Miltos from Kea was the highly prized, naturally fine, red iron oxide-based mineral of antiquity used as a pigment, in boat maintenance, in agriculture and in pharmacological preparations. The frequent reference of miltos in association with boats suggested not mere decoration. We found that some samples of Kean miltos were high in Pb. It follows that, if such samples of miltos, placed in an organic medium like pitch, were to be applied to a wooden surface like a ship’s hull, they would not have allowed the growth of a biofilm (on which barnacles attach themselves); as such they would have indeed acted as an efficient antifouling agent. 

Red Lemnian earth was frequently referred to in antiquity as Lemnian miltos; indeed, the similarity in colour is striking (see photo below); but compositionally they are quite distinct. Also the method of their use, as leachates (Lemnian Earth) vs powders (miltos) was quite different, underpinning their bioactivity.

We suggest that bioactivity of miltos, its effect on living organisms (biofilms, plants or humans) was the result of a interactions between mineral and its natural microbiome; it is the combined effect of the biotic and abiotic components that gave miltos, its  bioactivity.

The G-R world understood natural materials as an integral part of their ecological setting. They worked at enhancing their properties and tailored them to an intended  specific application.