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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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