The Conservation Problems of the Theatre of Eraclea Minoa (Sicily)
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1085 THE CONSERVATION PROBLEMS OF THE THEATRE OF ERACLEA MINOA (SICILY) ALAIMO, R. Isl. di Mineralogia e Petrografia., Via Archirafi, 36, 90123 Palermo (Italy) GIARRUSSO, R. C.E.P.A., Via Oretea, 23, Palermo (Italy) LAZZARINI, L. LAM.A., (1.U.A.V.), S.Polo 2554, 30125 Venezia (Italy) MANNUCCIA, F. L'ISOLA, Via Curcio 3, 98051 Barcellona, Messina (Italy) MELI, P. Sopr. BB.CC.AA., P.Diodoro Siculo 8, 92026 Agrigento (Italy) SUMMARY Eraclea Minoa was a small Greek colony situated on a low promontory by the sea between Selinunte and Agrigento (Sicily). Its theatre, built in the IV c.BC with a local biocalcarenite cemented by gypsum, was unearthed at the beginning of the fifties. Following excavation it was enclosed in a transparent perspex structure that respected its stepped shape. In spite of the intention to protect the very soft and delicate stone and maintain the original shape of the monument, the covering was fixed to the stone itself, thus damaging it and producing a strong green house effect much favouring the yearly growth of vegetation. This vegetation, and its improper control, produced such obvious damage in the last thirty years as to convince, in 1995, the local authorities responsible for the safeguarding of the monument to remove part of the covering, in order to study in detail the causes of deterioration of the stone, and test for alternative consolidating and protective materials. Minero-petrographic and chemical analyses made on several samples of stone have allowed for good characterisation and determination of the most important morphologies and causes of deterioration. The results so far attained indicate that alongside the damaging effects of vegetation, it is water which produces the strongest weathering effects by dissolving the gypsum of the stone and vehiculating it as a soluble salt. The best consolidating effect on a test area has been obtained by multiple applications of ethyl silicate followed by a protection with a water-based silicon water-repellent. INTRODUCTION Eraclea Minoa was a Greek subcolony of Selinunte, situated on a low promontory by the sea between its mother town and Agrigento in the SW of Sicily. The town had a long history, but it became a flourishing centre in the IV c. BC after the destruction of Selinunte and the Carthaginian conquest of that part of the island. Proof of this is also given by the construction toward the end of that century of a small theatre well placed facing the African sea. Unearthed in the fifties (De Miro, 1955 and 1958), it shows a cavea divided in 9 sectors (kerkides) by 8 stone stairs (limakes) and with 10 orders of seats (fig. 1). The orchestra, with an earthen ground, was separated from the cavea by a narrow canal (euripo} extending to the two extremes of the half-circumference, where there are two well-preserved walls (analemmata) 3,5 m high. No traces of the scene were found, leading to the hypothesis that the one existing in ancient times would have been made out of wood, as in phlyacic theatres. Apart from this, our theatre was made entirely of a local stone of very poor quality, which soon after its re-exposure to the atmosphere started to deteriorate heavily. The archaeologists responsible for the preservation of the monument, after a first unsuccessful! treatment of the stones with an acrilic resin (whose composition is not specified in the old reports), accepted in 1963 the project of the 1086 architect F. Minissi to cover the entire cavea by sheets of perspex (1 cm thick) shaped over the remains of the cavea (fig. 2). The idea was to protect the stones of the theatre from rain (which proved to dissolve the gypsum cementing the rock) and leave them visible. The project was realized in 1964, but in a way that unfortunately damaged and severely modified the original structure: more than 800 holes were carved on the seats to fix as many iron bars necessary to hold the plastic cover; concrete supports were cast at the two sides of the stairs for the same purpose; many stairs were covered by concrete steps; a new moat was opened between the stony cavea and its upper earthwork (fig. 3). Further, as could have been predicted, the protecting efficacity of the perspex cover did not result sufficient: water was able to reach the stone surface and to percolate through it; the cover produced a continuous green house effect with a diffused growth of vegetation accompanied by a strong superficial deterioration of the stone of the theatre. The bad state of preservation was intensified several times by repeated improper control of vegetation through mechanical removal of plants and grass done yearly in combination with the cleaning of the plastic cover (fig. 4). A first study on possible consolidation treatment of the stones of the theatre (Zava, et al., 1976) by mineralization with gypsum solutions, or by impregnation with ethyl silicate was not followed by any restoration intervention. It was only in 1995 that the Superintendency responsible for the safeguard of the theatre decided to examine the conservation problems of the monument more thoroughlyby removing the perspex from one sector of the theatre, thereby making possible appropriate laboratory . studies and in situ treatment tests. The results will be presented here. EXPERIMENTAL Some 30 samples have been examined for the characterisation of the stone of the theatre and the study of its deterioration. The stone has been characterised by studying several thin sections under the polarising microscope (OM) and trough diffractometric analysis (XRD, radiation Cu Ka with a graphite monocromator). The same analyses were done also on the rocks outcropping on the hills close to the theatre to find the origin of the stone used for the monument and then a supply of stone blocks for laboratory tests. Thin and polished sections of weathered stone samples were also studied. The soluble salts present as efflorescences in various parts of the theatre (mainly to the left of the cavea and on the right retaining wall) were identified by XRD; their presence in weathered stone samples has also been determined quantitatively by HPLC according to the reccomendation Normal 13/83. Small cubes (side =3 .1cm) of the same stone of the theatre quarried in a nearby hill have been impregnated by three commercial silicic esters (Wacker OH, Rankover, Tegovakon) following the reccomendations 4.2.1.1. (points a and b) of Normal 20/85. Once impregnated the cubes were left to season for 1 month; they were then submitted to various tests as follows (each test was done at least on 3 untreated and treated samples): - control of the distribution of the consolidant under SEM, with SE and BSE mapping of Si by EDS (Normal 8/81); - water absorption by total immersion (Normal 7/81); - water absorption by capillarity (Normal 11/85); - mercury porosimetry (Normal 4/80); - resistence to salt crystallization according to the RILEM-UNESCO test N° v, 16, using a 10% sol. of Na2S04 (2 h of immersion, 19 h of drying in the oven at 60 °c, 3 h of cooling in a dryer). The number of cycles performed was 15. 1087 RESULTS AND DISCUSSION The characterization of the stones of the theatre The rock used for the building of the theatre of Eraclea may be classified as gipsous biocalcarenite. It belongs to the upper Miocene (Messinian) which is largely represented in the central-southern part of Sicily by the so-called "Serie Gessoso-Solfifera". Our stone has been quarried in the lower part of the series which is characterised by 6-7 sedimentary cycles each one starting at the base with marls and ending with selenitic gypsum. In fact, right over the marls are normally lying several beds of a biocalcarenite with variable thickness, clay content and cement made of gypsum. The fine grain and good workability made this stone an easy choice for the Greek colonists, who used it for the theatre and other buildings in Eraclea. They also used a selenite, especially for the town wall and the foundations of their houses. Both lithotypes were quarried on the hills to the NE of the ancient town, as demonstrated by the good match of the results of the minero-petrographic analyses done on stone samples from the theatre and its outcroppings. A preliminary field survey on the environs of Eraclea has in fact led to the determination of some possible quarrying sites. Although no clear traces of quarry marks were found, the roughly stepped shape of some slopes suggests ancient exploitation. The stone of the theatre shows under the microscope a typical elastic and homogeneous fabric and an isotropic texture (fig. 5). The calcareous fraction is mostly composed of microfossils and bioclasts of 0.1-0.2 mm, not exceeding 0.5 mm. Among the former are largely prevailing the microphoraminiphera with Globigerinidae, Miogipsinae, Amphisteginae, Rotalidae, etc., whose shell is frequently filled by microcrystalline calcite and seldomly by limonites and glauconite. The shells mainly include echinoids, calcareous algae, bryozoans and molluscs. The intergranular gypsous cement accounts for 20-30% of the whole rock and is prevailingly formed by gypsum crystals of 0.2-0.5 mm, frequently showing the typical twining plane 100 characterising selenite. The absence of intragranular gypsum and its psammitic grain size point to a redeposition of elastic primary gypsum occurring during the diagenesis. A typical modal analysis of this stone gives the following results: calcite=71 %; gypsum=22%; glauconite=1 %; other components (quartz, feldspars, volcanic glass)=1 %; pores=5%. The other stone used in the theatre only for the "euripo" has been identified as the biocalcarenite of Agrigento.