Monument Future. Siegfried Siegesmund

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СКАЧАТЬ IN PRE-COLUMBIAN AND COLONIAL ARCHITECTURE OF OAXACA, MEXICO

Alexandra Kück1, Christopher Pötzl1, Rubén López-Doncel2, Reiner Dohrmann3, 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 Georg August University Göttingen, Germany

2 Geological Institute of the Autonomous University of San Luis Potosí, Mexico

3 Federal Institute for Geosciences and Natural Resources (BGR), Hanover, Germany

       Abstract

      Volcanic tuff was of great importance in the ancient culture of Zapotecs and Mixtecs as a construction material. The historical buildings in both Mitla and the historical center of Oaxaca were erected with a great variety of volcanic tuff rocks, many of which are not quarried anymore. These tuffs were compared and evaluated regarding their petrophysical properties and weathering behavior. Analyses of the petrography, pore space, water transport and water storage as well as mechanical properties were performed.

      The results of the investigations show that the rocks have high sensitivity to water, linked to high porosities and high amounts of capillary pores. Additionally, very variable behavior towards hydric expansion and salt bursting provokes different responses to weathering and decay. To protect the historical buildings in Oaxaca, it is therefore necessary to control the exposure to water or to find suitable conservation measures for the stones.

       Introduction

      The state of Oaxaca has a very long history of architectural construction and an important archaeological heritage. The Zapotecs and Mixtecs were the leading cultures in the region until the Aztecs invaded in 1428, and finally conquered by the Spanish conquistadors in 1521, occupying Oaxaca (Blanton et al., 1999).

      The convent of Santo Domingo de Guzman was built in the 16th century by the Dominican Order (Urquiaga 2000). The Cathedral of Oaxaca, also called ‘Catedral de Nuestra Señora de la Asunción’, was built from 1553 to 1733. There were several periods of reconstruction in the history of the cathedral, for example in 1696, 1723, 1891 and 1999 (Casanova and Pino 2004).

      The pre-Hispanic archaeological site of Mitla (about 45 km southeast of Oaxaca) is about 1800 years old and was first mentioned in the literature of the 16th century (García 2016; Bernal 1963).

      The historical center of Oaxaca de Juárez is UNESCO world heritage site known for its cultural tradition and its history of art and architecture. Both the city center of Oaxaca and the archaeological site of Mitla were built with a great variety of volcanic tuff rocks (Fig. 1).

      This study focusses on the deterioration behavior of the tuffs in both Colonial and pre-Hispanic architecture. A variety of tuff rocks have been tested regarding their petrography, pore space properties, water transport- and storage properties, mechanical properties and weathering behavior.54

      Figure 1: Historical buildings in Oaxaca. a): Church of Santo Domingo de Guzmán, b): Oaxaca Cathedral, c): Archaeological site of Mitla (‘El Palacio’).

       Sampling

      Cantera Verde Oaxaca (CVO), Cantera Amarilla Oaxaca (CAO) and Cantera Rosa Oaxaca (CRO) are samples from the city of Oaxaca de Juárez. Cantera Verde Etla (CVE), Cantera Amarilla Etla (CAE) and Cantera Rosa Etla (CRE) are used nowadays as replacement stones (originating 20 km north of Oaxaca de Juárez). The samples from Mitla are called MG (Mitla Gris) and MR (Mitla Rosa).

       Methods

      The archaeological site of Mitla, the church of Santo Domingo de Guzmán and the Oaxaca Cathedral were mapped for lithology, intensity of damage and weathering features (Fig. 2). For most walls a representative area was selected, and each rock was mapped individually to make a semi-quantitative analysis.

      In accordance to the German industrial norms, several laboratory tests were conducted for the analysis of the petrophysical and moisture parameters as porosity, density, capillary water absorption, water vapor diffusion, hygroscopic water sorption, 55ultrasonic velocity, rebound hardness and tensile strength. For the analysis of weathering properties, tests on the thermal expansion, hydric expansion and salt bursting were performed. The petrophysical properties were analyzed parallel (X-direction) and perpendicular to the bedding plane (Z-direction). A detailed description of the laboratory analysis can be found in (Siegesmund and Dürrast 2011).

      Figure 2: Mapping in Oaxaca and Mitla. a): Lithological map of Santo Domingo de Guzmán, b): Weathering features at the eastern side of the Oaxaca Cathedral, c): Intensity of damage at the southern side of ‘el palacio’ at the archaeological site of Mitla.

       Results

      Table 1 summarizes the test results for the petrophysical and moisture properties as well as the weathering properties of all samples.

       Mapping

      The historical buildings in Oaxaca were mostly built with CVO. Delicate parts like ornaments and window sills were built with the softer CAO. CVE was used as a replacement for CVO during different phases of restoration (Fig. 2a).

      The most important weathering features in Oaxaca are loss of components, loss of matrix and bursting. The base parts of the buildings often show scaling and flaking accompanied by salt crust formation. Upper wall parts and protruding areas are strongly affected by moisture and form a characteristic organic black crust (Fig. 2b). CVO is dominated by loss of components when pumice clasts dissolved. CAO and CVE are strongly affected by fracturing, when in contact with water (Kück et al. 2020b). CRO and CRE are the only samples which show bedding parallel scaling.

      In Mitla most of the building stones are original and mostly in good condition. The southern and eastern side of the buildings have the highest damage intensities linked to high temperature changes during the day of up to 20 °C (Fig. 2c). The northern side experiences a lot of moisture, but the temperature difference during the day is low (3 °C). The western side is the least weathered side.

       Petrophysical and moisture properties

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