Monument Future

Reflections on Decay Patterns

The reflection on probable material intrinsic and environmental factors that might cause, contribute or accelerate the two predominant decay patterns was important in order to gain a better understanding on their damaging potential and the rates of degradation.

It was assumed that on one hand, stone-intrinsic properties and style of quarrying have caused and accelerated these kinds of decay. The distinctive stratification of the stone varieties A1 and A3 combined with the unsuitable positioning of the steles – the layers are orientated at a right angle to the ground – might have favoured delamination. The vertically aligned layers can be a wear point and initiate cracking and fissuring.

It is also likely that the granite quarried was not totally sound. The hardness of the material makes it very difficult to quarry, especially when structural defects are lacking. So it is possible that pre-weathered stone slabs or boulders from the surrounding were used in first place for the erection of the complex monument. This was also underpinned by the inspection of the surrounding area which indicated no traces of possibilities for mining or quarrying. The pre-weathering in turn can adversely affect the subsequent degradation rate of the stone (Rodrigues 1980).

On the other hand, the hypothesis is proposed that thermal loads and temperature gradient play a significant part in the decay process.

The evaluation of meteorological and climate data from a weather station approx. 150 km south of the respective site (Saikhan-Ovoo, Dundgovi) showed that in summer maximum temperatures reach around 35 °C, while in winter temperatures can drop to minus 25 °C. Throughout the year the temperature difference between day and night is approximately 15 °C. The area counts almost 250 sunny days a year, often with clear cloudless skies. Combined with the lack of trees and therefore shadow in the steppe, this direct sunlight leads to a quick heating up of the stone. This in turn is assumed to result in a considerable temperature gradient within the stone and thermal stress due to a differential expansion of the rock minerals. Both factors might accelerate physical weathering and delamination (Rodrigues 1980).

In order to examine this assumption, the surface temperature on the top and the bottom of all faces of the main stele was measured in 2019 hourly with an infrared thermometer (Bosch Thermodetektor PTD 1) from sunrise to sunset. It is notable that particularly after sunrise (between 6:30 and 7:30) the surface of the east face heated up verifiably faster than the shaded west face. While 32 °C were measured on the top of the east face, the top of the west face had only 11.5 °C. Further, a perceptible difference of up to 4.1 °C could be measured from 8:30 to 14:30 between the top and the bottom of the East and South face. Over the measuring period the surface temperature varied from the minimum of 7.7 °C to the maximum of 44.6 °C.

Although the measurements only reflect heating and cooling during an average summer day, it is obvious that there have been thermal loads and a measurable temperature gradient between the front and back face of the stele since its erection 1,300 years ago. Particularly the gradient on the top might play a role in the delamination which is particularly distinctive in this area.

106Freeze-thaw-cylces, which considerably contribute to stone deterioration at our (European) latitudes, could be widely excluded as cause of damage in Mongolia. From September to November and middle of February to middle of April successive temperature transitions over the freeze point have to be expected in the Mongolian steppe. As there is hardly precipitation in these periods (less than 5 mm), damage caused by freeze-thaw-cycles can be neglected (Ruedrich, Kirchner and Siegesmund 2011).

Summary

The project provided the unique opportunity for the Austrian and Mongolian conservators and conservation scientists to study a stone monument from the 8th century, which has been continuously exposed to weathering.

Condition surveys together with extensive analyses of taken samples and three non-destructive field tests enabled to characterize the extent and type of decay. The findings showed a clear correlation between material instrinsic properties and prevalent decay patterns: while the stone varieties with layering mainly suffered from delamination, those without layering were rather affected by disintegration. They further revealed that particularly the human- and animal-shaped sculptures are, different to what was expected, heavily decayed and suffer from advanced disintegration.

Surface temperature measurements and the evaluation of meterological and climate data suggested that thermal loads and temperature gradients rather than freeze-thaw-cyles enhance decay and weathering of the site.

Based on the findings, the conservation treatment concept was developed and implemented. While the emphasis at the animal- and human-shaped sculptures was on the injection of fissures and partial consolidation, backfilling and micro pointing were carried out at the two steles.

Monitoring, survey, assessment and conservation of the heritage site have been carried out in a collaborative model-like approach to be utilized as a best practice example for future preservation efforts. In addition, the field tests, which were carried out for the first time at the respective site, provided important measurement data on pulse velocities and water absorptions of local stone varities to be used for comparative studies at similar sites in Mongolia.

References

Hoffmann A. 2006. Naturwerksteine Thailands: Lagerstättenerkundung und Bewertung. Dissertation Georg-August-Universität, Göttingen.

Kotwicz W., Samoïovitch A. 1928. Le monument turc d’Ikhe-khuchotu en Mongolie centrale. Rocznik Orientalistyzny, vol. 4, pp 60–107.

Rodrigues J. D. 1980. Decay of granite, Advanced study course, Science and technology of the environment for sustainable protection of cultural heritage. Relim.

Ruedrich J., Kirchner D., Siegesmund S. 2011. Physical weathering of building stones induced by freeze-thaw action: a laboratory long-term study. Environmental Earth Sciences, Volume 63, Issue 7-7, pp 1573–1586.

Svahn H. 2006. Non-Destructive Field Tests in Stone Conservation, Field and Laboratory Tests. Rapport från Riksantikvarieämbetet, Stockholm.

Vandevoorde D., Pamplona M., Schalma O., Vanhellemont Y., Cnudded V., Verhaeven E. 2009. Contact sponge method: Performance of a promising tool for measuring the initial water absorption. Journal of Cultural Heritage, Volume 10, Issue 1, pp 41–47.

Vandevoorde D., Cnudde V., Dewanckele J., Boone M. N., Verhaeven E. 2011. Contact sponge method: Performance compared with Capillary Rise, Karsten Tube and Mirowski Pipe. European Workshop on Cultural Heritage Preservation, EWCHP.

Vasconcelos G., Lourenco P. B., Alves C. A. S., Pamplona J. 2008. Ultrasonic evaluation of the physical and mechanical properties of granites. Ultrasonics 48(5):453–66.

Winkler E. M. 2013. Stone in architecture: properties, durability. Springer, Berlin-Heidelberg.

107

CHARACTERIZATION AND DETERIORATION ASSESSMENT OF KÖSSEINE GRANITE IN PALACIO SALVO, MONTEVIDEO, URUGUAY

Manuela Morales Demarco, Eloísa Cocorel

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.

Instituto de Ciencias Geológicas, Facultad de Ciencias, Universidad de la República, Uruguay

Abstract

In Uruguay, one of the most important buildings is Palacio Salvo. The present research is part of a diagnosis for the definition of an interdisciplinary project of intervention for the revalorization and conservation of its façades.

These activities started with the identification of the rock used as Kösseine Granite, of Wunsiedel, Germany. This rock is applied as polished pieces and most of them show, as deterioration features, rounding, missing parts, perforations with or without metallic inserts, often surrounded by oxide staining, deposits and soiling. Some plates eventually show up to 20 cm cracks in the edges related to splintering, chipping and two large plates are crossed by 80 cm fractures. Pieces used as steps show abrasion, rounding, and fractures in the edges causing missing parts.

Exterior and paving are among the most demanding applications of dimensional stones. Nevertheless, Kösseine Granite presents a good performance and its state of deterioration is low.

Keywords: Historical and cultural heritage, Kösseine Granite, Uruguay

Introduction

One of the most iconic buildings in Montevideo is Palacio Salvo, in the heart of the city: 18 de Julio Avenue and Plaza Independencia. Built between 1922 and 1928 by Mario Palanti, an Italian architect, it was commissioned by Salvo brothers.

In the declaration of Palacio Salvo as National Historical Monument, in 1996, it is clearly stated the relevance of its preservation and recovery (Presidential Resolution N° 1144/996, Uruguay). In this sense, an agreement was signed between the company owner of the building, Palacio Salvo S. A., and FADU (Facultad de Arquitectura, Diseño y Urbanismo – Universidad de la República) for a study for the conservation of its façades (IC-FADU, 2019). The ICG team (Instituto de Ciencias Geológicas, Facultad de Ciencias) was assigned a 3-month period the deterioration assessment of the granitic external façade of the ground floor.

Identifying the granitic façade

For the general registration of the rock applied and its deterioration assessment, the recommendations of the working group “Natural Stones and Weathering” (2019) of Aachen University were followed. It 108was not possible to map the entire external façade due to time constrains of the study, therefore only four sections of the building were mapped.

There was some controversy about the origin of the rock applied. In the Uruguayan small geological community, it was believed it was a facies of Pan de Azúcar Pluton. This pluton, studied in detail by Oyhantçabal (2005), is a km-size syenitic, quartz-syenitic and granitic circular intrusion, historically exploited with ornamental purpose (Morales Demarco, 2012). All efforts to localize the variety supposdly applied in Palacio Salvo in the area of the pluton were unsuccessful.

At the beginning of this study, 15 blueprints of the entire granitic façade were found by FADU in the General Archive of the Nation (Archivo General de la Nación). These blueprints have the localization and dimensions of around 1,000 pieces of the granitic façade of Palacio Salvo (the external façade and the façade of the passage, not studied in the present work), and some inscriptions written in German: Kösseine Granit Poliert, Grasyma Wunsiedel, Januar 1925.

The ICG team compare the dimensions of the pieces applied in Palacio Salvo, with those stated in the blueprints. The match is perfectly accurate. Simultaneously, a communication with the German company allowed the confirmation as the providers of the rock and the undoubtedly identification as Kösseine Granite (Figure 1).

Characterization of Kösseine Granite
Petrography

Due to the impossibility of sampling the rock in the buidling, as it is a National Historical Monument, a preliminary petrographic characterization was performed in situ. It is a very coarsed-grained leucocratic rock, yellowish white, to beige and greyish light brown. This is not the typical color of Kösseine Granite, but a yellowish variety that is also widely used in Germany.

The rock shows a porphyritic texture, due to the presence of alkali feldspar up to 3.5 cm, most around 2 cm, subhedral, frequently rounded. Some show zoning with a light brown core, sometimes euhedral, surrounded by a greyish white mantle. Most are grey. Inclusions of biotite (Strohmeyer, 2003) are very frequent, often evidencing growth rims. Occasionally, a translucent mineral can be observed, that gives the rock a pearl luster, this is probably plagioclase.

Figure 1: Right: Brochure of Palacio Salvo in construction with promotional poster of Grasyma (courtesy of Claus Wölfel).

Quartz is interstitial between feldspar and biotite, with light brown to beige color. Biotite shows sometimes hexagonal sections with dark reddish to coppery color. Muscovite is silver grey sometimes with greenish hues. Both minerals frequently form nests, and the biotitic sometimes show orange oxidation rims.

Garnet is present as an accessory mineral in mm euhedral to subhedral red crystals with vitreous luster. They occur occassionally as inclusions in alkali felspars, sometimes in matrix minerals.

Table 1: Mineralogical composition of Kösseine Granite.

The rock structure is characterized by the presences of elongated mafic enclaves, up to 20 cm with aspect ratio up to 1 : 20, some banded (mafic and felsic bands). Small rounded mafic enclaves are also present (2–3 cm). Felsic enclaves up to 40 cm 109ocurr either as fine to very fine-grained or as very coarsed-grained. All these structural elements disrupt the rocks décor. The rock is classified according to its mineral composition (Table 1) as a granite (Strohmeyer, 2003; Grimm, 2018).

Table 2: Physical properties of Kösseine Granite.

Table 3: Mechanical properties of Kösseine Granite.

Physical and mechanical properties

No physical or mechanical tests were performed, nevertheless, results of Strohmeyer (2003) and Grimm (2018) were considered (Tables 2 and 3).

Kösseine Granite shows a very good behavior against atmospheric agents, directly related to its physical properties: very low porosity, low water absorption, low water uptake coefficient and permeability. These properties show that the rock is practically impermeable, and consequently extremely stable to weathering conditions.

Mechanical properties for Kösseine Granite are typical of granitic rocks, even slightly higher. Comparing its UCS (uniaxial compressive strength) values to those of the statistical study of Mosch (2008), it can be observed that they are above the median. Flexural strength is also above the median of Mosch (2008), but the indirect tension is slightly below this parameter.

Abrasion resistance values are slightly higher than those reported by Morales Demarco (2012) for Uruguayan granitic rocks, but in the range of values given by Siegesmund & Snethlage (2011).

Figure 2: Ground floor plan (modified from FADU, 2019).

Figure 3: a. Perforation with metallic insert, oxide staining and star crack; b. Missing gap presumably a product of bursting or removal of metallic insert.