Natural red chalk in traditional old master drawings

In the second half of the 15th century, European artists added natural red chalk-raw unprocessed red ocher-to their limited selection of natural chalks for use as a drawing medium. This article presents the history, geology, chemical composition, sources, production, and physical characteristics of natural red chalk when used as a traditional old master drawing material. The historical record was examined to understand natural red chalk's relationship to the development of fabricated red chalks, including artificial red chalks. A total of five different specimens of raw unprocessed natural red chalks from France, Germany, and the United States were obtained and studied using optical microscopy and field emission scanning electron microscopy to observe their appearance, grain size, and particle morphology, as they relate to the medium's working properties as a drawing material. Their chemical composition was determined using x-ray diffraction analysis and scanning electron microscopy with energy dispersive x-ray spectroscopy. These results were compared with the physical characteristics and chemical composition of traditionally used natural red chalk documented in the historical record.