The building most interesting and enjoyable of San Quirico d’Orcia is undoubtedly the magnificent collegiate or parish church of Osenna (Saints Quirico and Giulitta), the church is starting out from the eighth century, but the current structure dates from the twelfth or thirteenth century. The oldest part seems to be that corresponding to the facade and in particular the main portal, the façade adorned with arches and a Gothic rose window is decorated with a beautiful Romanesque Portal of the twelfth century, preceded by a protiro with knotted columns resting on two lionesses.
Inside the protiro, there are five columns to the left and five to the right with capitals decorated with acanthus leaves and animals. The architrave of the portal has carved a bas-relief depicting a struggle between two monstrous figures. In the center of the lunette is carved in high relief a figure on the throne held an effigy of San Damaso, but in reality to be identified with the representation of San Quirico.
In the second half of the thirteenth century were undertaken extensive renovation work, which consisted of the addition of the transept and the two side doors along the Francigena, on the right side of the Collegiate church. Here there are two mullioned windows and the first of two lateral portals of Lombard type (13th century), topped by a protiro supported by two cusped caryatids resting on so many lions, probably built in 1288, it has been attributed to Giovanni Pisano who in those years (1287-1288) was engaged in the construction of the Duomo in Siena, on the cusp the portal is written in Gothic letters “Iohes”, interpreted as the name of Pisano or even as a memory of the title of the parish church dedicated to San Quirico and St. John the Baptist. A third Gothic Portal (dated 1298) is in the right head of the transept, built by the priest Lotto (from inscription on the lintel) which is probably also the also the manufacturer of the transept, the portal shows a combination of Gothic and Romanesque.
The interior has a Latin cross with three apses, with wooden trusses in the vaulted nave and transept cruises embers. On the left side in the third arch is the tomb of Count Henry of Nassau (died in San Quirico in 1415). In the left side of the transept is a triptych of Sano di Pietro (1406-1481), in the apse is a very old carved wooden choir attributed to the Sienese Antonio Barili, intarsie originally executed for the Duomo of Siena and later bought by the Chigi family. The end of the church has been completely altered with the demolition of the apse in 1653 to build the original chorus. The bell tower was rebuilt between 1798 and 1806.
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