Review paper

Cinnabar reviewed: characterization of thered pigment and its reactionsRenate Nöller

BAM-Federal Institute for Materials Research and Testing, Berlin, Germany

In the majority of cases, the red color of cinnabar on objects of cultural heritage is well preserved, thoughturning black is often claimed and has been the subject of investigations. To evaluate conditions for thestability of the pigment and understand the reactions, in this paper the problem is approached fromvarious viewpoints. First of all the natural form cinnabarite is compared with the artificially preparedpigment vermilion. This establishes a differentiation of types in terms of quality, depending on structuralimpurities. With regard to the pigment’s reactions influencing the discoloration, the most commonlymentioned environmental factors, such as radiation or halogens, are evaluated. In relation to varioususages, the pigment’s structural stability is then viewed in connection with adjacent pigments, glues, andthe substrate, which may lead to a brown or black coloration or even the release of mercury, whereas thecolor is preserved in most cases when used on lime or in ink and lacquer. Due to the materials’properties, attention is drawn to the fact that discoloration to a brownish-black is not necessarily a sign ofdamage and harmful reaction products, but may indicate good preservation of the painted material,provided that the mercury can be bound in the substrate.

Keywords: Cinnabar, Vermilion, Differentiation of cinnabar, Impurities, Color change, Photosensitivity, Halogens, Preservation

IntroductionThe red pigment cinnabar (HgS) has been used as apainting material for thousands of years in many cul-tures (Tanabe, 1943; Wang & Wang, 1999). The colorwas preferred because of its deep red, special gloss,good covering characteristics, and adhesive strength.Due to its low hardness and high density, it was thematerial most suitable for coloring surfaces.Furthermore, it is very resistant to oxidation or acidrain, which is why many very old objects still showan unaltered bright red color. In addition, cinnabaris well known for turning black or grey, but this doesnot always occur. The quality of the pigment is deci-sive for the reaction. In this respect, different typeshave been distinguished with determinations depend-ing on the origin.According to the US Department of Standards,

since 1962, cinnabar – synonymous with the mineralcinnabarite – has been considered the naturally occur-ring pigment, whereas the term vermilion has been

reserved for the artificial product used by artists(Gettens et al., 1972).In earlier times the color was not differentiated from

other reds also used for mixing, such as minium (redlead), or the organic colors cinnabaris, made out ofthe resin of the Dracaena tree, and vermilicum, madefrom louses of the Kermes oak. With increasingdemand and synthetic production of HgS in Europesince the seventeenth century, the term cinnabar(natural and artificial) became prevalent and wasused interchangeably with the term vermilion, whichwas used in France, Portugal, Catalan, Spain, andItaly. The vermilion from Spain, known on themarkets as a cleaned ground powder ideal for paint-ing, was declared as artificially produced. Cinnabarfrom China, imported because it was the best of all,showed impurities either from the natural environmentor artificially added.It is not always possible to distinguish analytically

the natural HgS, which sublimates easily and thensediments as a very fine powder, from synthetic HgS.This fact, together with the uncertainty of a specifichistory of use of the naturally occurring mineralpigment in works of art (Grout & Burnstock, 2000),leads to inaccuracy in determining the origin

Correspondence to: Renate Nöller, BAM-Federal Institute for MaterialsResearch and Testing, Workgroup: Analysis of Artefacts and CulturalAssets, Unter den Eichen 44-46, 12203 Berlin, Germany.Email: renate.noeller@bam.de

© The International Institute for Conservation of Historic and Artistic Works 2013DOI 10.1179/2047058413Y.0000000089 Studies in Conservation 2013 VOL. 0 NO. 0 1

(Eastaugh et al., 2008). Independent of its formationor usage cinnabar is, thus, still the most commonterm used for the red modification of HgS withuniquely defined material properties.How cinnabar nevertheless can be differentiated is

demonstrated in this paper by the electro-chemicalcharacteristics of impurities involved in the formationprocess. Under specific physical conditions theyexchange with the mercury or sulfur ions of the struc-ture. Their properties define the impurity activationenergy that is seen to be crucial for the stability ofthe color and its reactions.Research on why cinnabar darkens mainly points to

the influence of environmental factors, and photosen-sitivity is commonly referred to. In addition, theharmful reaction of the red color triggered by chlorideas an impurity has always been mentioned. For a longtime the hereby deduced blackening was attributed tothe structural transformation of cinnabar into meta-cinnabarite, the black modification of HgS (Kopp,1843–47; Eibner, 1914). However, this phase couldnot be detected with modern analytical techniquesand, therefore, alteration to amorphous HgS hasbeen proposed. More recent research has shed newlight on the reaction mechanisms. Chemical reactionsleading to Hg0 and mineralization products, such ascorderoite (α-Hg3S2Cl2) and calomel (Hg2Cl2)(Daniels, 1987; Keune & Boon, 2005; Istudor et al.,2007; Eastaugh, et al., 2008) have been analyzed andheld responsible for the color change.In addition, the present work also examines the

special conditions of the pigment in different culturalcontexts. The adjacent material, for example otherpigments, binding media, or substrates, can affectdecomposition of the pigment and present possibilitiesof binding with mercury as well. In such a case, wesee that cinnabar can also be a good medium for thepreservation of surfaces as it is capable of bindingmetal ions by amalgamation and the lignin ofwooden objects.The kinds of reactions caused by various factors are

crucial for conservation measures with painted objects.

Is the darkening of the red an indication of instabilitywith even the release of black metallic mercury? Andunder which conditions can it be expected, and howcan damage be prevented?

Differentiation of the color and its qualityCinnabarite in natureCinnabarite is a red mineral, a pigment found innature. The color is very stable and not soluble inwater, acid, nor alkaline solution. Observed are huesfrom light to dark red, depending on the deposit(Figs. 1–3) and its conditions for crystallization. Thedarker cinnabarite has larger grains; the lighter oneis purer and often formed by secondary sublimation.The properties of the light red mineral are perfectlysuited for its use as a pigment.

Cinnabarite is the most common ore of mercury. Ithas a hexagonal chain structure. Sometimes the blackmodification of HgS – metacinnabarite, which crystal-lizes in a less stable cubic crystal structure – is found inthe same place (Fig. 3). Stabilization of the blackmetacinnabarite by diadochy of Hg, Zn, FeS, Se(Dickson & Tunell, 1959) has been observed.

In addition to these modifications, some rarer min-erals and compounds of mercury are also found, which

Figure 1 Cinnabarite from Almadén, Spain.Figure 3 Cinnabarite with black meta-cinnabarite fromMexico.

Figure 2 Cinnabarite from Tarragona, Spain.

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can be interesting in respect to possible reaction pro-ducts after the decomposition of cinnabar; observedare white calomel (Hg2Cl2) crystallizing in a tetrahe-dral crystal structure, tiemannite (HgSe), onofrite(Hg(S,Se)), coloradoite (HgTe), livingstonite(HgS.2Sb2S3), kleinite (HgNH4-chloride), mosesite (ahydrous HgNH4-chloride), eglestonite (Hg4Cl2O), ter-linguaite (HgClO) and montroydite (HgO).The formation of the red cinnabarite is tied to vol-

canism near the surface, at low hydrothermal tempera-ture and pressure in the Sb-As-Se sequence.Crystallization takes place in fissures and cavities ofsandstone, limestone, or volcanic rock. Secondary for-mation is found in the oxidation zone of ore depositsor as sedimentation in rivers. It is associated withother sulfides and silicates such as chalcedony,calcium carbonates, and dolomite. Depending on itsgeological provenance, the natural conditions for itsstability can be considered in association with the fol-lowing elements: Sb, As, Se (Bi, Te, U, W) in theprimary hydrothermal formation, or Ag, Au, Cu, Fe,Pb, Sn, Cr, Zn as secondary formation in the oxidationzone of ore deposits. The minerals associated aremainly limestone (Ca,Sr,Ba(CO3)), sandstone, quart-zite (SiO2), pyrite (FeS2), orpiment (As2S3), realgar(AsS), and stibnite (Sb2S2).The elemental composition of different deposits is

the most characteristic fingerprint for determiningthe local origin of the mercury ore (Nöller, 2012)and an important factor in the red color and itsquality. So it is well known that cinnabarite fromcertain regions used as a pigment changes its color,whereas from other places it is very stable. In Europethe mineral cinnabarite was used for red colorationup to the sixteenth century. After this, the pigmentwas increasingly produced synthetically in industrialcenters (Resenberg, 2005; Radepont et al., 2011).

Vermilion the pigmentThe stability of vermilion or cinnabar as a pigment(Fig. 4) has to be seen also in dependence on thekind of treatment the mineral was submitted to whentaken from its natural environment. The followingtechniques are reported (Brachert, 1980): Cinnabariteis cleaned from associated mineral phases (limestone,quartzite, or muscovite), as well as from metals suchas Fe, Pb, Zn, or Se, by hydraulic separation inwater and/or nitric acid (Mildner, 1960). The redpowder is dried on chalk.In India, the Jaina text Citrakalpadruma (530 BC)

describes the cleaning of cinnabarite (hingula orsidura) by grinding the raw material in a mortartogether with water, sugar, and lime. The yellow sub-strate is then dried. This is repeated 15 times; in thefinal repetition, lime juice is added and the groundpaste is dried in the form of tablets. The text of

Silparatna from Sri Lanka also mentions the cleaningof cinnabarite (Gopinath Rao, 1918).The Romans were quite familiar with the synthetic

preparation of the pigment (Vitruvius Pollio, 1960).In the Middle Ages, cinnabar of good quality cameto Europe from China. Its light red color is due tothe addition of antimony (stibnite, Sb2S3). Vermilionproduced in the manufactories of Amsterdam inthe seventeenth century is known for containing theadditional elements Sb, Fe, Ca, or Pb. To reduce theprice, low-quality pigment was sometimes mixedwith various red colorants, such as brick (Freeman& Graichen, 1943; Gettens, 1954).

Dry process and wet process productionAside from many slightly different ways of preparingthe pigment and mixing it with other colorants,mainly two production techniques are described: thedry and the wet techniques (Bucholz, 1801).In the dry process, the elements Hg and S are pulver-

ized together and heated very slowly with lime on aniron pan. Iron has a reducing effect on the reactionof Hg with S and is a metal that does not react withmercury by amalgamation. The product is called‘Aethiops mineralis’; it is a HgS of black coloration. Ittransforms to the red phase by sublimation at 588°C,carried out industrially by heating it in clay bottleswith taps made out of iron. The black product is alsoheated in a watery solution of ammonia or potassiumsulfide (caustic potash). A light yellow-red vermilionis produced by stirring. An orange hue develops witha surplus of sulfur together with Al, K, and Si.Very important for the stability of the color is the

ratio of Hg to S. To remove sulfur, the substance iswashed in an alkaline solution (KOH with K2CO3)and water. Sulfur not bound in the pigment’s crystalstructure or in soluble sulfides as impurities will turnthe color from red to black. To eliminate a surplusof mercury, the pigment is treated with sulfur and

Figure 4 Cinnabarite prepared as pigment.

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soda (Na2CO3); or it is distilled, but this product issaid to turn dark easily (Wolff & Zeidler, 1934).The color produced in this way in Europe from the

eighth to the seventeenth century is brilliantly red andsaid to be resistant to ultraviolet (UV) radiation.The other production method for vermilion is the

wet process, which makes it out of a solution. Thismethod was introduced in Japan in 1609 (Kenjo,1980). Gottfried Schulz developed the following tech-nique in 1687 in Germany (Kopp, 1843–47). Mercurysulfide is precipitated from a solution of a salt ofmercury by gaseous H2S. Salts of Hg – HgCl2 orHg(CN)2 – are volatile and hardly dissociated inwatery solution. Injecting H2S into a solution withHg2+ produces a black precipitation (metacinnabar-ite). The solubility of HgS in water and acids is verylow (in water 1.6 × 10−54 mol/l). In a slightly under-saturated solvent, precipitation is reached by slowlyheating, so that the red vermilion develops from theblack formation of HgS with rising temperature.Pure Hg or HgO are used for synthesis by cleaning

the metal in a solution with sulfur and heating it to40–60°C. The resulting very finely dispersed precipi-tate is then filtered.The advantage of the wet process technique is that

costs are reduced, as no stove for high temperaturesis necessary and the product does not need to beground. It is even possible to obtain variations of thered color intentionally. Thus, in the nineteenthcentury in Idria in what now is Slovenia, more than18 different colorations of vermilion were produced(Resenberg, 2005). By this time, however, the redmercury sulfide had already dwindled in importanceas cadmium red took its place.

Color typesThree types of cinnabar are commonly distinguished,the natural one and the synthesized ones producedeither in the dry or in the wet process. The syntheticproducts, especially vermilion produced in the wetprocess, are said to be less stable and to differ fromthe natural ones by the absence of other mineralphases and their small grain size. The quality or stab-ility of the color is determined by the same impuritiesas found in natural cinnabar or by influencing physicalfactors during the production, such as the specificweight (Eibner, 1914; Feller, 1967) or the specialatmosphere and duration of the reaction process(Kenjo, 1980). When analyzing the pigment, variouselements besides Hg and S can be detected, corre-sponding to the specific conditions during thegenesis of the mercury deposits. Also, after cleaning,Sb, Fe, Pb, Ca, K, Na, or Cl can be present in thenatural pigment or are added later in the productionprocess.

Trace elements influence cinnabar’s electronic struc-ture and binding strength, resulting in a more or lessred coloration. Sensitively triggered by environmentalfactors, they lead to energetic changes in the crystal’sstructure (Potter & Barnes, 1978). However, they arenot necessarily responsible for later discoloration, ashas often been suggested for the pigment producedin the wet process (Grout & Burnstock, 2000).Diffusion and binding of ions can inhibit or catalyzethe darkening of the color or facilitate stabilizationor the process of alteration. That is why types andqualities of cinnabar are distinguished by traceelements incorporated in the crystal’s structure.

As seen above, in nature Sb (As, Se, and Te) is verycommon in primary ores, Fe and Pb in secondaryones. Calcium is often found in host rocks associatedwith cinnabar. Antimony can easily be exchangedwith mercury. This is due to the nearly identical sizeof their ions and their similar electronegativity value.In cinnabar, Hg2+ is bound in a disturbed octahedronwith two nearer covalently and four more ionicallybound electrons farther away (Pauling, 1963).Antimony is sometimes stabilized as Sb2+ in an octa-hedron, but more often it is bound as Sb3+ in a tetra-hedron. Incorporated in cinnabar as an impurity byexchange of Hg2+, the more tetrahedrally formedlattice positions then have a positive charge surplus.A reduction to Sb2+ is preferable to the reduction ofHg2+, which would be visible as a color change.Incorporated in the crystal’s structure, antimony thuscan compensate for lattice defects and hinders thereduction of mercury.

Iron reacting to red iron oxide is very important fora reducing atmosphere during synthesis, so that noHgO will be present at temperatures between 300and 600°C. The red HgO is less stable than HgS andthus impurities later turn out to be brownish-black.Red lead oxide is also added not only to influencethe coloration, but also to prevent the oxidation ofsulfur and thus stabilizing cinnabar. Lead is oftenmentioned combined with unaltered vermilion(Grout & Burnstock, 2000).

Production in the dry process is carried out withlime, which results in an alkaline milieu during themineralization. The preparation of the pigment withCa, K, and Na is very useful for eliminating asurplus of sulfur. These elements ensure stoichiometricpurity, making them important for the color.

Traces of Cl− as an impurity – being a negativelycharged strongly polarizing ion – influence the stab-ility of cinnabar directly (McCormack, 2000). Aboveall at the surface, after decomposition of HgS toblack mercury directly upon UV radiation, the Hgions may form white calomel (Hg2Cl2). So its elimin-ation after the production in the wet process is verynecessary.

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From red to brown or black – deterioration orstabilizationDarkening by radiationPliny already noted that cinnabar is photosensitive(Plinius, 2007). It irreversibly blackens on walls whenexposed to sunlight, as in Pompeii and Herculaneum(Fig. 5). Whereas inside the house the red color

painted on polished stucco ‘shines brilliantly, outsidethe house it becomes ugly and blackens within thirtydays’. Even moonlight might be harmful to thecolor, as observed on stucco in peristyles/exedrae(Vitruvius, 1960) (Table 1).Blackening induced by light has been noted at

various times (Cennini, 1922; Brosset, 1936, Istudoret al., 2007). The discoloration has been explainedby phase transition to black metacinnabarite, by oxi-dation of the sulfur, or by other chemical reactions(Grout & Spring, 2002).Cleaning with a laser (Q-switched YAG laser) can

also cause discoloration (Echert et al., 2000; Sobott,2003). This is due to the energy of the radiationapplied. If it is higher than the color-specific energyof cinnabar, which is due to light absorbed at590 nm (Garcia Moreno, 2005), it activates photoche-mical processes (Schnell et al., 2001).The blackening produced by cleaning is also

explained mechanically as a result of a pressure-induced phase transition to metacinnabarite (Fuchs& Jacek, 2001). Darkening due to free mercury onthe surface of the pigment after cleaning is observed,if the metal did not react to other compounds.X-rays, even if applied for a long time, do not

change the red color of cinnabar (Petertil, 1933).

Figure 5 Wall painting with color change to gray-black, Herculaneum, Italy.

Table 1 Discoloration of cinnabar

Due to Reaction product Reference

Light/UVradiation

Hg, metacinnabarite(Hg2+ and Hg+)

Cennini (1922);Brosset (1936);Plinius (2007);Grout and Spring(2002)

Laser Hg, metacinnabarite,depolymerizationHgxSy

−

Echert et al. (2000);Sobott (2003)

Synthesis/preparation

Remnants of Hg, HgO,or S (K, Na, Cl)

Wolff and Zeidler(1934)

Salts/halogens HgCl2 up to Hg2Cl2(Hg2+ and Hg+)

Grout andBurnstock(2000); Saundersand Kirby (2004)

Humidity Mobility of salts orhalogens

Fuchs and Jacek(2001)

Amalgamation With Cu: HgCu+ ,CuS, natural patina

Heumann (1874)

Polished wood Bond with lignin Echard (2003); Lvet al. (2012)

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Reaction with halogensAs mentioned above, chlorine may be incorporatedduring the natural or synthetic formation of cinnabar.Electron microprobe analysis shows that photosensi-tive cinnabar contains noteworthy concentrations ofchloride (0.04 mole %), whereas non-photosensitivecinnabar does not (<0.01 mole %) (Grout &Burnstock, 2000; McCormack, 2000). Even traces ofchlorine in the air may influence the discoloring onwall paintings with altered conditions of humidity(Saunders & Kirby, 2004). Especially in sunlight, halo-gens on the colored surface can facilitate the darkeningof cinnabar (Daniels, 1987).Chloride ions act as catalysts in the photo-electro-

chemical process. Cinnabar then readily undergoesstructural transformation becoming black due to freemercury Hg0 and sulfur S0 with accumulated chlorides(Keune & Boon, 2005). The black mercury will reactfurther with chloride, probably from an externalsource, to form white mercury chloride products.Sulfide is oxidized to sulfur dioxide, which canescape. Degradation products such as photosensitiveterlinguaite (Hg2ClO), corderoite (α-Hg3S2Cl2), orkenhsuite (γ-Hg3S2Cl2) have been detected by micro-scopic X-ray diffraction (Cotte et al., 2006;Radepont et al., 2011). The reaction product HgCl2is a yellowish-white compound with covalent, stronglydirected binding forces. Further reduction turns it intowhite calomel (Grout & Burnstock, 2000). In its tetra-hedral structure, the valence of mercury is one andtwo. Because of its low binding forces, calomel canbe easily decomposed by further photochemicalreaction.Cinnabar is reportedly not suited for frescoes in

northern countries (Malaguzzi-Valerj, 1973).Compounds of halogens as impurities as well as thealkalinity of the plaster cause damage in associationwith absorption of water or OH−, leading to themobilization of the incorporated salt ions (Fuchs &Jacek, 2001). Halogens in water then easily catalyzethe reaction on the surface of cinnabar. The sulfur ofHgS can also be oxidized by acidic organic materialand be dissolved, forming complexes with Hg ions.Mercury reduced to Hg+ then easily combines withCl−, the free sulfur leads to SO2

+ and in the presenceof Fe and CaO to FeS+ and CaSO4. The reactionprocess could be detected on a blackened wall paintingon calcite mortar. Here different degradation processesalso showed red cinnabar distributed unchanged indeeper layers within a sulfated coating of gypsumcolored black by organics (Cotte et al., 2006).

Reaction with pigmentsPliny mentions cinnabar’s incompatibility with othermetal-sulfide pigments and with white lead(PbCO3)2·Pb (OH)2. White lead is reduced by sulfides

to black PbS. This, as well as reactions with ceruse(PbCO3) or ochre (Fe2O3·H2O), happens to cinnabarin an acidic environment (Riederer, 1977; Grout &Spring, 2002). Cinnabar prefers reducing metals withlow binding strength. That is why reactions with mala-chite and azurite have also been observed on wallpaintings in the Far East. In these pigments, Cu isbound to OH, facilitating amalgamation withmercury when it is reduced.

In an alkaline milieu, cinnabar stabilizes colors,especially when it is mixed with lead oxide. Thus,tempera wall paintings in Agra, India still show thered of cinnabar near other pigments (Nagpall, 1964).The mixture with red lead detected on mural paintingsin eastern Turkestan, Jericho (Edwards et al., 1999),and Rome is very common. Red or yellow ironoxides, chalk, and the clay mineral illite are alsoadded; this is the same stable mixture used as earlyas in Pergamum in Asia Minor. A special mixturewith soot (carbon black) is found on mummies fromEgypt.

In China as well as in India, a mixture of the red andyellow pigments of arsenic (realgar As4S4 and orpi-ment As2S3) is also common. It was later used byLucas Cranach (†1553), for example. The stability ofits red is explained by its origin from China, wherethe dry process was used to synthesize it (Grout &Burnstock, 2000).

In oil paintings, Pierre Renoir (†1919) often usedcinnabar mixed with lead white to create a rose color-ation, mainly for skin. Together with calcined crystalglass (30% Pb), this coloration is very stable.Leonardo da Vinci (†1519) mixed 99% lead whitewith 1% HgS to create a special brightness in thebrownish layers of Mona Lisa’s face (Elias & Cotte,2008). A shining effect has been obtained by usingthe red of cinnabar under gold leaf or precious gem-stones. Other red colorants, for example kermes orSn- and basic Cr-oxide, are also used together withcinnabar. The mixture with watercolors was avoided,because cinnabar easily darkens with humidityespecially in the presence of halogens.

Cinnabar was a favorite pigment for many famouspainters including Titian (†1576), Botticelli (†1510),Leonardo da Vinci (†1519), Raphael (†1520),Rubens (†1640), Rembrandt (†1669), Goya (†1828),Seurat (†1891), Gauguin (†1903), and Monet(†1926). The color is stabilized with oil (turpentineoil in Rubens’ case) or with fish glue (Kittel, 1960).But discoloration is also reported (Kopp, 1843–47),for example, when cinnabar is used with egg white(albumen), probably due to the latter’s sulfurcontent. Paintings showing an ugly color change areknown to have been restored to brightness byrubbing with warm water, urine, vinegar, spirit, redwine, saffron or other materials.

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In binding mediaTo stabilize the color, the surface of a painting wasoften polished or treated with resin. If the resin pene-trates deep enough to enclose the surface structure,photochemical reactions are prevented (Cennini,1922; Dörner & Hoppe, 1985). Cinnabar is preservedwhen used in varnish or glazes made out of madder,kermes, or cochineal lakes (Bucholz, 1801) or in abrown glazing paint made out of bitumen and resin.Adding organic gum or other substances can play animportant role in stabilizing it. With linseed oil it isreported to be less stable (Grout & Burnstock, 2000).According to Vitruvius, some Roman wall paintings

were treated with a layer of Punic wax-oil after drying,still showing the red unchanged (Fig. 6). Candle waxwas heated in an iron pot over a charcoal fire till itbecame fluid. Mixed with oil, it was brushed ontothe wall and rubbed with a linen cloth (VitruviusPollio, 1960).Used in ink, in color for stamps, or in lacquer for

seals (Fig. 7) or wooden objects (Yi et al., 2000), thered is mostly unchanged. Reported are lacquersmade out of Brazil wood (caesalpinia echinata) withcinnabar as an additional colorant.

Influence of substratesDue to its high density (8.1 g/cm3) and low hardness(2.25 Mohs), cinnabar – especially when obtained asa fine sized sublimate – has good tractability, so that

relatively little binder is necessary on porous materialof metal, stone, bone, wood, cloth, or paper.Depending on the surface structure and treatment,and hence chemo-physical properties, however, thekind of support influences reactions notably.For example, Chinese vessels of the Shang dynasty

(2000 BC, Tao Tie style) made out of copper or brassand painted with cinnabar show a blackish-browncolor due to deterioration. Mercury readily formsamalgams and reacts with copper to produce Hg–Cuand a patina of CuS at the surface (Heumann, 1874).This prevents deeper corrosion.Objects made out of iron do not react by amalgama-

tion. Painting them with lead red or cinnabar – bothnamed ‘minium’ by the Romans – protects themfrom oxidation and the red is preserved. This color-ation was also used to imitate bronze. The Romansused iron flasks to transport mercury, knowing thatno reaction would take place.Being a symbol of blood and life, red colors were

often used in burial ceremonies. So cinnabar is foundin Neolithic graves, for example in Spain (Martin-Gilet al., 1995). Human bones were colored red, andnot only for religious reasons. Cinnabar sustainsbonding with the calcium of the bones in an alkalinemilieu, so that decay is prolonged and the colorremains unchanged.Cinnabar is usually stable in wall paintings (see

above) and preserves the lime of the plaster. Thismade it a favorite priming color for wall paintings inthe Buddhist caves of Asia. It also protects the wallsagainst mildew and mold fungus.Cinnabar has been used to color wooden objects.

For example, the famous violins made by Stradivari(†1737) in Italy were polished with red cinnabar(Echard, 2003), reportedly not only for coloration.Reaction to HgCl2 at the surface serves as a fungicide.HgCl2 is the oldest means applied to protect woodenobjects. Leonardo da Vinci (†1519), for example,used it to conserve the frames of his paintings.The sublimate bonds with the wood, as Hg has a

high adsorption capacity on lignin. A study on this

Figure 7 Cinnabar used to color lacquer seals.

Figure 6 Cinnabar polished with wax. Wall painting Pompeii,Italy.

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by X-ray absorption spectroscopy showed changes ofHgCl2 to the monodentate complex -C-O-HgCl andthe bidentate complex -C-O-Hg-O-C with increasingalkalinity (Lv et al., 2012). This confirms that HgSand HgCl2 are good means for protecting wood, asreduction to free mercury does not take place and adeeper infiltration of harmful substances is prevented.Here a color change in the red cinnabar, which is fre-quently used in panel paintings, indicates the chemicalbinding of mercury with the wooden support.On paper, ink made out of cinnabar remains red in

most cases. But here also a metallic gray-black colorindicating free mercury was found on an old manu-script protected under glass. Changed pressure andtemperature conditions between the panes of glassassociated with the thickness of the ink and its badinfiltration in a special paper has to be consideredresponsible for this harmful structural transformation.Mercury serves as an antiseptic with antibacterial

function in medicine or as a paint known as antifoul-ing for boats. In museums, organic material such asfurs and feathers were treated with fungicides contain-ing mercury and arsenic (Sioris, 2001), which howeverturned out not to be an appropriate measure.Mercury’s poor binding strength with these organicsupports and its sensitive temperature dependencycan easily lead to sublimation and contaminatedrooms. Release of mercury has also been reportedfrom cinnabar on a textile coat (Chancay culture1000–1200 AC, Peru) during its restoration(Rouleché, 1986).

AcknowledgementI am thankful to my colleague Stefan Röhrs, RathgenResearch Laboratory Berlin State Museums, for hishelpful comments. The mineral samples were kindlyprovided by Angela Ehling, Federal Institute forGeosciences and Natural Resources, Berlin; Ralf-Thomas Schmitt, Museum of Natural History,Berlin; Susanne Herting-Agthe, Technical Universityof Berlin, Mineralogical Collections.

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