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Lead seals
Electrochemical oxidation is a process that degrades all metals, including lead, when exposed to fluctuating environmental conditions. In specific conditions of environmental temperature it creates a very thin patina that is initially made up of a mixture of oxides. The oxides change into lead carbonates (cerussite or hydrocerussite) when they come into contact with the carbon dioxide in the atmosphere and in the presence of polluting agents the mixture can determine the diffusion of anglesites or sulfides (galena).
The corrosive action of acids is often enhanced by the same storerooms, containers or showcases where concentrations of harmful substances and incompatible environmental conditions prevail.
Relative humidity is one of the parameters causing active corrosion of lead. In our experience reducing the rate of humidity to around 40% markedly reduces deterioration. It is clear that if the causes of deterioration are not eliminated, the item will pulverize contingent with the different ageing process present in each seal.
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Lead seals.
Intervention methods using chemical processes
In the past lead seals used to be treated with consolidating agents in an attempt to arrest deterioration. Lead bulls treated with protective varnishes resisted corrosion longer than non-treated ones, but once the varnishes lost their protective capacity and the harmful vapours came into contact with the lead, deterioration set in. The use of consolidating varnishes to achieve at least palliative conservation is valid, but today electrolytic reduction is the real and proper protocol provided by chemical sciences to eliminate this serious problem. Our laboratory uses this method and other chemical procedures.
Treatment aims at remetalising the parts affected by corrosion to achieve adequate consolidation of the metal. Furthermore, the seals are immersed in electrochemical baths that restore consistency to the corroded seals.
Seals are archaeological artefacts per se even when no document is present, though they are generally attached to the writing support by cords. The presence of textile fibres complicates intervention as chemical treatments always foresee the immersion of seals in baths that are potentially hazardous for the threads. In our laboratory this problem has been successfully tackled by identifying and performing localised operations or specific interventions to protect the cords and writing supports.
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