Claus was known for his negligent attitude towards his health. In particular, he often tasted his chemicals and new compounds and tested the strength of the acids by dipping a finger in them and touching his tongue with it. Once he severely burned his mouth while tasting one of the new ruthenium compounds that he had synthesized. When he isolated osmium tetroxide – a rather toxic chemical – he described its taste as astringent and pepper-like. In April 1845, he was poisoned by osmium tetroxide vapors and had to stop working for two weeks. In 1852, Klaus moved from Kazan back to the University of Tartu and assumed position of professor in pharmacy. He died in Tartu in 1864.