Series editors' preface - Wood Tools

696 Conservation of Furniture
agents most commonly used to remove corrosion
products from brass are EDTA and DTPA.
Uncorroded metal is not attacked by the
chelating agent because it does not exist in the
ionic form. However, if these materials are
used at an acidic pH, the acidic conditions can
etch the metal. These materials are discussed
in section 11.5.5.
DTPA and EDTA have been used for the
removal of corrosion products because they
form complexes with many metal ions. DTPA
has been used to remove corrosion products
from brass because it is believed to have a
slower and more controllable rate of reaction.
The action of chelators on metal ions is pH
dependent, thus the pH of chelating solutions
should be buffered to the desired (preferably
alkaline) pH. Adding sodium hydroxide to
EDTA (free acid) will raise the pH of the
solution and, if pH is raised high enough, will
affect both the displacement series and EDTA’s
effectiveness at picking up a given metal ion
(see Figure 11.22c). Complexes with divalent
metals (e.g. Cu 2+ , Fe 2+ ) may be displaced from
the chelator and redeposited on the surface in
acidic conditions. Some trivalent metal
complexes are stable even in acidic solutions.
In some cases different metals or alloys are
present on the same object, e.g. ferrous metals
and brass may be present in the same component.
Table 11.10 and Figure 11.21 can be
used to compare the chelating agents and to
predict the selectivity of the chelating solution.
At a pH of 5, for example, EDTA would selectively
chelate Fe(III) ions over Cu(II) ions,
whilst at a pH of 10, EDTA would selectively
chelate Cu(II) over Fe(III). This selectivity is
also dependent on the molar ratios of the
chelating agents. At a pH of 10, for example,
EDTA would chelate Cu(II) ions, but when
these were no longer available would chelate
other ions (Zn, Mn, Fe(III), Ca, Mg) etc. This
is the reason for using repeated applications
of a less concentrated solution, rather than
using a single strong solution for an extended
period.
The technique used to apply the chelating
agent can affect the result. Problems may
occur with the formation of copper
complexes, so it is important to clear such
complexes from the surface as they are
formed. It is therefore important to use fresh
solution on each swab and to discard used
swabs rather than re-immersing them in the
cleaning solution. Whether metal is swabbed
or dipped, it is important to watch for
unwanted reactions. Any change in the colour
of the cleaning solution indicates that the
solution should be changed immediately.
It is possible that an overly strong solution
of any given chelating agent may simply
provide a high concentration of an electrolytic
solution that will encourage dissolution
reactions on the surface of the metal itself.
Although at present this phenomenon has not
been studied, limiting the concentration of
chelating solutions used to clean metals to 5%
or the overall conductivity of a cleaning
solution to 15 000–20 000 microsiemens may
represent a safer corrosion removal strategy.
15.3.10 Ormolu
Ormolu is a term used to describe gilt brass
and is particularly applied to gilded mounts on
furniture, such as those found on eighteenth
and nineteenth century French furniture.
Eighteenth century mercury gilding techniques
have been considered by Goodison (1974) and
Chapman (1994). Thornton (2000) has
discussed the wide range of materials that
have been used to imitate gold as well as
unintended gold-coloured corrosion products
and thin-layer interference patterns that may
produce gold-like surfaces. Comparatively little
has been published on the conservation of
ormolu, however useful information may be
found in Drayman-Weisser (2000), Fiorentino
(1994), Sawada (1993) and Scott (1983).
Under normal conditions gold does not
corrode and alloys with a gold content of
more than 50 atomic per cent gold are usually
corrosion resistant (Figure 15.11). In binary
gold-copper alloys, for example, this corresponds
to 75.6 wt% gold and 24.4% copper
(Scott, 1983). In the context of furniture, the
metal mounts onto which the gold is applied
are usually brass with a high copper content.
Matthew Boulton, for example, who produced
ormolu in England in the late eighteenth
century, recommended an alloy of about 95%
copper and 5% zinc (Chapman, 1994). Mercury
gilding produces a porous gold layer. The
galvanic effect of cathodic gold in contact with
anodic brass or bronze can accelerate corrosion.
In the presence of moisture, copper