The Time of Castles Project: Characterization of Mortar Raw Materials for ¹⁴C Dating in Medieval Castles of Northern and Central Italy

The transformation of medieval castles in Italy, especially between the 11th and 12th centuries during the process of incastellamento, represents a crucial yet complex phase in European history. The transition from early wooden fortifications to durable stone structures is often poorly documented and broadly dated. PRIN 2020 CASTLES Project integrates archaeology, history, geology, conservation science, and physics in a multidisciplinary framework aimed at building a new and more precise chronology of medieval castle construction in northern and central Italy. The project focuses on 25 castles in Piedmont, Liguria, and Tuscany, combining historical and archaeological evidence with archaeometric analysis of masonry to refine the dating of construction phases and improve understanding of building practices. For this purpose, radiocarbon dating (¹⁴C) is used as an absolute dating technique. In this study, the ¹⁴C method was applied for dating ancient mortars of masonry castles, targeting both inorganic material (binder and lump calcite) and organic inclusions (charcoal). However, selection of the inorganic datable fraction and elimination of potential contamination proves to be a challenge for the international radiocarbon community. To select the most suitable mortar binder for ¹⁴C dating, a key step in the research is mortar sampling and the characterisation of the raw materials used in mortars. To minimise mortar contamination, samples were carefully taken with consideration for the archaeological structure and the preservation of the masonry. Once the mortar samples were selected, penetrometric and carbonation tests were carried out on site. Subsequently, mineralogical, petrographic, and chemical analyses were performed using XRPD, OM, SEM–EDS, OM-CL, and ATR-FTIR. These analyses enabled the identification of binder types, aggregate composition, and hydraulic properties. Air-hardening lime mortars proved to be the most reliable, whereas natural hydraulic and magnesium-rich limes often introduced complications. Further analyses were performed on powders of binder-rich portions or lumps, using non-destructive techniques, such as XRPD, OM-CL, ATR-FTIR [1]. More than 120 mortar samples were analysed, from which 63 powder samples were selected and prepared for ¹⁴C dating. Mortar radiocarbon results were cross-checked with charcoal dating and archaeological data, confirming the reliability of the methodological procedure. Several case studies from Tuscan castles demonstrate strong coherence between mortar characterization and radiocarbon ages, validating the approach. On a larger scale, the project is defining chronological trends that reflect different phases of castle development, from early fortifications to fully developed lordly residences and village centers. Overall, the project offers a reproducible and multidisciplinary framework that significantly improves the chronological reconstruction of medieval castles in Italy and provides a model applicable to similar contexts elsewhere.

 

[1] Calandra S., et al. 2024, A new multi-analytical procedure for radiocarbon dating of historical mortars, Sci Rep, 14(1), 19979.