“From blue to red: first evidence of heat treatment in the production of Minoan serpentinite vases through non-invasive study and experimental petrology”

The intentional heat treatment of stone to alter its appearance remains a largely understudied practice in archaeology, and its identification in the archaeological record is often challenging. By combining portable and non-invasive X-ray fluorescence (pXRF) and magnetic susceptibility (pMS) analyses with controlled heating experiments on local serpentinite used in Minoan (Bronze Age Crete) contexts, this study presents the first documented evidence of intentional heating in the production of stone vases. It also proposes a replicable analytical framework broadly applicable, yet particularly suited to ultramafic lithologies. It focuses on a unique assemblage from the late Protopalatial period at Quartier Mu, Malia (Crete, 1800–1700 BCE), where twenty-five serpentinite vases exhibit a distinct red coloration. Macro-petrographic observations and pXRF analyses confirm that the red vases consist of serpentinite and show no trace of added pigments. pMS values are significantly lower in the red serpentinite vases than in the blueish (unheated) ones, which is consistent with the effect of heating samples of Cretan serpentinite in our experimental results. In our high-temperature heating experiments, the red coloration is driven by the natural transformation of magnetite into low-magnetic iron oxides at temperatures above 700 °C under oxidising conditions. The application of this thermal threshold, combined with contextual evidence showing no signs of large-scale burning, allows us to reject the hypothesis of accidental firing. These findings provide new insights into Minoan stone-vase production, identifying heat treatment as a deliberate technological choice at Quartier Mu. More broadly, the methodology illustrates how experimental petrology and non-invasive techniques can together highlight ancient heat-related practices while preserving the artifacts.