Pewter artefacts

Tin and pewter artefacts with low lead contents will form a non-protective Sn02 layer on the surface in well-aerated seawater environments. Hence, artefacts recovered from the surface of the seabed, will be almost completely mineralised. The shape of the artefact may still be maintained by the tin oxide, but it will be very fragile and must be handled with great care. In anaerobic conditions, however, a very protective sulfide film will form, which virtually inhibits further corrosion. Artefacts recovered from these sites are usually in excellent condition and require minimum conservation. 

The anodes that have been used include stainless steels, mild steel, lead and platinised titanium, while typical electrolytes for ferrous materials have been 0.5 M sodium hydroxide, 0.2 M sodium carbonate, 0.5 M sodium sesquicar-bonate and tap water. For bronze cannons recovered from the Mary Rose, both sodium hydroxide and sodium carbonate electrolytes were employed while pewter artefacts (plates) from the same ship were treated in similar electrolytes or in a 0.5% solution of EDTA as a sodium salt in alkaline solution. 

As mentioned above for pewter objects, the corrosion products may maintain the shape of the object even though no metal is left. One of the best examples of this is observed in cast irons. The iron phase (ferrite) corrodes to form the same corrosion products as already stated above. The graphite flakes in the cast iron are inert and trap the corrosion products and the shape is maintained. This is called graphitisation of cast iron and almost all cast iron artefacts recovered from archaeological sites have graphitised layers on their surface. 

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See also in sourсe #XX -- [ , ]

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