Replication of the conversion of goethite to hematite to make pigments in both furnace and campfire

The intentional conversion of brown ochres (goethite) to red (hematite) by heating is well documented in cultures deep into our past, and it has been thoroughly studied in the laboratory by modern analytical techniques. In this study, we replicate the heating of natural goethite in wood fires, and we document the colors of the resulting powders by visible reflectance spectra, Munsell colors, and photography. We compare these results to goethite heated in the lab in a furnace. To our knowledge, this is the first controlled attempt to replicate the conversion of goethite to hematite in a fire to make pigment powder, to characterize the colors, and to compare results in a fire to those in the laboratory. We show that with a little care goethite is easily converted in a wood fire into hematite whose colors match those when goethite is heated in a furnace. We find that the heating of solid lumps rather than powder makes it easier to avoid contamination in the fire. The color of the resulting hematites is independent of the temperature within the temperature range offered by wood fires. The ease of the process supports the early discovery and rapid dissemination of the technology observed by archaeologists. Finally, this study demonstrates that the conversion of synthetic goethite differs significantly from natural goethite, thus synthetic does not offer a realistic alternative when replicating ancient processes.

Automated summary

The intentional heating of natural goethite to convert it into hematite has been successfully replicated in both laboratory and fire settings, with similar colors produced in both. Heating solid lumps instead of powder helps avoid contamination in fire, and the resulting hematite colors are consistent within the temperature range of wood fires. This supports the rapid dissemination of technology observed by archaeologists, and highlights the significant differences between natural and synthetic goethite in terms of color conversion processes.