Two painted plaster masks and an effigy head are among many objects from a late 15th-century ritual assemblage recovered from San Lázaro Pueblo (LA92), New Mexico. During consultations with Native American descendants of San Lázaro residents, the assemblage was identified as belonging to a prehistoric medicine society. In accordance with an agreement for respectful study, small fragments from these artefacts, as well as experimental raw material preparations, were studied using SEM–EDS (scanning electron microscope–energy dispersive X-ray spectrometry), CHN (carbon–hydrogen–nitrogen) analysis and FTIR (Fourier transformed infra-red) spectroscopy. The artefacts are made of gypsum plaster, which shows microstructural layering reflecting manufacturing phases and techniques. Green and red paints were made from copper- and iron-bearing mineral phases, mixed with organic binding agents, whereas black paint is carbonaceous. The pigmenting agent present in grey paint is a mixture of amber-coloured gypsum and a small amount of carbonaceous matter. These three artefacts are part of the only reconstructable plaster sculptural assemblage known from the prehistoric United States Southwest, and analysis results provide new insight into the technology associated with an important prehistoric sculptural tradition.