conservation

gilded bronze conservation

 Gilded Bronze Conservation

 England is among one of many countries that has been exposed its gilded bonze to severe pollution.  Many gilded bronzes in the UK, and in London in particular, have been irreversibly damaged and pitted by numerous corrosion spots that have appeared since the industrial revolution.  David Scott from the Getty, a leading expert in the field of corrosion of copper alloys and treatment explains the black spots on many gilded objects very well:

 “Atmospheric pollutants such as sulphur dioxide can cause local dezincification (bronze mounts are made of brass, an alloy of mainly copper and zinc) leaving traces of zinc sulphite on the surface of the object, and these traces are rapidly oxidised to zinc sulphate.  Zinc sulphate is highly deliquescent, therefore even slightly elevated relative humidity can cause localized spots of condensation.  The zinc sulphate solution thus formed will dissolve more sulphur dioxide, which will be oxidised to sulphur trioxide, facilitated by the copper ions.  In the presence of the increasingly strong concentration of sulphuric acid in the droplets, together with atmospheric oxygen, more copper-zinc alloy will dissolve………….”

 Extract from Copper and Bronze in Art, corrosion, colorants, conservation by David A. Scott, the Getty Conservation institute, Los.Angeles, USA.

 Thanks to the Clean Air Act, pollution in London is now significantly lower with fewer atmospheric pollutants around that could cause new corrosion spots.  Unfortunately, where there are existing spots of corrosion these will continue to spread thanks to England’s damp climate. The photos below illustrate deep corrosion spots spreading inside the brass. 

  

High magnification photo showing a cross-section of two corrosion spots on brass spreading beneath the surface and causing the distinctive “pitting” of the surface.  The brass metal is white on this photo.

 

If left untreated, corrosion will continue to spread beneath the surface of the brass causing irremediable damage to the object.  Careful cleaning techniques to address corrosion-related problems have been developed during recent years in our workshop.  This is opposed to the more traditional cleaning techniques which tend to use very strong cleaning products which can in turn damage the object.  Our objective is to use stable, neutral PH products and non-abrasive techniques, for example high pressure steam so that corrosion spots can be safely removed from most bronzes.  This technique avoids causing damage to the contrast of matt and burnished areas. Gel techniques also enable the safe and careful cleaning of localised areas of bronze which may require further attention.  All of our techniques aim to preserve the patina on the back of the furniture mounts.  Great care is taken that newly conserved pieces of gilded bronze should acquire an agreeable aspect.  Newly cleaned gilded bronze can look too new or shiny in many collections and pieces are often treated using a technique known as “mise en couleur de l’or”, an 18th century technique of applying coloured wax which can impart a golden tone improving dramatically the visual impact of the finished object.

 

 

Cleaning of gilded bronze using steam

 

 Typical example of gilded mounts before and after conservation.  The preferential corrosion seen before (top) on this original Boulle mount was carefully removed using techniques described above..  In many cases, gilded bronzes do not need to be re-gilt.

 

 

 

Again, please do not hesitate to contact us with regard to your gilded bronze, whether  they are furniture mounts, clocks or sculpture.  The problem of corrosion is very serious and should not be left untreated.