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Current Research on Artificial Arsenic Sulphide Pigments in Artworks: Ashort Review

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Current Research on Artificial Arsenic Sulphide Pigments in Artworks: Ashort Review CONSERVATION OF CULTURAL HERITAGE 903 doi:10.2533/chimia.2008.903 CHIMIA 2008, 62,No. 11 Chimia 62 (2008) 903–907 ©Schweizerische Chemische Gesellschaft ISSN 0009–4293 Current Research on Artificial Arsenic Sulphide Pigments in Artworks: AShort Review Günter Grundmann*a and Mark Richterb Abstract: Ageneral re-examination of the artificial arsenic sulphide pigments orpiment, realgar and alacranite using polarised light microscopy (PLM), scanning electron microscopy (SEM) combined with energy and wavelength dis- persive X-ray analysis (EDX, WDX) and X-ray diffraction analysis (XRD) revealed the following results: wet-process precipitation products of artificial orpiment consist of golden yellow,amorphous, spherical particles ranging from 0.2 to 2 µmØ.Dry-process roasting and/or sublimation products with an arsenolite/sulphur mixtureconsist mainly of amorphous golden yellow oval drops and spherules of amorphous arsenic sulphide glass (g-AsxSx)with a smooth surface ranging from 1to20µmØinassociation with arsenolite crystal aggregates and members of the alacranite solid solution series (As8S8)–(As8S9). Abright lemon yellow to orange-red arsenic sulphide pigment on a sixteenth/seventeenth-century South German polychrome recumbent sculptureand aseventeenth-century Dutch painting attributed to Rembrandt’sstudio has been identified as an artificial orpiment produced with dry-process (roasting/sublimation) methods. The recumbent figurecontains (i) yellow,orange or red-brown amorphous splintery fragments and spherules of arsenic sulphide glass of up to 25 µm Ø, (ii) yellow,brown or red-brown crystalline spherules and splintery fragments of alacranite, (iii) lemon yellow to red-brown agglomerates of arsenic sulphide glass, and (iv) colourless irregular fragments or octahedral crystals of arsenolite. This type can be classified as ‘complex artificial orpiment’. The arsenic sulphide pigment in the yellowish paint layers of the Rembrandt studio painting proved to be very uniform bright yellow spherules (max. 9 µmØ)ofarsenic sulphide glass, which can be classified as ‘purified artificial orpiment glass’. Adeep redhistoric arsenic sulphide cake from the collection of the Landesmuseum Joanneum (Graz, Austria) called ‘Realgar’ turned out to be an amorphous arsenic sulphide glass matrix (approx. g-As2S3)filled with numerous redcrystal aggregates of alacranite (approx. As8S9)inthe form of spherules, which fill gas bubbles and pores with an average 3 µmØ(max. 20 µmØ). Avery fine-grained light redarsenic sulphide powder called ‘Realgar’ from an archaeological excavation of ahistoric arsenic smelter near Strassegg (Steiermark, Austria) turned out to be amember of the alacranite solid solution series (As8S8)–(As8S9) in the form of idiomorphic crystals and complex twinned crystal aggregates (average 10 µmØ,max. 30 µmØ). All red-colored crystals or crystal aggregates of artificial arsenic sulphides that have been called ‘realgar’ in the past areinfact members of the alacranite solid solution series. The occurrence of pararealgar as asecondary photo- chemical reaction product is not exclusively linked with realgar,but can also form on members of the alacranite solid solution series (As8S8)–(As8S9). Keywords: Alacranite ·Artificial arsenic sulphide pigments ·Orpiment ·Painting ·Polychromy ·Realgar Introduction incentive to replace the bright yellowpig- ment with an equally homogeneous artifi- The minerals orpiment and realgar belong cial product in Antiquity.The production to asmall group of sulphides with anon- numbers of arsenolite smelters (production metallic character.Both were considered of synthetic arsenolite =arsenic trioxide = important pigments and were in use until As2O3)aswell as the distribution and eco- *Correspondence: Dr.rer.nat. G. Grundmanna the end of the nineteenth-century.[1] The nomic use of their products revealed that aTechnische Universität München natural occurrences, however, neversup- hundreds of tonnes of artificially prepared Lehrstuhl für Ingenieurgeologie Arcisstr.21 plied enough material in order to be able yellowand red arsenic sulphides, besides D-80333 München to even roughly meet the large demand of the main product artificial arsenolite, must Tel.: +49 89 289 25831 the golden yellowand red colorants. The have been available during aperiod of Fax: +49 89 289-25852 foliaceous, mica-liketough consistencyof more than 350 years ending with World E-Mail: [email protected] [2] bTechnische Universität München the coarse orpiment required agreat deal WarII. Beyond this its use as aversatile Lehrstuhl für Restaurierung, Kunsttechnologie und of effort during pulverising and grinding. colorant is documented in historical docu- Konservierungswissenschaft Its sparse occurrence as adeposit and the mentary sources (e.g.treatises, handbooks, DFG Research Project ‘Polychromy’ Oettingenstr.15 problems encountered by the preparation manuals, encyclopaedias and pharmacy D-80538 München of apure product might have been the true price-lists). According to Wallert[3] and CONSERVATION OF CULTURAL HERITAGE 904 CHIMIA 2008, 62,No. 11 FitzHugh[4] only orpiment, realgar,para- length of up to several millimetres. Prior to namic reasons it appears improbable that ar- realgar and alacranite (see Popova et al.[5]) the first description of alacranite by Popova tificial orpiment crystals with the properties have been identified on cultural property. et al.[5] it wascalled ‘natural and synthetic of natural orpiment can develop in arsenic Wallert[3] and FitzHugh[4] also supplied the arsenic sulphide’ realgar and classified as sulphide cakes which solidify rapidly. [11] first approaches for the identification of ar- ‘alpha-As4S4’byClark, as ‘beta-As4S4- tificial orpiment and realgar in modern art unnamed’by Porter and Sheldrick[12] and as [13] technology.Compared to realgar the very ‘As2S2,15’byKothiyal and Gosh. Howev- Microscopic, Diffractometric similar and in nature very rare arsenic sul- er,Bonazzi et al.[14] discovered that the pre- and Chemical Characteristics of phide alacranite wasidentified with X-ray fixesalpha and beta were used inconsistent- Artificial Arsenic Sulphides diffraction (XRD) and X-ray fluorescence ly in these works, and therefore designated (XRF) in various historical samples of the natural low-temperature form (realgar) Atotal of eight varieties of artificial [4] artificial arsenic sulphide pigments. Al- as ‘alpha-As4S4’and the natural high tem- arsenic sulphides from historic and mod- though the morphology,chemical composi- perature form[11] and/or synthetic high tem- ern synthesis products were studied with [12] tion and crystal structure of natural arsenic perature form as ‘beta-As4S4-unnamed’. polarised light microscopy, scanning elec- sulphides are well known, their artificial In addition the investigations of Bonazzi tron microscopy, X-ray diffraction as well manufactured equivalents are largely con- et al.[14] also clarified for the first time the as electron beam microprobe methods and sidered unexplored. Even the newest ency- complicated structural relations between compared to natural orpiment. Five syn- clopaedias and books for the determination the natural and synthetic products using the thesis test series for the preparation of ar- of artists’ pigments contain contradictions general formula As4S4 and the natural alac- tificial orpiment were carried out under at- in viewofX-ray diffraction and the chemi- ranite sensu stricto As8S9.According to a mospheric conditions to produce on the one cal and microscopic characteristics of ar- comparison of the elementary cell volumes hand precipitations with the wet-process senic sulphides. In manycases this litera- of afurther newdiscovery of alacranite sen- methods using (I) –thioacetamide and (II) ture does not clearly differentiate between su stricto (Katerˇina Mine, Czech Republic) –hydrogen sulphide, and on the other hand IMA-provedmineral species names (veri- Bonazzi et al.[14] come to the conclusion cakes and/or sublimates with the dry-proc- fied by IMA =International Mineralogical that acontinuous solid solution series must ess methods using (III) –natural orpiment, Association), which are naturally in acrys- be present between the high temperature (IV) –natural orpiment and additional syn- talline state (e.g. realgar), and their isotropic form (‘beta-As4S4-unnamed’) and the min- thetic arsenolite, and (V) –substance mix- [6] [2] amorphous artificial equivalents. eral alacranite sensu stricto (As8S9). This tures of synthetic arsenolite and sulphur. In this article the basic criteria for the makes it possible to assume that manyifnot The wet-process precipitated product distinction of the natural and artificial ar- all of the substances that were earlier called obtained using hydrogen sulphide (II) re- senic sulphides orpiment, realgar and alac- ‘alacranite’, ‘artificial realgar’, ‘rotes Ar- vealed golden yellow, spherical arsenic ranite are presented and made possible with senglas’ or ‘Rotglas’ are in fact members of sulphide particles with an average size of the discovery and investigation of historical the alacranite solid solution series (As8S8)– 0.2 µm(maximum 0.4 µm), which are in an and modern reference samples. (As8S9). If the cell volumes between these order of magnitude smaller than the golden twocrystal structures shift, then the dimen- yellow, spherical arsenic sulphide particles sions of the unit cells and accordingly the obtained using thioacetamide (I) with an The Arsenic-Sulphur System diffraction angles (2 theta, Cu-K-alpha) as average size of
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