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Anne-Marie Guimier-Sorbets, Christine Kondoleon,Henri Lavagne, Demetrios Michaelides,

Per Jonas Nordhagen, Carla Salvetti

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SOMMARIO

Editoriale/Editorial 9Abbreviazioni 11

Claudia Angelelli, I mosaici delle terme di Otricoli. A proposito didue disegni recentemente ritrovati 15

Federico Guidobaldi, Silvia Pedone, Il viraggio delle scelte deco-rative nei rivestimenti pavimentali e parietali in età costantiniana: dagliantefatti agli esiti 35

Bente Kiilerich, Optical colour blending in the Rotunda mosaics atThessaloniki 163

Per Jonas Nordhagen, Form and function in the mosaic Maria Reginafrom the oratory of John VII (a.d. 705-707) in the Old St. Peter’s 193

OPTICAL COLOUR BLENDINGIN THE ROTUNDA MOSAICS

AT THESSALONIKI

Bente Ki ilerich

In the early Byzantine mosaics in the Rotunda of St George at Thessaloniki, sometesserae are arranged so that the colours blend optically. This method, which hasits scientific origin in Greco-Roman optics, is related to the principles of nine-teenth-century pointillist and divisionist painting. Based on observations in situand subsequent studies of digital photographs, I discuss two instances of opticalblending, namely its use in the interwoven decoration (tablia) of the soldier saints’cloaks, and in the wings of angels. In order to explain the colouristic effects, Ibring into the discussion the phenomena of contrast and assimilation and recentneurobiological research on luminance. It is concluded that by means of thecolour blending technique, the artists achieved astounding results with respect tohue, luminance, texture, three-dimensionality and variety.

Keywords: Vault mosaics, optical blending, colour, luminance, Rotunda Thessaloniki,early Byzantium.

The mosaics in the Rotunda

he mosaics in the Rotunda at Thessaloniki were probably commis-sioned by Emperor Theodosius I when the building, presumably

erected ca. 310 by Emperor Galerius, was converted into a church around380/390 AD.1 The church rises to an inner height of nearly 30 m, and theinternal diameter is ca. 26 m. Rich and sumptuous mosaics arranged inthree concentric zones adorned the cupola. In a medallion at the summitwas an image of Christ in the guise of Sol Invictus, appearing in a long flow-ing robe against the shimmering light of large silver tesserae. Unfortunate-ly it is only fragmentarily preserved. A lost middle zone had a band ofwhite-clad angels dancing in rhythmic motion. The better preserved lowerzone – more than 7 m high – shows splendid golden architecture setagainst a golden ground.2 In front of this architecture there stand saints

1 The chronology of the conversion is disputed, and suggested dates for the mosaics rangefrom the reign of Constantine, Bakirtzis et al. 2012, pp. 48-127 to that of Justinian, Fourlas2012, pp. 177-195, 364 (5th-6th c.). A Theodosian date is upheld by, e.g., Matthiae 1962, pp. 196-199 (ca. 400-425); Bianchi Bandinelli 1970, pp. 258-259, 438 (Theodosius I); Torp 1991; Nasrallah 2005; Kiilerich 2007, p. 322 with further references in n. 3; Torp 2011.

2 For excellent colour photos, see Bakirtzis et al. 2012, pp. 48-127.

T

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who are more than life size. Of the twenty original saints, fifteen cleric andlaic martyrs are preserved.1 Of interest to the present study of opticalcolour blending are the soldier martyrs. They are dressed in inner and ou -ter tunics and cloaks (chlamydes). The decorations (tablia) woven into thesecloaks stand out for their sophisticated rendering of colour in a pointillistor divisionist technique.2 This technique can be seen in a few other partsof the decoration; especially fine is the effect achieved in the wings of thefour angels who carry the medallion of Christ in the cupola’s summit.

Objectives and methods

The present study of the mosaics is based on autopsy and digital imagerecordings.3 The colours recorded by the digital camera may differ some-what from the colours observed in situ by Hjalmar Torp in the 1950s and onwards, and by Torp and the author in more recent years. In addition tothe essential relativity of colour and the subjectivity of colour perception,there is the further problem of colour designation.4 It is difficult to find anobjective way to describe colour. In connection with ancient mosaics, theMunsell chart or other colour systems are only of limited use.5 Mosaictesserae in a given hue are not easily fixed at a specific value because thechemical composition of the glass, its colour agents and fluctuations in heatall tend to result in non-homogenous areas of colour.6 In fact, even withina single tessera, the hue may vary slightly. Colour-measuring, that is chro-mometrics, may seem a more promising resource.7 However, although achromameter can register hues, since the mosaic medium is such that thehue differs imperceptibly from one tessera to the next, a very large numberof readings would be required to give a significant mean result. Moreover,it must also be taken into account that the mosaic surface is not smooth, asthe individual tesserae are deliberately set into the surface at varying angles,sometimes even with the rough rather than the polished side up. In the

1 Kiilerich 2007. 2 Kiilerich 2012a, p. 185f.3 Photos of the martyrs were taken from scaffoldings in May 2009 with a Lumix Panasonic

DMC-TZ1 digital camera with a Leica lens. The medallion and the angels were photographedfrom the ground in April 2011 with an Olympus SP-800UZ with 30× optical zoom. Enhancingof some images and conversion into greyscale were done with Presto! Page Manager 7.15: ArcSoft PhotoStudio 5 for Canon. I am grateful to then ephoros Melina Païsidou for grantingaccess to the scaffolding for three days in 2009, and for permission to take photographs of themosaics.

4 James 2000, p. 38, gives examples of one scholar singling out twelve colours, while another named fifty-seven in the very same mosaic.

5 Munsell 1900 and later editions. As Gage 1990, p. 538, points out, colour chips can onlyapproximate the nuances encountered in artefacts themselves.

6 Wypyski 2005; James 2006.7 Strudwick 1991 advocates the use of this method for Egyptian paintings.

optical colour blending in the rotunda mosaics 165

Rotunda the mosaics are set into an uneven and slightly undulating mortarground on an inclined surface that follows the cupola curvature. These various deviations from the straight and regular were made to reflect lightfrom variant angles and to create a lively and vibrant surface.1 Indeed, in themosaic medium, the aesthetic is not primarily a matter of colour, but oflightning.2 In spite of colour constancy, viewed under different illumina-tions, the appearance of the mosaics will also differ. The present study, then,is based on visual observation and the subjective registration of the perceived colours at close range and as they blend when seen at a distance.

The Tablia of the Chlamydes

Singled out for examination here are the tablia – the large decorations wo-ven into the cloaks (chlamydes) – of six soldier-martyrs in the Rotunda,namely Onesiphoros, Anonymous, Basiliskos, Therinos, Leon and Eukar-pion. (The tablion of the seventh soldier, Priskos, is almost totally de-stroyed and therefore left out). Based on their garments, the saints can bedivided into two sub-groups: four soldiers, who are dressed in whitechlamydes, and a second smaller group consisting of Leon and Eukarpion,who wear purple cloaks.3 The tablia are woven into the cloaks, thus of thesame material but differently coloured. One tablion is placed along thefront edge, another along the edge of the backside of the cloak. All areshaped as parallelograms, but the cloak folds cause them to look less regular and appear narrower at the bottom edge than at the top. The sizesof the represented tablia differ somewhat but extend roughly from the upper part of the chest to the knee.

The tesserae of the tablia are arranged in even vertical rows. These areeither single or double monochrome rows, or two or more rows with twoor three colours set in a chequerboard pattern. At times a monochromerow in colour A meets a two-colour row in colour A + B in a comb-setting,resulting in a denticulated effect within the tablion. The sizes of the tesseraein the tablia are uniform, measuring ca. 7-8 by 7-8 mm, but being ‘hand-crafted’, they deviate in form from a regular square; this irregularity ischaracteristic for the medium and gives ancient mosaics a particularly lively quality lacking in modern mass-produced mosaic cubes. The tesseraeare set with a density of 130 per 10 cm2.4 The interstices show the neutral

1 For a technical study of the Rotunda mosaics, see Torp in print (a)2 For the relation between colour and light in Byzantium, see James 1996.3 For the garment in general, see Kolb 1973; Gerszke 2010.4 Measured in Onesiphoros’ tablion. The sizes of the tesserae differ in various parts of the

mosaics: golden background: 90-110 tesserae per 10 cm2; a face: up to 360 tesserae per 10 cm2(Leon). I am grateful to Hjalmar Torp for information on these details, recorded by him in situ.

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grey of the mortar, but originally the ground was certainly coloured.1 Thesix tablia will be described here in order to give an idea of the colour com-binations and designs and the variations among the figures.2

Onesiphoros

Blue; light blue; two rows of light blue/orange chequerboard; two tothree rows of red/blue chequerboard; light blue/orange; light blue;blue/red; black; blue; light blue; light blue/orange; red/blue; black; blue;two rows light blue; light blue/orange; ca five rows of diagonal setting ofalternating light blue/orange chequers and red/blue chequers; lightblue/orange; light blue; blue/red; black; blue; two rows light blue; tworows light blue/orange chequer; red/blue; blue; red/blue; black; blue;red/blue interspersed with orange/light blue.

Onesiphoros’ tablion is particularly important because it is fully pre-served. It shows one of the more complex designs with many colours in adelicate setting. The diagonal pattern combining light blue/orange andblue/red down the central vertical fold triggers an impression of a dyna -mic surface and a slightly spiralling effect, as in a silken weave. There isdominance of light blue/orange, set off by black and light blue (Fig. 1).

Anonymous

Blue; light blue; two rows of light blue/orange chequerboard; light blue;two rows of blue/red chequerboard; black; blue; blue/orange; red/blue(spreading from one to two rows); light blue; two rows of light blue/ orange chequerboard; four rows of red/blue chequerboard with some diagonal setting of orange/blue chequers; orange/blue; light blue; red/blue; blue/black; red/blue; light blue; orange/blue; black; two rows ofred/blue chequerboard; light blue; two and more light blue/orange rowsin chequerboard; many rows of red/blue chequerboard.

Anonymous’ tablion is almost fully preserved. Its complex setting is mostclosely comparable to that of Onesiphoros. Some tesserae are set at a dia -gonal, albeit sparingly, almost hesitantly. The general impression is of slen-der stripes or folds. Three main systems have been used: red/blue, orange/blue and blue. Orange is abundant. There is quite an elegant transition fromorange/blue to red/blue. Clear blue is set in denticulation (Fig. 2).

1 Torp in print (a) and (b).2 All descriptions are from left to right and by vertical rows of tesserae. Colour terms are

simplified for sake of argument and clarity. Thus the hues are described simply as blue, lightblue, dark blue, orange, red, violet and black. It must be noted that each hue encompassesmany shades, and, in the Rotunda decoration as a whole, it is possible to pick out more thanten shades of blue, etc.

optical colour blending in the rotunda mosaics 167

Basiliskos

Black; black/violet; violet/red; three rows of red/blue in chequerboard;light blue/orange; light blue; black; violet/red; red/blue/black; black;

Fig. 1. Thessaloniki, Rotunda. Onesiphoros’ tablion (photo author).

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blue; blue/red chequerboard in four rows; orange/light blue chequer-board in four rows; light blue; black; blue/red chequerboard in two rows;orange/blue; black; light blue; orange/light blue chequerboard in fiverows; blue/red chequerboard in five rows.

Fig. 2. Thessaloniki, Rotunda. Anonymous’ tablion (photo author).

optical colour blending in the rotunda mosaics 169

Basiliskos’ tablion is damaged at the outer left, right and lower extremi-ties, but its main part is preserved. The general structure is vertical.Around the middle, the setting is partly mirror-reversed. Six main hueshave been employed: black, violet, red, orange, light blue and blue. Lightblue sets apart areas dominated by light orange from those dominated bydark red/blue. There is almost equal stress on red/blue and orange/blueareas, but the latter are slightly more dominant. Light blue and black areused to delineate sub-areas (Fig. 3).

Therinos

Light blue; light blue/orange; blue/orange; medium blue/red chequer-board in three rows (spreading to further rows of these colours); darkblue; blue; blue/orange; blue/red; dark blue; light blue; light blue/orange(forming denticulation); blue/orange/red chequerboard in two to threerows; red/blue chequerboard in three rows; dark blue; light blue; multiplerows of light blue/orange in chequerboard; multiple rows of blue/red atthe right side.

Tesserae are missing in places, but the tablion of Therinos is generallywell preserved. The pattern is related to that of Basiliskos: it is a simple,

Fig. 3. Thessaloniki, Rotunda. Basiliskos’ tablion (photo author).

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mainly vertical setting. Hues are slightly lighter than for Basiliskos, andneither violet nor black are used. It is noticeable that orange/blue che-querboard areas melt gradually into red/blue areas. Denticulation is used.The rows are slightly irregular. A red/blue area at the left is set off from alight blue/orange area at the right by a dark blue vertical line (Fig. 4).

Leon (purple)

Dark blue; two blue rows; blue/red; blue; black; three rows of blue/redchequerboard; black; three blue rows; three rows of blue/red, alternatingwith blue/orange chequerboard; two blue rows; black; three blue rows;two rows blue/orange; two blue rows; black; dark blue; two blue; twoblue/orange chequerboard; four dark blue/red chequerboard; four rowsof blue/orange chequerboard.

Most of the middle part of Leon’s tablion is destroyed. There are alsomissing parts at the lower edge, and a large diagonal crack is due to earth-quake damage. Still, the preserved upper part is sufficient to reconstructthe general colour design. Blue to dark blue are prominent. The domi-nance of blue is presumably on account of the surrounding colour beingpurple rather than white. In order to make the tablion stand out in thecontext of a larger purple area, it had to have a bluish rather than a red-dish tint. Areas dominated by blue are set off from areas where red is

Fig. 4. Thessaloniki, Rotunda. Therinos’ tablion (photo author).

optical colour blending in the rotunda mosaics 171

prevalent. Orange is used sparingly, as if to suggest light glittering on thesurface (Fig. 5).

Eukarpion (purple)

Dark blue; blue; two rows of blue/red chequerboard; red/violet; threerows of violet/dark blue chequerboard; black; blue; red/violet; violet/dark blue; dark blue; blue; six rows of blue/red chequerboard; three rowsof red/violet chequerboard; red/blue; two rows of blue/black chequer-board; black; blue; two rows of blue/red chequerboard; red; blue; twoblue/red in chequerboard; black/blue; blue; four and more rows of blue/red chequerboard, tapering off.

Eukarpion’s tablion is well preserved and has only a few missing spots.Red, blue, violet and black tesserae are set in two-colour combinations, in-terspersed with single colour rows. A red/blue chequerboard systemdominates the regular design. The setting is rhythmic: A + B, B + C, C,etc., forming gradual transitions. Of particular interest is that Eukarpionprovides the only instance of a tablion without orange tesserae. This is sig-nificant because one might have imagined that orange would have beenchosen as a contrast to the purple cloak. A possible explanation is that be-

Fig. 5. Thessaloniki, Rotunda. Leon’s tablion (photo author).

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ing placed in the western part of the building’s interior, Eukarpion wasmore directly lit by sunlight than the other martyrs under discussion.1Thus it may have been a conscious choice to leave out orange tesserae inhis tablion (Fig. 6).

The characteristics of the tablia

As the above examination bears out, each of the six tablia varies slightly.Common to all is the chequerboard setting in vertical rows. Some have astrict vertical structure; others (Onesiphoros and Anonymous) display amore complex design including partly diagonal setting. By means of red,orange and different shades of blue, and occasional black and in two in-stances violet (Basiliskos, Eukarpion), the artists have optically created var-ious shades of purple, ranging from a red-purple to a deep bluish purple.These colours come into being when the saints are viewed from a distance

1 The influence of sun light is difficult to estimate precisely due to the fact that the situationhas changed considerably from late antiquity to modern times, where tall buildings cut outmuch daylight. Measurements of reflected light taken at various points in the frieze are alsohampered by the presence of the scaffolding and protective nettings, see Iliadis 2001 and 2005.

Fig. 6. Thessaloniki, Rotunda. Eukarpion’s tablion (photo author).

optical colour blending in the rotunda mosaics 173

of at least 4 to 5 meters.1 Thus from ground level, at some 20 meters dis-tance where it is impossible to resolve the colours of individual tesserae,the general impression is that of viewing a purple inset.

The tablia decorate either a white or a purple garment. The soldiersaints’ white cloaks were originally speckled with silver tesserae (translu-cent tesserae topped with a thin layer of silver foil which is protected by athin top layer of translucent glass). Due to the loss of most of the silver,or because it has turned dark, it is difficult to visualize the original impactof the shining silver, yet it must have been very strong (Fig. 7). The quan-tity of light reflected from a surface varies with the level of absorption andthe angle of refraction of the light as it enters the mineral. The reflectivityof minerals ranges from less than 5 % to approximately 95 % in native sil-ver (it is obviously less in silver foil overlaid with glass).2 Thus the reflec-tivity of silver is higher than that of gold. In the Rotunda, the architecturalzone inhabited by the saints is made almost entirely of golden tesserae(gold foil in the same ‘sandwich’ technique as the silver cubes). It fills thebackground and is the main ‘colour’ of the pictured buildings. However,for the medallion at the summit of the cupola, the artists chose a silverground, plausibly to make Christ stand out from the surroundings and ap-pear all the more illuminated.3 It is also worth noting that in the garments,where silver is plentiful in combination with white, golden tesserae havebeen used only sparingly.

In Leon and Eukarpion’s tablia, both of which are surrounded by pur-ple, blue has been employed more extensively in narrow vertical bandsthat separate areas of chequerboard (Fig. 8). This is why the tablia take ona bluish tint. It is possible that the chequerboard system was introduced tomake an alternative purple that would stand out against the purple cloaks,and that it was then further developed in the other garments. Visual ex-periments would then have included orange and greater variation in set-ting of tesserae.

The wings of angels

In the Rotunda, the large central medallion with the theophany of Christis encircled by a frame composed of silver stars on a blue ground, a lus-trous wreath of flowers and fruit, and finally a multi-coloured rainbow.Four hovering angels, whose fingertips barely touch the edge of the rain-bow, support the medallion (Fig. 9). The curly-haired, haloed angels are

1 As registered in 2009 by viewing the figures at various distances on the scaffolding, and atvarious times of the day, before and after noon.

2 Rapp 2009, p. 40. 3 For silver in mosaics, see Nordhagen 1974.

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dressed in white flowing robes that are regrettably mostly lost.1 Neverthe-less, it is clear that the robes are embellished with silver and purple cuff

1 The typology of the angel is close to that of the Sarigüzel sarcophagus in Istanbul, ca. 380-400, Kiilerich 2002.

Fig. 7. Thessaloniki, Rotunda. Onesiphoros (photo author).

optical colour blending in the rotunda mosaics 175

Fig. 8. Thessaloniki, Rotunda. Leon (photo author).

Fig. 9. Thessaloniki, Rotunda. Cupola:medallion with Christ, rainbow and angels (photo author).

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bands. As in the tablia of the martyrs, the purple hue derives from opticalblending: in one angel (northeast), the coloured bands are set mainly withdark blue and red tesserae in a chequerboard pattern. For the cuffs of thetwo other remaining angels (southeast and southwest), the setting is

Fig. 10. Thessaloniki, Rotunda. Cupola, detail of flying angel (photo author).

optical colour blending in the rotunda mosaics 177

varied slightly: in one, red is juxtaposed with dark blue, while in the other,red, purple, blue and black in even rows create a purple splendour. Enhanced by the shimmer of silver tesserae in the bands next to them, theangels’ cuffs acquire an effect of shining purple silk.

The angels’ large spread wings, spanning about 5 m from tip to tip, con-stitute a particularly excellent example of the colour blending technique.Due to the viewing distance of about 27-28 m, it must have been quite de-manding for the artists to calculate and work out how best to achieve thedesired effect. Viewers standing at floor level were originally supposed togain the impression of fluffy purple wings; today the wings appear largelyin brownish shades. However, by zooming in on the angels and enhancingthe photos digitally, it becomes clear that just as in the tablia and the cuffs,the purplish hue is actually created optically. It follows only a slightly dif-ferent setting than in the garments (Fig. 10). As Hjalmar Torp observedwhen studying this part of the mosaics in situ from scaffolding, the tesseraeare set according to a sophisticated system. The wing quills and feathersare set with various blues, some slightly greyish blue, others dark blue (ul-tramarine), in addition to violet hues ranging from light to reddish. Thetesserae are placed in even parallel rows running perpendicular to the wingquill, alternating in predominantly blue and reddish violet stripes. Con-tour lines in dark blue and black appear only sparingly to distinguish indi-vidual sections of the wings. The plumage is given shape and volume bymeans of colour modelling. On the central part of the wings the tesseraeare in lighter tones of glass. The pale hues serve as highlights and help cre-ate an impression of light hitting the wings. The light falling on the angelsmust be understood as emanating from the theophany of Christ-Sol in themedallion above.

In addition to its visual and aesthetic impact, the colour blending in thisinstance has a symbolic meaning. Since angels are by nature asomatoi, spir-itual beings, there is an innate contradiction and ambiguity in the visualembodiment of the heavenly messengers. Trying to solve this ambiguity,it is apparent that the artists in the Rotunda strove to create images that asfar as possible expressed the angels’ airy and insubstantial nature. The sub-tle non-naturalistic colour blending sustained the notion of the angels asweightless beings.

Optical Factors

Optical blending in antiquity

The optical technique employed for the angels’ wings and cuffs and forthe saints’ tablia (and for a few other selected textile features) seems un-paralleled in early Byzantine wall mosaics. If one considers, for sake of

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comparison, the purple tablia of the chlamydati in the imperial panels inSan Vitale, Ravenna, or the angels’ wings in that church’s apse, thesehave been rendered in a more ‘conventional’ manner in a graduatedscale of purplish and violet hues. In fact, rather than being a Byzantinemethod, optical colour blending has its origin in antiquity. As pointedout by Hjalmar Torp, many technical details in the Rotunda mosaics notleast the chequerboard pattern can be traced back to Greco-Roman art.1A simple chequerboard design is quite common in floor mosaics. Simi-larly, the setting in parallel stripes is used for rendering fish skin in orderto give an impression of the diffracted light of the scales.2 Unfortunately,due to the accidental survival of material, it is difficult to trace a deve -lopment of the chequerboard and stripe settings leading to the pointwhere a deliberate use was made of these techniques with the aim ofobtaining colour mixture.

Scientists in antiquity were certainly familiar with the principles of op-tical blending. Aristotle named three alternative ways of mixing colour,one of which was that ‘colours can mix by laying small areas of colour sideby side’ (Arist., De sensu 439b 20ff; 440a 31ff). The treatise De coloribus, for-merly ascribed to Aristotle, but now accepted as the work of one of hispupils, plausibly Theophrast, also discusses colour and the interaction be-tween light and colour.3 Speaking of plumage, the author notes that‘when exposed to the light, it has a purple (halourges) colour.’ With lesslight, the feathers become dark (zopheron) and take on a dark violet hue(orphnion). When there is much light and this is mixed with black, theplumage becomes red (phoinikion). When the feathers are bright and shin-ing, the appearance changes to a fiery-red (phlogoeides) (Ps.-Arist. Col. 792a24-29). With the emphasis on purple, violet, red and the importance oflight, it sounds like a description of the angels’ wings in the Rotunda.

A particularly important source for optical colour mixing was Ptole-my’s Optika, written in Alexandria ca. 150-175 AD.4 In the second book ofhis work, Ptolemy explains the reason for colour blending, indicating that‘because of distance […] the sight is not strong enough to perceive theparts individually’ (ii.95).5 In fact, ‘the individual angles which containvarious colours, would be insensible, and it would appear, by the compre-hension of parts that cannot be distinguished individually, that the per-ception of each of them is gathered into one perception, for the colour of

1 Torp in print (a) and (b).2 Andreae 2003, pp. 126-159, esp. p. 150, p. 156, emblema from Casa del Fauno, late second

century BC. 3 Gottschalk 1964; Ferrini 1999.4 Preserved in a Latin edition, translated from the Arab; Lejeune 1956.5 For a discussion of Ptolemy, see Smith 1988, who argues for Aristotelian influence, p. 200f.

optical colour blending in the rotunda mosaics 179

the whole object will be unified and differ from that of the individualparts.’ (ii, 95, 6-10).1 Much the same idea is echoed around 200 AD byAlexander of Aphrodisias, who writes that ‘the same colour seen up closeand from afar appears different’.2 The continued interest in optics in lateantiquity is proved by treatises such as Theon of Alexandria’s late fourth-century commentary on Euclid’s Optics. Possibly in the fourth century,too, Damianos of Larissa composed an Optical Hypothesis.3

Instances of Hellenistic optical and illusionist ideas being transmitted tothe late antique period can be seen in the mosaic known as the Asarotosoikos, ‘The Un-swept Room’ (Plin. NH 36, 184). Originally devised by Sososof Pergamon in the second century BC, the picture is preserved in severalRoman versions, the most important being the one signed by Herakleitos(Museo Gregoriano Profano). Variations on this motif were made in vari-ous parts of the Mediterranean, e.g., at Aquileia and in Tunisia, some oflate antique date.4 Alexandria was equally famous for outstanding mosaicwork in the Hellenistic period. A particularly fine mosaic dating fromaround 150-100 BC was found during the construction of the new Bibliothe-ca Alexandrina. It depicts a seated dog and is surprisingly naturalisticallymodelled in minute tesserae.5 Recent excavations in Israel and elsewhereprove that by ca. 150/100 BC mosaicists in many parts of the empire wereproficient at laying exquisitely tesselated floors.6 The same, undoubtedly,applied to the decoration of walls; in fact the wall was the obvious placefor intricate mosaic work, as surviving Roman examples indicate. A casein point is the large first century AD nymphaeum façade at MassaLubrense.7 That these technically advanced works do not make direct useof optical blending is probably due to the fact that they were meant to beviewed at comparatively close quarters, whereas a certain distance is re-quired for optical blending to work. Be that as it may, the artists of late an-tiquity could beneficially draw on centuries of practical experimentationand had become proficient in various mosaic techniques. They had ac-

1 Singuli autem anguli qui continent diversos colores, fuerint insensibiles, apparabit ex comprehen-sione partium que non discernuntur, cum omnium sensibilitas congregabitur, quod color totius rei situnum, alter quam singularum partium, Lejeune 1956, p. 60; Lejeune, p. 17*, suggests that thispassage ii, 95, 4-10 may refer to mosaics. English translation adapted from Gage 1999, p. 78f.

2 Ganson 2003, p. 383.3 Schöne 1897, p. 4, l. 17-20. 4 Werner 1998, pp. 260-275.5 Guimier-Sorbets 1998; Ead. 2004, pp. 15-34, at pp. 15-17. Andreae 2003, pp. 26-61: Alexan-

dria, Pergamon and Delos.6 Fragments from Tel Dor, Israel excavated in 2000, Stewart, Martin 2003, pp. 132-146;

Wootton 2012.7 Budetta 2005-06, glass paste and limestone tesserae; alternating large exedrae and small

rectangular niches give the impression of a garden, as preserved also in nymphaea from Pom-peii and Herculaneum.

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quired knowledge of how to exploit illusionist effects and – also – the op-tical workings of colour.

Simultaneous contrast versus assimilation

The colour blending technique used in the Rotunda brings to mind the artof neo-impressionist painters and to some extent appears as pointillism ordivisionism avant la lettre.1 The mosaicists must have faced some of thesame problems as those encountered by the painters. Around 1884Georges Seurat devised the technique of placing small dots of paint closeto one another which came to be known as pointillism, or in his own ter-minology chromoluminarism.2 Paul Signac found that Seurat’s paintingswere divided into dots that were so small they might lead to the unwantedresult of a greyish tone when the colours fused.3 In order to avoid blendingthe colours in a way that led to a loss of liveliness, Signac experimentedwith the size and density of the dots and devised a system of larger dots,which came to be known as divisionism. Signac’s aim was to use local con-trasts of different hues to intensify colours and to bring about a slightshimmering of the light.

The painters were inspired by the upsurge in colour science and colourtheory, which took place in the course of the nineteenth century.4 Mostimportantly, the chemist Michel-Eugène Chevreul, director of dyeing atthe Manufacture royale des Gobelins, worked out a law of simultaneous con-trast.5 This law was based on the circumstance that colours are affected bysurrounding colours: when the eye sees two contiguous colours simulta-neously, the colours will appear as dissimilar as possible in the strength oftheir colour (§ 16). Each of two contrasting surfaces, juxtaposed, will exaggerate its apparent difference in the direction of the other surface’scomplementary colour. Accordingly, if red is next to blue, the red will take

1 Demus 1949, p. 383, makes a general comparison between the colour-dot style of earlyByzantine mosaics and nineteenth-century pointillism: ‘this technique of mosaic was meantfor the distant view. Looked at from a distance the colour-dots appear as modelled forms’. Un-fortunately he does not discuss specific instances.

2 Gage 1987; Lee 1987; Ferretti Bocquillon 2008 (a), 2008 (b); Roque 2010. The paint-ing style is sometimes included under néo-impressionism, a term coined by the critic FélixFénéon in 1885, or post-impressionism, a term introduced by another critic, Roger Fry, in 1910.

3 Ratliff 1992; Maffei, Fiorentini 1995, pp. 133-136; colour reproductions in FerrettiBocquillon 2008 (a), 2008 (b).

4 Chevreul 1839; Fechner 1870; Blanc 1867; Helmholtz 1867; Bezold 1874; Fechner1876; Rood 1879; Henry 1888; for an overview of their work, see Kemp 1990, pp. 306-319. Ques-tions of colour in history, Kemp 1990, pp. 259-322; Gage 1990, 1993, 1999.

5 De la loi du contraste simultané des couleurs et de l’assortiment des objets colorés, 1839. It wastranslated into English and German soon after.

optical colour blending in the rotunda mosaics 181

on a yellowish tint, and the blue a greenish tint.1 When complementaries,e.g., red and green, are seen next to one another, their colour effect is ineach case intensified, since the simultaneous use of contrasts reinforcesthem. However, as Seurat and Signac also experienced, for colours thatmutually enhance each other, the very opposite result might occur whencomplementary colours are too close together. The effect depends on thesize of the juxtaposed elements. Thus if the elements are too small or toothin, rather than enhance the intensity of the individual colours, they willfuse and lead to assimilation.

Assimilation, or colour spreading, was discussed by the meteorologistWilhelm von Bezold.2 In assimilation the laws of contrast are reversed. Inone of Bezold’s plates, a blue meander on a red ground appears a darkerblue when lined with black, but a lighter blue when lined with white. Thesame applies to the red ground; both the darkness and the lightness ‘spread’.Similarly, viewing a black pattern on a blue ground next to a white patternon the same blue ground produces the distinct impression of two differenthues, a dark and a light blue, while in fact there is but one blue (Fig. 11).3The opposing principles of contrast and assimilation are related. Indeed,much the same configurations may, depending on factors such as size, distance and lightning, lead to either assimilation or contrast – or neither.

These principles also apply to mosaics. Tesserae of a given colour willlook different depending on what colour they are set next to. In the simul-taneous contrast where a blue and orange chequerboard is juxtaposedwith a red and blue chequerboard, the light surroundings (orange) lightenthe blue, while the relatively darker surroundings (red) darken the blue. Inmost tablia, red tesserae make the blue appear darker, while orange makesthe blue appear lighter. The difference between the blue gradations is sub-tle and the eye is easily tricked. The mosaic artists also made clever use ofintersecting or dividing lines to make adjacent areas look either darker orlighter (Fig. 12). This is possibly consistent with Bezold’s observation thata line separating two coloured surfaces may almost neutralize their effecton each other.4

Luminance

In recent years neuroscientists have made advances in research on opticalphenomena related to those phenomena addressed by Chevreul and Be-

1 Kemp 1990, p. 306f; Roque 2009; Roque 2011. 2 Bezold 1874.3 Bezold 1874; discussed by Evans 1948, p. 181, pl. xi, and Burnham 1953 with fig. 1. The

illustration is also reprinted in Gombrich 1961, p. 309, fig. 251. Factors such as viewing distanceand manner of viewing must also be taken into account.

4 Burnham 1953, p. 378.

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Fig. 11. The Bezold effect: colour assimilation (Bezold 1874, after Evans 1948, pl. xi).

optical colour blending in the rotunda mosaics 183

zold. Discussing the optical effects of impressionistic paintings, the neuro-biologist Margaret Livingstone proposes that colours blend as a functionof isoluminance. If the dots of paint (or the tesserae of a mosaic) are ap-proximately isoluminant – that is, of the same luminance or lightness – thecolours will blend.1 In an image, the blending occurs within each region,with each part of the image defined by the average luminance of thebrushstrokes or dots. To find out whether an area is isoluminant, one canseparate colour and lightness by turning the image into a greyscale ver-sion; areas of isoluminance will then appear homogenous with little vari-ation between individual hues.

As experiments with turning the mosaic photos into greyscale show, inplaces the colours appear to be approximately isoluminant (i.e., of approx-imately the same lightness). In Basiliskos’ tablion, the light blue and orangetesserae appear to be truly isoluminant, while the red next to the medium

1 Livingstone 2002, p. 173, pp. 176-178. I am grateful to art historian and neurobiologist PerOlav Folgerø for discussions on neurobiological and neurophysiological matters. His lectureson neuroaesthetics at the University of Bergen gave important insight into the fascinating fieldof current research.

Fig. 12. Thessaloniki, Rotunda. Tessera setting in Eukarpion’s tablion (photo author).

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blue tesserae in the chequerboard are approximately isoluminant. The sameapplies to Onesiphoros and Anonymous’ tablia. This brings about the senseof a scintillating surface, somewhat as in the sun in Monet’s Impression. Lesoleil levant from 1872, where the orange sun is the same luminance as theblue clouds. Livingstone’s explanation of the effect of impressionist paint-ings is that low to isoluminant luminance contrasts create an illusion ofmotion.1 When juxtaposed, colours of the same or near luminance cangenerate a sense of vibration and scintillation. This effect was especiallyimportant in glass mosaics. In the diagonally set parts of the tablia of One-siphoros and Anonymous, isoluminant orange next to light blue under op-timal viewing conditions make for a slightly scintillating effect, as if thispart of the dress is suggested to reflect the light differently from the rest;one gets the impression of silk lightly caught by a breeze (Fig. 13).

Colour Blending in the Mosaics

When seen at a distance, the eye cannot perceive the red and blue tesseraeas separate entities, so they fuse and look purplish. That the red and blue,at times interspersed with a little black, assimilate and blend to becomepurplish is hardly surprising; more surprising is the mixture of the com-plementaries orange and blue, where contrast would be expected. Expe -riments conducted with different viewing distances for the material inquestion show that where orange/blue are set in comparatively large areas, they tend to produce an effect of a brownish colour (e.g., the angels’wings, Therinos’ tablion). In Therinos’ case the problem seems to be that,somewhat as in Signac’s complaint of Seurat’s paintings, the opticallyachieved colour verges on becoming indistinctive and blurred rather thanstanding out in a clear hue. In other instances (e.g., Basiliskos’ tablion)where the area of orange/blue chequerboard is comparatively narrow andalternates with a red/blue area of approximately equal size, the blue sub-ordinates itself to the orange. A further effect is that the orange, as in a si-multaneous contrast, accentuates and increases the effect of purple in ad-jacent areas. It may be suggested that loss of form perception withisoluminance or low luminance contrast could be the reason why thetesserae are not perceived as separate unities but blend and melt into one.Seen from a distance, the tesserae are simply too small to distinguish indi-vidually, so they are bound to blend – as noted already by Ptolemy and other early scientists.

As for the Rotunda mosaics, it is questionable whether the early Byzan-tine viewers at some 20 m distance could actually perceive optical subtleties

1 Livingstone 2002, p. 66, p. 153.

optical colour blending in the rotunda mosaics 185

such as the intended scintillation. Most of the effect would undoubtedlyhave been lost to the eye in the same way that one cannot, from the floor,distinguish the minute designs on the soldiers’ shoulder decorations (Fig.14). Possibly the technique of optical mixture was first used in residentialwall mosaics that had more adequate viewing distances. But in the Rotun-da the effects were still taken into account, as if designed for an ideal viewerwho stood at some 5 to maximum 10 m distance, where the illusion is mostpowerful.1 Actually, this ideal viewer was not an earthly, but a heavenly being. Furthermore, in spite of some effects being undoubtedly lost to thehuman eye, the optical mixture is successful inasmuch as purple hues doappear when the figures are seen at the optimal distance.

The colour blending technique employed for selected motifs in the Ro-tunda served several purposes with regard to hue and luminance. More-over, the technique increased the impression of three-dimensionality andvariety. These visual effects are largely interdependent, but they will beoutlined separately here:

1 One can compare the triumphal columns, the finer details of which were also problematicto make out from a distance, Brenk 2005.

Fig. 13. Thessaloniki, Rotunda. Part of Onesiphoros’ tablion (photo author).

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– Colour

Purple was the most highly praised colour in antiquity.1 The apprecia-tion of purple hues is substantiated by the large vocabulary – porphyreos, porphyroeides, halourges, ostreios, thalassion, orphninon, purpureus, violac-cea, amethystinus, ianthina – that exists for the purple-violet scale.2

In the Rotunda, the mosaicists rendered purple by various means: forlarger areas the common method was to set various shades of brown-ish-violet glass tesserae. These were used for the paenulae of the clerics(brownish-violet interspersed with among other hues olive and beige),and for the chlamydes of Leon and Eukarpion. Eyes and hair were alsomade of purple-violet glass (in the hair it was interspersed with othercolours such as yellow and red).3 But for the tablia a different technique,that of optical blending, was chosen. Except for a small percentage inBasiliskos and Eukarpion’s tablia, purple glass is basically absent; insteadpurple hues are created by the optical mixing of blue, red, orange and

1 Longo 1998; Bradley 2009, pp. 189-211; Kiilerich 2012 (b), pp. 12-14.2 Blum 1998, pp. 20-41. 3 Kiilerich 2007, pp. 334-336.

Fig. 14. Thessaloniki, Rotunda. Onesiphoros’ shoulder decoration showingmale figure (photo author).

optical colour blending in the rotunda mosaics 187

black in different proportions. By means of this method, the chromaticeffects become more intense. So, while the paenulae prove that it wasquite possible to render purple by using glass tesserae in purplish hues,the overall result would perhaps have been less intense, as purple glassmay tend to become brownish.1 It can therefore be proposed that onereason for using colour blending was to achieve more saturated purpleand subtler gradations of hues.

For the angels’ wings, the colour purple had the further advantage ofevoking light, since porphyreos also means splendid and gleaming.Rather than using a solid colour to represent purple, the blending tech-nique helped suggest that the heavenly beings were creatures of light.

– Luminance

A main point in mosaics of gold, silver and coloured glass is of courseto evoke light and luminosity. As an expression of heavenly light, it is notonly an aesthetic but a religious and symbolic matter: an area investedwith light symbolized divine presence.2

In the angels’ wings, light effects reflected divine illumination. In-coming light that hits tiny irregularities in the feathers’ surface is dif-fracted. In nature, this can be seen most spectacularly in peacock fea -thers.3 In order to achieve a heightened brilliance, the mosaicists tookadvantage of luminance effects. By setting small areas with isoluminant(equal value) tesserae, the artists attained an intensified luminosity. Theoptical effect of the juxtaposed tesserae gave the impression of shimmer-ing angels’ wings in gentle motion, and scintillation in the surface ofsilken tablia.

– Texture (Silk)

The mosaicists were required not only to render a specific dress colour,but also a particular quality of fabric, namely silk.4 Because of its prism-

1 Fiori 1992, p. 51: ‘Il colore passa al marrone se è presente anche in piccola quantità il ferroe manca, o ci sono solo tracce, di manganese’. In wall mosaics from Amorium in Asia Minor,there are no violet glass tesserae, except for a very dark, almost black hue, Wypyski 2005, p. 183,187; see also James 2006.

2 Boorsok 2000, for the importance of the mosaic medium’s brilliancy to express divineillumination.

3 Livingstone 2002, p. 23. It may be worth noting that the Rotunda mosaicists did not useoptical blending for rendering peacocks and other birds. Plausibly the birds were set by otherartists who worked in a different tradition.

4 It is uncertain whether imperial soldiers’ cloaks were made of silk or of linen. However,no matter what the saints wore as soldiers of the imperial court, inhabiting the heavenlycourt, they were undoubtedly imagined and pictured as if dressed in the finest possible fabric,namely silk.

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like structure, silk cloth refracts light at different angles. When lightplays upon it, the surface of a silk garment appears to ‘glisten, trembleand graduate in colour.’1 While the lustrous sheen can be simulated byproficient artists in oil paint, it is much more difficult to give an impres-sion of silk in the mosaic technique.

The finest silks were coloured purple, which, due to the rarity of themurex shell, was the most expensive clothe colour in antiquity.2 Ge -nuine sea-purple dyes, blatta, oxyblatta and hyakintha, were reservedfor the imperial household (Cod.Iust. iv, 40, 1).3 Other terms for purple-coloured silk include alethina, true colour, and oxys, meaning bright.While it is difficult to assign these terms to specific hues, at least it maybe assumed that the various optically achieved colours in the saints’ gar-ments could correspond to textile choices. It is therefore tempting to as-sociate hyakintha (bluish purple?) with Eukarpion and Leon’s tablia; oxy-blatta (bright purple) with those of Onesiphoros and Anonymous, andblatta (purple) with those of Basiliskos and Therinos.

For the angels’ wings, the technique was particularly effective for ren-dering the specific texture, density and weightlessness of feathers andplumes.

– 3-D effect

In the tablia, the juxtaposing of contrasting colours – especially in thesetting off of a relatively light area by a dark blue or black vertical stripe– made the dark area recede into the background whilst pushing thelight area forward, thus creating a slight three-dimensional effect sug-gestive of garment folds. It is argued therefore that the colour setting al-so serves to indicate depth and to differentiate receding and precedingplanes, that is, to suggest a plastic modelling of the surfaces.

In the angels’ wings, the setting gives an impression of the variouslayers of feathers and plumage and intimates depth by means of subtlecolour modelling.

– Differentiation

The subtle differentiation of the individual tablia by colour combina-tions, form and design is sufficient to distinguish one figure from theother simply by looking at his tablion. This tendency to differentiate isin keeping with the general presentation of the saints, whose physiog-nomies vary considerably with regard to represented age and hair and

1 Hallett, Johnston 2010, p. 114. 2 Longo 1998.3 Kiilerich 2007, pp. 335-336; Kiilerich forthcoming.

optical colour blending in the rotunda mosaics 189

beard fashions.1 Of those dressed in white, the fairest faces, One-siphoros and Anonymous, have the lightest tablion hues; while the twowith darker hair, Basiliskos and Therinos, similarly have somewhatdarker colours in their tablia. Finally, for the purple-clad Leon and Eu-karpion, the challenge of juxtaposing one purple area next to anotherled to the choice of a bluer tone for the tablia in order not to dull thepurple cloak, but make the two parts of the dress clearly distinguishable.

For the rendering of the angelic apparition, variation was also re-quired, inasmuch as wings are composed of variously sized feathers andplumes and fine fluffy down that in nature display many forms andshades.

Conclusion

For the purple tablia of the soldier saints’ chlamydes, for the cuffs of the an-gels’ tunicas and for the feathers of the angels’ wings, the artists who de-signed the Rotunda mosaics refined a technique of optical colour blend-ing, one and a half millennia before a related technique was reinvented byneo-impressionist painters. The scientific basis was grounded in the studyof optics in the Aristotelian tradition, although the artists’ skills were plau-sibly practical more than theoretical.

By adopting this elaborate and time-consuming method rather than amore conventional setting with various purple hues, the Rotunda artistsachieved astounding results with regard to colour, luminance, texture,three-dimensionality and variety. The artists could make a visual impressionof splendid purple hues and purple gradations from light to dark; a scintil-lating and light-infused surface; shimmering feathers in angels’ wings; theimpression of iridescent silk; almost weightless plumage; a three-dimen-sional effect in the plastic colour modelling of folds and fea thers; and differ-entiation in each tablion’s colour and design. In effect, the early Byzantineartists exploited colour perception and successfully tricked the eye.

University of Bergen(bente.kiilerich@lle.uib.no)

Bibliographical abbreviations

(See also the shortened forms listed on the first pages of the volume, pp. 11-13)

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composto in carattere dante monotype dallafabriz io serra editore, p i sa · roma.

stampato e r ilegato nellatipo grafia di agnano, agnano p i sano (p i sa) .

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