Archaeological Desalination
The preservation of archaeological iron artifacts has always posed a significant challenge for conservators throughout the years. The acidic and moist conditions prevalent in Northern and Western Europe contribute to the aggressive corrosion of iron artifacts, often leading to their complete deterioration before excavation. Even when these artifacts manage to survive in the ground, their exposure to above-ground conditions introduces new threats, such as moisture, oxygen, and fluctuating temperatures, triggering renewed degradation.
One of the main factors contributing to the degradation of iron finds is the presence of soluble salts in the soil. The rusting of the iron objects underground is an electro-chemical reaction, which attracts the negatively charged salt ions and causes them to become electro-chemically bound to the corrosion products. This promotes further corrosion, which in turn attracts more salts, accelerating the corrosion process even further. This cycle will continue until there is either no metal, no oxygen, or no moisture left in the burial environment. But these issues can rapidly re-appear when the artefacts are excavated and exposed to the air, as this exposes them to much higher oxygen levels.
Above: Iron finds, before treatment (left) and after desalination treatment (right)
The most common way to preserve iron artefacts is to store them in a dry environment, thus eliminating moisture from the equation. It is a relatively inexpensive solution that allows for a large number of artefacts to be preserved at the same time. The downside is that it requires regular upkeep and maintenance. Another method of minimising corrosion is to try and remove the dangerous chloride salts altogether. Since they are chemically bonded to the iron, the salts cannot be simply dissolved and washed away. Conservators use specific alkaline solutions to soak the objects in. This provides another source of negative ions for the iron corrosion products to bind with, thus releasing the salts to diffuse into the solution. It is a slow and complex process that can take months. Some aspects of the treatment are still being researched and tested further, but desalinated finds have shown much improved survivability, compared to non-desalinated ones.
At AOC we performed alkaline desalination for two objects in particularly bad condition, which were showing signs of weeping – a severe form of iron corrosion. Following the three-month stabilisation treatment, the objects were cleaned and coated to further improve their appearance and protect against future corrosion issues.
Above: Desalinating iron finds - during treatment in alkali solution
Below: Iron finds - after desalination treatment
