The construction methods, problems and conservation techniques specific to zinc statuary.
The production of zinc statuary began around 1830, when relatively pure zinc became available. However, the fashion for cast zinc statues did not last long because they proved very fragile.
Most zincs were sand cast or slush cast (a method of casting in which metal is spun or agitated in the mould so that a thin shell of metal is formed). Large sculptures were made up of many small castings pieced together by soft soldering, occasionally supported by iron armatures for added strength.
Zinc statuary was either painted to imitate bronze or stone, or electroplated with copper and then lacquered to resemble bronze. All these finishes were intended to be both protective and decorative.
There are several inherent problems with zinc sculpture. Cast zinc is brittle, so zinc statues will easily break if struck or mishandled.
Rusting iron armatures will damage zinc sculptures. Signs include splits in the zinc, rust stains on the metal or pedestal, or partial collapse of the statue.
Early repairs to zinc statues involved rudimentary materials such as lead solder, plaster or cement. These will invariably fail, initiating corrosion around casting defects and areas of physical damage.
When original surface coatings fail, a complex sequence of different forms of corrosion occurs. Copper-plated zinc and soldered joints are prone to serious galvanic corrosion, which causes deep pits to develop rapidly in the zinc and leads to structural failure.
Galvanic corrosion: Corrosion resulting from the current flow between two dissimilar metals in the presence of an electrolyte. Behaviour is determined by the relative position of the two metals in the electrochemical (or ‘galvanic’) series: the less noble metal (lower in the series) becomes anodic, suffering increased corrosion; the more noble metal (higher in the series) becomes cathodic, suffering less corrosion. The relative surface area and degree of polarisation of the two metals determines the extent of corrosion. Also known as ‘bimetallic corrosion’.
The treatment methods for zinc are particularly complex. Consult a conservator who has relevant experience working on historic zinc statues.
Chemical cleaning should be avoided. Air-abrasive techniques combined with hand cleaning can be effective but must be carefully controlled. Ultra-high-pressure (2000–3000 bar) cold water cleaning can be successful but the equipment is very expensive. Generally, it is hired and operated by a specialist contractor under the instruction of a conservator.
Cleaning may expose the remains of original copper plating, which should be recorded and preserved.
If the statue is structurally damaged, it may be necessary to carefully dismantle it and then reassemble it using the original fabrication techniques. In some cases, heavily corroded elements may need to be recast.
Replicating original colours and textures requires specialist experience to select and apply the best modern protective coating systems.
The delicate zinc sculpture should be protected from physical damage and the surface coatings should be regularly maintained.