Caring for Your Metal Objects

Silver egg steamer or boiler, probably designed by Christopher Dresser

Silver

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

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

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

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

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.

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.

Iron

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.

Steel

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

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.