Evaluation of handheld X-ray fluorescence spectroscopy results of Roman copper alloy brooches by using archaeological typology

As part of the Late Roman research project in Belgium and the Netherlands, 187 Roman copper alloy brooches were analyzed by means of handheld X-ray fluorescence (hXRF) spectroscopy in order to explore the relationship between composition, production organization and change over time. The selected brooch type is called ‘the crossbow brooch’, which is an artifact that is closely associated with the Late Roman elite, frequently occurring in many portraits on mosaics, sculptures and fresco’s from the 4th to the 6th century. The biography of the crossbow brooch, however, starts in the 3rd century as a simple military object and develops into one of the most significant symbols of Roman state authority in the 5th and 6th century. The hXRF spectrometer provided a non-destructive, mobile, quick and inexpensive way of analyzing these brooches that were part of valued museum and archaeological collections. The samples were selected to cover the entire chronological, geographical and stylistic variation of these brooches in the study area. Each object was measured in three to five locations to compensate the heterogeneity of the copper alloy and the geometry of the object. The compositional results revealed a continuous variation that crosses copper alloy boundaries with no evident groups or trends, as is often the case for archaeological data sets. To move beyond a black box approach that provides little insights, the typological information was added to interpret the compositional data. An archaeological typology is constructed based on observed variations that represent expressions of choices made by craftsmen and workshops. In other words, typology contains information that can help explain compositional variation caused by different manufacturing techniques or production centers, changes in consumer demands, as well as factor in aspects of chronology and regionality. Consequently, the division of the compositional data in the six types of crossbow brooches allowed to distinguish patterns that indicated changes in production organization related to social context. The combination of typology and compositional data allowed to characterize the different phases in the crossbow brooches’ life history, which in its turn contributed to the larger narrative of the rise to power of the military elite in the Late Roman period