Pictorial remanent magnetization retrieved from the Santo Domingo Convent (Isthmus of Tehuantepec, Oaxaca, Mexico): An attempt to directly date mural paintings based on geomagnetic field variation

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Highlights

  • A detailed magnetic mineralogy and archaeomagnetic investigation was performed on thirty-two red mural painting samples.

  • Titanium-poor titanomagnetite seems to act as responsible for major part of pictorial remanent magnetization.

  • The possible elaboration date of the painting layer under study is estimated between AD 1684 and 1744.

  • Despite low success rate, the described methodology deserves to be considered for dating of wide range of wall paintings.

Abstracts

A detailed magnetic mineralogy and archaeomagnetic investigation was performed on thirty-two red mural painting samples belonging to the colonial Santo Domingo Convent in Oaxaca (Southern Mesoamerica). Systematic magnetic experiments included continuous thermomagnetic curves, hysteresis cycles, acquisition of isothermal remanence and step-wise alternating field demagnetization of the pictorial remanent magnetization (PiRM). Titanium-poor titanomagnetite seems to act as responsible for major part of pictorial remanent magnetization while heating and cooling curves show marked irreversibility and high thermal instability. Although all samples were demagnetized by means of alternating fields up to 90 mT, only seven of them provided reasonably well-grouped paleodirections yielding following mean values: Inclination = 35.2°, Declination = 2.3°, α95 = 4.8° and k = 161. According to the probability density function obtained by means of the global geomagnetic model SHA.DIF.14 K, the possible elaboration date of the painting layer under study is estimated between 1684 CE and 1744 in accordance with historical documents. Despite low success rate, the described methodology deserves to be considered for dating of wide range of red mural paintings worldwide.

Introduction

Mural and cave paintings are widely distributed archeological elements throughout our planet. Beside their importance, still no direct dating methods are available. Although Beck et al. (2020) succeeded to date some specific artworks containing 14C in inorganic pigments, this kind of situation is probably unique or at least very unusual.

Remanent magnetization of mural (and probably cave) paintings is usually carried by antiferromagnetic hematite grains, present mainly in the red color pigments. Chiari and Lanza (1997) demonstrated that when the red pigment is applied to the wall, the grains are free to move and they align their magnetic moment with the Earth's magnetic field. When the painting is dried, the magnetic minerals maintain their orientation, acquiring a ‘pictorial’ (PiRM) remanent magnetization (see also the review paper of Goguitchaichvili et al., 2016). Thus, the mural paintings may act as an alternative source of information about the geomagnetic field secular variation (Chiari and Lanza, 1997, Chiari and Lanza, 1999; Lanza, 2009; Goguitchaichvili et al., 2016).

The magnetic study of paintings is still very scarce because of the great difficulties related to accurate laboratory measurements. Moreover, the main problem resides in the sampling procedure since it is difficult to separate the painting from the wall because of high adhesion. Based on a few studies achieved until now, Goguitchaichvili et al. (2016) pointed out that most of murals may retain their remanent magnetization for centuries and demonstrated the viability of pictorial remanence as a realiable archaeomagnetic tool. In this study, we analyzed 32 oriented samples that belonged to two different red paintings in colonial Tehuantepec Convent (state of Oaxaca, Southern Mexico). Both layers are stratigraphically and physically distinguishable. Thus, it is improbable that both layers are present in the same specimen. The present study is part of the global restoration project of the Dominican complex “Templo y Convento de Santo Domingo” in Santo Domingo Tehuantepec, Oaxaca, which was seriously damaged by the September 2017 earthquakes.

Section snippets

Location, context and samples

The Isthmus of Tehuantepec is located to the east of the state of Oaxaca, between the Gulf of Mexico and the Gulf of Tehuantepec. It borders on the north with Veracruz and on the east with Chiapas. The southern portion is called the Isthmus region in the state of Oaxaca and corresponds to the Tehuana coastal plain of the Pacific. It covers an area of 2,062,410.12 ha. Most of the Isthmus belongs to the physiographic province “Central American Cordillera” and is divided into two main

Laboratory experiments

The magnetic mineralogy of samples was determined using a Variable Field Translation Curie Balance. The experiments were carried out in the laboratory of the National Archeomagnetic Service, Mexico. These experiments are aimed to identify the main magnetic carriers, their domain state and thermomagnetic stability. The measurements included: a) isothermal remanent magnetization (IRM) progressive acquisition curves, b) magnetic hysteresis cycles recorded up to 0.8 T, c) so-called Back-Field

Main results, discussion and concluding remarks

Continuous thermomagnetic experiments show frankly unstable behavior judging from the heating and cooling curves (Fig. 3a, b and c) which are highly irreversible. Close to almost pure magnetite phase or Ti-poor titanomagnetite is detected during the heating cycle showing a quite marked Hopkinson type effect. Magnetite creation during the laboratory heating from about 400 °C is evident and most probably due to the presence of some paramagnetic minerals. Corresponding hysteresis and isothermal

Author statement

We explicitly declare that this manuscript is original, has not been published before and is not currently being considered for publication elsewhere. We confirm that the manuscript has been read and approved by all co-authors. We further confirm that the order of authors listed in the manuscript has been approved by all authors. This work is leaded by Avto Goguitchaichvili. All authors participated in the field trip, laboratory measurements and data analysis.

Declaration of Competing Interest

We explicitely declare that there is no conflict of interest.

Acknowledgments

This work was supported by the CONACYT project n° 252149 and the DGAPA-PAPIIT grant n ° IN101920. We thank the Alfredo Harp Helú Oaxaca Foundation for the facilities to collect oriented samples. We thank the Alfredo Harp Helú Oaxaca Foundation for the facilities tp collect oriented samples.

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