
samples.

2.2.2 SR-μPXRD

SR-μPXRD data were collected at the microbranch of
beamline ID13 at the ESRF. Historical cross section
samples were mounted perpendicular to the X-Ray beam.
The incident beam energy was 13.0 keV (λ = 0.950799 Å).
The beam was focused using compound refractive lenses to
2.2 x 2.4 μm. Flux through the sample was approximately
2.1 x 10¹¹ photons/s. Regions of interest (ROIs) of variable
dimensions were identified over sample cross sections
and scanned with 1x1 μm step size. 2D diffraction data in
transmission geometry were collected at each point using a
single photon counting detector (Dectris Eiger 4 M; 2070
× 2167 pixels; 75 x 75 μm pixel size; 20 ms dwell time)
[49–51].

2D XRD patterns were subjected to azimuthal integration using the PyFAI software package and dedicated Jupyter notebooks developed at ESRF (https://gitlab.esrf.fr/loic.huder/juno) to obtain 1D 2θ versus intensity diffraction patterns at each point in the ROI in .h5 file format [49, 52]. This data was then analysed in PyMCA (version 5.6.5)[53] using the ROI imaging feature.

Average diffraction patterns from the entire sample
region and subregions were extracted and a wavelength
conversion was applied using PowDLL (version 3.1)[54] to
facilitate comparison to reference data collected at the CuKα
wavelength. Detector alignment was corrected by applying
a further scaling factor in OriginPro by aligning sample
patterns to a calcite reference spectrum (calcite was present
in all samples). The broad underlying peaks attributed to
the embedding resin were removed by subtracting a spline
baseline, and then pattern matching was carried out using
DIFFRAC.EVA software (version 5.2) and the ICDD PDF-2
database (2021). Data visualisation employed PyMCA’s ROI
imaging tool and the results of pattern matching.

Rietveld refinement was carried out in TOPAS-Academic
V7 using fundamental parameters. A pattern extracted from
an area of pure resin was used to fit the broad underly-
ing background, and reference CIF files from the Ameri-
can Mineralogist Crystal Structure Database were used to
fit phases [55]. Preferred orientation was addressed using
either March-Dollase or spherical harmonics, and a rigid
body model for NO₃⁻ was used to carry out structural refine-
ment of rouaite.

2.2.3 SR-μXANES

SR-μXANES data at the Cu and Cl K edges were collected
at beamline ID21 at the ESRF [56].

Undulator-generated X-Rays were monochromatized
using a Si (111) double crystal monochromator. The
incident X-Ray beam was 62° to sample surface. Incident
intensity was monitored by a photodiode detector
upstream of the sample. Downstream of the sample,
another photodiode detector measured the intensity
of the transmitted beam for analysis of the reference
powder samples. A silicon drift detector (Bruker, 80 mm²
collimated active area) detected fluorescence photons
(28° from surface of sample) for μXRF mapping and μ
XANES spectra collected in fluorescence mode [49, 50,
56, 57]. Analysis was carried out under vacuum, which
reduces scattering effects and reabsorption of XRF by air,
and also has been documented to limit reactions between
the metal centre and gases [58, 59].

The system was calibrated using Cu metal foil and NaCl powder references at the edge jump energy (8.979 keV at Cu K-edge, 2.828 keV at Cl K-edge, taken as the edge inflection point determined using the first derivative maximum). The beam was focussed by Kirkpatrick-Baez mirrors to 0.35(v) x 0.9(h) µm at the Cu K-edge and 0.5(v) x 1(h) µm at the Cl K-edge. Prior to data collection, XRF deadtime was evaluated for each sample and maintained ideally between 10-40%. Flux was reduced using Ti 25 µm/Al 100 µm (Cu K-edge) and Al 6 µm (Cl K-edge) attenuators for study of historical samples.

Bulk XANES reference spectra were collected in transmission mode using an unfocussed beam (300 x 300 µm) with 100 ms dwell time. The energy range was 8.93–9.25 keV for the Cu K-edge (0.5 eV step) and 2.80–2.92 keV (0.25 eV step) for the Cl K-edge. All XANES reference spectra are presented in Supplemental Information (Fig. S5).

Locations for μXANES point spectra were selected based on μXRF maps (fluorescence mode) with excitation energies 8.98 keV (Cu K edge, flux 1.1 x 10⁹ photons/s, dwell time 0.05s) and 2.88 keV (Cl K edge, attenuated flux 3.8 x 10⁶ photons/s, dwell time 0.05s). XANES point spectra at the Cu K-edge (8.95–9.25 keV, dwell time 0.05s) and Cl K-edge (2.80–2.92 keV, dwell time 0.1s) were collected in fluorescence mode for historical and reference samples, using the focussed beam. Point spectra were collected every 3-6 µm from the surface of the sample to the bulk in order to track Cu and Cl speciation as a function of the distance to the painting surface.

XRF spectra were fitted using PyMCA and in-house
scripts, and elemental distributions were visualised using the
PyMCA ROI imaging tool. Initial XANES data extraction
and preprocessing was carried out using in-house scripts,
followed by normalisation and principal component analysis
clustering in Orange3 (Python application) using in-house developed workflows. Linear combination analysis (LCA) fitting was carried out in Larch (version 0.9.72) using the Larix interface. Correction for overabsorption was done prior to LCA fitting (sample T1 Cu K-edge only) using weighted stoichiometry for verditer pigment in polyester resin (styrene base). The FLUO algorithm, integrated into Larch, was used for the correction Newville [60].

## 3 Results and discussion

Results of a multimodal analysis by μPXRD, μXRF, and μXANES of samples M9 and T1 are presented. Rouaite is identified by multiple analytic methods in both samples, in conjunction with verditer/azurite in sample M9 and in conjunction with a basic copper chloride in sample T1. μPXRD and μXANES not only enable detection and spatial localisation of phases but also provide strong evidence for the presence of rouaite as opposed to its extremely similar polymorph gerhardtite.

The phases associated with rouaite in M9 and T1 have important implications for interpretation of these results. Discussions of rouaite's use as a standalone pigment and

its appearance as a byproduct of verditer production are therefore presented in the context of the analytical results from M9 and T1. Analytical challenges and potential explanations for its paucity in previous studies of painted works follows on from this.

### 3.1 Sample M9: Rouaite detected alongside verditer

Sample M9 is comprised of two fragments of painting (named fragments 1 and 2), both small cross sections containing three distinct layers, one of which appears greenish-blue and contains a copper blue pigment (iv). Optical and SEM micrographs are shown in Fig. 2. A lower layer of lead white (iii) and a calcium-based ground layer (i/ii) are also evident. The copper containing particles are morphologically ambiguous between verditer and azurite. Particles are small and lack the typical spherical shape and fractured or lobed structure of synthetic verditer as well as the jagged and highly irregular particle morphology of natural azurite. Chlorine was not present in elemental analysis to a significant degree-although it was detected as a minor species in the EDX sum spectra (Fig. S1), elemental analysis of M9 showed below average chlorine abundance compared to the

<figure><img src="image_6.png" /><figcaption>**Fig. 2 a-b)** Optical microscope images of cross section sample M9, comprised of two fragments from same region of painting scheme: white ground layer (i), grey ground layer with possible soot particles (ii), bright white layer (iii), medium blue Cu-containing layer (iv). **c-d)** SEM images (BSE detector) of sample M9 with layers ii-iv labeled (lower ground layer is not shown)</figcaption></figure>

<figure><img src="image_7.png" /></figure>
<figure><img src="image_8.png" /></figure>

**Fig. 3 a)** Optical microscope image showing sample M9 (fragment 1) with mapped ROI outlined. **b)** µPXRD map of calcite (magenta), verditer/azurite (green), rouaite (blue). **c)** PXRD diffractograms extracted from map data shown for a region in which rouaite peaks were localised (black) and a region in which verditer/azurite peaks were localised (dark blue). Reference data for rouaite (red), gerhardtite (teal), malachite (purple), and azurite (blue) are provided

for comparison [55]. Peaks matching rouaite are marked by a *, and peaks matching verditer/azurite are marked by a +. A grey box highlights a major rouaite peak present in M9 and shifted in the gerhardtite reference data. **d)** Sample diffractogram from rouaite-containing region is presented with calculated fit using rouaite (top, blue) and gerhardtite (bottom, green). Residuals are plotted below

rest of the sample set and colocalisation with copper was minimal (11.2 ± 0.4 wt.% Cu, 0.2 ± 0.1 wt.% Cl.).

Several mineral phases were identified by µPXRD mapping of fragment 1 and localised to specific regions of the cross section sample.

The overall ROI studied is shown in Fig. 3a. Major mineral phases excluding those containing copper are presented in Supplemental Information (Fig. S3); hydrocerussite and cerussite (lead white) localise to layer iii, while the ground layers i/ii contains primarily calcite. The relatively common degradation product weddellite (calcium oxalate) is also present primarily in the top layer of the sample (iv).

Two copper mineral phases were identified in layer iv, shown in Fig. 3. The main component of layer iv showed a good match with azurite/verditer, which are indistinguishable by PXRD. More importantly, rouaite was also identified based on pattern matching, localised to a few regions in the middle of the paint layer (Fig. 3b). PXRD diffractograms from rouaite and azurite/verditer containing regions were extracted and are presented in Fig. 3c alongside reference data for rouaite, gerhardtite, malachite, and azurite/verditer.

Reference data for other species present such as calcite and quartz are not presented in Fig. 3c for easier readability; Fig. S3 and Table S1 summarise the contributions from all species indexed to the fits shown in Fig. 3d.
The experimental diffractogram shows a good match for rouaite, although there are several reference peaks absent from the experimental pattern; this is likely related to the very significant preferred orientation and particle size effects observed (discussed in section 3.4). Peaks corresponding to azurite/verditer and calcite are also clearly present. This is unsurprising as the measurements in transmission mode will detect signal throughout the cross section perpendicular to the polished face, and both can therefore be expected in the sampled volume as minor components.

The diffraction pattern for gerhardtite is presented in Fig. 3c as well, but shows a poorer match with the sample data. Fitting from fundamental parameters (Fig. 3d) further compares the agreement between M9 and rouaite/gerhardtite. The residuals are comparable, with rouaite showing worse agreement to peak shape but better agreement to the pattern overall with fewer negative peaks. These results suggest that the phase present can be identified as rouaite, although not unambiguously, suggestive of synthetic origin. However, it is challenging to distinguish between polymorphs with very similar PXRD patterns, especially when several other phases are also present and orientation effects may be influencing peak intensities [61]. The possibility of using the sensitivity of local structure and environment of μXANES to further shed light on the paint composition was therefore investigated.

XANES spectra from sample M9 collected at the Cu K-edge are presented in Fig. 4a. The best LCA fit came from references for azurite/verditer, malachite, and rouaite, and there is no clear trend in spectra versus sample depth. Figure 4b shows the phase fractions versus sample depth

for M9, again showing azurite/verditer as the primary component with varying amounts of malachite and rouaite present. Specifically, rouaite is included in the LCA fit only for spectra collected midway through the paint layer, consistent with the μPXRD mapping in which it is sporadically present.

The best fit is identified when malachite is included as a component. The XANES spectra for these three compounds are very similar, so the presence of malachite was investigated by fitting the PXRD data. The results suggest that the best fit is obtained when malachite is included to fit the PXRD diffractograms as well, although the relevant peaks are broad and not especially well resolved, and primarily malachite containing regions cannot be isolated from azurite/verditer in the map. The combination of XANES and PXRD analyses therefore enables confidence in malachite's identification in sample M9.

## 3.2 Sample T1: Rouaite detected alongside copper hydroxychloride

Sample T1 is comprised of a number of cross section fragments 1-4 showing the same layer structure, shown in Fig. 5. Three to four distinct layers are present, and the top layer (iii/iv) contains a copper blue pigment. The vivid red layer (i) contains iron (hematite), followed by a grey Ca-based ground (ii). Layer iii/iv is quite thick (50 μm) and in some samples appears to contain two distinct regions (noted by brackets in Fig. 5b, d); visually, however, it is difficult to distinguish layer iii from layer iv. Regions of Cu pigment were identified by SEM-EDX, and exhibited unusual morphology

<figure><img src="image_9.png" /><figcaption>**Fig. 4** a) μXRF map of region of sample M9 showing locations of μXANES data collection in Cu-containing (blue) area of sample. To aid interpretation, Pb (green) and Ca (magenta) signals are also shown. b) XANES spectra at the Cu K-edge collected from M9. Reference spectra for azurite, malachite, and rouaite are presented for comparison. Spectra were collected in a line from the surface of the</figcaption></figure>

sample to the bulk, and four representative datasets with their best fits are presented. c) Phase fractions from LCA fits versus sample depth for spectra collected at the Cu K-edge. The dominant phase is azurite/verditer, with some contribution from malachite as well as a region in the middle of the layer containing rouaite
<figure><img src="image_10.png" /><figcaption>**Fig. 5 a-c)** Cross section sample T1, optical microscope images: vivid red layer (i), grey ground layer (ii), bright pale teal layer (iii), upper bright pale teal layer (iv). Note that (iv) is not clearly visible in all cross section fragments. **d)** SEM image (BSE detector) of sample T1 (fragment 4) with layers i-iv labeled. **e)** µXRF map of a region of sample T1 (fragment 3) showing Cu (blue), Cl (green), and Ca</figcaption></figure>

(magenta) signals. Cl is observed to be concentrated at the top of layers iii/iv. Note that the excitation efficiencies for the Cl and Cu signals differ; Cl is excited much less efficiently at the energy used in this case (just above the Cu K-edge), explaining its patchy appearance in the µXRF map
with indistinct, almost stringy 'particles' (Fig. 5d). The atypical morphology is not characteristic of verditer or azurite. Colocalisation of copper with chlorine in this sample was very significant, especially near the top of the cross section (exposed to environment). Chlorine signal decreases moving from the surface to the bulk, suggesting that two (or more) different Cu compounds may be present, one of which being a copper chloride (Fig. 5e).

Fragments 3 and 4 from sample T1 were studied by μ-PXRD mapping. The results of analysis of fragment 3 are presented here and are consistent with those of fragment 4. Major mineral phases excluding those containing copper are presented in Supplemental Information (Fig. S4) and are consistent with the use of a red hematite containing pigment in layer i such as red ochre and a grey ground layer containing calcite and gypsum. As in M9, weddellite is present primarily in layers iii/iv towards the painting surface. These results are typical for wall painting/mural palettes [62, 63].

μPXRD mapping revealed two distinct layers containing Cu phases, a lower layer of rouaite and an upper layer of a basic copper chloride ($Cu_2(OH)_3Cl$). Significantly, verditer/azurite is not present in this sample. The two layers are of similar thickness, 20-30 μm. Figure 6b presents a three-colour map of calcite, rouaite, and basic copper chloride alongside the corresponding optical micrograph with mapped ROI.

Diffraction patterns extracted from the upper and lower regions are presented in Fig. 6c-e. The lower layer clearly shows a good match with a reference pattern of rouaite, while the upper layer shows a good match with several polymorphs of $Cu_2(OH)_3Cl$. It is interesting to note that in between the regions containing primarily $Cu_2(OH)_3Cl$ and rouaite, there is a region with generally decreased diffraction intensity which appears to correspond to the transition between the two species.

Figure 6c and d show reference diffractograms for rouaite and gerhardtite compared to the lower layer of sample T1.

<figure><img src="image_11.png" /><figcaption>**Fig. 6** a) Optical microscope image showing sample T1 with mapped ROI outlined. b) μPXRD map of calcite (magenta), basic Cu(II) chloride (green), rouaite (blue). c) PXRD diffractograms extracted from the lower blue layer (black) and upper blue layer (blue) of sample T1. Reference diffractograms for rouaite (red), gerhardtite (teal), clinoatacamite (yellow), and atacamite (purple) are presented for comparison</figcaption></figure>

[55]. Peaks in the lower layer pattern matching rouaite are marked by a *, and peaks in the upper layer pattern matching clinoatacamite are marked by a +. d) Sample diffractogram from lower blue layer (grey) is presented with calculated fit using rouaite (top, blue) and gerhardtite (bottom, green). Residuals are plotted below.
The sample pattern is much better represented by rouaite, while several significant gerhardtite peaks are not observed (2θ = 25.2, 34.1, 34.6). Rietveld refinement on diffraction data extracted from rouaite containing regions of T1 was carried out in TOPAS to further investigate the possible presence of gerhardtite, with results presented in Fig. 6e. While it is possible to fit gerhardtite with reasonable residuals, fewer peaks are accounted for and the quality of the fit is not as good as the fit when rouaite is used instead. The presence of rouaite rather than gerhardtite strongly supports a synthetic origin for the pigment, possibly from a failed blue verditer synthesis attempt.

The basic copper chlorides atacamite and clinoatacamite have very similar diffraction patterns, and the difficulty of identifying specific polymorphs is compounded by the presence of other phases and the influence of preferred orientation on peak intensities. Minerals from the atacamite family are common alteration or degradation products of azurite and malachite, especially in wall paintings and murals [64, 65]. Medieval synthetic recipes producing purely atacamite have also been documented [18] The specific identity of the basic copper chloride is therefore of interest, since it could provide information about the origin and mechanism of formation in the sample. Further analysis by µXANES was therefore carried out on sample T1 to characterise its composition.

Cu and Cl K-edge XANES spectra were collected in a line from the surface to the bulk, approximately perpendicular to the painting surface (Fig. 7b). Data was collected along different trajectories to avoid resampling areas that may have undergone local alteration by the X-Ray beam. Spectra with associated LCA fits are presented in Fig. 7c (Cu K-edge) and d (Cl K-edge), and phase fractions versus depth from surface are presented in Fig. 7e and f.

At the Cu K-edge, we see a good match with primarily atacamite with some copper(I) oxide at the surface. The proportion of atacamite decreases moving into the bulk, where rouaite is the primary species identified. There is a region of intermediate character in which atacamite and rouaite both contribute to the LCA fit, suggesting a region of transition between the two species.

Similarly, Cl K-edge data also shows a good fit with primarily atacamite (as well as embedding resin, rabbit glue, and a small amount of CuCl). We note that although medium analysis has not been carried out on these samples, the use of size/glue in Tudor wall paintings is expected; Cl K-edge XANES spectra collected from a modern albeit traditionally prepared rabbit skin glue showed a good fit as a component of T1 [1].

The analysed distance from the surface for Cl XANES is smaller than for Cu XANES, due to observed drop in Cl signal around 35 µm from surface (Fig. 7f). This corresponds well with the Cu K-edge shift from atacamite to

rouaite shown in Fig. 7e. The good quality LCA fits support the phase identifications based on µPXRD, confirming that the likeliest polymorphs present are atacamite at the surface and rouaite in the bulk. The significance of the spatial distribution of atacamite is discussed further in section 3.5.

### 3.3 Verification of historical synthetic production method

Identification of rouaite in combination with verditer in sample M9 (approximately c. 1600 or early seventeenth century) strongly supports hypotheses about historical verditer synthesis using the refiners' method [23]. The starting material of aqua fortis (nitric acid), rather than sulfuric or hydrochloric acid, is the source of nitrate ions incorporated into rouaite. As rouaite is vanishingly rare in nature and commonly produced synthetically, it is highly unlikely that it would be found as an associate mineral.

This evidence strongly suggests that the copper carbonate in sample M9 is synthetic verditer, perhaps from a not entirely successful synthesis leading to the atypical morphology, and highlights the limitations of visual analysis for characterisation of minor phases in cross section samples (although this remains a useful tool for identifying verditer in many cases). The pigment in M9 is visually a medium blue, so evidently rouaite is present to a small degree and does not visually impact the scheme. Complete conversion to verditer is not necessary to produce a satisfactory blue colour, and it is possible that more samples irrespective of colour could contain rouaite as well.

This also problematises the assumption that green verditer was chemically analogous to malachite, as malachite was not conclusively identified by PXRD in any of the samples studied. In sample M9, there is some evidence that malachite is present in addition to rouaite and verditer, but cannot be clearly isolated from verditer in the map data. It is therefore difficult to draw firm conclusions about its presence, although this is the subject of further work. Additionally, we note that in some experiments studying blue verditer synthesis, malachite was produced alongside rouaite, and therefore may also be present as a synthetic byproduct. It was not identified in other samples studied.

It is also important to note that the rarity of synthetic malachite in these samples does not mean that it did not exist—it has been previously reported in multiple sources—but rather that rouaite could be a chemical fingerprint for the refiners' verditer synthesis. Modern green verditer references studied, however, have been analogous to malachite, so it is likely that modern production processes differ significantly from those 'failures' producing green pigment during the early modern period.
<figure><img src="image_12.png" /><figcaption>**Fig. 7** a) Optical microscope image of T1 showing ROI for μXANES data collection. b) μXRF map of ROI of sample T1 showing locations of Cu (·) and Cl (+) K-edge μXANES data collection in Cu-containing (blue) and Cl-containing (green) areas of sample. To aid interpretation, the Ca (magenta) signal is also shown. c) XANES spectra at the Cu K-edge collected from T1. Reference spectra for rouaite, atacamite, and CuO are presented for comparison. Spectra were collected in a line from the surface of the sample to the bulk, and four</figcaption></figure>

representative datasets with their best fits are presented. d) XANES spectra at the Cl K-edge collected from T1. Reference spectra for rabbit glue, embedding resin, atacamite, CuCl, CuCl₂, and CaCl₂ are presented for comparison. Spectra were collected in a line from the surface of the sample to the bulk, and three representative datasets with their best fits are presented. e) Phase fractions from LCA fits versus sample depth, Cu K-edge. f) Phase fractions from LCA fits versus sample depth, Cl K-edge

<figure><img src="image_13.png" /></figure>
Sample T1, comprised of rouaite with a significant upper layer of copper hydroxychloride, dates from approximately 1600 and represents a relatively early scheme painted during the period in which verditer use and synthesis is documented. The use of rouaite presumably intentionally as the main blue-green pigment in the area sampled suggests that artists did make use of less desirable materials rather than disposing of them. It is possible that a fugitive organic yellow may have been used with rouaite to produce a green paint initially, fading to the observed teal, and that in this case as in the miniature where rouaite was alongside masficicot to paint a green doublet, artists intentionally used the green hue of the material [33]. However, we did not find any evidence of this being the case, and it is also possible that the painter simply took advantage of the unusual colour of the pigment alone.

Further study of pigments suspected to be verditer would benefit from analysis capable of detecting rouaite; based on this small study of only seven samples in which two instances of rouaite have been found, it is likely to be much more common than previously known.

**3.4 Orientation and conformation effects**

Analysis of rouaite-containing samples by μPXRD proved challenging. The influence of preferred orientation (preferential crystal alignment along a particular plane) on diffraction peak intensities is well documented for pigments and other materials, and is especially significant when the effect of directional paint application and pigment particles settling during the drying process is taken into account [61, 66–72]. Bulk PXRD data collection minimises orientation effects by two methods: sample rotation and reduction of particle dimensions. When working with embedded cross sections, however, the sample material is not homogeneous and rotation is not possible. Particles also cannot be ground more finely. Pigment particles will settle in specific orientations in liquid binder prior to drying and will therefore show orientation effects in resulting μPXRD diffractograms, particularly when the particle dimensions are similar in size or larger than the beam size, as is the case in this work [61]. Although it would be possible to mitigate preferred orientation by increasing the beam size, this would adversely affect the lateral resolution attained, and therefore is also undesirable.

Figure 8 presents phase maps of the three main rouaite peaks (2θ = 12.85, 25.8, 33.5) in M9 (a) and T1 (b). The regions of maximum intensity for each peak are found to be localised to the same general sample region, but do not overlap precisely. We propose that this variation is due to preferred orientation effects on relative peak intensities, as there is good agreement between sample data and reference data and a mis-identification of phase therefore is not likely.

Some high angle peaks in the sample diffractogram, though, are missing or poorly resolved when compared to reference data. This, too, supports the notion that preferred orientation effects are significant. Figure 8c presents phase maps of three major peaks of calcite in T1, and similar, though less significant, spatial variation in maximum intensities is observed.

We observe that the variation in location of maximum intensity is most pronounced for rouaite particles, which is to be expected due to their tabular crystal habit (hydrocerussite, also tabular, has also been shown to exhibit strong preferred orientation in cross section samples) [45, 71]. Patterns extracted from several regions of T1 were fitted, and the best fit was achieved by refining the structure of the rouaite unit cell as well as accounting for preferred orientation (Fig. 8d). Specifically, refinement of the positions of the atoms in the nitrate groups using partial nitrate occupancy gave an excellent quality fit, and the exact atom positions varied among patterns. The resulting structures showed the nitrate groups bound doubly to the copper atoms, a slight variation from the expected structure, likely due to allowing for incomplete nitrate occupancy in the fitting [45, 55]. The mobility of these nitrate groups has been previously reported and highlights the sensitivity of X-Ray diffraction to small variations in crystal structure [45].

**3.5 Possible degradation of rouaite in wall painting samples**

The presence of a basic copper chloride located at the surface of sample T1 is suggestive of degradation processes. While it is possible that this layer was original to the scheme and applied after the rouaite paint layer, the visual similarity in optical images does not support the idea of intentional application of a basic copper chloride as it does not influence the colour of the paint. The increased labour and material costs of applying a second layer of paint in this context and period is unlikely, especially if it did not significantly change the visual and aesthetic impact of the scheme.

It is more likely that the upper layer results from reaction of rouaite with chloride ions present in the atmosphere or the paint mixture. The localization to the surface of the sample and the presence of a zone in between the pure chloride and pure rouaite support this interpretation.

As previously mentioned, it is likely that the schemes presented here are carried out in aqueous binder such as glue/size rather than in a drying oil [1]. When azurite/ verditer is bound in aqueous media, it is vulnerable to weathering and alteration to other copper compounds [48, 73], although the discolouration and darkening observed in oil binder is not present [74–76].
<figure><img src="image_14.png" /><figcaption>Fig. 8 a) μ-PXRD map of sample M9 showing three rouaite peaks (2θ = 12.85, 0 0 1, magenta; 2θ = 25.8, 0 0 2, green; 2θ = 33.5, 1 2 0, blue). b) μ-PXRD map of sample T1 showing three rouaite peaks (2θ = 12.85, magenta; 2θ = 25.8, green; 2θ = 33.5, blue). c) μ-PXRD map of sample T1 showing three calcite peaks (2θ = 29.5, magenta; 2θ = 35.5, green; 2θ = 39.5, blue). d) Sample diffractogram</figcaption></figure>

from lower blue layer of sample T1 (grey) is presented with calculated fit using rouaite accounting for preferred orientation (top, blue) and without accounting for preferred orientation (bottom, green). Residuals are plotted below. Note that the experimental wavelength is 0.9508Å and the background has been fitted

These results suggest that rouaite may be particularly prone to attack and degradation by chlorine present in other sample layers, forming basic copper chlorides. While it is possible that atacamite's origin in this case is as an intentionally used pigment, this is unusual based on the presence of rouaite, the lack of visible difference in hue between the two phases, and the poorly crystalline atacamite particles.

As rouaite is known to be metastable and can be readily converted to other mineral phases, it is not unreasonable to hypothesise that commonly observed alteration reactions for copper mineral species will occur here as well, and possibly more rapidly. For instance, anion replacement reactions forming copper hydroxychlorides and copper hydroxysulfates from rouaite have been documented (Equation 1):[45]

$$Cu_2(OH)_3NO_3 + NaCl \rightarrow Cu_2(OH)_3Cl + NaNO_3 \quad (1)$$

Other studies of degradation of basic copper carbonates (azurite, malachite) point to the possible role of oxalic acid (and formation of weddellite) in facilitating reactions by decreasing pH [63]. The identification of weddellite in M9 and T1 means that these conditions are possible (as already mentioned, weddellite is a very common species to observe in wall painting samples) [76]. One possible reaction between azurite and oxalic acid in the presence of chlorides offers a plausible route for conversion of azurite to copper hydroxychloride (Equation 2), although most literature discussion of azurite degradation is limited to identification of degradation products rather than discussion of reactions involved and their favorability [18, 64, 65, 76–79]. Work on copper corrosion products as well as studies of paintings
suggests that formation of nantokite (CuCl) may occur as an intermediate reaction in the formation of copper hydroxychlorides (Equation 3) [80]. The reactions presented here therefore offer plausible routes for azurite as well as rouaite degradation, but further study is needed to determine whether intermediate species are indeed formed and the relative favourability.

$$4\text{Cu}_3(\text{OH})_2(\text{CO}_3)_2 + 6\text{NaCl} + 3\text{H}_2\text{C}_2\text{O}_4 + 10\text{H}_2\text{O} \rightarrow 6\text{Cu}_2(\text{OH})_3\text{Cl} + 8\text{H}_2\text{CO}_3 + 3\text{Na}_2\text{C}_2\text{O}_4 \quad (2)$$

$$4\text{CuCl} + \text{O}_2 + 4\text{H}_2\text{O} \rightarrow 2\text{Cu}_2(\text{OH})_3\text{Cl} + 2\text{H}^+ + 2\text{Cl}^- \quad (3)$$

Rouaite's metastability and propensity to react further is a possible explanation for its absence in literature. Apart from degradation or conversion to other copper minerals such as chlorides or sulfates, identification of rouaite in historical samples may be hampered by the difficulty of detecting light elements such as nitrogen, the low abundance in some samples necessitating analysis with a low detection limit, and the similarity of rouaite to synthetic malachite, copper chlorides, and verditer when examined by optical and scanning electron microscopy.

## 4 Conclusion

Rouaite, a basic copper nitrate found to be a significant byproduct of the refiners' synthesis of blue verditer, has been identified in historical wall painting samples for the first time to our knowledge. Table 2 presents a summary of the analytic results for both samples studied.

In two samples, M9 and T1, we detected rouaite using SR-μPXRD and SR-μXANES. We show that its polymorph, gerhardtite, fits the experimental data collected more poorly; this is therefore highly suggestive of a synthetic origin for the material [27].

Identification of rouaite alongside blue verditer (sample M9) further supports its presence in paintings as a synthetic byproduct, while identification of rouaite in the absence of blue verditer (sample T1) suggests that in addition to this rouaite may have been used as a pigment in its own right.

The detection of rouaite in two samples from a relatively small sample corpus suggests that this material may have been much more widely present as a pigment and as a synthetic byproduct than previously understood. In light of this, the highly sensitive and phase specific analyses available at facilities such as the ESRF are extremely valuable for accurate characterisation of samples that cannot be conclusively understood by more accessible techniques such as SEM-EDX and optical microscopy. In addition to the historical information gleaned from specific phase identification and spatial localisation, it may also inform preservation and conservation considerations.

The detection of a relatively thick layer of basic copper chloride at the surface of a sample containing rouaite in the absence of verditer suggests that this painting may have undergone significant degradation. This raises intriguing and concerning questions about the stability of rouaite. In this work, we put forward a hypothesis about differential reactivity observed between rouaite and verditer/azurite, and we point to the need to further study rouaite's susceptibility to degrade or alter in the presence of common species such as chlorides, sulfates, and volatile organics. Further analysis would be beneficial to not only understand the fundamental reactivity of the material but also to optimise preservation of paintings and murals in which it is present.

The different contexts for use in two wall painting samples from approximately the same time in the East of England offer novel insight into the integration of synthetically produced copper pigments into the painter's palette in the early modern period. Rouaite, identified here as a synthetic byproduct and an original pigment, may have been more

<table><caption>**Table 2** Summary of analytic results</caption><thead><tr><th>Sample</th><th>SEM-EDX</th><th>μPXRD</th><th>μXRF</th><th>μXANES</th><th>Presumed origin</th></tr></thead><tbody><tr><td>M9</td><td>Small particles, mixed spherical/irregular; chlorine minimal</td><td>Verditer/azurite, rouaite, malachite, calcite, quartz (minor), weddellite (minor)</td><td>Cu detected in blue layer, chlorine not abundant</td><td>Best LCA fit: azurite, malachite, rouaite</td><td>Synthetic byproduct of blue verditer</td></tr><tr><td>T1</td><td>Indistinct particle morphology, chlorine abundant especially at surface</td><td>Rouaite (bulk), atacamite (surface), calcite, quartz, gypsum, weddellite (minor)</td><td>Cu, Ca, Cl (surface)</td><td>Best LCA fit: Atacamite/CuO (surface), rouaite (bulk), minor contribution at Cl K-edge from other copper chloride species</td><td>Original pigment</td></tr></tbody></table>
significant to the early modern palette than previously understood. In combination with blue verditer, it offers a possible fingerprint for the refiners' synthesis method. However, this work also suggests that rouaite may be prone to alteration, which poses challenges for its detection and evidences the value of material analysis for preservation purposes.

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1007/s00339-024-07954-1.

**Acknowledgements** The authors would like to thank Dr. Giulio Lampronti at the University of Cambridge for assistance and advice regarding diffraction data analysis. The authors also acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities. We would like to thank Dr. Victor Gonzalez for assistance accessing ID13 beamtime (proposal HG-172, pilot of a BAG project supported by the European Union's Horizon 2020 research and innovation programme under grant agreement No 870313, Streamline, DOI: 10.15151/ESRF-ES-1171320945) and Dr. Jiliang Liu, Ida Fazlic, and Victor Poline for on-site assistance. We acknowledge the support of the ESRF team with beamtime at ID21 (proposal HG-225, DOI: 10.15151/ESRF-ES-1304131335, 10.15151/ESRF-ES-1520459969).

E.P.'s PhD funding has been provided by the Gates Cambridge Trust. This work was supported, in whole or in part, by the Bill & Melinda Gates Foundation [OPP1144]. Under the grant conditions of the Foundation, a Creative Commons Attribution 4.0 Generic License has already been assigned to the Author Accepted Manuscript version that might arise from this submission.

**Author Contributions** Theoretical conception: EP, AK, MTLC; data collection, processing and interpretation of results: EP, SC, CH, MC; data presentation: EP; writing: EP; editing: EP, CH, MC, AK, MTLC. All authors contributed to the final revision and reading of the manuscript.

**Funding** E.P.'s PhD funding has been provided by the Gates Cambridge Trust.

**Data availability** The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

**Declarations**

**Conflict of interest** The authors have no Conflict of interest to declare.

**Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

**References**

1. A. Kirkham, Sixteenth- and Seventeenth-Century Secular Wall Paintings in Suffolk. PhD thesis, The University of East Anglia (2010)

2. F. Delamare, *Blue Pigments: 5000 Years of Art and Industry* (Archetype Publications, London, 2013)

3. H. Howard, *Pigments of English Medieval Wall Painting* (Archetype, London, 2003)

4. R.D. Harley, The interpretation of colour names. The Burlington Magazine **110**(785), 460–461 (1968)

5. R.D. Harley, Artists' Pigments C.1600-1835: a Study in English Documentary Sources, 2nd ed. edn. Technical studies in the arts, archaeology and architecture. Butterworth Scientific, London (1982)

6. R. Boyle, Certain Physiological Essays and other Tracts (1669): Two Essays, Concerning the Unsuccessfulness of Experiments. In: Hunter, M., Davis, E.B. (eds.) The Works of Robert Boyle, Vol. 2: The Sceptical Chymist and Other Publications of 1661. London : Pickering & Chatto, (1999)

7. C. Merrit, The Art of Refining, Communicated by Dr. Christopher Merrit. Philosophical Transactions (1665-1678) 12, 1046–1052 (1677)

8. R. Dossie, The Handmaid to the Arts, 2d ed., wi edn. Printed for J. Nourse, London (1764)

9. J. Harris, Lexicon Technicum Or, an Universal English Dictionary of Arts and Sciences: Explaining Not Only the Terms of Art, but the Arts Themselves. Vol. I. Printed for Dan. Browne, Tim. Goodwin, John Walthoe, John Nicholson, Ben. Tooke, Dan. Midwinter, and Tho. Ward, London (1716)

10. C.G. Juste, All that is gold does not glitter: a technical, historical and iconographical study of the central panel of the Great Chamber fireplace, Charterhouse, London, and the figure of Rowland Buckett. Bulletin of the Hamilton Kerr Institute **6**, 66–82 (2016)

11. J. Kirby, *Trade in Painters' Materials in Sixteenth-century London* (Archetype Publications, London, 2010)

12. J. Kirby, The painter's trade in the seventeenth century: Theory and practice. National Gallery Technical Bulletin **20** (1999)

13. I.C. Bristow, *Architectural Colour in British Interiors 1615–1840* (Yale University Press, New Haven, CT, 1996)

14. K. Davies, *Artisan Art: Vernacular Wall Paintings in the Welsh Marches, 1550–1650* (Logaston Press, Woonont, Herefs, 2008)

15. H. Hughes, "...with a feather edge of a Goose-Quill": experimental archaeology at the Little Castle Bolsover. Architectural finishes in the built environment, 123–133 (2009)

16. A. Van Dyck, M. Jaffé, *Van Dyck's Antwerp Sketchbook* (Macdonald I & Co., London, 1966)

17. P. MacTaggart, A. MacTaggart, Refiners' Verditer. Studies in Conservation **25**(1), 37–45 (1980)

18. S. Švarcová, D. Hradil, J. Hradilová, Z. Čermáková, Pigments-copper-based greens and blues. Archaeological and Anthropological Sciences **13**(11), 190 (2021). https://doi.org/10.1007/s12520-021-01406-0

19. R.E. Wilson, *Two Hundred Precious Metal Years; a History of the Sheffield Smelting Company Limited, 1760–1960* (E. Benn, London, 1960)

20. R.J. Gettens, E.W. Fitzhugh, Azurite and Blue Verditer. Studies in Conservation **11**(2), 54–61 (1966). https://doi.org/10.1179/sic.1966.008

21. R.J. Gettens, E.W. Fitzhugh, Malachite and Green Verditer. Studies in Conservation **19**(1), 2–23 (1974)

22. N. Eastaugh, V. Walsh, T. Chaplin, R. Siddall, *Pigment Compendium: A Dictionary and Optical Microscopy of Historical Pigments* (Butterworth-Heinemann, Oxford, 2008)

<figure><img src="image_15.png" /></figure>
23. E.H. Purdy, S. Critchley, A. Kirkham, M. Casford, Illuminating the problem of blue verditer synthesis in the early modern english period: chemical characterisation and mechanistic understanding. *Heritage Science* **12**(1), 138 (2024)

24. D. Bersani, P.P. Lottici, A. Casoli, D. Cauzzi, Pigments and binders in "Madonna col Bambino e S. Giovannino" by Botticelli investigated by micro-Raman and GC/MS. Journal of Cultural Heritage **9**(1), 97–102 (2008) https://doi.org/10.1016/j.culher.2007.05.005

25. G. Heydenreich, A Note on Schifergrün. Studies in Conservation **48**(4), 227–236 (2003)

26. P. MacTaggart, A. MacTaggart, Blue Pigments (2007). https://academicprojects.co.uk/blue-pigments/ Accessed 2023-10-05

27. C.H. Yoder, E. Bushong, X. Liu, V. Weidner, P. McWilliams, K. Martin, J. Lorgunpai, J. Haller, R.W. Schaeffer, The synthesis and solubility of the copper hydroxyl nitrates: gerhardite, rouaite and likasite. *Mineralogical Magazine* **74**(3), 433–440 (2010). https://doi.org/10.1180/minmag.2010.074.3.433

28. A. Loon, L. Speleers, The use of blue and green verditer in green colours in the mid-seventeenth-century paintings of the Oranjezaal, The Hague. In: Spring, M. (ed.) Studying Old Master Paintings: Technology and Practice, pp. 260–268. Archetype Publications, (2011)

29. F. Vanmeert, N. De Keyser, A. Van Loon, L. Klaassen, P. Noble, K. Janssens, Transmission and Reflection Mode Macroscopic X-ray Powder Diffraction Imaging for the Noninvasive Visualization of Paint Degradation in Still Life Paintings by Jan Davidsz de Heem. Analytical Chemistry **91**(11), 7153–7161 (2019)

30. L. Keith, Giulio romano and the birth of jupiter. National Gallery Technical Bulletin **24**, 38–49 (2003)

31. N. De Keyser, G. Snickt, A. Van Loon, S. Legrand, A. Wallert, K. Janssens, Jan Davidsz. de Heem (1606-1684): A technical examination of fruit and flower still lifes combining MA-XRF scanning, cross-section analysis and technical historical sources. *Heritage Science* **5**(1), 38 (2017) https://doi.org/10.1186/s40494-017-0151-4

32. R. Sperber, J. Stenger, Canaletto’s Colour: the inspiration and implications of changing grounds, pigments and paint application in the artist’s English period. British Art Studies (2) (2016) https://doi.org/10.17658/issn.2058-5462

33. L. Burgio, A. Cesaratto, A. Derbyshire, Comparison of English portrait miniatures using Raman microscopy and other techniques. Journal of Raman Spectroscopy **43**(11), 1713–1721 (2012). https://doi.org/10.1002/jrs.4133

34. V. &.A. Museum, Portrait of an unknown man, called Sir Arnold Breams (1999). https://collections.vam.ac.uk/item/O17388/portrait-of-an-unknown-man-portrait-miniature-hilliard-nicholas/ Accessed 2023-10-05

35. A. De Haro, M. Córdova, C. Rua Landa, C. Huck-Iriart, G. Sira-cusano, M.S. Maier, E. Tomasini, Methodologies for the Charac-terization and Identification of Natural Atacamite as a Pigment in Andean Colonial Painting. Heritage **6**(7), 5116–5129 (2023). https://doi.org/10.3390/heritage6070272

36. T. Kosec, H.O. Ćurković, A. Legat, Investigation of the corrosion protection of chemically and electrochemically formed patinas on recent bronze. Electrochimica Acta **56**(2), 722–731 (2010). https://doi.org/10.1016/j.electacta.2010.09.093

37. V. Hayez, T. Segato, A. Hubin, H. Terryn, Study of copper nitrate-based patinas. Journal of Raman Spectroscopy **37**, 1211–1220 (2006). https://doi.org/10.1002/jrs.arXiv:arXiv:1011.1669v3

38. M. Crippa, V. Bongiorno, P. Piccardo, M.M. Carnasciali, A Char-acterisation Study on Modern Bronze Sculpture: The Artistic Pati-nas of Nado Canuti. Studies in Conservation **64**(1), 16–23 (2019). https://doi.org/10.1080/00393630.2018.1492253

39. T.E. Graedel, Copper Patinas Formed in the Atmopshere-II. A Qualitative Assessment of Mechanisms. Corrosion Science **27**(7), 721–740 (1987)

40. J.D. Chelaru, L.M. Mureșan, L. Barbu, T. Kolozsi, S.C. Pînzaru, T. Tamas, Characterization of the corrosion products formed on Michael the Brave's equestrian statue in urban atmosphere. Materials Today Communications **31**, 103565 (2022). https://doi.org/10.1016/j.mtcomm.2022.103565

41. A. Privitera, A. Corbascio, G. Calcani, G. Della Ventura, M.A. Ricci, A. Sodo, Raman approach to the forensic study of bronze patinas. Journal of Archaeological Science: Reports **39**, 103115 (2021). https://doi.org/10.1016/j.jasrep.2021.103115

42. R.G. Herman, C.E. Bogdan, P.L. Kumler, D.M. Nuszkowski, Preparation and characterization of hydroxycarbonate precursors that yield successful alcohol synthesis catalysts. Materials Chemistry and Physics **35**(3–4), 233–239 (1993). https://doi.org/10.1016/0254-0584(93)09137-B

43. S. Guru, S. Kumar, S. Bellamkonda, R.R. Gangavarapu, Synthesis of CuTi-LDH supported on g-C3N4 for electrochemical and photoelectrochemical oxygen evolution reactions. International Journal of Hydrogen Energy **46**(30), 16414–16430 (2021). https://doi.org/10.1016/j.ijhydene.2020.08.102

44. B. Liu, One-dimensional copper hydroxide nitrate nanorods and nanobelts for radiochemical applications. Nanoscale **4**(22), 7194–7198 (2012). https://doi.org/10.1039/c2nr32439c

45. T. Stanimirova, S. Dencheva, G. Kirov, Structural interpretation of anion exchange in divalent copper hydroxysalt minerals. Clay Minerals **48**(1), 21–36 (2013). https://doi.org/10.1180/claymin.2013.048.1.02

46. A. Kirkham, North Mymms Park Mansion, Hertfordshire: Interim Report on the Painted Decoration in the Mymms Room and the Worthies Room. Technical report (2013)

47. C. Krekel, K. Polborn, Lime blue - A mediaeval pigment for wall paintings? Studies in Conservation **48**(3), 171–182 (2003). https://doi.org/10.1179/sic.2003.48.3.171

48. C. Cardell, A. Herrera, I. Guerra, N. Navas, L. Rodríguez Simón, K. Elert, Pigment-size effect on the physico-chemical behavior of azurite-tempera dosimeters upon natural and accelerated photo aging. Dyes and Pigments **141**, 53–65 (2017). https://doi.org/10.1016/j.dyepig.2017.02.001

49. M. Cotte, V. Gonzalez, F. Vanmeert, L. Monico, C. Dejoie, M. Burghammer, L. Huder, W. Nolf, S. Fisher, I. Fazlic, C. Chauf-feton, G. Wallez, N. Jiménez, F. Albert-Tortosa, N. Salvadó, E. Possenti, C. Colombo, M. Ghirardello, D. Comelli, E.A. Clerici, R. Vivani, A. Romani, C. Costantino, K. Janssens, Y. Taniguchi, J. McCarthy, H. Reichert, J. Susini, The “Historical Materials BAG”: A New Facilitated Access to Synchrotron X-ray Diffrac-tion Analyses for Cultural Heritage Materials at the European Synchrotron Radiation Facility. Molecules **27**(6) (2022) https://doi.org/10.3390/molecules27061997

50. M. Gomez Lobon, M. Ghirardello, E. Juncosa Darder, C. Palomino Cabello, M. Bauza, M. Cotte, A. Burnstock, A. Nevin, S.R. Amato, F.C. Izzo, D. Comelli, A study of cadmium yellow paints from Joan Miró's paintings and studio materials preserved at the Fundació Miró Mallorca. *Heritage Science* **11**(1), 1–16 (2023). https://doi.org/10.1186/s40494-023-00987-4

51. N. Oriols, N. Salvadó, T. Pradell, N. Jiménez, M. Cotte, V. Gonzalez, S. Butí, Carbonation of fresco mural paintings with a dolo-mitic mortar. Cement and Concrete Research (2022). https://doi.org/10.1016/j.cemconres.2022.106828

52. G. Ashiotis, A. Deschildre, Z. Nawaz, J.P. Wright, D. Karkoulis, F.E. Picca, J. Kieffer, The fast azimuthal integration Python library: PyFAI. Journal of Applied Crystallography **48**, 510–519 (2015). https://doi.org/10.1107/S1600576715004306

53. V.A. Solé, E. Papillon, M. Cotte, P. Walter, J. Susini, A multiplatform code for the analysis of energy-dispersive X-ray fluorescence
spectra. Spectrochimica Acta - Part B Atomic Spectroscopy **62**(1),
63–68 (2007). https://doi.org/10.1016/j.sab.2006.12.002

54. N. Kourkoumelis, PowDLL, a reusable .NET component for interconverting powder diffraction data: Recent developments. In: O'Neill, L. (ed.) ICDD Annual Spring Meetings, Vol. 28, pp. 137–148 (2013). https://doi.org/10.1017/s0885715613000390

55. B. Lafuente, R.T. Downs, H. Yang, N. Stone, The power of databases: the RRUFF project, in *Highlights in Mineralogical Crystallography*. ed. by T. Armbruster, R.M. Danisi (W. De Gruyter, Berlin, 2015), pp.1–30

56. M. Cotte, E. Pouyet, M. Salomé, C. Rivard, W. De Nolf, H. Castillo-Michel, T. Fabris, L. Monico, K. Janssens, T. Wang, P. Sciau, L. Verger, L. Cormier, O. Dargaud, E. Brun, D. Bugnazet, B. Fayard, B. Hesse, A.E. Pradas Del Real, G. Veronesi, J. Langlois, N. Balcar, Y. Vandenberghe, V.A. Solé, J. Kieffer, R. Barrett, C. Cohen, C. Cornu, R. Baker, E. Gagliardini, E. Papillon, J. Susini, The ID21 X-ray and infrared microscopy beamline at the ESRF: Status and recent applications to artistic materials. Journal of Analytical Atomic Spectrometry **32**(3), 477–493 (2017). https://doi.org/10.1039/c6ja00356g

57. M. Yuan, J. Bonet, H. Castillo-Michel, M. Cotte, N. Schibille, B. Gratuze, T. Pradell, The role of tin and iron in the production of spanish copper red glass from the 19th to the 20th century. Boletin de la Sociedad Espanola de Ceramica y Vidrio (2023). https://doi.org/10.1016/j.bsecv.2023.07.005

58. L. Monico, M. Cotte, F. Vanmeert, L. Amidani, K. Janssens, G. Nuyts, J. Garrevoet, G. Falkenberg, P. Glatzel, A. Romani, C. Miliani, Damages Induced by Synchrotron Radiation-Based X-ray Microanalysis in Chrome Yellow Paints and Related Compounds: Assessment, Quantification, and Mitigation Strategies. Analytical Chemistry **92**(20), 14164–14173 (2020). https://doi.org/10.1021/acs.analchem.0c03251

59. L. Bertrand, S. Schöeder, D. Anglos, M.B.H. Breese, K. Janssens, M. Moini, A. Simon, Mitigation strategies for radiation damage in the analysis of ancient materials. TrAC - Trends in Analytical Chemistry **66**(2015), 128–145 (2015). https://doi.org/10.1016/j.trac.2014.10.005

60. M. Newville, Larch: An analysis package for XAFS and related spectroscopies. Journal of Physics: Conference Series **430**(1), 7 (2013). https://doi.org/10.1088/1742-6596/430/1/012007

61. N. Salvadó, T. Pradell, E. Pantos, M.Z. Papiz, J. Molera, M. Seco, M. Vendrell-Saz, Identification of copper-based green pigments in Jaume Huguet's Gothic altarpieces by Fourier transform infrared microspectroscopy and synchrotron radiation X-ray diffraction. Journal of Synchrotron Radiation **9**(4), 215–222 (2002). https://doi.org/10.1107/S0909049502007859

62. H.G.M.Edwards, D.W. Farwell. The conservational heritage of wall paintings and buildings: An FT-Raman spectroscopic study of prehistoric, Roman, mediaeval and Renaissance lime substrates and mortars. Journal of Raman Spectroscopy 39(8 SPEC. ISS.), 985–992 (2008) https://doi.org/10.1002/jrs.1917

63. M. Pérez-Alonso, K. Castro, J.M. Madariaga, Investigation of deg-
radation mechanisms by portable Raman spectroscopy and thermo-
dynamic speciation: The wall painting of Santa María de Lemoniz
(Basque Country, North of Spain). Analytica Chimica Acta **571**(1),
121–128 (2006). https://doi.org/10.1016/j.aca.2006.04.049

64. A. Lluveras, S. Boularand, A. Andreotti, M. Vendrell-Saz, Degradation of azurite in mural paintings: Distribution of copper carbonate, chlorides and oxalates by SRFTIR. Applied Physics A: Materials Science and Processing **99**(2), 363–375 (2010). https://doi.org/10.1007/s00339-010-5673-5

65. G. Cavallo, Alteration of azurite into paratacamite at the St. Ales-
sandro Church (Lasnigo, Italy). Conservar Património 9(9), 5–11
(2009) https://doi.org/10.14568/cp9_2

66. D. Hradil, P. Bezdička, J. Hradilová, V. Vašutová, Microanaly-
sis of clay-based pigments in paintings by XRD techniques.

Microchemical Journal **125**, 10–20 (2016). https://doi.org/10.1016/j.microc.2015.10.032

67. C. Nunes, A. Mahendrasingam, R. Suryanarayanan, Quantification of crystallinity in substantially amorphous materials by synchrotron X-ray powder diffractometry. Pharmaceutical Research **22**(11), 1942–1953 (2005). https://doi.org/10.1007/s11095-005-7626-9

68. M. McGrath, The Materials and Techniques of Irish Medieval Wall-paintings. Journal of the Royal Society of Antiquaries of Ireland **117**(1987), 96–124 (1987)

69. E. Possenti, C. Colombo, C. Conti, L. Gigli, M. Merlini, J.R. Plaisier, M. Realini, G.D. Gatta, What's underneath? A non-destructive depth profile of painted stratigraphies by synchrotron grazing incidence X-ray diffraction. Analyst **143**(18), 4290–4297 (2018). https://doi.org/10.1039/c8an00901e

70. V. Gonzalez, M. Cotte, F. Vanmeert, W. Nolf, K. Janssens, X-ray Diffraction Mapping for Cultural Heritage Science: a Review of Experimental Configurations and Applications. Chemistry - A European Journal **26**(8), 1703–1719 (2020). https://doi.org/10.1002/chem.201903284

71. V. Gonzalez, T. Calligaro, G. Wallez, M. Eveno, K. Toussaint, M. Menu, Composition and microstructure of the lead white pigment in Masters paintings using HR Synchrotron XRD. Microchemical Journal **125**, 43–49 (2016). https://doi.org/10.1016/j.microc.2015.11.005

72. J.V.A. De Boer, An examination of particle size distributions of azurite and natural ultramarine in some early netherlandish paintings. Studies in Conservation, 233–243 (1974)

73. D. Saunders, J. Kirby, The Effect of Relative Humidity on Artists' Pigments. National Gallery Technical Bulletin **25**, 62–72 (2004)

74. M. Price, A Renaissance of Color: Particle Separation and Preparation of Azurite for Use in Oil Painting. Leonardo **33**(4), 281–288 (2000). https://doi.org/10.1162/002409400552667

75. R. Punis, A. Zoleo, Cu(II)-binder complexes in azurite and malachite pictorial mixtures: An EPR study. Microchemical Journal **200**(March), 110303 (2024). https://doi.org/10.1016/j.microc.2024.110303

76. A. Coccato, L. Moens, P. Vandenabeele, On the stability of mediaeval inorganic pigments: A literature review of the effect of climate, material selection, biological activity, analysis and conservation treatments. Heritage Science **5**(1), 1–25 (2017). https://doi.org/10.1186/s40494-017-0125-6

77. S. Švarcová, D. Hradil, J. Hradilová, E. Kočí, P. Bezdička, Microanalytical evidence of origin and degradation of copper pigments found in Bohemian Gothic murals. Analytical and Bioanalytical Chemistry **395**(7), 2037–2050 (2009). https://doi.org/10.1007/s00216-009-3144-7

78. L. Dei, A. Ahle, P. Baglioni, D. Dini, E. Ferroni, Green degradation products of azurite in wall paintings: Identification and conservation treatment. Studies in Conservation **43**(2), 80–88 (1998). https://doi.org/10.1179/sic.1998.43.2.80

79. J.B. Sharkey, S.Z. Lewin, Conditions governing the formation of atacamite and paratacamite. The American Mineralogist **56**, 179–192 (1971)

80. D.A. Scott, A Review of Copper Chlorides and Related Salts in Bronze Corrosion and as Painting Pigments. Studies in Conservation **45**(1), 39–53 (2000)

<figure><img src="image_16.png" /></figure>