Series editors' preface - Wood Tools

318 Conservation of Furniture
Table 8.1 Ferrous corrosion products
Name Formula Colour
Goethite (alpha ferric oxyhydroxide) -FeO(OH) Yellow ochre
Akaganeite (beta ferric oxyhydroxide) -FeO(OH) Dark red-brown (burnt sienna)
Lepidocrocite (gamma ferric oxyhydroxide) -FeO(OH) Bright orange-red
Haematite -Fe 2O 3 Red ochre
Magnetite (ferrosoferric oxide) FeO.Fe 2O 3 Shiny dark-grey
(also called fire scale, mill scale) or 8[Fe 3O 4]
Ferric chloride FeCl 3 Red-brown
Ferric oxy-chloride FeOCl Red-brown
Vivianite (basic iron phosphate) Fe 3(PO 4) 2.8[H 2O] Blue
Jarosite KFe 3(OH) 6(SO 4) 2 Lemon yellow
Natrojarosite NaFe 3(OH) 6(SO 4) 2
Goethite is common but non-destructive. The other oxyhydroxides will sometimes convert to this under natural conditions.
Akaganeite occurs only on chloride-contaminated objects and is unstable and destructive. Its lattice structure randomly incorporates
Cl ions. As it converts to goethite and lepidocrocite it releases Cl ions to catalyse further corrosion. Akaganeite often forms columnlike
structures that exert great pressure on overlying iron layers or coatings. Lepidocrocite is common and ubiquitous on terrestrial
site artefacts, but uncommon on artefacts from a marine environment. Haematite is a very common ore, but is uncommon on
artefacts, forming only under anaerobic conditions. The oxyhydroxides convert to haematite when strongly heated, and the presence
of haematite on an object can usually be taken as a sign that it was heated after it had rusted. Magnetite is a mixture of ferric and
ferrous oxidation state oxides. It is the product which gives rise to temper colours and mill scale when iron and steel are heated. It
can also be formed by the reduction of other corrosion products. It is stable, and when present in compact and even layers is
protective and even decorative (fire blue). Large pieces of naturally magnetic magnetite were known as lodestones. When chloride
contaminated ferrous objects are subjected to high humidity, ferric chloride hydrates and forms tears of highly acidic ferric chloride
(known as weeping iron). When the tears dry out they resemble hollow spheroidal ‘insect eggs’ under the microscope. Ferric oxychloride
has been reported as a major component of corrosion layers on iron objects from a marine environment. Vivianite forms
on iron artefacts that have corroded in anaerobic conditions in contact with phosphorus containing materials such as bone and
leather. It can easily be mistaken for traces of blue paint. Jarosite and natrojarosite are stable brightly coloured products that do
not present a danger to artefacts.
corrosive species. Active corrosion layers are
often visible as powder or flakes on the
surface of the object. Examples of active corrosion
may be found on ferrous metals in the
presence of salts and lead in the presence of
organic acids. Guidelines for identifying active
corrosion have been published by the CCI
(1989). Active corrosion will lead to further
loss of material and damage to the object and
should be removed where possible. The treatment
of corrosion is in part dependent on the
relative molar volume of the corrosion
products in comparison to the original metal.
In the case of ferrous metals and lead, the
corrosion products are larger in volume. Once
corrosion has occurred there will be disruption
of the surface and a loss of metal where
the surface has corroded. In some cases, metal
ions migrate beyond the original surface to
form an overlying corrosion crust, which can
in some instances be safely removed to
expose original detail. Some copper corrosion
products are smaller in volume than the metal
and can retain decorative detail in the corro-
sion layer. In such cases, aesthetic considerations
aside, it is usually acceptable to stabilize
and coat the corroded surface.
The majority of metals are reactive and tend
to combine with other elements such as
oxygen, sulphur and chlorine to form metal
corrosion compounds (Brown, 1991). Common
corrosion products for iron, copper, silver
and lead are listed in Tables 8.1–8.4.
Metallurgists use the term oxidation to
describe corrosion reactions even where other
elements are involved because oxygen from
air or water is so often involved in corrosion
of metals. It is interesting to note that since
virtually all of the oxygen in our atmosphere
is the result of photosynthesis, trees have been
at war with metals since they first evolved! In
oxidation, surface metal ions combine with
oxygen or another reactive species to form
corrosion products. Moisture is usually present
in the form of an adsorbed layer from water
vapour in the ambient atmosphere, forming an
electrolyte in which charges can be easily
transferred.