Asmosia III Athens: Transactions of the 3rd International Symposium of the Association for the Study of Marble and Other Stones used in Antiquity

THE STUDY OF MARBLEAND OTHER.

STONES USED IN ANTIQUITY

Edited byYannis Maniatis, Norman Herz and Yannis Basiakos

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The brown waterproofing plaster of the ancient cisterns in Laurion and its weathering and degradation

by G. Papadimitriou and J. Kordatos

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Many hundreds of water cisterns, dating at least from the 4th century BC, are situated in Laurion, in the ancient industrial areas of the mines, ore dressing plants and metallurgical installations, which have served in the classical antiquity for the production of silver from silver bearing ores of lead (Fig. 1 ).

The cisterns are of important dimensions, ranging in capacity from some hundreds to two or three thousand cubic meters. They served for the storage of the rainwater, necessary for the continuous opera tion o f the ore concentration plants during the whole year.

The internal surface of the cisterns, as well as of other installations where water was stored or circulating, is covered with a lining consisting of a thin layer of a highly efficient waterproofing plaster of characteristic brown colour, applied on a substrate of a usual lime mortar. In most cases, this special brown plaster is conserved in good

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AbstractMany hundreds of water cisterns, dating at least from the 4th century BC, are situated in Laurion, in the ancient industrial areas of the mines, ore dressing plants and metallurgical installations, which have served in the classical antiquity for the production of silver from silver bearing ores of lead.

The cisterns are of important dimensions, ranging in capacity from some hundreds to two or three thousand cubic meters. They served for the storage of the rainwater, necessary for the continuous operation o f the ore concentration plants during the whole year.

The internal surface of the cisterns is covered with a thin layer of a highly efficient waterproofing plaster of characteristic brown colour, applied on a substrate of a usual mortar. In many cases, this special brown plaster is conserved in good condition to the present time. However, the weathering associated to the direct exposure to the sun and the dryness caused by the fires of the surrounding pine forests during the last decade, provoke the continuous degradation of the mortars and the gradual ruin of the cisterns.

The plaster and its properties have been the subject of some studies in the past. In a previous investigation of one of us, dating from 1978, we have emitted some hypothesis concerning the fabrication and application of the brown plaster on the walls of the cisterns. At the present time, the availability of new analytical tools, especially the scanning electron microscope associated to EDS microanalysis, incited us to rev isit the problem . A part from the confirmation of our previous results in relation to the

Fig. 1. A system o f two cisterns in Souresa - Laurion. The smaller one served fo r the separation o f mud and slimes from the rain w ater co llec ted in w inter from the surrounding area. The overflow, pure water, was conducted to the larger cistern, which served as a water reserve for the needs o f the ore dressing plant during the year.

Fig. 2. The interior wall o f a large water cistern, almost fu ll o f water, lined with the waterproofing mortar and the special brown plaster.

preparation of the plaster material and its application, new information concerning the constitution of the brown plaster has been obtained. The thickness of the plaster and its bonding to the substrate have been examined. The microstructure of the plaster and the distribution of its constituents - the glassy material with the surrounding calcite - was thoroughly studied. Microanalysis allowed to identify two types of plasters, according to the raw materials used and depending mainly on the location of the cisterns. Furthermore a preliminary examination of the degradation o f the p laster allowed to delineate the mechanism of its alteration and to suggest the causes of its accelerated deterioration in the last years.

Introduction - Previous Work

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condition to the present time, as it is seen in Figure 2, showing a cistern almost full of rainwater.

Before we get into the details o f the present investigation, we shall present an overview of the results obtained earlier on the hydraulic linings of the ancient cisterns in Laurion.

The waterproof lining as a whole and more particularly the brown plaster have been the subject of investigations in the Laboratory of Physical Metallurgy of the National Technical University of Athens in the 1970s (Badecas, 1974; Conophagos and Badecas, 1974; Conophagos and Papadimitriou, 1978).

Helen Badecas in her doctoral thesis (Badecas, 1974) found that the surface of the Laurion cisterns is covered in most cases with two different layers of mortar. The inner one, usually having a thickness of 2-5 cm, is an ordinary mortar. It consists of sand and gravel, taken from the surrounding mining area, and of lime as a binder. The

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Fig. 3. Schematic representation o f the lining. A usual lime mortar is applied on the stone-wall o f the cistern and is subsequently covered by a special waterproofing plaster, with a characteristic brown colour.

outer layer is a special hydraulic plaster of dark brown colour, which according to macroscopic observation, has a thickness of about one to two millimetres. The chemical analysis, presented as a mean analysis and expressed in the form of oxides, shows about twenty nine percent PbO, eighteen percent MnO and smaller amounts of S1O2 , Fe2Cb, AI2O3, CaO, MgO, ZnO, each of them of the order of five to six percent. Based on X-ray diffraction on one specimen of the brown plaster which showed the presence of PbCCh, PbO, Pb4Si04 and a small quantity of S1O2, the author concluded that the material was probably a mixture of litharge (PbO), slag, powder of ores and quicklime.

As a continuation of the above work, Conophagos and Badecas measured the coefficient of water permeability of the dark brown plaster and, in most cases, found it equal to zero, a value better than that obtained for cement nowadays (Conophagos and Badecas 1974).

Conophagos and Papadimitriou have studied the brown plaster using m etallographic techniques and X-ray diffraction. They have revealed some important features of the structure of the plaster, allowing to suggest the preparation method of the material and the method of its application on the substrate (C onophagos and Papadimitriou, 1978).

Fig. 5. Same as Figure 4, but the depth o f the impregnation is very irregular, extending between 1 and 2.5 mm into the substra te, under the compact layer o f the plaster.

Fig.4. A metallographic section through the plaster and mortar: ‘c ’ is the brown plaster, o f a thickness o f 0.15 mm;‘b ’ is the usual lime mortar (substrate) impregnated by the

plaster material, and ‘a ’ is the original (not impregnated) lime mortar. The depth o f the impregnation is uniform, about 1 mm from the surface.

Fig. 6. SEM micrograph o f a section, showing at right the layer o f the plaster and under it the substrate (mortar consisting o f lime and sand). In the plaster the black regions are calcite and the white litharge.

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The brown waterproofing plaster o f the ancient cistern in Laurion and its weathering and degradation

According to their results the thickness of the brown plaster is smaller than that of one to two millimetres suggested previously, but the porous substrate (the ordinary lime mortar) is impregnated to a depth of about two millimetres, giving macroscopically the impression of a much thicker layer. Figure 3 gives schematically the profile of the cistern wall with its lining, as suggested in the work of Conophagos and Papadimitriou (1978).

The presence of the impregnation in the substrate, suggests that the plaster was applied as a very fluid suspension, most probably using a brush.

The authors, based on X-ray diffraction either on powder

Fig. 7. Optical micrograph o f the surface o f the plaster. The irregular white aggregates are from glassy litharge and the surrounding black material is calcite in the form o f fine veins, having crystallised from the lime used for the preparation o f the plaster suspension.

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or on the surface of a number of specimens, have observed the systematic absence o f reflections, except o f that belonging to calcite. This situation should correspond to an amorphous condition of the main constituents of the brown plaster, PbO, MnO, etc., which were determined by analysis.

The direct use of ground litharge (PbO) and ores for the preparation of the plaster, as it was suggested previously by Badecas (1974) and Conophagos and Badecas (1974), could not explain the absence o f X-ray reflections, because numerous specimens of ancient litharge (PbO) examined as well as the iron/ manganese ores of the surrounding mining area are well crystallised and give well defined and sharp diffraction patterns. It was, therefore, concluded that the raw material used for the preparation of the suspension was in a glassy condition. This could be achieved if the raw materials (litharge and ores) were melted together in a crucible or furnace and then poured into water in their melted state, resulting in a granulation and rapid solidification. From this practice bits of an amorphous glassy substance were obtained, which were subsequently ground into a fine powder. This powder was then added to an emulsion of slaked lime and thoroughly mixed, in order to form a very fluid suspension, which was applied on the porous substrate with a brush (Conophagos and Papadimitriou, 1978).

A brief overview of this work was, also, being presented by Conophagos (1982) during a sem inar on ‘Early Pyrotechnology’ in the Smithsonian Institution.

At the present time, the availability of new analytical tools, especially the scanning electron m icroscope associated to EDS microanalysis, incited us to revisit the problem. Apart from the elucidation of some controversial

Fig. 8. X-ray diffraction on the plaster material Only the diffraction lines corresponding to the calcite are present, whereas the amorphous - glassy constituent does not diffract. A small quantity o f S1O2 is also observed.

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Description CaO PbO S1O2 FeO MnO ZnO A I2O 3 MgO K20

Souresa valley

1 section 40.47 45.22 10.21 1.72 - - 2.37 - -

2 section 30.63 53.3 12.57 0.77 - - 2.67 - -

3 section 34.96 48.79 12.85 1.42 - - 1.97 - -

4 external surface 61.11 31.37 4.42 1.27 - - 1.84 - -

5 calcite 47.40 6.50 9.07 6.17 0.37 20.40 5.68 4.17 0.25

6 litharge 3.38 38.83 5.98 10.34 20.16 16.14 3.28 - 0.26

7 calcite 46.45 19.81 5.55 4.44 3.38 14.33 3.30 2.05 0.48

8 litharge 1.01 39.22 3.12 8.22 25.84 16.43 1.35 0.80 0.22

9 surface 4.84 83.65 7.37 1.53 - - 2.62 - -

10 litharge, deeper 22.23 72.75 3.88 0.55 - - 0.58 - -

Demoliaki Mining Area

11 section 18.10 16.70 21.01 7.51 6.23 19.28 9.59 ma 1.01

12 section 17.26 20.12 14.65 10.18 9.22 15.61 7.08 1.43 0.83

Table 1. EDS - microanalysis on the special brown waterproofing plaster o f Laurion. Analyses 1 to 4 and 11 to 12 give the general composition o f plasters from the Souresa and Demoliaki mining areas, respectively. Analyses 5,6 and 7,8 are pairs o f analyses on the calcite and on the glassy constituent (mainly litharge) o f two samples o f plaster o f Souresa. Analyses 9 and 10 are from the surface and from a deeper layer o f the plaster in a superficially corroded sample from Souresa.

or uncertain results obtained previously on the real thickness of the plaster and its bonding with the substrate, new information concerning the constitution of the brown plaster and the materials used for its preparation has been obtained through extended metallography and micro­analysis. In addition a preliminary examination of the mechanism of degradation of the plaster has been done.

Experimental ProcedureSamples of the plaster and its substrate were taken from several cisterns from the Souresa valley and from the

Fig. 9. The external surface o f the plaster after removing the calcite constituent by deep etching in a solution o f acetic acid. This acid dissolves very effectively the calcite but has no effect on the glassy material, allowing to observe the irregular form o f the glassy aggregates and some round drops o f glassy litharge.

mining area of Demoliaki. The samples were characterised by X-ray d iffraction using a Philips d iffraction spectrometer and cobalt, iron and copper Ka-radiation.

Optical and scanning electron microscopy and electron probe microanalysis were used for the examination of samples on their surface and in sections. The electron microscope used was a Jeol-6100 EM associated with a Tracor EDS microanalysis system.

Results and DiscussionMicrostructure o f the MortarFigures 4 and 5 are metallographic sections through the thickness of the waterproofing lining: in the interior lies the usual lime mortar, and on the surface the dark brown plaster, applied closely on the substrate. One can clearly observe the thin but very compact layer of the plaster of a thickness of only 0.1 to 0.2 mm. Under this, the extended impregnation of the substrate, advancing one to two millimetres deep in the porous substrate, is observed. This impregnation is irregular and its depth depends also on the presence of the inert constituents of the substrate, pieces of sand, minerals etc. The pieces of the inert materials are of angular form. Their determination shows that they are ground minerals, consisting of fluorite, calcite, quartz, lead and zinc carbonates, rejected as tailings during the concentration of the silver bearing ores. Figure 6 is a SEM micrograph and shows clearly the about 200-microns thick plaster and the mortar of the substrate, which becomes also very compact and impermeable up to the depth of the impregnation. Figure 7 is a micrograph of the external surface of the plaster and shows that the material consists of two different constituents or phases. The massive aggregate with irregular boundaries (in white) is the glassy phase and it is surrounded and impregnated by the second constituent

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The brown waterproofing plaster o f the ancient cistern in Laurion and its weathering and degradation

(in black), appearing as a network of fine veins. The latter was originally slaked lime, that transformed later to calcite under the action of the carbon dioxide contained in the atmospheric air. The corresponding X-ray diffraction on this surface is given in Figure 8 and shows only diffraction lines

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Fig. 10. Typical EDS analysis on plasters, a) EDS-analysis on a sample taken from a cistern in the Souresa valley. It consists mainly o f PbO and CaO. There is a small quantity o f iron (about 1.5% FeO). Silica o f the order o f 10-13% and alumina 2-2.5%) are also present, b) EDS-analysis on a sample taken from a cistern from Demoliaki, another mining area in Laurion. It shows a variety o f chemical constituents: CaO, PbO, ZnO and S1O2 o f the order o f 15- 20%o, MnO, FeO and ΛΙ2Ο3 o f the order o f 10%. Apparently, not only litharge, but mainly various ores have been used fo r the preparation o f the plaster. c)EDS-analysis on the surface o f a strongly corroded sample from the Souresa valley. It shows an increased content o f PbO in relation to the analyses taken in the interior o f the section. This is because the brittle calcite constituent has been removed preferentially, due to the weathering conditions.

corresponding to the calcite, whereas the amorphous - glassy constituent does not diffract. It is worth mentioning that in a number of about fifty specimens examined by x- ray diffraction, only two of them showed evidence of small quantities of crystallised constituents like PbCh, PbO and PbC03, in their surface. Apparently these products have formed during the weathering on the external surface of the plaster, as they cannot be detected in its deeper layers.

Figure 9 shows the external surface of the plaster after removal of the calcite constituent by deep etching in a solution of acetic acid. This acid dissolves very efficiently the calcite but has no effect on the glassy material, allowing to observe the irregular form of the glassy aggregates, which have a spongy appearance with their pores impregnated by the slaked lime.

EDS Microanalysis o f the MortarThe scanning electron m icroscopy com bined to microanalysis is an ideal tool for studying the composition of the brown plaster as a whole and at different depths from the surface. More particularly it allowed to detect compositional variations in different areas of the plaster and to identify its constituents unambiguously.

Our study showed that either pure litharge or a mixture of litharge and iron/manganese ores have been used for the preparation of the raw material of the plaster. Table 1 gives some typical analyses taken in the scanning electron microscope using EDS microanalysis. The results are expressed as oxides. Most samples taken from cisterns in the Souresa valley consist mainly of PbO and CaO, as it is implied by the analyses No 1 to 4 of Table 1. There is a small quantity of iron (about 1.5% FeO), but neither manganese nor zinc, which are usual constituents of the ores of Souresa, are present in significant amounts. The presence of silica of the order of 10-13% and alumina 2- 2.5% is difficult to explain.

They might come from the slaked lime, but most probably they might have been introduced into the glassy material during the grinding stage, from the abrasion wear of the grinding tools used. Indeed, a volcanic rock rich in silica (tracheite) has been widely used in Laurion for the fabrication o f the grinding devices, described by Conophagos in his work on Laurion (Conophagos, 1980). Concerning the relative quantities of lime and litharge used in the suspension, the analyses show that they were used in almost equal proportions, as it is indicated by the compositions Nos 1 to 4 of Table 1.

The samples taken from cisterns from Demoliaki, another mining area in Laurion (analyses 11 and 12 of Table 1), as well as a small number of the Souresa valley (analyses 5 to 8 of Table 1 ), show a variety of chemical constituents: CaO, PbO, ZnO and S1O2 of the order of 15- 20%, and MnO, FeO and AI2O 3 of the order of 10%. Apparently, not only litharge, but also various ores have been used in this case for the preparation of the glassy constituent (Fig. 10b).

It is evident that a large quantity of litharge was needed for the plaster during the construction of the cisterns, so when this m aterial was in shortage, the ancient metallurgists needed to replace it by manganese ores.

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Fig. 11. SEM micrograph on the surface o f the plaster, showing a complex network o f cracks, associated with the removal o f the calcite by the weathering conditions.

The Degradation o f the PlasterThe chemical analyses on the surface of the Souresa plasters show in general an increased content of PbO in relation to the analyses taken in the interior of a section. This is because the brittle calcite constituent has been removed preferentially, due to the weathering conditions, presumably by the strong temperature changes. A typical example is the analysis No9 given in Table 1 (corresponding Figure 10c), where the PbO content is 83.65% and the CaO content only 4.84%. In the same sample, the analysis of the glassy phase in the interior of the plaster is that shown by the analysis No 10 of Table 1, and is richer in CaO (22.23 %).

The removal of the calcite, which is an effective binding agent between the glassy aggregates of litharge, leads to a complex network of cracks on the surface of the plaster, as it is shown in Figure 11. In general these cracks follow the calcite constituent and penetrate deeper into the plaster,

Fig. 12. SEM micrograph on a section, showing the same type o f superficial cracks as in F ig .ll, which follow the calcite constituent and penetrate deeper into the plaster and then into the substrate.

Fig. 14. A crater on the surface o f the plaster, containing corrosion products, among them spherica l drops o f litharge, as it has been verified by their point analysis.

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Fig. 13. Pieces o f a sublayer o f the plaster on the surface, surrounded by cracks, are entirely removed and the corrosion advances to the interior in the form o f a crater.

Fig. 15. A 200-microns thick layer o f plaster made by lith­arge and consisting o f three successive sub-layers, which were presumably applied by a brush on the substrate. The sublayers have a very good adherence between them and the inner one is very well adhered to the substrate. The impregnation o f the substrate is, also, distinguished.

282

The brown waterproofing plaster o f the ancient cistern in Laurion and its weathering and degradation

finally reaching the substrate, as it is seen in Figure 12. In some cases, pieces of the plaster are entirely removed from the surface (Fig. 13) and the corrosion advances to the interior in the form of a crater. Figure 14 shows such a deep crater containing corrosion products, among them spherical drops of litharge, as it is verified by their point analysis.

It is worth mentioning that stronger degradation is observed in plasters containing not only litharge, but other m ineral constituents , too. In general, the p lasters containing only litharge seem to be more resistant to the weathering conditions. Their thickness is more uniform and small, compared to that o f plasters made with mixtures of litharge and other ores. The small thickness may be advantageous to the resistance of the plaster and to its adherence to the substrate during alternating dilatation and contraction with tem perature changes, as it is generally accepted for coatings. Figure 15 shows a 200- microns thick layer of plaster made with litharge and consisting of three successive sub-layers, which were presumably applied by a brush on the substrate. The layers have a very good adherence between them and the inner layer is very well adhered to the substrate . The im pregnation o f the substrate by litharge is, also, d istinguished. The p laster is very com pact and no important degradation has occured on its surface, in the opposite of the layers presented in Figures 11 to 14.

The strong temperature changes are presumably the more dangerous factor affecting the degradation of the plaster. The dryness is perhaps another factor accelerating the corrosion and degradation of the plasters, because the presence of water moderates the temperature changes and preserves a relatively constant moisture in the environment.

At the end of the 1970s the cisterns in the Souresa valley were protected by the shadow of a beautiful and

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dense pine forest. Most of the cisterns were in good condition and preserved the rain water of the winter during the whole year.

In the summer of 1986 a fire has ruined the environning forests and contributed to the dryness of the valley. Since that time most of the cisterns are dry and their degradation seems accelerated. Not only the brown plaster, but also the

Fig. 16. The wall o f a cistern having completely lost its waterproofing lining. The cistern is fu ll o f water in the winter period, but it looses its content early in the spring.

substrate looses its adherence with the stonewalls of the cisterns, accelerating the destruction of these admirable technical monuments (Fig. 16).

ConclusionsSamples o f the brown waterproofing plaster and its substrate from the Souresa valley and from the mining area of Demoliaki have been thoroughly examined through X-ray diffraction and optical and scanning electron microscopy associated to electron probe microanalysis. It is concluded that:

The layer of the plaster consists of multiple sublayers, has a thickness of only 0.1 to maximum 0.2 mm and is very compact. The porous mortar of the substrate is impregnated with the material of the plaster up to a depth of 1 to 2 mm, becoming also dense and compact. The impregnation ensures a continuity and very good adhesion of the plaster to the substrate.

The plaster consists of two closely interpenetrating constituents: a glassy material and calcite in a network of fine veins, surrounding and impregnating the former. The removal of the calcite constituent by deep etching allowed us to observe the irregular form of the glassy aggregates, presenting a spongy appearance. The quantities of lime and litharge in the plaster are in almost equal proportions.

The X-ray diffraction shows only reflections corres­ponding to the calcite, the glassy constituent being in an amorphous state. In a number of about fifty specimens examined, two of them show evidence of small quantities of crystalline phases (Pb02, PbO and PbC03) only on their exterior surface, apparently as a result of alteration due to weathering. In close correlation with the location of the cisterns, two different kinds of plaster were found: those prepared using only pure litharge and those prepared using mainly iron/manganese ores which are eventually mixed with some small quantity of litharge. Some silica and alumina are present in the pure litharge plaster and they might have been introduced into the glassy material during the grinding stage, from the abrasion wear of the grinding tools used.

The chemical analysis on the surface of the plasters shows an increased PbO content in relation to the deeper layers, due to the preferential removal of calcite from the weathering conditions. This causes a network of cracks penetrating deeper into the plaster and eventually reaching the substrate. Subsequently, pieces of the plaster are detached from the surface and the corrosion advances to the interior in the form of a crater. Plasters containing only litharge apparently are more resistant to the weathering conditions than plasters prepared mainly from Fe/Mn ores.

The strong temperature changes and the dryness are presumably the more dangerous factors affecting the degradation o f the plaster. As a m atter o f fact the degradation was accelerated in the last decade, since a fire has ruined the environning forests and contributed to the dryness of the valley.

National Technical University o f Athens, 42 Patission str., Athens

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cisterns, Doctoral thesis, Athens, Nat. Techn. Univ. of Athens (in Greek with abstract in English).

Conophagos, C. and Badecas, H. 1974, The Water cisterns ofore washers in Laurion and their special hydraulic mortar, Proceedings o f the Academy o f Athens, 49, 251-261, Athens (in Greek with summary in English).

Conophagos, C and Papadimitriou, G., 1978, Sur l’enduitimpermeable a l’eau des anciennes citernes grecques du Laurium, Comptes Rendus de l ’Academie des Sciences de Paris, 287, 79-82, Paris (in French).

Conophagos, C., 1980, Le Laurium Ancien et la Technique deproduction de ΓArgent, Athens, Ekdotike Hellados.

Conophagos, C., 1982, Concrete and Special Plaster Water­proofing in Ancient Laurion, In Early Pyrothech- nology , the Evolution o f the First Fire Using Industries (eds T.A.Wertime and S.F.Wertime), 117- 123, Washington, D.C.: Smithsonian Institution Press.

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