Monument Future

Conclusions

The tuffs are broadly used in the monuments of Europe, Asia and North-America. The physical properties of the studied mostly acid tuffs are very different. Despite the high porosity (from 17 to nearly 37 vol%), tuffs can have relatively high tensile strength in dry conditions. This is significantly reduced with water saturation. The loss in strength can be one-fourth of the original one, but for most of the studied tuffs, it is half or two-thirds of the initial dry value. Not only water saturation but also freeze-thaw cycles and salt crystallization lead to a loss in strength or even an ultimate disintegration. There are tuff lithologies that can survive more than 100 salt crystallization cycles or 90 freeze-thaw cycles, while others disintegrate after few cycles. These differences in durability are attributed not only to porosity but also to pore-size distribution and micro-fabric, and are influenced by formation processes such as welding. Despite these findings and variations of durability, the usage of tuffs is still common, but at several sites the tuff elements show signs of deterioration.

Acknowledgements

ÁT acknowledges the financial support of National Research, Development and Innovation Fund of Hungary (K 116532). CP was supported by the German Federal Environmental Foundation (AZ20017/481).

References

Germinario L., Török Á. (2019). Variability of technical properties and durability in volcanic tuffs from the same quarry region – examples from Northern Hungary. Engineering Geology, 262, 105319.

López-Doncel, R. A., Wedekind, W., Cardona-Velázquez, N., Gon-Zález-Sámano, P. S., Dohrmann, R., Siegesmund, S. Pötzl, Chr. (2016): Geological studies on volcanic tuffs used as natural building stones in the historical center of San Luis Potosí, Mexico. Proceedings of the 13th International Congress on Deterioration and Conservation of Stone, vol. I, 107–115 pp., Glasgow. ISBN: 978-1-903978-59-7

Pötzl, C., Dohrmann, R. and Siegesmund, S. (2018a). Clay swelling mechanism in tuff stones: an example of the Hilbersdorf Tuff from Chemnitz, Germany. Environmental Earth Sciences, 77 (5), p. 188.

Pötzl, C., Siegesmund, S., Dohrmann, R., Koning, J. M. and Wedekind, W. (2018b) Deterioration of volcanic tuff rocks from Armenia. Constraints on salt crystallization and hydric expansion. Environmental Earth Sciences, 77 (19), p. 660.

Stück H., Forgó L. Z., Rüdrich J., Siegesmund S., Török Á. (2008). The behaviour of consolidated volcanic tuffs: weathering mechanisms under simulated laboratory conditions. Environmental Geology, 56, 699–713.

Török Á. (2007). Geology for Engineers. (in Hungarian with English summary) Müegyetemi Kiadó, Budapest, Megyetemi Kiadó, Budapest, 384 p.

Wedekind, W., López-Doncel, R., Dohrmann, R., Kocher, M. and Siegesmund, S. (2013). Weathering of volcanic tuff rocks caused by moisture expansion. Environmental Earth Sciences, 69 (4), 1203–1224.

137

THE CATHEDRAL-RUINS OF ZVARTNOTS AND AVAN (ARMENIA) – A COMPARATIVE STUDY ON DECAY AND RESTORATION

Wanja Wedekind¹, Emma Harutyunyan², Siegfried Siegesmund¹

IN: SIEGESMUND, S. & MIDDENDORF, B. (EDS.): MONUMENT FUTURE: DECAY AND CONSERVATION OF STONE.

– PROCEEDINGS OF THE 14TH INTERNATIONAL CONGRESS ON THE DETERIORATION AND CONSERVATION OF STONE –

VOLUME I AND VOLUME II. MITTELDEUTSCHER VERLAG 2020.

1 Geoscience Centre of the University of Göttingen, Goldschmidtstr. 3, 37077 Göttingen, Germany

2 National University of Architecture and Construction of Armenia, Teryan 105, 0009 Yerevan, Armenia

Abstract

The Zvartnots Cathedral was built from basalts and tuffs in the 7th century. Since the year 2000, it has been listed on the UNESCO World Heritage list. The Avan Cathedral is even older and was built in the 6th century, mainly from tuff. Both monuments are now ruins and have been restored several times with different materials. This applies primarily to the restoration mortar used.

The tuff material used for both cathedral ruins is comparable.While damage to Avan Cathedral is dominated by cracks and static problems, the Zvartnots ruin is primarily damaged by widespread salt contamination.

In this study, the restoration history of both cathedral ruins were worked out and the petro-physical properties of the building stones were investigated. The historical and modern mortars used in both structures were analysed and examined in the laboratory. Onsite object investigations were carried out, which includes damage mapping, surface hardness, electrical conductivity and the sampling of salts. The investigation clarify the high quality and weathering resistance of the historical mortars.

Introduction

The Zvartnots and Avan Cathedrals are two outstanding examples of the high point of Armenian medieval architecture (Fig. 1a). Today both buildings are only preserved as ruins.

Zvartnots Cathedral

The ruins of Zvartnots Cathedral are located near Echmiadzin, about 17 km west of Yerevan. The complex consists of the ruins of a round cathedral, which was built in the mid 7th century and dedicated to Saint Gregory. Beside the cathedral there are also the ruins of the palace of the Catholic Nerses III, who officiated from 641 to 662.

The church of St. Gregory in Zvartnotz was destroyed in the 10th century and is considered to be the oldest and largest tetrakonchos in the Caucasus Region. The structure has been imitated several times with similar forms. The four conches of the central building were surrounded by a circular corridor, the diameter of which was 37.7 meters, with a presumed height of the triple-tiered cylindrical structure of around 45 meters (Fig. 1a).

The Gregory Church is mentioned in several medieval sources and is one of the best-known architectural symbols of the Armenian Apostolic Church. The Armenian Apostolic Church is the oldest “self-sufficient” state church in the world.

138Archaelogical investigations and restoration

After the first excavation work in 1901, a systematic archaeological investigation began in 1904 and was lead by Toros Toramanian (1864–1934). He presented a three-stage reconstruction model, that initially raised doubts, but was accepted by most experts as probably the right one in 1906. This model is considered a pilot project of reconstructive restoration work in Armenia (Fig. 1a).

From 1958 to 1967 the remains of the wall were being secured. Further restoration measures were carried out in the 1980 s, with large areas of cement-containing mortar and suspensions being used in both cases (Fig. 2f). The most recent restorations were carried out under the direction of Grigor Nalbandyan using cementless materials.

Avan Cathedral

The Catholic Cathedral of the Holy Mother of God of Avan is located in the Avan Yerevan district on the outskirts of the capital of Armenia. The Avan Cathedral is the oldest surviving church within the city limits of Yerevan.

The church was built in the late 6th century, between 591 and 602. Construction began shortly after the Byzantine-Sasan War ended (572–591), which led to the expansion of direct Byzantine rule over all western regions of Armenia.

Figure 1: a) The cathedral ruin of Zvartnots and the reconstruction model by Toramanian with the floor plan of the buildung. b) The cathedral ruin of Avan, a hypothetical reconstruction and the floor plan of the church.

The architectural historian Toros Toramanian (1864–1934) made the assumption that the Avan Church once had five domes (published in Maroutyan 1976). A single larger dome in the middle and four smaller domes over each corner of the church, over the round corner chambers. Thus, Avan would be the first such example of a church with five domes. The church has a four-pass floor plan with an octagonal central bay over which there was probably originally a dome (Fig. 1b).

A low, arched door leads from the west wall into the church and is surrounded by an ornate covering with three-quarter columns, which are decorated with capitals and bezels. A vischap stone from pagan times was used as a lintel above the main portal. Another door on the north wall, which was probably built at a later date, after the church was built, leads to the Catholic Palace. The church is also of immense architectural importance as the prototype of the much better known masterpiece, the St Hripsime in Ejmiatzin, Armenia.

Restoration history

In 1940–1941, 1956–1966 and 1968 restoration work was carried out on the church. Of note here is the oldest restoration from 1940–41, which was carried out according to the then most modern and still current principles of restoration theory (Brandi 2006). Compatible dry slaked lime mortars were used, which also has to be emphasized in terms of materials.

Damage forms and deterioration

The damages observed at Avan Cathedral are dominated by cracks and crackles of single ashlar, back-weathering, scaling and static problems of the top of the single building walls and pillars (Fig. 2c). At the Zvartnots ruins, the damage is primarily caused by widespread salt contamination associated with back-weathering and rounding (Fig. 2d, 4).

Historical photographs from the beginning of the 20th century show, that at some parts of the original walls even the historical whitish plaster was 139preserved (Fig. 2e). Today all the plaster surfaces are destroyed. Many of the stones are damaged by typical weathering forms associated with the presence and crystallization of salt (Fig. 2d, 4).

Figure 2: a) The remains of medieval architectural structures in Sanahin, Armenia shows the huge amounts of mortar within the building structure. b) Cross section of a wall at the Avan Cathedral. c) The critical static situation at the top of the walls at Avan Cathdral. Restoration mortar from the 1941 campaign is exposed in the lower parts of inner masonry. d) Widespread salt contamination and weathering at the Zvartnots Cathedral. e) Huge remnants of the inner masonry and preserved plaster on some walls at the Zwartnots Cathedral shown in the museum of the site (early 20^th century). f) Cement mortar injections done by restoration in the 1960 s also documented in the museum.

Methods of investigations

Onsite investigations were done by comparable observations, damage mapping, electrical capacity and conductivity measurements on single building stones.

The electrical conductivity and capacity is basically dependent on the material and is also influenced by the moisture content of the material and the presence of ionic compounds. The used device (surveymaster/protimeter) measures moisture in buildings and other related structures both on (conductivity) and below (capacity) the surface.

Sampling of the drilling powder was also done. Field studies were done on a southwestern wall of the Zwartnotz ruins and the westen front facade of the Avan Cathedral.

This study concentrates on the historical mortars of the Zvartnots Cathedral (ZM), the Avan Cathedral (AM) and the restoration mortar (RM) from 1941 used for Avan (Fig. 4). From these three mortars an analysis of their aggregate/binder ratio and the grading curve of its aggregate were done in a mobile laboratory. Petrographical and fabric analyses of the building materials were performed on thin sections under a polarization microscope and cathodoluminescence (CL) microscopy. Hydrostatic weighing on the samples was carried out to acquire the particle and bulk density as well as the porosity (DIN 52102). Mercury intrusion porosimetry was used to acquire the pore radii distribution (Fig. 3d, h, l).

Results
Methods of construction

Double shell masonry structures are characteristic of many ruins of the rich cultural heritage in Armenia. The building stones that form the exterior and interior masonry are held together between the two masonry shells by lime mortar and quarry stone masonry. The percentage of both structural elements in rising walls is about 50 % (Fig. 2a, b and e).

The inner shell of the masonry has a very solid and durable mortar. This mortar seems to literally glue the two wall shells together and seems to be the reason why many church buildings, as ruins, have still been partly preserved (Fig. 2a).

The mortars

The mortars of both historical buildings are lime mortars with volcanic aggregates with a high amount of glass-rich, amorphous material. These are characterized by a predominantly dense and binder-rich, light and dense matrix surrounded by aggregates of different sizes and color (Fig. 3a–3h). In contrast, the restoration mortar of the Avan Cathedral shows single quartz grains, feldspar crystals and sub-rounded inclusions clearly identified as lime accumulations (Fig. 3i). The historical Avan mortar (AM) mostly shows reddish, large pumice-rich grains (Fig. 3e). They reach a size up to 5 mm. The Zvartnots mortar shows more basaltic and andesitic fragments of sand particle size (0.063–2 mm). The matrix of both historical 140mortars is extremely dense and shows a firm connection to the grains (Fig. 3b, c and f and g). The Zvartnots mortar shows larger accumulations (–1 mm) of crystallized lime, which show multicolored effects in polarized light (Fig. 3b). These accumulations show a distinct red color under cathodoluminescence (CL) and could be a dolomitic lime. The matrix of the historical mortar shows a slight dark reddish structure, that can be identified as calcite-silica compounds (3c). A similar slight dark red color of the matrix could be observed for the mortar of the Avan Cathedral, while only light reddish lime accumulations are visible in CL-light (3g). The lime inclusions within the restoration mortar (RM) shows a similar intense light reddish color but no crystallization (Fig. 3k). The binding cement of the mortar is made from fine crystals, showing a multicolored birefringence in polarized light (Fig. 3j). These crystals are identified as calcium hydroxide or portlandite, which only forms single grain contacts. The blue glowing areas are probably claystone fragments (Fig. 3k) or alteration products (Fig. 3c).

Figure 3: The three investigated mortars. a–d) The mortar of the Zvartnots Cathedral (ZM), a) hand specimen, b) thin section under polarized light and c) under cathodoluminescence. d) The pore size distribution. e–h) The mortar of the Avan Church (AM) and i–l) the restoration mortar of Avan (RM) in the same order.

The pore size distribution of the three mortars is different. In the Zvartnots mortar smaller capillary active pores dominate the pore structure attaining 73 %. The microporosity (0.001–0.1 µm) reaches 20 % for the ZM and 28 % for the AM. Both mortars are characterized by a bimodal pore size structure, while the finer pores dominate (Fig. 3d, 3 h). The restoration mortar of the Avan Cathedral attains an amount of micropores of 64 %. A bimodal pore size structure is also formed in this case, dominated by small pores as well (Fig. 3l).

The historical mortars are lime mortars and show different binder contents. In the case of the original Avan Cathedral mortar, the lime content is 60 %, and only 25 % for the Zvartnots Cathedral. However, it must be taken in mind that the percentages are by weight and not by parts by volume. The Zwartnots mortar has a comparatively heavy aggregate mainly from basalt, which is why it also has the highest density of all mortars (Tab. 1). Therefore, the binder content appears relatively low. The binder-aggregate ratio of the restoration mortar (RM) shows a lime content of 34 %. In terms 141of the grading curve, there are huge differences between the historical mortars and the restoration mortar of the 1941 work. The historical mortars show a falling aggregate content from 0.5 mm to 63 µm and only a small amount of fine sands (Fig. 4). In contrast, the restoration mortar (RM) is dominated by fine sands with a clear domination of the grain size > 5 mm.

Figure 4: The grading curve of the three mortars.

Volcanic building stones

The tuffs, which were used in the Yerevan region in historical times, belong to the varieties of the Hoktemberyan Brown (HB) and the Hoktemberyan Red (HR) or the Hoktemberyan Black (HBl). Basalt is partly found at the Zvartnots Cathedral and can be compared with the Armenia Black (AB) variety. A detailed description of all building stones is given in Pötzl et al. 2018.

The porosity of the building stones ranges between 14.1–35.6 %, the particle density between 2.46 and 2.93 g/cm³ (Tab. 1). The porosity of the different mortars differs between 31.3 and 47.6 %, with densities between 1.2 and 1.7 g/cm³. Their particle densities range between 2.2 and 2.6 g/cm³ (Tab. 1).

Table 1: Porosity and density of the investigated stone and mortar samples. * Pötzl et. al 2018

Onsite investigations

The results of the mapping of the south-west wall of the Zvartnots complex show, that back-weathering, concentrated at the lower part of the wall is the main damage form covering more than 13 square meters of ashlar and more than 70 % of the whole study object (Fig. 5c).

These damages can be clearly associated with the effloresences of salts. Using test stripes, high amounts of sulfate (> 1,600 mg/l), chloride (1,000 mg/l) and nitrate (500 mg/l) could be detected. Loss of material, rounding and scaling only took place at around 3.5 m² of the investigated wall. Biological growth is not observable (Fig. 5c).

In a comparison of both buildings, which were built with comparable rock materials, values between 0–15 RF and 0–200 % WME speak for a largely intact condition.

For the Zwartnots cathedral electrical conductivity and capacity shows critical values in all measurement points by reaching more than 15 (% WME) in case of the conductivity and more than 400 (RF) for the capacity.

At the Avan Cathedral crack formation is the predominant weathering form, following by back-weathering and scaling. In Figure 6, these damage forms are depicted at one half of the façade, where also no relevant salt weathering was found. The electrical conductivity and capacity show critical values only in the lower part of the building, which is also partly effected by rising dampness (Fig. 6).