Development of rust stains on the skin due to contact with a gun

In order to study the conditions for the formation of so-called rustmarks on the skin after contact with weapon steel tests to provoke rustmarks were performed on corpses and living bodies. These tests were only successful under the condition of a firm contact with the weapon steel and a certain minimum contact time between the weapon and the skin. The experiments showed that the critical parameters for the appearance of rustmarks are, first, the humidity of the skin and the environment, respectively, as well as the contact time, and second, the state of the weapon surface (greased or ungreased). Both the ambient temperature and the pH value of the skin (alkaline or acid) are irrelevant to the formation of rust. The longest time period until rustmarks appeared was observed on dry skin in contact with a greased weapon, namely 22 hours on a corpse and 170 min on a living subject. When using a greased gun humidity accelerated the formation of rust. An ungreased surface of the weapon also resulted in faster rust formation. The minimum time necessary for the formation of rust determined under the most favourable circumstances was 135 min for a corpse and 27 min for a living person. During the early postmortal interval the rustmark may therefore be another piece in the jigsaw towards determining the time of death on suicides committed with firearms.