History of polysulfides and their role in the evolution of chemical knowledge: from ancient Egypt medicine to the discovery of oxygen

Before the introduction of modern chemical nomenclature, polysulfides were known as ‘theion hudor’ (divine water), ‘hepar sulphuris’ (liver of sulfur), ‘dia sulphuris’ (synonymous to hepar sulphuris), and ‘liver of sulfur’. These compounds were used for industrial and medicinal purposes already in the ancient world and are still at the cutting edge of modern research in the areas of energy storage, environmental chemistry, and geochemistry. In the Graeco-Egyptian school of alchemy, polysulfides played an important role for coloring of metals with golden patina which was viewed as a step to their transmutation to gold. During the Middle Ages, sulfur was purified by first reacting it with carbonates of alkali metals to form polysulfides and then decomposing the polysulfides with acid. Phlogiston theory, which represented the prevailing understanding of the redox reactions in the XVIII century, was used to explain transformations involving polysulfides. Due to their ability to react quantitatively with oxygen, polysulfides played an important role in determining the composition of the atmosphere at the end of the phlogiston era.