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Conservation of an iron window grille

This window grille was found in 1965, lying under the debris of a buried Roman building at Hinton St Mary, Dorset. It is made of eight 25 mm by 3 mm flat iron bars with a two-piece pointed cross at each junction of the bars.

When iron is buried like this, it may absorb salts from the soil. The salts can lie dormant and cause further corrosion long after excavation. During burial, thick concretions of rust build up on the surface, and at the same time the object can be weakened by laminations or splits. This is what had happened to the window grille. What was originally a very strong piece of ironmongery, designed to withstand intruders, was now much more fragile.

When the grille was first conserved, rust and soil concretions were removed with hand tools and the iron was soaked in hot distilled water to remove salts. After drying, the grille was soaked in hot wax. This was to consolidate the iron, repair it and protect it from moisture. However, after about twenty years, outbreaks of orange powdery corrosion showed that the wax was no longer doing its job. Salts were still at work underneath, and breaks were occurring which could only be repaired with more wax. The wax itself was blurring the detail of the object.

Wax is difficult to remove completely, especially from an object as large and complex as this one. The conservator found an ingenious method however, which proved successful. The grille was covered with soft paper tissue. Fuller's earth and polyvinyl acetate (PVAc) emulsion was mixed into a thick paste and applied as a poultice over the whole surface. The grille was then heated in an oven at 95 degrees Celsius for twenty-four hours. The wax melted and was drawn into the poultice. The iron was now clean and stable, and adhesive could be used to make neater and stronger repairs. The detail too, is much sharper, allowing us to appreciate the Roman blacksmith's workmanship.