Artcare - Auckland Art Gallery

MetalsThe preservation of metals is a very complexfield. Metal artefacts are made from awide variety of elements and these areprone to many deterioration processes.Except for the basic cleaning methods mentionedhere, it is inadvisable to treat metalartefacts yourself and professional adviceshould be sought.Causes of DeteriorationHigh humidity is one of the greatest causesof deterioration of metals as it aids chemical degradation.Atmospheric pollution is also a cause of deteriorationfor some metals (such as silver which, in Rotorua, isinvariably tarnished by the naturally high content ofhydrogen sulphide in the atmosphere), and lead-basedmetals, such as pewter, which are attacked by organicacid vapours. Chlorides (such as salt from the sea) causean especially destructive type of corrosion process,known as bronze disease, to affect copper and its alloys.Cleaning of metals also causes some deterioration,as many polishing solutions are abrasive and removemetal particles. Because of this, polishing should not bedone too often. It may be necessary to coat the metal sothat tarnishing does not occur.The metals most likely to be encountered in NewZealand are iron and its alloys, silver in various forms,and copper and its alloys.Iron and its Alloys—iron in its various forms isreadily attacked by oxygen, in the presenceof moisture, to form rust.Iron objects should be carefully examinedto determine the extent of deteriorationand whether the corrosion is stillactive. Objects showing signs of activecorrosion need specialised conservationtreatment.The surface of iron artefacts may becovered with a mixture of dirt, grease andloose rust; this will not provide an effectivebond for coatings applied over it. Such deposits can becleaned by a trained conservator using mechanical orchemical techniques.Surface coatings help prevent further corrosion andtherefore great care has to be taken in selecting andapplying the appropriate coatings. Home remediesagainst rust and metal corrosion more often than nothasten the deterioration of the artefact rather thaninhibit it.Silver—unless they are kept polished, silver artefactswill gradually tarnish with a layer of black silver sulphide.To remove tarnish, use a commercially availablesilver dip such as Goddard’s. To avoid retarnishing, a protectivecoating has to be applied. Waxes or lacquers canbe used as coatings. Waxes are easier to use, as a singlebreak in a lacquer coating can allow corrosion to recurand the entire layer must be removed before anothercoating is applied. Wax coatings can be reapplied over49