Series editors' preface - Wood Tools

692 Conservation of Furniture
stable complexes with ferric iron (Fe(III)). It
should be remembered that the more acidic
the solution, the more likely the base metal
will be attacked and etched. Between a pH of
8 and 10 EDTA can form stable complexes
with ferrous iron Fe(II). Above a pH of about
10, hydroxyl ions successfully compete to bind
Fe 2+ ions, thus EDTA complexing of such ions
drops off markedly as pH rises beyond this
point. DTPA complexes most strongly with
Fe(III) between a pH of 5 and 7, whilst the
DTPA-Fe(II) complex formed at a pH of 10 or
above is significantly more stable than that
formed by EDTA-Fe(II) (Figure 11.22b).
Enzymes have also been used to remove
light rusting from iron objects. One commercial
product is Rust Biox. Removal is quite
slow and removal is less even than with the
acid cleaners, but enzymatic cleaners are
useful where sensitive coatings or associated
material is present (e.g. tin-types).
Some organic amines are excellent corrosion
inhibitors, and may have the ability to reduce
some iron corrosion products. Ethylenediamine
and triethanolamine (TEA) have both been
used, but TEA is less toxic and appears to
work well. Archaeological iron has apparently
been stabilized by TEA soaks. Amines can be
thickened to a gel and applied as a poultice,
or can be added to wax coatings to passivate
iron surfaces (Argo and Turgoose, 1985).
Thioglycolic acid has been found to etch
uncorroded iron, has an unpleasant odour and
is no longer used. Electrolytic reduction has
been used to strip corrosion products from
ferrous metals but its success is dependent on
operator skill. Thermal reduction has been
used successfully to remove chlorides and
reduce corrosion products on large iron
objects recovered from marine environments,
whilst plasma reduction has been used to
reduce corrosion products on small archaeological
objects.
Coatings
Any coating on heat or chemically blued iron
will change its visual properties. Such surfaces
should be kept clean and uncoated. Iron
components of furniture were not usually
waxed or lacquered unless as a side effect of
finishing the adjacent wood. Polished and
clean iron may be coated with a lacquer or
waxed. If corrosion is likely to be a problem
in the future it may be appropriate to apply a
corrosion inhibitor as part of a coating
process. Paraloid B72 or cellulose nitrate
lacquers may be used if required. Brewer
(1991) found wax coatings were less permeable
to moisture than Paraloid B72, however
waxes may be difficult to remove from pitted
iron surfaces. A combination of a coating of
B72 followed by a wax coating may reduce
permeability to moisture and ensure the future
removability of a wax coating.
A hot wax application provides good
penetration and is less likely to be damaged
than a lacquer layer. The component is heated
with a heat gun until a mixture of microcrystalline
wax, made to a paste consistency with
petroleum benzine or white spirit, melts into
and saturates the surface. Care should be
taken not to overheat the metal and thereby
compromise metallographic evidence. Excess
wax is removed by buffing with a soft cloth
and the component left to cool. A higher shine
can be obtained by buffing the wax after it
has cooled.
15.3.9 Brass and bronze
In modern usage, brass refers to an alloy of
copper and zinc, whilst bronze refers to an
alloy of copper and tin. Such nomenclature
does not necessarily reflect historic practice.
The Latin, and hence medieval, term for brass
was auricalcum (gold copper). Many historic
copper alloy objects contain both zinc and tin,
and so the term ‘copper alloy’ is prudent if the
exact composition is not known. Brass components
in furniture may have been polished
or matte, patinated or lacquered. Copper
alloys that were stamped, rolled or hammered
during manufacture may become embrittled
over time. They are vulnerable to damage –
for example if levered up, they may snap.
Casting bronzes may contain 10–18% tin,
though a common alloy composition for
‘bronze’ statuary is 85% copper and 5% each
of zinc, tin and lead.
Stabilization
Stabilization treatments are designed to retain
all of the corrosion products and render them
stable. Stabilization treatments may alter the
appearance of the metal. The decision to
undertake a stabilization treatment should take