All that glitters is not gold: Unraveling the material secrets behind the preservation of historical brass.

• Non-degraded centuries-old brass objects are rare. • Brass sequins appear pristinely preserved in a series of 16th-century altarpieces. • Key information on the factors affecting the reactivity of brass has been obtained. • The manufacturing process has a central role on the stability of these objects. Brass is a relatively stable alloy but it tends to tarnish over time due to the interaction with the atmosphere. Thus, it is rare to observe centuries-old brass objects untouched by the passing of time. For this reason, the pristine appearance of hundreds of brass sequins in the Enclosed Gardens of Mechelen (reliquary altarpieces produced between 1530 and 1550) is remarkable. In this study, the chemical and metallographic characterization of such unexpectedly well-preserved objects is presented. The results revealed the reason for their stability to be a combination of high-quality materials (i.e. medium Zn content, low impurities) and optimal surface properties (i.e. high homogeneity, low roughness), indicating the high level of expertise of the craftsmen who produced them. Novel fundamental insights on the historical manufacturing method of metallic sequins were also obtained. [ABSTRACT FROM AUTHOR]