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 photochemical 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 1 µm(maximum 2 µm). well as the similar d-values (lattice plane The dry-process synthesis products The As-S system includes three con- distances) also change significantly.The obtained from various starting materials gruently melting stable crystalline phases optical properties of the alacranite solid so- including natural orpiment (III, IV) are at arsenic/sulphur ratios of 4:3, 4:4 and lution series (As8S8)–(As8S9)and realgar generally amixture of colourless, octahe- [7] 2:3. The natural mineral equivalents are (As4S4)are almost identical. Thus it is not dral arsenolite crystals (As2O3)and yellow dimorphite (As4S3), realgar or pararealgar possible to allocate these arsenic sulphides to brown glassy arsenic sulphide particles (As4S4), and orpiment (As2S3). The arsenic accurately without proper X-ray diffraction (As2S3)with achemical composition iden- sesquisulphide, As2S3,occurs in three and/or crystal structural analytical data. tical to natural orpiment. This mixture often [7] crystalline polymorphs: α, β and γ. The In the phase diagram of the binary Asx- contains crystalline lamellar-foliated relicts [15] [7] β and γ polymorphs are only stable under Sx system (Massalski, Emelina et al. ) of natural orpiment. Natural orpiment pos- high pressure conditions. The α-phase is the transition temperature from liquidus to sesses apronounced layer-lattice structure equivalent to the mineral orpiment and un- solidus of the arsenic sulphides realgar and similar to the minerals talc, graphite and dergoes no polymorphic transformations orpiment is around 318 ºC, whereas, for molybdenite. The crystalline character of during heating under normal conditions. dimorphite Iand II the transition tempera- natural orpiment is verifiable both with po- With amelting temperature at 580 ±3K ture is around 200 ºC. Emelina et al. (2007) larised light microscopy(due to the anisot- (318 ºC) realgar and orpiment are reported concluded that crystalline As4S3,As4S4and ropyeffects and interference colours) and to sublimate congruently,which means that As2S3 sublimate congruently and that two X-ray diffraction methods (XRD). Natural during sublimation its chemical composi- liquid-vapour azeotropes exist in the As-S orpiment particles are characterised by tion does not change.[7] system.[7] This azeotropic behaviour could their typical angular and lamellar-foliated The natural arsenic sulphide realgar is be an explanation for the often observed morphology.Ductile deformation struc- [8–10] the low-temperature form of ‘As4S4’. complexmixture of amorphous arsenic sul- tures observed on single particles include The red mineral alacranite, which is very phide glasses, alacranite and dimorphite Iin compressed corners, edges and tight folds, similar to realgar (idealised chemical for- artificial arsenic sulphide cakes. The high which sometimes result in complexcreased mula As8S9,type locality Alacran mine in viscosity of arsenic sulphide melts, which structures. Kamchatka) is in comparison rarely found have nearly the same chemical composition Artificial orpiment glass, manufac- in nature and occurs only in microscopical- as natural orpiment, drastically reduces the tured by the dry-process roasting method ly small quantities.[5] Derivedfrom burning nucleation rate of artificial orpiment during exclusively with finely pulverised natural coal beds or pit fires the tabular alacranite rapid cooling. The behaviour of this type of orpiment (III), normally forms ayellowish- crystals, which often occur together with molten arsenic sulphide suspends the new brown, glassy mass or cakeand/or subli- the mineral arsenolite (As2O3)can reach a growing orpiment crystals. Forthermody- mates whose glass proportion appear iso- CONSERVATION OF CULTURAL HERITAGE 905 CHIMIA 2008, 62,No. 11 tropic and amorphous using polarised light at athickness of 10 µm) are overlapped by cial orpiment on works of art which specifi- microscopy(PLM) and X-ray diffraction internal reflections, which are especially cally includes asixteenth century (?) South (XRD). The sublimate consists of yellow visible on the edges of thin splinters when German polychrome wooden sculpture and smooth, round drops and spherules, as is the viewed under crossed polarisers. X-ray dif- aseventeenth century Dutch painting attrib- case with the other sublimates of artificial fraction analysis of orange-yellow, mica- uted to Rembrandt’sworkshop.[16] orpiment. ceous, crusty fragments on the orange-red In the case of the polychrome recum- Artificial arsenic sulphide particles ob- alacranite crystal surfaces of the Harry von bent sculpture the analysis of the spherulitic tained by the dry-process methods of roast- Eckermann specimen resulted in parareal- particles and splintery fragments within the ing and sublimating amixture of artificial gar. Yellowfragments of this transition crust orange-yellowpreparatory layer with SEM- arsenolite and natural orpiment (IV) are showthe typical optical characteristics of EDX revealed twodifferent compositions made up of the same components as de- pararealgar.These pararealgar flakes on the (Type 1 and Type 2). Light microscopy(cross scribed in the previous cases. alacranite are interpreted as asecondary sections)andpolarisedlightmicroscopy(dis- Artificial arsenic sulphide particles pro- photochemical reaction product resulting persed samples) revealed that the spherules duced by sublimation with arsenolite and from alight-induced phase transformation of Type 1 and Type 2 exhibit adistinct colour sulphur (V) consist mainly of amixture of as is already known by natural realgar. zoning structure from core to rim as well as colourless transparent arsenolite crystal ag- Furthermore, modern, red to orange four segments of ablack cross appearing in- gregates, often as sharp-edged octahedra, yellowartificial sulphides cakes from the side the outer rim of the spherules (Fig. 1). and golden-yellowtoyellow-brown, highly production of Chinese arsenic smelters Another important discovery of the orange- refractive,ovaldrops and spherules with a were identified as members of the alacran- yellowpreparatory layer of the polychrome smooth surface. With the aid of polarised ite solid solution series with subordinations sculpture are the red-brown spherules and/or light microscopythese glassy particles gen- of dimorphite Iand II.[2] After exposure to splintery (anisotropic, crystalline) fragments erally appear isotropic and are X-ray amor- amixture of daylight and artificial light for of alacranite that have falsely been described phous. several months the colour of the red sur- in the sources as ‘artificial realgar’ or ‘red The products of the various dry-process face altered from light red to orange-yellow. orpiment’. methods revealed yellowspherical particles The penetration depth of the colour change In the case of the painting attributed to with an average size of 4 µm(maximum reached approximately 0.3 mm after one Rembrandt’sworkshop it is striking that all 12 µm). The sublimated spherules can year and afine grained orange-yellowpho- artificial orpiment particles are spherules only be clearly distinguished from those tochemical reaction product with micaceous without anytraces of arsenolite. These par- of the wet-process methods if (apart from flakes of up to 1mminsize detached from ticles are almost uniform in size, which may clumped aggregates) the particle size of the the surface. The orange-yellowflakedevel- suggest that only acarefully washed and yellowamorphous arsenic sulphide com- opment exhibiting the optical characteris- levigated sublimate seems to have been used ponents is more than 2 µm. All particles tics of pararealgar wasalso confirmed by as apigment. The spherules exhibit identi- smaller than 2 µmlook so similar to each X-ray powder diffraction as pararealgar. cal optical properties (colour zonation, four other that neither the use of polarised light segments of ablack cross appearing inside microscopy(PLM) nor scanning electron the outer rim) as the spherulitic particles microscopy(SEM) can consistently distin- Evidence of Artificial Arsenic identified in the orange-yellowpaint layer guish between the products of the dry and Sulphides in Painting and of the recumbent figure. Striking is the al- wet-process methods. Polychromy most identical chemical composition of It appears improbable that both the dry these spherules with the Type 1 particles of and wet-process synthesis tests with roast- On the basis of these newresults Richter the recumbent figure of St. Alto, which sug- ing and/or sublimation under atmospheric et al. securely identified adry-process artifi- gests the use of avery similar dry-process conditions can produce crystal types with method to produce amorphous arsenic sul- the characteristic unisotropic properties of phide spherules. The use of artificial orpi- natural orpiment. All synthesis products of ment has until nownot been found before in the dry-process methods where natural orpi- Rembrandt’spalette, or in those of his pu- ment (As2S3)was not used as astarting ma- pils/assistants. The occurrence of orpiment terial are generally amixture of crystalline wasonly reported in Rembrandt’swork arsenolite (As2O3)and amorphous arsenic once, in Simeon’sSong of Praise from 1631 sulphide glass (g-As2S3)with achemical (The Hague, Mauritshuis, inv. Nr.145), but composition corresponding largely to natu- here the natural variant wasused.[17] The ral orpiment. These products sometimes finding of synthetic orpiment in this paint- contain traces of members of the alacranite ing is even more remarkable, since until this solid solution series (As8S8)–(As8S9). point it has only sporadically shown up in Orange-red lentil-shaped crystals of up old master paintings. Interesting is another to 5mminlength on ahistoric sample of recent discovery by Wallert and Dik of ball- ‘artificial realgar’ from the collection of the Fig. 1. Polychrome recumbent figureofSt. Alto, shaped arsenic sulphide particles in ayellow Technische Universität München (special mitreband, paint cross section with layer build-up layer in aseventeenth century painting by collection of the Swedish mineralogist Dr. of oldest polychromy.Layer build-up from bottom Jan Davidsz De. Heem (Still Life of Flowers Harry vonEckermann, 1886–1969) ana- to top: 1) bright lead white ground; 2) bright and Fruit with Tazza and Birds,oil on can- lysed by X-ray diffraction turned out to be yellow artificial orpiment layer with spherules vas, early 1670s, private collection), which amember of the alacranite solid solution (preparatory layer or mordant of gilding); 3) wasinterpreted as adeterioration product. gilding; 4) various overpaintings. The largest series (As S )–(As S ). Microscopic fea- [18] Neither X-ray diffraction (XRD) nor 8 8 8 9 spherule measures approx. 30 µm. Incident tures of the translucent to clear transparent light, dark field. Photographer: Dipl.-Rest. electron diffraction analysis could identify [18,19] crystal fragments include aclear pleochr- Mark Richter,Technische Universität München, anyphases in acrystalline state. This oism from orange-red to red-brown with an Lehrstuhl für Restaurierung, Kunsttechnologie example of aspherical amorphous arsenic extremely high refractive index.[2] The in- und Konservierung, DFG Research Project sulphide can be interpreted as adry-process terference colours (light green to light blue ‘Polychromy’. product as described above. CONSERVATION OF CULTURAL HERITAGE 906 CHIMIA 2008, 62,No. 11

glass refer specifically to the solidified arsenic sulphide particles, which are based on the recent thesis by Georgiev[20] and the study by Grundmann and Rötter.[2] Newinvestigations on ahistorical sample of an artificial arsenic sulphide termed ‘Realgar’; the so-called German ‘Rotglas’ from the mineral collection of the Landes- museum Joanneum in Graz, Austria (Figs. 2–5) turned out to be amixture of extremely fine-grained alacranite spherules in an amorphous arsenic sulphide glass matrix and is interpreted as an authentic historical sample of artificial ‘realgar’. The spherulitic alacranite fills the pores of solidified gas bubbles in the amorphous arsenic sulphide cake. According to the museum records this Fig. 5. SEM image of the surface of the artificial deep red artificial arsenic sulphide cakewas arsenic sulphide cake (Specimen number 07223, Fig. 2. Fragment of artificial arsenic sulphide cake produced in the famous arsenic smelter of Joanneum collection in Graz, Austria). The ‘Realgar,Rotglas’ (scale bar in cm) produced in Muldenhütten near Freiberg(Saxony, Ger- surface revealed gas bubbles, which aremore the famous arsenic smelter of Muldenhütten many) before 1867. This specimen was or less filled by alacranite spherules (approx. near Freiberg, Saxony (Germany) before1867. documented in 1867/68 as part of the inven- As8S9)developing in radial crystal aggregates This specimen was documented in 1867/68 as tory (No. 07223) of the Joanneum collec- protruding from the fracturesurface of the arsenic sulphide glass matrix (approx. As S ). part of the inventory of the Joanneum collection tion in Graz, Austria. The sample consists 2 3 in Graz, Austria. Photographer: Daniel Modl, Photographer: Dipl.-Min. Klaus Rapp, Analytical of macroscopic homogeneous, deep red, Consultant, Munich. Landesmuseum Joanneum, Graz. glassy fragments with conchoidal fractures on the surfaces. Microscopic investigations revealed adistinct fluidal texture outlined ranite spherules resulted in an average com- by the numerous gasbubbles, which are position of 31.9 (+/– 2%) weight %sulphur more or less filled by alacranite spherules and 68.1 (+/– 2%) weight %arsenic, which (Figs. 3–5). These clearly exhibit undulous largely corresponds to the composition of extinction with light green to light blue- alacranite sensu stricto (As8S9). greyinterference colours at athickness of Microscopic and X-ray powder diffrac- 10 µm(under crossed polarisers), which re- tion analyses of another historic smelter sults from radial grown crystal aggregates sample of artifical ‘realgar’ from an archae- (Figs. 3and 4). Rare opaque inclusions are ological excavation near Strassegg (Steier- an association of iron sulphide (pyrite?) and mark, Austria) by the Bundesdenkmalamt zinc sulphide (sphalerite?). Element analy- and Landesmuseum Joanneum (Graz, Aus- ses (SEM-EDX) of the amorphous arsenic tria) resulted in an authentic member of the Fig. 3. Artificial arsenic sulphide cake Realgar sulphide glass matrix resulted in an average alacranite solid solution series. The sample ‘Realgar,Rotglas’, polished thin section with a chemical composition (resulting from three consists of numerous very small light red thickness of 10 µm, transmitted light 1Polar, measurements) of 37.1 (+/– 2%) weight crystals and crystal aggregates of up to 30 width of image: 0.12 mm. Round orange-red %sulphur and 62.9 (+/– 2%) weight % µmindiameter.Microscopic features of particles of alacranite floating in an orange- arsenic, which largely corresponds to the the translucent to clear transparent crys- redmatrix of arsenic sulphide glass (approx. composition of orpiment glass.[2] The alac- tals include adistinct pleochroism from g-As S ). Photographer: Dr.Günter Grundmann, 2 3 orange-red to red-brown with an extremely Technische Universität München, Lehrstuhl für Ingenieurgeologie. high refractive index. There are also indica- tions that this particular smelter produced an extremely fine grained, bright-yellow, amorphous artificial arsenic sulphide (aver- The very complexmixture of various age size 0.5 µm) with wet-process methods. constituents of artificial orpiment identified Further investigations are in progress. in the orange-yellowpreparatory layer of the polychrome recumbent figure can be classified as the product of multiple synthesis Conclusions and Outlook procedures e.g. roasting and/or sublimating and grinding. Therefore, it should be called The comparative studies of historical complexartificial orpiment.Inthe case of and modern synthetic arsenic sulphide pig- the Rembrandt studio painting it is sug- ments as well as the descriptions and reci- gested that this very uniform and pure type Fig. 4. Artificial arsenic sulphide cake ‘Realgar, pes used to manufacture arsenic sulphide of spherulitic arsenic sulphide glass should Rotglas’, polished thin section with athickness pigments in historical documentary sources be classified as purified artificial orpiment of 10 µm, transmitted light crossed polarisers, have proventobesignificant for the iden- glass.Itshould be noted that both terms pu- width of image: 0.12 mm. Round particles with tification and abetter understanding of this light green to light blue-gray interference colours rified artificial orpiment glass and complex and undulous extinction arealacranite (approx. very complexand rarely found colorant. artificial orpiment refer specifically to the Due to the great complexity of the artifi- As8S9). Photographer: Dr.Günter Grundmann, components present in each sample. The Technische Universität München, Lehrstuhl für cial arsenic sulphide components which are terms orpiment glass and arsenic sulphide Ingenieurgeologie. caused by anumber of reasons including CONSERVATION OF CULTURAL HERITAGE 907 CHIMIA 2008, 62,No. 11 the choice of starting materials, production with realgar,but can also form on mem- [8] H. T. Hall, ‘The system Ag-Sb-S, Ag- method (condition of formation), prepara- bers of the alacranite solid solution series As-S, and Ag-Bi-S: Phase relations and mineralogical significance’, PhD Thesis, tion procedures (purification, classification (As8S8)–(As8S9). etc.)and the preparation of the pigment for Brown University,Providence, Rhode painting (e.g.pulverisation, grinding) it is Acknowledgements Island, 1966. We are very grateful to Daniel Modl [9] G. W. Roland, The Canadian Mineralogist to be expected that the future analyses of ar- 1972, 11,520. tificial arsenic sulphides in historical sam- (Abteilung Archäologie, Landesmuseum Joan- neum, Graz, Austria) and Dr.Hans-Peter [10] R. Blachnik, A. Hoppe, U. Wickel, Z. ples (e.g.mineral collections) and works of Anorg. Allg.Chem. 1980, 463,78. art will result in awide range of compo- Bojar (Abteilung Mineralogie, Landesmuseum Joanneum, Graz, Austria) for providing the [11] A. H. Clark, Amer.Mineral. 1970, 55, sitions. It should be pointed out that until spectacular historical samples of artificial 1338. nowartificial arsenic sulphide pigments arsenic sulphides. We are also very grateful [12] E. J. Porter,G.M.Sheldrick, J. Chem. prepared by the wet-process methods have to Dipl.-Min. Klaus Rapp (Munich) for his Soc. Dalton Trans. 1972, 13,1347. not been identified in artworks.[4] analytical support with SEM-EDX. This study [13] G. P. Kothiyal, B. Gosh, J. Crystal Growth Grundmann and Rötter suggested that is supported by the research project ‘The 1976, 32,29. only arsenic sulphide particles molten and Polychromy of Baroque and Rococo Sculptures [14] P. Bonazzi, L. Bindi, F. Olmi, S. Menchetti, and Retables in Germany’ (Technische Eur.J.Mineral. 2003, 15,283. solidified from natural orpiment (demon- [15] T. B. Massalski, ‘Binary AlloyPhase strated by natural orpiment inclusions in the Universität München) which is funded by the Deutsche Forschungsgemeinschaft (DFG). Diagrams’, Eds. T. B. Massalski, H. glassy masses) are to be referred to as orpi- Okamoto, Vol. 1, ASM International, ment glass (g-As2S3, isotropic and X-ray- Received: October 3, 2008 1990. amorphous).[2] All other solidified arsenic [16] M. Richter,G.Grundmann, A. van sulphide particles derivedfrom the cake [1] C. Rötter, Restauro. Zeitschrift für Loon, K. Keune, A. Boersma, K. Rapp, and/or sublimate of an arsenolite and sul- Kunsttechniken, Restaurierung und ‘The occurrence of artificial orpiment phur mixture (using different ratios) and/or Museumsfragen 2003, 6,114. (dry process) in northern European natural orpiment (without natural orpiment [2] G. Grundmann, C. Rötter,‘Artificial painting and polychromy and evidence inclusions) are to be called arsenic sulphide Orpiment’: Microscopic, Diffractometric in historical sources’, in Carolin Rötter et al. Auripigment –Studien zu dem glass (g-As S ,isotropic and X-ray-amor- and Chemical Characteristics of x x Synthesis Products’, Carolin Rötter et al. Mineral und den künstlichen Produkten phous). This terminology is based on the Auripigment –Studien zu dem Mineral =Orpiment –Studies on the mineral very recent thesis by Georgievwho studied und den künstlichen Produkten =Orpiment and the artificial products [= Materialien the binary chalcogenides including the As– –Studies on the mineral and the artificial aus dem Institut für Baugeschichte, Ssystem.[20] The successful use of Raman products [= Materialien aus dem Institut Kunstgeschichte und Restaurierung mit spectroscopybyGeorgievisinour case ex- für Baugeschichte, Kunstgeschichte und Architekturmuseum der Technischen Uni- tremely significant since it shows that this Restaurierung mit Architekturmuseum der versität München, Eds. M. Schuller,E. method is an excellent option for the future Technischen Universität München, Eds. Emmerling, W. Nerdinger], Anton Siegl, differentiation between natural arsenic sul- M. Schuller,E.Emmerling,W. Nerdinger], Fachbuchhandlung GmbH, München phides and arsenic sulphide glasses. Anton Siegl, Fachbuchhandlung GmbH, 2007,pp. 167–192. [17] P. Noble, A. vanLoon, Art Matters From the observations and results ob- München, 2007. [3] A. Wallert, Maltechnik-Restauro90,1984, Netherlands Technical Studies in Art tained from the newly discovered histori- 4,45. 2007, 4,19. cal and modern samples of red artificial [4] E. W. FitzHugh, in ‘Artists’ Pigments [18] A. Wallert, J. Dik, Zeitschrift für arsenic sulphide crystals and/or amor- –AHandbook of Their History and Kunsttechnologie und Konservierung phous glass cakes it can be concluded that Characteristics’, Ed. E. W. FitzHugh, Vol. 2007, 21,38. most if not all of the so-called ‘artificial 3, 1997,pp. 47–79. [19] L. Sheldon, S. Woodcock, A. Wallert, realgar’, ‘Rotglas’, ‘red arsenic’, ‘roter [5] V. I. Popova,V.A.Popov, A. Clark, V. O. ‘Orpiment overlooked-expect the unex- Arsenik’, ‘künstliches Realgar’, ‘Rubin- Polyakov,S.E.Borisovski, Zap. Vses. pected in 17th century workshop prac- glas’, ‘Rauschrot’, ‘Rubinschwefel’, ‘rotes Mineral. Obshchest. 1986, 115,360, in tice’, ICOM Committee for Conservation Rauschgelb’, ‘red orpiment’, ‘rotes oper- Russian. 14th Triennial Meeting, The Hague, 2005, poster,p.529. ment’etc. will turn out to contain members [6] N. Eastaugh, V. Walsh, T. Chaplin, R. Siddal, ‘Pigment Compendium: Optical [20] D. G. Georgiev, ‘Molecular structure of the alacranite solid solution series with MicroscopyofHistorical Pigments’, and intermediate phases in group Vbi- the end members (As8S8)–(As8S9). It was Elsevier-Butterworth Heinemann, 2004, nary chalcogenide glasses’, PhD Thesis, also found that the occurrence of parareal- p. 129. University of Cincinnati, Cincinnati, garonartworks and historical samples (e.g. [7] A. L. Emelina, A. S. Alikhanian, A. V. 2003. collections) as asecondary photochemical Steblevskii, E. N. Kolosov, Inorganic reaction product is not exclusively linked Materials 2007, 43,95.