Miltos: the elusive red pigment

Ancient Greek and Roman texts tell that a red powdered mineral, known as miltos (μίλτος), was used for several unrelated applications[1]. Its use is attested to in Mycenaean clay tablets, inscribed in Linear B as mi-to-we-sa and dated around the second millennium BC.
The variety of applications for which it was used was broad: it was used as a pigment, as a cosmetic, in ship maintenance, agriculture, and medicine. It was precisely this diversity that intrigued the scientists[2].

The ancient texts made it clear that miltos could be found (and mined), in only three places in Graeco-Roman world: Kea in the Cyclades, Lemnos in the northeast Aegean, and Cappadocia in Turkey.

The team examined miltos samples at source. The fact that the substance contained a lot of iron-oxide, which gave it its colour, was uncontroversial, but the researchers were keen to know what else was in each sample to see if the additives meant that different mines produced miltos suited for its different uses.
The scientists analysed five samples: four obtained from Kea, and one originally from Lemnos that had been collected during the sixteenth or seventeenth century and is currently housed in the Pharmacy Museum of the University of Basel in Switzerland. No Turkish miltos was available.

The results show that, yes, different samples from different mines have potentially different abilities – the result of both small amounts of various chemicals embedded with the iron oxide and the microbial communities that live in them.

One of the samples from Kea, for example, was found to have 'exceptionally high' lead levels. This could explain a 360 BC Greek inscription decreeing that Kea miltos could only be exported to Athens because of its value not just for decoration but also because of its role in boat maintenance.

The high lead levels mean that Miltos from Kea would make a very effective anti-fouling agent, preventing the growth of bacterial colonies and barnacles on boat hulls. Another sample from Kea had significant quantities of zinc, arsenic and copper, making it ideal as the base ingredient for a biocidal boat paint. Until very recently all marine antibacterial paints contained zinc and copper.

There are several references in Greek and Roman literature to the presence of miltos on farms. It could be mixed with pitch or resin, and probably was used to ward off plant diseases or as a fertiliser. Key were the microbial communities that lived in them.

The researchers found that such applications were certainly not without merit, and that antibacterial effects varied quite widely across the sample range.

The sample from Lemnos, for instance, was found to contain traces of titanium dioxide, a known antibacterial compound. Interestingly, the samples high in lead were not particularly effective, perhaps, the scientists suggest, because lead is toxic and its effect therefore dose dependent.

The different effects produced by the different samples supports the idea that not all miltos was the same.

[1] Photos-Jones: From mine to apothecary: an archaeo-biomedical approach to the study of the Greco-Roman lithotherapeutics industry in World Archeaology - 2018. See here.
[2] Photos-Jones et al: Greco-Roman mineral (litho)therapeutics and their relationship to their microbiome: The case of the red pigment miltos in Journal of Archaeological Science: Reports – 2018. See here.

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