Deciphering the Roman red dust

From ancient Greek and Roman source texts it is possible to conclude that in the classical world a mineral, a powder known as miltos, was something of a wonder substance.

Miltos – referred to in the works of writers such as Theophrastus, Dioscorides and Pliny – was red, fine-grained, and made up mostly of iron-oxide.

By the time Theophrastus, a Greek philosopher and proto-botanist, wrote about it in the third century BCE, it was already a mineral validated by antiquity. Its use is attested to in Mycenaean clay tablets, inscribed in the script known as Linear B and dating from the second millennium BCE.

The variety of applications for which it was used was broad indeed. According to a team of researchers writing in the Journal of Archaeological Science: Reports, it was used “as a pigment, as a cosmetic, in ship maintenance, agriculture and medicine”.

It was precisely this diversity that intrigued the scientists, led by Effie Photos-Jones from the University of Glasgow in Scotland.

The ancient texts made it clear that miltos, unlike some other types of mineral, could be found, and mined, in only a few places in Graeco-Roman world – namely Kea, in the Cyclades, Lemnos in the northeast Aegean, and Cappadocia in Turkey. This specificity meant identifying the substance was simple: it was the red dusty stuff found at the mine sites, and easily matched, therefore, with older samples held in museums and galleries.

This kind of visual confirmation, however, did little to answer the question of why it was used in such a broad range of fields. It was a question further complicated by the habit of ancient writers of using different reference conventions to those employed by modern academics. The same name, for instance, could be used for both a raw mineral, freshly dug out of the ground, and its refined end product, obscuring potentially vital differences.

Photos-Jones and her colleagues therefore opted to examine miltos samples at source, from the mine sites mentioned in the texts. 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 – additional chemicals and resident micro-organisms – to see if the additives meant that different mines produced miltos optimally suited for different uses.

The short answer was they did.

To make their findings, 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 samples were subjected to a battery of tests, including X-ray defraction, geochemical analysis, dynamic light scattering to probe nanoparticle structure, DNA sequencing to discover microbiological components, and anti-microbial tests.

Of course, as Photos-Jones and her colleagues note, there is no way of being 100% sure that the miltos samples analysed have the exact same composition as those used by Greeks and Romans 2000 years ago, but, absent evidence to the contrary, it serves as a working hypothesis.

The results show that indeed 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 Kea samples, for example, was found to have “exceptionally high” lead levels. This, say the researchers, could explain a 360 BCE 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 the powder, once mixed into an organic medium, would make a very effective anti-fouling agent, preventing the growth of bacterial colonies on boat hulls – coatings of slime, which, in turn, provide substrates for populations of larger organisms such as barnacles.

Another Kea sample had significant quantities of zinc, arsenic and copper, making it ideal as the base ingredient for a biocidal boat paint. Until very recently, many marine antibacterial applications contained zinc and copper.

“Bacteria-generated biofouling is hugely detrimental to the shipping industry even today, and they would have been exceptionally so in antiquity since they considerably reduce speed of movement at sea,” the researchers write.

There are several references in Greek and Roman literature to the presence of miltos on farms. Applications appeared to vary – it could be mixed with pitch, for instance, and used, the authors assume, to ward off plant diseases, or applied directly to the roots of trees.

In this second instance, its purpose was as a fertiliser. Photos-Jones and her colleagues found that, on the basis of lab tests at least, some of the miltos samples were ideally suited to such a role. Key was the microbial communities that lived in them.

“In scrutinising the ecological microorganisms associated with [some of the samples]”, they write, “it can be demonstrated that many bacteria identified in the samples are linked with nitrogen-fixation in the soils, and iron is a major component of the enzymes and organic compounds associated with that process.”

Although, of course, the existence of bacteria was unknown in the classical world, there are several references to the use of miltos in treating disease or wounds – essentially, using the stuff as an antibacterial agent.

The researchers found that such applications were certainly not without merit, and that antibacterial effect 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.

Other samples, however, were found to be effective against either gram-positive or gram-negative bacteria – the effect being ascribed not to mineral content but to microbiology.

The different effects produced by the different samples supports the contention of the researchers, and the implications of ancient writers, that not all miltos was the same. Users accessed the product from different mines depending on what they wanted it to do.

The findings also potentially explained a slightly puzzling reference in the works of Theophtrastus. The writer noted that an artificial sort of miltos could be made by heating yellow ochre until it turned red, but the result was considered “inferior”.

Theophrastus did not spell out exactly why this was so, but Photos-Jones and colleagues suggest that it was because heating the powder would have killed most of the microorganisms living in it, thus rendering it pretty, but pretty ineffective as well.

Still, the fake miltos would still have been useful as a pigment. And decoration was a perennially popular application for the stuff, regardless of its provenance.

All up, then, miltos was indeed a Graeco-Roman wonder substance.

“To conclude, we suggest that bioactivity of miltos, its effect on living organisms  …  was and is the result of a complex network of interactions between mineral and its microbiome, the organic and the inorganic component contributing, more or less, to the total bioactivity of the miltos sample,” the researchers conclude.