 Other references suggest that the first mention of aqua regia is in Byzantine manuscripts dating to the end of the 13th century.Free hydrochloric acid was first formally described in the 16th century by Libavius, who prepared it by heating salt in clay crucibles.Other authors claim that pure hydrochloric acid was first discovered by the German Benedictine monk Basil Valentine in the 15th century, when he heated common salt and green vitriol, whereas others argue that there is no clear reference to the preparation of pure hydrochloric acid until the end of the 16th century.