Metal has been used extensively by a multitude of cultures throughout history. If you think your object has significant monetary value, or it has historical, aesthetic or sentimental value and you wish to pass it on to future generations, consider having it valued, insured and professionally conserved.
After identifying the type of metal your object may be made of, the next step is to examine your piece in good light. Use a padded surface if possible, for example thin foam or cardboard covered with blotting paper, to prevent scratching of the object or the table surface. Then look out for the following:
Additional materials used in manufacture
- Other materials such as ivory or wooden handles etc. In some cases, for example decorative plaques, knife blades or the silver stoppers in toilet sets, the silver part is attached with resin or plaster which can dissolve if dipped in water or in contact with solvents.
- Original coatings, for example some metal pieces were given a final coating of tinted oil or wax (historic manuals for making bronzes describe such coatings) varnish or lacquers, such as the tinted shellac used on 19th century brass to imitate gilding. In other cases waxes, resin or ink were used to accentuate engraving. Inks and resins are easily removed using mild solvents.
- Original patination, for example the brown or green patination on bronzes, the peacock blue of blued steel, or the varied and subtle effects achieved by, for example, Japanese metalsmiths.
Contrasting or precious metals can be used for decorative effect and will need to be identified. The main techniques are:
- Inlaying, in which the design is incised into the receiving metal (often with an undercut hold the inlaid metal in place), and the metal shapes or wire is pushed into the incised design.
- Overlaying, in which crosshatched lines are incised on the receiving metal; metal wire or sheets are laid on the surface and hammered on - the crosshatching acts as a key to hold design in place. Overlaying is more fragile and easily damaged than inlaying.
Beware impersonators - as well as platings, many metal alloys have been used to imitate precious metals. Examples include:
- High zinc brasses (such as pinchbeck), for example this chatelaine, used to imitate gold in jewellery.
- Nickel silver (60% copper alloyed with 22% nickel and 18% zinc) has a silvery-yellow appearance and was often silver plated, but could also be used as a silver substitute.
- Spelter, used in the 19th century to imitate bronze. Spelter is mainly composed of zinc, a silvery white metal. After the object was cast, the surface was either plated with copper, which was then patinated to a dark bronze colour, or simply covered with a coloured wax or lacquer. Spelter was used for statuettes and for decorative elements on clocks, and can be hard to distinguish from bronze at first glance
- Electrotypes, also known as electroforms. A method of creating a metal copy of an object particularly popular in the 19th century. Electrotypes were most commonly made in copper, which could then be patinated, coated, or silvered or gilded to make a copy of the original. Electrotypes are lighter than the original object, and usually show a distinctive bubbly interior surface where gases have been released during the electroforming process. These two indicators can, however, be disguised, by adding lead weights inside the body of an object, and by covering the interior surface with soldered-on plates.
- Cold cast bronze - this is a modern resin mixed with metal powder to make imitation decorative bronze objects. It was developed in 1958 by Tiranti's.
If in doubt about an object's composition or surface finish, leave your piece uncleaned or consult a conservator before you start.
Common problems associated with metals
Chemical damage (corrosion)
All metals, with the exception of 24 carat gold, suffer the effects of corrosion. Corrosion is the process by which a metal is gradually damaged by a chemical reaction. The result of the process is a corrosion product which takes various forms. Mild corrosion will form a corrosion product which causes the metal to appear dull or change the colour of the surface. More aggressive corrosion processes will cause formation of powdery crusty layers or spots to form on the surface.
Corrosion products can be intended, in which case terms like patina and oxidation are used - or not intended e.g. tarnish, rust or verdigris.
Generally, if the corrosion products are shiny and firmly attached they do not represent an immediate problem. The more powdery, flaky or soft the corrosion products, the more active the corrosion process, and the more urgently treatment is required. In good conditions, metal objects can survive with little change for centuries - in poor conditions they can be irreparably damaged or destroyed within a matter of years.
Some corrosion product layers offer a degree of protection against further corrosion ('passivating layers'), e.g. black oxidation on iron, whilst others do not, e.g. orange rust on iron or black tarnish on silver. Check your metal items for signs of corrosion regularly, ideally when dusting or cleaning. No corrosion is completely inactive - even passivating layers, which slow down the corrosion process, do not prevent it entirely.
Corrosion will be caused or accelerated by:
- damp/high relative humidity
- pollution (external sources - sulphur dioxide produced by car exhaust, fires, smoking; internal sources - organic acids given off by wood and wood-based products, fresh paints and lacquers)
- Many general household cleaning products contain corrosion-inducing chemicals. Aerosols can leave tiny droplets which are invisible to the naked eye, but, given time, may damage a coating or the metal itself
- display or packing materials that give off sulphur or acidic vapours (e.g. wool or felt linings, some plastics, such as PVC or rubber, newspaper and some adhesives)
- contact with acidic parts, such as leather components (in combination with high relative humidity)
- food residues (e.g. acidic residues from vinegar or fruit juices; salt left in salt cellars) residues from metal cleaning products (e.g. proprietary metal polishes that contain ammoniates)
- dirt and dust accumulation (which can attract moisture and hold it on the surface)
- galvanic corrosion - where two metals are in contact or are alloyed, one may corrode faster than the other
- coatings that do not completely cover a surface - the uncoated areas will corrode faster than if no coating had been applied
The oil on your hands contains acids, lipids, sugars and proteins, as well as residues from items you have handled previously. If not wiped off metal objects, fingerprints can cause localised corrosion that causes the print to be permanently etched deep into the surface. This is very difficult to remove.
Objects handled and cleaned on a regular basis are not at so much risk of this problem, as the individual fingerprints will at least be smeared over the surface, even if they are not completely removed. The lamp illustrated here was handled once without gloves and then put into storage. The chemicals in those fingerprints have been allowed to eat into the surface without being smeared in any way, and so the distinctive fingerprint can be seen as dark corrosion products. Each dot is an individual pit going into the surface.
Metal objects can be damaged through use or accidents. Parts can break off, or become detached when rivets fail. A common cause of damage is poor storage, where objects are piled on top of each other, causing scratching and, in extreme cases, deformation of a light object by a heavy object placed on top of it.
Repairs to antique metal objects which involve re-shaping should be referred to a professional conservator. Metals tend to become embrittled by age and can crack or fracture if not handled very carefully.
Cleaning metal objects
Please read the Basic Guidelines page before attempting to clean your object.
Please note that this page was updated on 1st December 2015.