Microscopic techniques to study Roman renders and mural paintings from various sites

M A T E R I A L S C H A R A C T E R I Z A T I O N 6 0 ( 2 0 0 9 ) 5 8 6 – 5 9 3

Microscopic techniques to study Roman renders and muralpaintings from various sites

Johannes Webera,⁎, Walter Prochaskab, Norbert Zimmermannc

aInstitute of Art and Technology, Conservation Sciences. University of Applied Arts Vienna, AustriabDepartment of Applied Geosciences and Geophysics, Chair of Geology and Economic Geology, University of Leoben, AustriacInstitute for Studies of Ancient Culture, Austrian Academy of Sciences, Austria

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +43 1 71133 4825E-mail addresses: [email protected]

[email protected] (N. Zimme

1044-5803/$ – see front matter © 2008 Elsevidoi:10.1016/j.matchar.2008.12.008

A B S T R A C T

Article history:Received 23 May 2008Received in revised form28 November 2008Accepted 3 December 2008

Based on recent research on mural paintings from a number of ancient Roman sites, i.e.Ostia, Ephesos, Carnuntum and Saalfelden, the contribution presents the results obtainedbymicroscopic techniques used to study similar paints from different sites as well as paintsof different artistic quality, style and date from one and the same site.It is shown that careful microscope investigations are capable of detecting the stratigraphicsequence of plaster layers, the way of preparing and applying themortars, the rawmaterialsused, and the means to produce the desired surface in terms of colour and texture. Theanalyses focus on petrographic thin-sections observed in different modes of illumination.Pseudocoloured images produced from characteristic sample micrographs are presented inorder to better illustrate the relevant features.The preparatory — arriccio-plaster layers always contain aggregate from local sources,preferentially unwashed river sands with enhanced amounts of silt. For the finish —intonaco-plaster layers, the aggregate was produced by crushing selected stone material oflight colour; when available or appropriate, marble was employed, but other stones can befound as well. For high quality paints, a number of intonaco layers were applied alwaysusing the same raw materials at reduced grain sizes and eventually admixing pigments.While this was the usual way to produce a smooth surface, a few examples were foundwhich indicate grinding of the hardened intonaco prior to painting.Themicroscopic aspect of the paint layers clearly point to the application of a lime-pigmentcolour onto a dry or semi-dry surface, i.e. no true fresco technique was used.

© 2008 Elsevier Inc. All rights reserved.

Keywords:Roman wall paintingPaint techniqueMicroscopy

1. Introduction

The technique used by ancient Romans to render walls andcover them with paintings has been a matter of interest formany researchers. The main contemporary written sources,i.e. Vitruvius and later Pliny, give valuable information on thepreparation of lime andmortars, their application on the wall,and the final steps to prepare an appropriate ground to thepainting [1,2]. The paint technique itself is also described by

; fax: +43 1 5321447 4829c.at (J. Weber), walter.prormann).

er Inc. All rights reserved

those authors, but their indications are too vague to allow for aclear understanding of all steps involved. This fact, as well asthe large range of periods, styles, locations and architecturalcontexts encountered when dealing with Roman murals,necessitates the study of samples by modern techniques.

Through thepast fewdecades, theuse ofmicroscopemethodsfor the analysis of renders, plasters and paintings has receivedincreasing attention. Elsen [3] has given a comprehensive andcritical review of these techniques as opposed to traditional wet

[email protected] (W. Prochaska),

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http://dx.doi.org/10.1016/j.matchar.2008.12.008

Table 1 – Investigated archaeological sites.

Site Administrationin charge

Reference Acronym

Ostia –Domus of theBucranium

Soprintendenza peri Beni Archeologicidi Ostia/Universityof Lyon

[4] O-DB[5]

Ostia —Santuario Bona Dea

Soprintendenza peri Beni Archeologicidi Ostia

O-BD

Ostia —Casa delle Ierodule

[6] O-CI

Ephesos —Terrace House 2

AustrianArchaeologicalInstitute/AustrianAcademy ofSciences

[7] E-HEE-HH

Carnuntum Archaeologie ParkCarnuntum

[8] C-CA

Saalfelden Salzburg MuseumCarolinoAugusteum

[9] S-SA

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chemicalmethodsofanalysis. It is clear that, fromtheappropriatecombination of light optical and electron microscope investiga-tions of ancient mortars and paints, artistic and handicrafttechnologies can be studied with regard to details that can notbe assessedbyothermeans. Someof thesedetailsmaybeof greatinterest to e.g. archaeologist involved, because theymay indicatethe source of raw materials and illuminate various aspects ofprocessing, working and applying these materials. Such studiesmay therefore improveknowledgeofhowaconstruction sitemayhave been organised in past epochs.

Theaimof thispaper is topresent insights intoancientRomantechniques of plastering and painting, gained by traditionalmicroscope methods for a number of examples from severalancient Roman sites. For the sake of a clearer visual impression,pseudocolour images produced from relevant micrographs areused to illustrate the findings. The samples had been received bythe archaeological services in charge of these sites during thepastfour years. Some of the studies, i.e. Ephesos and Ostia, have beenperformed on a relatively large scale; they are not yet completed.The other studies were of a narrower range. However, all of theresults presented here are based on a large enough number ofsamples to ensure their representativeness.

2. Questions of Interest

The major questions of interest to archaeologists whenpainted architectural surfaces are examined by scientificmeans are: which materials and technologies were employed,to what extent do they differ from place to place or from oneperiod to another within one and the same archaeological site,and; what are the similarities between similar paintings fromdifferent sites. In this context, all information on the natureand manufacture of the raw materials — binders, fillers andpigments — and on their provenance is of importance as wellas all details regarding the technical and artistic procedure tomix and apply them onto the wall.

3. Investigated Sites

The analyses presented in this paper refer to four excavations ofdifferent size, geographical position and age. Three of them, i.e.Ephesos, Ostia and Carnuntum, represent excavations of impor-tant cities or towns situated in the centre of the Roman Empire(Ostia) or in the periphery (Ephesos and Carnuntum), respec-tively. The fourth site (Saalfelden) stands for an isolated buildingin a rural environment. In the case of Ostia and Ephesos, muralsfromseveral periodswere investigated. In thisway, the study canbe seen as an attempt to cover a range of different conditions ofRomanwall painting but does not attempt to provide a completepicture of the range of techniques employed by the Romans.

Table 1 lists the sites and the administrative bodies incharge of them.

3.1. Ostia

Ostia, known today as Ostia Antica, was the port city of Rome,situated some 25 km from the capital. Ostia is placed on the

alluvial soils of the Tiber river, in an area of volcanic origin,yielding a material known for its use in so-called Romanconcrete and mortars. Travertine is another type of rockoutcropping in the surroundings, where it has been quarriedsince ancient times. Marbles do not occur in the region, butthey were shipped to Ostia from different places of theMediterranean on a large scale, and stocked and eventuallyworked there before transport to Rome [10].

The town of Ostia was of growing importance from the 2ndcentury BC and was enlarged and largely rebuilt from the 2nd(AD) to the early 3rd century A.D. Three sites have beeninvestigated so far by the authors: Domus of the Bucranium, alarge private home at the place of the Scuola di Traiano, datingfrom the 2nd to the 1st century B.C. and decorated with interiormural paintings of the 2nd Pompeian style; further on, theSanctuary of Bona Dea with paintings of the 4th Style, originatingfrom the late 1st century A.D.; and finally the paintings of the so-called Casa delle Ierodule, an insula in the function of anapartment building, dating from the 2nd century A.D.

3.2. Ephesos

Ephesos on the west coast of Anatolia is well-known as one ofthe most important ancient cities in Asia Minor. Its geologicalsurroundings are characterized by epi- and mesozone meta-morphic rocks, including a number of different silicate rocksandmarbles. Pure whitemarbles which were extensively usedfor architecture and sculpture occur in several strata of limitedthickness — some tens of meters — approximately 15 kmnorth of Ephesos. No volcanic rocks exist in that area.

Ephesos experienced continuous prosperity between the3rd century B.C. till the end of the 3rd century A.D. Its TerraceHouses, blocks of flats (apartments) of different use andpurposes from living to representation, form one of the largestand finest interiors preserved in the Eastern Roman Empire.The wall paintings there which have been studied by theauthors were executed in the Classical Roman period inseveral stages from the 1st to the 3rd century A.D. However,recent findings of earlier painting fragments from the 2nd or

Table 2 –Major material characteristics of the investigated wall paintings.

Arriccio aggregate Intonaco Paint layer pigment

thickness aggregate layers

OstiaO-CB volcanic 5 mm marble 2–3 (last one or two pigmented) cinnabarO-BD volcanic 4–5 mm travertine 2 cinnabarO-CI volcanic 5 mm volcanic 1 red ochre

EphesosE-HE metamorphic silicate rocks 7 mm marble 2 (last one sometimes pigmented) cinnabarE-HH metamorphic silicate rocks 1–2 mm marble 1 red ochre

CarnuntumC-CA quartz 8 mm fossil limestone 2–3 red ochre

SaalfeldenS-SA dolomitic limestone 2 mm quartz 2 (last one pigmented) red ochre

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1st century B.C., i.e. from the late Hellenistic period, and intype matching closely the Pompeian 2nd Style of Ostia, havebeen also included in the present study.

3.3. Carnuntum

Carnuntum, the former capital of the Roman province ofPannonia and situated east of Vienna, was an important civiland military town between the 2nd half of the 1st and themiddle of the 4th centuryA.D. Stretched along to the river banksof the Danube, the site sits on glacial sediments bedded on

Fig. 1 –Example of a detail of a thin-section of a pigmented intonaa, b: transmitted light without resp. with crossed Nicols; c, d: inc

Tertiary fossil limestones which are exposed in several placeswithin the area. No marbles occur in the region of Carnuntum.

Recent excavations in Carnuntum brought to light a largenumber of render fragments carrying wall paintings. Theirexact dating is still doubtful. Some of the fragments have beenstudied by the authors.

3.4. Saalfelden

Saalfelden, a small town in the Austrian county of Salzburg, isthe place where a Roman villa has been excavated. Saalfelden

cowith paint layer, viewed in differentmodes of illumination;ident light with resp. without reflecting background.


is situated in a valley which cuts perpendicularly throughseveral geological zones of the Eastern Alps, comprisingdifferent types of metamorphic rocks and schists. The townitself is surrounded by mountains formed by Mesozoic lime-stones, many of them of dolomitic nature.

The Saalfelden villa dates from between the end of the 2ndand the first half of the 3rd century A.D. The paintings whichhad covered the vaulted ceiling and adjacent portions of thewalls have been analysed as an example for rural provincialinterior decoration (Table 2).

Fig. 2 –Pseudocolour images produced from thin-section microgrlayers in samples representative of the sites investigated. (a) keyDomus of the Bucranium, (c) Ostia – Santuario Bona Dea, (d) OstiHellenistic, (f) Ephesos – Terrace House 2, Classical Roman, (g) Sa

4. Materials and Methods

The analyses presented in this paper were based on largeenough samples to include all relevant plaster layers togetherwith the paint in one single piece. This is of utmostimportance, because a separation of layers is inevitablyconnected with a loss of information. If a sample is too thickto be prepared as a single cross-section, two or more sectionswith an overlap of layers should be produced. In view of this, a

aphs, illustrating the appearance and composition of plasterto aggregate types and binders (black and white); (b) Ostia –

a – Casa delle Ierodule, (e) Ephesos – Terrace House 2, Latealfelden, Roman villa, (h) Carnuntum.


typical sample had a surface size of 1 to 2 cm2 and a thicknessof 1 to 2 cm.

Thin-sections were prepared from samples vacuum-impregnated in epoxy resin to which usually a blue dye wasadded. Instead of covering the sections with a glass slide, theyjust coated them with a resin, which could be easily removedwith acetone. In this way, the thin-sections could eventuallybe used for analyses in the scanning electron microscope.

The thin-sections were studied under the light opticalmicroscope at different conditions of contrast and illumina-tion. It was useful to look at the sections not just in thetransmitted-light mode with parallel and crossed polarisers,respectively, but additionally under incident light. This wasdone in twoways, i.e. using a white reflective background, andputting them onto a transparent glass carrier, respectively. Inthis way, two different images providing different informationcould be obtained just for the incident-light mode.

By employing all of the above described modes ofillumination and without changing the position of the sample(Fig. 1), a series of micrographs at low to intermediatemagnification were produced. With the different methods,different phenomena of interest can be observed. In order tobetter demonstrate the nature of plaster and paint layers, itwas decided to combine the observed features in pseudo-coloured images by editing the micrographs using standardsoftware.

In addition, polished sections were prepared mainly tostudy paints and pigments. They were analysed by lightoptical and scanning electron microscope (SEM) meansemploying an energy-dispersive X-ray analytical device (EDX).

5. Results

5.1. Plaster Layers

The basic way of plasteringwas found to be similar in all of thesites. A first preparatory layer, called arriccio, was applied onthe walls. Its thickness may vary according to the structure ofthe masonry. The following intonaco layer is again of varyingthickness; in some sites it was applied in several steps,sometimes adding pigment or coloured aggregate to the lastapplication.

Both arriccio and intonaco are based on lime.Representative pseudocoloured micrographs are given in

Fig. 2.

5.2. Arriccio Layers

As a rule, local sands have been added as aggregates. Thus,their petrographic composition varies from site to sitereflecting the local geological conditions, as does the rangeof grain sizes and the degree of roundness. River sands prevailin most of these layers, and significant amounts of fine grainsin the silt size, well dispersed within the bindermatrix, appearfrequently, indicating that no sieving or similar processing ofthe sand had been performed. This makes the arriccio mortarslook more brownish in colour. From the viewpoint of binderanalysis by means of SEM/EDX, frequently high amounts of Si

and Al pretend hydraulic limes, but careful analysis of smallspots as well as of lime lumps eventually present show thatalways pure lime had been used and the silicate is due to thefinely dispersed portion of the aggregate.

The usually wide distribution of grain sizes and the leannature of the mortars used to prepare the arriccio is the mainreason that shrinkage cracks are rarely observed in theselayers, while most of the pores appear as air pockets,indicating relatively wet mixtures of mortar and no specificexertion of pressure with the trowel in the course ofapplication. This is also confirmed by the structure of the ar-riccios' surface, which is always uneven.

The fact that the arricciomortar had always been left to dryand set at least for a day or two before the following layer wasapplied is very obvious from a thin layer of precipitation ofcalcium carbonate present in virtually all samples, and by theobservation that no cracks are propagating into the intonaco.

5.3. Intonaco Layers

Mixing and application of the intonaco mortar had beenperformed more carefully than for the underlying arriccio.This can be understood by a number of observationswhich aresummarized below.

A significant characteristic of these layers is their thick-ness, which varies between 1–2 mm and as much as 8 mm.Applied on the arriccio which provides a more or less smoothsurface, the thickness of the intonaco is independent of thestructure of the masonry but is rather inherent to the specifictechnique of the period in a given site or area. It appears thatthick intonacos usually consist of several layers which fre-quently are similar in composition and can therefore beidentified, if ever, only by careful microscopy (e.g. O-BD,Table 1). They have been applied wet-in-wet and onlyexceptionally the interface between these layers had beensmoothed, which helps to provide good adhesion. Themasonstended to use finer aggregates at reduced ratios of aggregate-to-binder the closer they came to the finish.

In some cases, the last application forming the ground of thepaint was employing a pigmented mortar mixture which wasprepared by adding small amounts of fine ochre and sometimeseven cinnabar to the lime. This technique can be best observedwith the O-DB samples where this mixture has been used toobtain a smooth surface on a relatively rough underlay.

Thechemical compositionof thepigmented layer's binders ischaracterized by increased amounts of silica and alumina, but itis not yet clearwhether this is a consequence of the clay contentcontaminating theochrepigment, or of thedeliberateadmixtureof some clay, or rather to the use of slightly hydraulic lime.

As a rule, to prepare the mortar for the intonaco, carefullyselected aggregate of a light or even white colour wasadmixed; its petrographic nature depends not just on theavailability of local material but also on the quality of a givenpainting. Thus, intonacos of high quality Second PompeianStyle paintings in Ostia (O-CB) always contain white marblefragments as the only aggregate, a material obviouslyobtained from the local factories working on importedmarble.It is interesting to note that those marble aggregates whichcan sometimes be found in the arriccio mortars, are well-rounded as opposed to the angular grains of the intonaco, and

Fig. 3 –Pseudocolour images produced from thin-section micrographs, illustrating the appearance at the surface of intonacolayers shown in Fig. 2 and the interface to the paint layers in the sections presented in Fig. 2. (a) key to aggregate types andbinders (color); (b) Ostia – Domus of the Bucranium, (c) Ostia – Santuario Bona Dea , (d) Ostia – Casa delle Ierodule, (e) Ephesos –Terrace House 2, Late Hellenistic, (f) Ephesos – Terrace House 2, Classical Roman, (g) Saalfelden, Roman villa, (h) Carnunutum.(For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)


thus indicate a totally different source, i.e. marbles from theItalian peninsula which have been transported and sedimen-ted by the river.

Other painted Ostia intonacos originating from later periodsused either travertine (O-BD) or even volcanic rock fragmentssimilar to the arriccio (O-CI) as aggregates.

Ephesos painting intonacos, on the other hand, employedmarble fragments throughout the centuries, obviously asa consequence of the fact that white marbles occur in theclose vicinity of that site. The most significant difference

between Hellenistic and Classical Roman intonacos lies in thethickness of this layer, i.e. changing from about 7 mm in theformer to 1–2 mm in the latter plasters. It must be mentionedthat the arriccio mortars of all those samples never contain anyfragment of pure marble but eventually just siliceous marbles,which occur in different strata of the metamorphic formationsin the area of Ephesos.

For provincial paintings such as the ones of Carnuntumand Saalfelden — both sites lack marbles in their localproximity — different types of aggregate were used according


to light-coloured rock material of local origin: fragments of afossil limestone in the Carnuntumsamples, or of fine quartz inSaalfelden plasters, respectively.

Depending on its mechanical properties following thepetrographic nature, the shape of the intonaco's aggregatetends to bemore angular than in the arriccio. It is clear that therespective fragments were derived from crushed stones ratherthan from river sands.

The ratio of aggregate to binder in the intonaco mortarsvaries markedly from one site to the other. A typical intonacomortar would have less aggregate and a narrower grain sizedistribution compared to the arriccio. Thick mortars usuallyexhibit shrinkage cracks running perpendicular to the surface,despite the obvious effort made to press and smooth themupon their application.

5.4. Paint Layers

The technique of painting is another matter of interest. Due tosurface decay phenomena, the original aspect of paint layerscan not always be retrieved. However, for the paintingspresented here, it must be assumed that smooth surfaceswith at least some degree of gloss had been achieved.

The interface between a paint layer and its base is howeverwell preserved in most of the samples and can provide goodinformation about the procedure to produce paints with asmooth surface.

Typical features are illustrated in Fig. 3.The approach to prepare a smooth surface to paint on

differs between the paintings investigated:The O-DB paintings, which show the highest gloss and

deepest colour hues, (for red paints cinnabar was used as theonly pigment) were put on a very smooth base. As alreadymentioned, the smoothness of these underlays was producedby applying thin layers of pigmented lime with some fine-grained marble powder that was worked with a suitable tooluntil they attained their appropriate surface finish.

A similar, though less careful, procedure was employed inthe case of the Saalfelden paints S-SA, where quartz powderwas admixed with tiny mica instead of marble, and thesurface was left less smooth compared to the O-DB group.

The other paints investigated lacked that kind of fine-grained and pigmented base; they are laid directly on the in-tonaco layers with their relatively coarser aggregate, butusually higher binder portions enabled creation of rathersmooth surfaces. Just for the E-HE group the microscopicfeatures — fragments of marble frequently reaching the veryinterface with the paint — yield evidence that the lastintonaco layer was ground after setting. Hence, there is nolayer of precipitated lime on top of this base, because it hasbeen removed during the grinding process.

Thepaint itself is of pasty appearance in all of the samples. It isbased on either fine red ochre of good quality, or cinnabar. Nomixtures of the two pigments were found. Lime is the main andprobably only binder used to fix thepigments; to date, all attemptsto identify organic compounds have not been successful.

The boundary between intonaco and the paint layer isalways sharp enough to exclude any true fresco application, aresult which is confirmed by the paint's consistency. Thecolours were hence prepared by mixing the pigment with

some lime andwater and the paint was laid on the carbonatedbase. Its shrinkage cracks are bridged by the paint layer.Eventually the colour was liquid enough to penetrate into pre-existing cracks. This technique is frequently referred to aslime-secco-painting.

Care was taken to detect even small amounts of claypossibly admixed to the colour in order to allow for betterpolishing. A lack of silica and alumina in EDX-analyses,however, would seem to exclude this hypothesis.

6. Discussion

The observations presented in this paper indicate that thematerials used to prepare a wall's surface for paintings mayhave significantly differed from what is described by Vitru-vius. In particular, when mixing mortars for plastering,preference was given to local sands rather than using marblefragments, as long as such sands could fulfil the need toproduce a pale, smooth and compact enough surface to painton. As far as the application of more than one layer to preparethe intonaco is regarded, however, Vitruvius' advice was largelyfollowed. Moreover, the admixture of pigments to the mortarof the last intonaco layer, as observed in some of the samples,exceeded that advice in elaborateness. In places where thepaint layer is worn away, such a technique is likely to leavebehind a coloured surface which could be easily misinter-preted as pigments having penetrated the intonaco from thepaint layer. Thus, a true fresco technique could be erroneouslyassumed in place of the lime-secco technique which was infact revealed by all of the studied samples.

Acknowledgements

The authors would like to thank all those who enabled thepresent study by supplying themwith samplematerial, exportpermits and all necessary information, namely St. Falzone,Chr. Gurtner, F. Humer, F. Krinzinger, S. Ladstaetter, Th.Morard and B. Tober.

The scientific input of T. Bayerova and E.-M. Maurer isgratefully acknowledged.

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[3] Elsen J. Microscopy of historic mortars — a review. Proc 10thEuroseminar on microscopy applied to building materials.Paisley, Scotland; June 21–25 2005.

[4] Morard Th. Le plan de la Domus aux Bucranes et son systèmedécoratif: pavements – parois peintes – stucs – plafonds. In:Perrier B, editor. Villas, maisons, sanctuaires et tombeauxtardo-républicains: découvertes et relectures récentes. Actesdu colloque international de Saint-Romain-en-Gal enl'honneur d'Anna Gallina Zevi – Vienne, February 8–10 2007.Roma: Edizioni Quasar; 2008. p. 55–79.


[5] Terrapon N. Les enduits peints du péristyle et de l'oecus 101de la Domus aux Bucranes: observations techniques. Ibidem;2008. p. 81–109.

[6] Falzone S, Panariti F, Pellegrino A. Il progretto divalorizzazione e musealizzazione della Casa delle Ierodule adOstia. In: Morandini F, Rossi F, editors. Domus romane: dalloscavo alla valorizzazione. Atti del Convegno di studi – Brescia2003, Milano; 2005. p. 223–31.

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Microscopic techniques to study Roman renders and mural paintings from various sites