Galicia preserves several late and early medieval churches (10th to 5th centuries) which in recent years have been systematically studied to try to obtain an absolute dating of their oldest phases (Sánchez-Pardo and Blanco Rotea, 2014). Thus, the building materials of Santa Comba de Bande (Bande), Santa Eulalia de Bóveda (Lugo), the basilica of La Ascensión and os Fornos, San Martiño de Pazó (both in Allariz), Santa Eufemia de Ambía (Baños de Molgas), Igrexa Vella de Santa Comba de Louro (Valga), Santa Martiño de Armental (Vilasantar), among others, have been dated using two techniques in the case of mortars: Optically Stimulated Luminescence (OSL) and Carbon 14 (14C). Two of these buildings, Santa Eulalia de Bóveda (Lugo) and San Breixo de Ouvigo (Os Blancos) present a complex construction sequence and a chronological and functional problem that is of interest for a dating project such as the one we are dealing with here.
Santa Eulalia de Bóveda is a two-storey building. The N wall of the upper one is barely left, as it was razed to the ground when the parish church was built in the 18th century. The lower one is a half-buried vaulted room with an apse at the W and a narthex at the E, which had a swimming pool. Its vaults preserve paintings depicting poultry. Since its discovery in 1926 it has been the subject of multiple interpretations and interventions, although researchers do not agree on its ascription and functionality. It has been considered a Roman nymphaeum, a Visigothic church, and Asturian… and recently a Roman funeral monument (Montenegro Rúa, 2005). The most generalised proposal is that initially it was a temple of pagan water worship (3rd century), which at the end of the 4th century was Christianised, with the paintings and the separation into three naves. From then on it would be a Christian church, being mentioned in the 8th century in Odoario’s dubious will (Núñez Rodríguez, 1978: 134-138).
In 2007, part of the team that participated in this project carried out a study of the monument that consisted of its documentation with a 3D scanner recovering its general volumetry; the reading of the facings that allowed the identification of up to 5 construction phases; or the study of its mortars, for its characterization and dating. Initially it would be a vaulted, semi-buried building with a swimming pool that would occupy most of the classroom. In a second phase the hall would be divided into three naves, the pool would be made smaller and the interior would be decorated with paintings and marble plates. In a third phase, a second floor would be built or renovated using some Roman materials (Blanco-Rotea et al. 2009).
Sample collection area in Santa Eulalia de Bóveda (perspective by Anxo Rodríguez Paz)
In San Breixo de Ouvigo (Os Blancos) we can find the remains of an old chapel, of which only the S wall, part of the N wall and the headboard, and the footboard are still standing. It would be a small temple with a single nave and a rectangular floor plan with a compartment at its E end, to delimit the apse. The site was excavated in 1972 and between 1977 and 1981 by Rodríguez Colmenero (1985). For the author it is a late Roman and medieval site consisting of a small rectangular building with an extensive necropolis in its surroundings. He distinguishes five phases: a first possible Roman building that disappeared at the end of the 6th century; in a second phase the current building is built, which had stuccoed interiors; in a third phase (8th-11th centuries) the walls are reinforced and the building is stuccoed again; in a fourth phase a well is opened for a silo, destroying the NE corner of the building; and a fifth phase of abandonment and ruin of the place, caused by the construction of a larger church in the 12th century in Covelas (Rodríguez Colmenero 1985). In the necropolis, a late Roman tomb of tégulas has been identified, another later one with mortar walls and interior plastering, using the same technique as that employed in the second phase of the building; and burials with stolae tombstones (one of them with an epigraphic date of the year 909), anthropomorphic tombs excavated in the rock and vertical slab graves. In the excavations, pieces of late Hispanic Terra sigillata decontextualized and numerous medieval ceramics were documented.
In Santa Eulalia de Bóveda the problem of the chronological assignment of the construction phases is due to the fact that the type of decoration, of pictorial motifs or the typology of the building presents similarities with other architectures belonging to a wide chronological range, from the 3rd to the 9th century, so in order to know its true chronology it is necessary to date the construction materials of each of the possible phases. In 2008, bricks were dated from various construction phases (Blanco-Rotea et al. 2009). The result indicated that they were ancient bricks of Roman origin. Joint mortars were dated by OSL. As these were the first works, we now know that the selection of the sample was not the correct one, nor was the quantity of the sample insufficient to carry out the necessary analyses. The results provided relatively modern ages. The oldest samples can be dated to the seventh and ninth centuries, all the others corresponding to the period between the fourteenth and eighteenth centuries.
St. Breixo d’Ouvigo, sampling. Left: plan made by Colmenero (1985); right sample collection (PHOTO: Mario Fernández Pererira)
San Breixo de Ouvigo is an interesting case since the rigging differs from that documented in other early medieval buildings analysed, as well as the type of mortar containing lime in all these possible stages (as opposed to the earthenware mortars that we generally document between the 8th and 11th centuries), which could lead us to consider the chronological difference with respect to these other constructions as a starting hypothesis. In order to contrast the proposed chronologies it is also necessary to date the building materials. Here, only lime and sand mortars would be possible, the objective being, above all, to date the second phase. Mortar samples were taken in 2016 and dated by radiocarbon, but the separation of archaeological and geological calcite was not carried out. They provided very different results with very wide ranges in one case, as one of them corresponds to the 9th-13th centuries and the other to the 14th-15th centuries. If these chronologies are confirmed, it would be an important change in the understanding of late antique architecture in Galicia and the NW of the peninsula, as one of its main representatives would become a pre-Romanesque building dated in the 10th and 10th-12th centuries respectively.
It is proposed to perform a characterisation and cross dating by OSL and 14C of several lime mortar samples in both buildings. The aim of the project is to obtain a chronology through two absolute dating techniques applied to mortars, but this requires prior characterisation. In the OSL dating process, it will be possible to determine whether the samples contain quartz in sufficient quantities, the heterogeneity of the samples and their content in certain trace elements and radioisotopes, which is an essential requirement for dating.
In the case of radiocarbon dating it is necessary to know how much calcite they contain and of what type (geological or archaeological) and to analyse their chemical composition and isotopic fractionation. A series of samples obtained previously in both buildings that have been reserved will be used, and other new ones among which it is intended to date materials deposited in the Provincial Archaeological Museum of Lugo, since they correspond to fragments fallen from part of the vaults of Santa Eulalia de Bóveda. Up to the present there are no published works that have applied cross-dating of OSL and radiocarbon in lime and sand mortars, so this project would be a pioneering work at world level.
Dr. Jorge Sanjurjo-Sánchez, Dra. Rebeca Blanco-Rotea y Dr. José Carlos Sánchez-Pardo
Dr. Jorge Sanjurjo-Sánchez (IP) (Profesor Contratado Doctor, Universidade da Coruña); Dra. Rebeca Blanco-Rotea (miembro del equipo) (Universidade de Santiago de Compostela); Dr. José Carlos Sánchez-Pardo (miembro del equipo) (Universidade de Santiago de Compostela)