Testing Dioscorides' medicinal clays for their antibacterial properties: the case of Samian Earth

Earths were medicinal clays reported by authors of classical antiquity. Today, there are no archaeological samples thereof, at least with irrefutable proof that they were used as medicines. Therefore, their mineral identity and potential pharmacological actions have to be investigated on the basis of field work, laboratory testing and analysis as well as scrutiny of the relevant ancient texts. This paper focuses on Samian Earth, from the island of Samos in the East Aegean, used in antiquity as an eye salve and for the cleaning of cloth. Previously, Samian Earth has been equated with kaolin but we suggest its composition was probably more closely associated with the local borates to be found in the proximity of bentonitic clays. Microbiological testing of hydrated layered silicates (kaolin and bentonite) from Samos and Melos in the Aegean in association with different borates (colemanite, tincalconite) was carried out against common pathogens, like Staphylococcus aureus and Pseudomonas aeruginosa. Experimental results showed that even a few hundred ppms of boron/boric acid in the Samos bentonitic clays can have a significant effect on the reduction of bacterial colonies. This observation corroborates the assumption that the medicinal properties of the earths were integrally associated with the mineralogy of specific localities within the volcanic islands of their origin.