Thermal decomposition of synthetic argentojarosite—Implications for silver production in medieval times

The thermal decomposition of argentojarosite has been studied by thermogravimetric, spectroscopic and infrared emission techniques. Dehydroxylation occurs in three stages at 228, 383, 463 °C with the loss of 2, 3 and 1 hydroxyl units. Loss of sulphate occurs at 548 °C and is associated with a loss of oxygen. At 790 °C loss of oxygen only leaves metallic silver and hematite. Changes in the structure of argentojarosite are also followed by the infrared emission spectra. The intensity of hydroxyl stretching vibration approaches zero by 400 °C confirming the loss of hydroxyl units by this temperature. At this temperature changes in the infrared spectra of the sulphate units occurs. The intensity of the sulphate stretching vibrations approaches zero by 750 °C. Thermal decomposition of argentojarosite is important in understanding silver production in ancient and medieval times. This work shows that temperatures of around 750 °C are required to produce metallic silver.