Caring for Your Metal Objects
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.
Click on an image below to find out more about how to identify different types of metals.
Silver egg steamer or boiler, probably designed by Christopher Dresser
Silver was once the most common precious metal in domestic use. As a general rule, European silver can be identified by its hallmarks. 'Pure' British silver, which in fact contains a small percentage of copper and other metals is either 'Sterling' (92.5% silver; hallmark is a walking lion [lion passant]) or Britannia silver (95.8% silver, hallmark is figure of Britannia). Other marks include a date letter, a maker's mark, an assay office mark (test of metal or ore to determine ingredients and quality) or a duty mark (in use between 1784 and 1890). If Sterling or Britannia silver is used in alloys with a lower silver content, it is marked '925'.
Silvering refers to applying a thin layer of silver onto another material. There are four common types of silvering:
1. French Plate - silver leaf and a mordant (adhesive) are applied in a similar way to traditional gilding.
2. Sheffield Plate - a thin layer of silver fused to a block of copper and then shaped as one metal into dinner services, snuff-boxes and candlesticks. Sheffield plate is sometimes stamped with maker's marks giving more detail about the object. It is common to find the plating has worn away, especially on corners or edges, revealing the copper underneath.
3. Close Plate - iron or steel dipped in molten tin with a very thin silver foil fused to the surface. Nutcrackers and candle snuffers are the most commonly found objects in close plate. They sometimes have maker's marks but these are difficult to trace.
4. Electroplate - uses electricity flowing through a bath of electrolytic salts to deposit a layer of silver onto another metal. Electroplating was introduced on a commercial scale in the 1840s and has its own marks, for example EPNS (electroplated nickel silver) or EPBM (electroplated Britannia metal) and many others. Maker's marks include crowns, deliberately similar to marks used on sterling silver.
Gold brooch, about 1860. Museum no. M.29-1991
Gold is traditionally the most precious metal. Pure or 24 carat gold is generally too soft to be used for making objects, and so gold is usually alloyed with other metals to add strength and/or to change its colour. Gold is most commonly seen as yellow, white or rose (pink) coloured, although other colours such as green gold are sometimes found. Generally speaking, there are five hallmarks associated with gold: a crown mark; a number stamp for carats (before 1798 only 22 carat was marked, from 1798 18 carat was marked, and from 1854 15, 12 and 9 carat were marked); a date letter; a maker’s mark and an assay office mark. The carat number shows the amount of gold in the alloy. So if pure gold is 24 carat, 9 carat gold contains under half gold; the rest being other materials.
Some objects have a thin outer layer of gold. There are four types of gilded surfaces on metals:
1. Gold leaf applied with a mordant (adhesive), for example on exterior iron railings. This type is rarely seen on decorative art objects.
2. Mercury amalgam gilding, the most common form of gilding until the 1840s. A paste made of scraps of gold dissolved in mercury was applied to the object, which was then heated until most of the mercury evaporated, leaving a gold layer fused to the metal surface.
3. Electrogilding became the most common gilding method post-1840s (see Silver). Until 1940 only 24 carat gold, which was very susceptible to scratching and wear, could be plated on in this way. In 1940 a method of depositing 22 carat gold was developed.
4. Rolled gold. A ‘sandwich’, with gold or gold alloy as the ‘bread’ and gilding metal/silver/nickel in between, is rolled so that it fuses together and is then fabricated into objects.
Copper dish or charger, John Pearson
Copper and its alloys, including brasses and bronzes, are ubiquitous in functional items and the decorative arts. Copper is usually red/pink in colour. Oxidation, a corrosion process, produces a dark red-brown patina. Copper saucepans, brass door fittings and bronze sculptures all have copper as their main constituent. Copper is also present in small quantities in silver, gold, Britannia metal and many other alloys.
In modern usage, brass refers to an alloy of predominantly copper and zinc, whilst bronze refers to an alloy of predominantly copper and tin. However, many historic copper alloy objects contain both zinc and tin, and so the term 'copper alloy' is often used if the exact composition is not known. Copper and its alloys were often patinated, which can also cause confusion.
Brass box, 17th century. Museum no. IS.2068-1883
Brasses are an alloy of copper and zinc. There are a wide variety of mixtures, for example yellow brass contains roughly 65% copper and 35% zinc whilst copper alloyed with zinc and tin is called gunmetal. Brass could be gilded, or coated with a tinted varnish to imitate gold.
Copper pieces could be 'brassed' by exposure to zinc vapour at high temperatures, or by boiling in hydrochloric acid and zinc amalgam. Iron could be electroplated or molten dipped in molten brass.
Bronze bust, Auguste Rodin
Bronze is an alloy of copper and tin and varies in colour from golden brown to dark brown to dark green. Statuary bronze also contains zinc and often some lead, which helps the molten bronze to flow better.
Pewter clock, Archibald Knox
Tin and pewter
Tin is a very soft, silver coloured, corrosion-resistant metal. It has been used decoratively, in alloys and as a coating on other metals such as copper and iron.
Pewter, sometimes called 'poor man's silver', is an alloy or mixture of metals consisting primarily of tin. Adding metals such as copper and antimony makes pewter harder and more durable. The best alloys contain over 90% tin.
Before 1800 lead was allowed in 'lay metal', a lower-quality alloy used for measures and boxes. Soft lead made casting easier, but was poisonous, so it was forbidden in 'flatware' such as plates, dishes and porringers. Tests do reveal small amounts of up to 2%, but modern pewter contains no lead. Most pewter is cast in moulds.
Britannia metal is often mistaken for pewter. It was developed in response to the Sheffield plate industry and it borrowed much of the technology: most was stamped into shape using steel dies. By the 1850s, the use of Britannia metal superseded pewter for cheap mass-produced items. Britannia metal and modern pewter, containing antimony and no lead, are virtually indistinguishable.
Marks on both old pewter and Britannia metal vary. Marks on pewter consist of a maker's mark, a secondary mark such as a crown or a pseudo hall mark, and labels such as 'London' (never Sheffield), 'superfine', and 'English block tin', among others. Britannia metal marks sometimes show the maker's name, town of manufacture and a catalogue number. Common Britannia metal manufacturers include Ashberry, Dixon, Broadhead and Atkin, Vickers, Wolstenholme, and Yates. From the 1860s, Britannia metal was often silverplated and was then usually stamped EPBM (Electroplated Britannia metal).
Iron and steel necklace, about 1820-30. Museum no. 96-1906
Iron and steel
Iron and steel are generally regarded as utilitarian metals, valued for their strength, but they have also been used for decorative purposes. Iron is converted into a metal (pig iron) by smelting iron-bearing ores, whilst steel is an alloy of iron and carbon. Modern steels contain additional elements introduced to give particular qualities, such as high tensile strength, increased hardness or corrosion resistance (e.g. stainless steel).
A magnet can usually be used to identify iron and steel as it will only stick to metals containing iron, although there are some modern steels which are not magnetic.
Historically, there were two main forms of iron - wrought iron and cast iron.
1. Wrought iron was the most commonly used form of ferrous metal for everyday objects (with the exception of weapons and blades) until the development of the steel industry in the 19th century. It is still available today for producing decorative ironwork. Wrought iron is tough, flexible and malleable, and also comparatively corrosion resistant. It was used for making a variety of objects from kitchen items to architectural pieces such as elaborate gates and balconies.
2. Cast iron, known in Europe from the late 14th century, is very hard and brittle. It has a dull grey porous surface that may have bumps and ridges, which are cast marks. It was first used for objects such as firebacks and, as casting technology improved, items such as railings, stoves, and weight-bearing architectural items. Cast iron reached its apogee in the 19th century, when even jewellery was made from cast iron, and British companies such as Coalbrookedale were able to mass produce cast iron objects such as benches and hatstands.
Until the 19th century, steel was only produced in small quantities in Europe and was used mainly for weapons and blades, and for specialist items such as watch springs, high status locks and keys. From the late 18th century onwards, steel began to replace wrought iron as the most commonly used metal for utilitarian items.
The 'cut steel' technique, which began in the 16th century and was most popular in the 18th century, was used for small items of personal adornment such as jewellery, buttons, shoe buckles and sword hilts. Small, highly polished, faceted steel studs were screwed or riveted onto a shaped steel backplate, for example this cut steel button.
Lead medal, 1517. Museum no. 172-1867
Lead is a soft malleable metal that has been used for a variety of objects, ranging from outdoor statuary and garden ornaments to children's toys (painted lead soldiers). Lead was used for roofs and drainpipes because it is resistant to moisture. In good conditions, lead develops a dull grey oxide layer on the surface. If it is badly stored and exposed to acidic vapours, it can develop a whitish powdery corrosion product which is toxic. For this reason, care should be taken when handling lead.
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 indentified. 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 developed in the 19th century. Electrotypes were most commonly made in copper, which could then be patinated, coated, or silvered or gilded to make an exact copy of the original. Electrotypes are lighter than the original object, and 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 Tirantis.
If in doubt about an object's composition or surface finish, leave your piece uncleaned or consult a conservator before you start.
Having identified your object, you can then move onto to basic cleaning.
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 tarnish 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.