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In antiquity, therapeutic empiricism attributed medicinal properties to animal products, plants, minerals and metals, including the soil of specific geographical locations.

The therapeutic use of certain earths and metals is thoroughly documented in the works of Hippocrates, Dioscorides and Galen and was still practised in the eighteenth century by eminent physicians such as Sir Hans Sloane. Mercury and arsenical compounds have also been widely used since antiquity, the latter finding application in our times in the successful treatment of acute promyelocytic leukaemia.


This paper presents the scientific investigation of three Lemnian sphragides (terra sigillata, stamped earth), a famed medicinal clay in antiquity, dated to the sixteenth–seventeenth centuries, and presently in the Museum for the History of Pharmacy, University of Basel. The three specimens are compared with clays from the purported locality of its extraction, at Kotsinas, NE Lemnos, Greece. The study suggests a local origin for the Basel samples; it also demonstrates, for the first time, that the three Lemnian sphragides have a significant antibacterial effect against Staphylococcus aureus, a common Gram-positive pathogen, but have no such effect against Pseudomonas aeruginosa, a Gram-negative microorganism. Clay samples from the purported locality of extraction showed no antibacterial effect against S. aureus. Subsequent analysis with ultra-performance liquid-chromatography mass spectrometry (UPLC-MS) revealed the presence of organic constituents in one sphragis which were absent from a sample of modern clay. A fungal secondary metabolite is proposed here as the active ingredient but other factors may also play a role. The ongoing investigation into the bioactivity of some medicinal clays might aid in the re-evaluation of Belon’s statement included at the start of this paper, namely, that the Lemnian earth worked only because people in the past wished it to work.

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