PESTICIDES ARE PROVEN TO DESTROY ANCIENT ARTIFACTS

An ancient Roman copper bowl after conservation works displayed at the Sandwich Guildhall Museum. Photo: Luciana da Costa Carvalho et al, Scientific Reports, 2022

Agriculture is often called the main enemy of archaeology. Fields ploughed over decades leave the cultural layer in tatters, the boundaries of archaeological sites are erased forever, plough and harrow pull artifacts out of the ground, turning them into the trophy of local residents.

Sometimes the opposite happens, when agricultural machinery, on the contrary, saves artifacts from looting by diggers, when, for example, the mound has long been destroyed by ploughing, but the burials and artifacts themselves are below the depth to which the soil is ploughed.

However, as researchers from Oxford University write in Scientific Reports, those artifacts that have not been reached by a farmer's plough or a digger's shovel can be reached by insidious organochlorine pesticides, and nothing good will come of it.

In the autumn of 2016, not far from the village of Wingham in the county of Kent in the UK, metal detector enthusiasts discovered a copper bowl in the ground. Archaeologists called to the place of the discovery carefully extracted the artifact together with the surrounding layer of soil, and at the same time made sure that there were no other objects or burials next to it. The copper bowl was dated to the middle I - early V centuries and sent to a museum, but before that it was at the hands of chemists. They helped us learn something interesting about the recent history of the Roman artifact.

There were characteristic traces of greenish copper corrosion on the surface of the bowl. It seemed that there was nothing strange that the copper, which had lain in the ground for more than a thousand and a half years, turned a little green, but the researchers determined that the bowl began to "rust" quite recently.

The green patina on the copper bowl consisted of minerals from the atacamite group – copper hydroxychlorides. It means that it was chloride ions that helped the copper to oxidise (just as metal objects and structures rust much faster when in contact with road salt). The soil where the bowl was found was characterised by a low concentration of chlorides from inorganic salts, but chemists found chlorobenzenes, in particular, hexachlorobenzene. These substances and their derivatives were used as pesticides in the treatment of agricultural fields for quite a long time. Even after the prohibition of their use, their concentration in the soil still remains at a high level, sufficient to accelerate the oxidation of metals.

Archaeologists have already noted that artifacts found in the period before the widespread "chemicalisation" of agriculture are often in a noticeably better-preserved condition than those found nowadays. Now we have managed to confirm these assumptions, linking the result of damage to the artifact with a specific anthropogenic source – no one else but farmers could produce such quantities of chlorobenzenes in the fields of good old England.

Materials of the article were taken from the journal "Science and Life" (No.10 October, 2022, RF) and the report "Scientific Reports" of Oxford University (UK).

For reference, hexachlorobenzene (perchlorobenzene) is a persistent organic pollutant.

Hexachlorobenzene is an organochlorine compound. It was produced in the form of light gray powder. It is formed as a by-product of the process of production of some chemicals and the same processes that are the source of dioxins and furans. It was used as an insecticide and fungicide (destruction of fungi harmful to food crops). In combination with other preparations it was used for the treatment of crop seeds.

Eng: hexachlorobenzene

CAS number: 118-74-1

Formula: C6Cl6

Molecular weight: 284.78 g/mol

Density: 1.5691 g/cm³

Ignition temperature: 11 °C

Water solubility at 20 °C: insoluble