Glass backed with a tin–mercury alloy, commonly called the amalgam mirror, was the dominant mirror used from the 15th century until the beginning of the 20th century. In this work, a qualitative analysis of the crystalline phases of the alloy surface of ancient mirrors was done using the grazing incidence X-ray diffraction technique (GID). Their morphologies were studied by scanning electron microscopy (SEM), and the elemental analyses were done by energy dispersive X-ray spectrometry (EDX). X-ray photoelectron spectroscopy (XPS) was used to characterize the atomic composition of the surfaces. Our results indicate that the amalgam is composed of a binary alloy of tin and mercury (Hg_0.1Sn_0.9). Mercury is volatile and slowly evaporates, leaving finely divided particles of tin that are easily oxidized, forming romarchite (SnO) and cassiterite (SnO₂).