Conserving Museum Objects
Sarah Morton ACR
Business Unit Manager & Objects Conservator
Oxfordshire Museums Service
Pure iron is a silvery white, relatively soft metal, it has a strong tendency to
oxidise so is alloyed with other elements (usually carbon).
Naturally occurring iron ore is smelted to produce solid metallic iron.
Heating with a carbon containing fuel reduces the iron oxides back to metal.
Wrought iron is a fairly pure iron that contains glassy inclusions of slag and
has a fibrous structure.
Steel refers to iron alloyed with carbon, usually less than 2 wt%. The
addition of carbon increases hardness and strength.
Cast iron refers to iron alloyed with carbon usually at 2-4 wt% and 1-3 wt%
silicon. Other elements are added to alter specific properties.
Pure copper is light red-pink and is ductile and malleable. It is non-magnetic
and can be easily soldered.
Copper is found naturally in the metallic state
Pure copper is soft so is often alloyed with other elements to make it
harder. Brass is an alloy of copper and zinc, bronze is an alloy of copper
and tin, gunmetal contains zinc and tin.
Lead can be added to an alloy to improve machinability.
Gold is a precious metal and naturally occurs in a relatively pure form. It is
soft, easy to work and does not tarnish under most conditions.
To make gold harder and more useful it is commonly alloyed with copper
and silver. Gold alloys vary in colour depending on the amount of each
Gold can be applied as a thin layer to the surface of another material, this
process is called gilding.
Many countries make it a legal requirements the gold and gold alloys are
identified by a small stamp. This practice of hallmarking was established in
England in 1300.
Pure silver is white in colour but is highly sensitive to tarnishing by sulphur
Pure silver is soft, ductile, malleable and easily scratched. It is often alloyed
with copper to make is harder and more useful.
As with gold many countries make it legal requirement for silver alloys to be
hallmarked. Sterling silver was introduced in Britain in 1238 with a
specification it should not contain less than 92.5 wt% silver and is still used
around the world as a silver standard.
A thin layer of silver can be used to plate other metals. Fused plate (known
as Sheffield plate) was delveloped in the 1740s but was replaced by
electroplating in the mid C19th.
Lead is the heaviest common metal. Bluish grey in colour it is non-magnetic
and very soft.
Lead has a low melting point and is often alloyed with other metals to make
low-melting alloys that are easy to cast.
Lead-tin alloys were widely used for soldering other metals together.
Many lead compounds are intensely coloured and have been used as
pigments in paint.
Because of environmental and health hazards lead is much less widely
Tin in its pure form is silvery white in colour, non-magnetic, soft and
Tin can be easily rolled and cold-worked but is too soft for most uses so is
commonly alloyed with other metals. One of the most common being the
lead-tin alloy pewter (lead is replaced by antimony in modern pewter).
Tin can also used to plate copper alloys and copper cooking vessels were
often tinned to prevent food being tainted with copper.
Iron can also be tin plated, originally achieved by wiping or hot dipping
modern tin plate is produced by electroplating.
Zinc is a non-magnetic, bluish-white metal. Zinc ores are difficult to smelt
and the metallic properties of zinc were not recognised until the C17th.
Zinc has a low strength and is subject to creep.
Zinc-aluminium alloys are used for die-casting and zinc-copper alloys for
Zinc can be used to galvanise iron or steel to protect it from rusting
(because the zinc corrodes preferentially).
Nickel is a hard, corrosion-resistant, silvery white metal. It can be polished
to a bright finish and is often alloyed with other metals.
The element nickel was discovered in 1751 and is usually recovered from
nickel sulphide ores.
Nickel has often been used for coins due to it’s corrosion resistance.
Nickel silvers are copper rich alloys that contain nickel and zinc. They do
not contain any silver but could be electroplated with silver (EPNS). Monel
metals are also nickel copper alloys.
Stainless steels are iron based alloys to which various amounts of nickel
and chromium have been added.
Electroplating of nickel onto other metals has been carried out since the
Pure aluminium is a highly reflective, silvery-white metal that is light and
can be easily formed, machined and cast.
Its corrosion products formed are colourless or white and form a protective
oxide film that renders it resistant to further corrosion.
Aluminium is found as an ore, the principle ore being bauxite. Pure
aluminium was isolated in 1872 but not widely used till 1886 when the
electrolytic process made producing aluminium easier and more affordable.
Aluminium has a wide range of uses and small quantities of various
elements can be added to produce alloys with special properties.
Aluminium can be used to plate iron to produce aluminized steel commonly
used in roofing.
When a metal reacts with its environment the compounds that
form as a result of the process are known as corrosion
Corrosion on metal objects may be valued for its colour, beauty
or stability. It may also mask the objects intended surface or
even weaken the objects physical structure.
We must determine if the corrosion is desirable, undesirable or
IMMUNE: Metal does not react with electrolyte and there
is no corrosion. Metal is said to be stable.
ACTIVE: Metal reacts with environment and corrosion
products are soluble enough to diffuse away from
PASSIVE: Metal has reacted with the environment and
the corrosion products have formed a protective film on
the surface reducing the corrosion rate.