Series editors' preface - Wood Tools

690 Conservation of Furniture
magnification, taking care to remove rust
without scratching the surface (Figure 15.9).
This technique is very time-consuming. Larger
scale problems, for example tubular steel
furniture on which the whole finish has been
affected, may require the use of mild abrasives
such as talc, fuller’s earth or jeweller’s rouge.
The removal of abrasive particles after treatment
is often problematic and residues are
potentially visually obtrusive. Proprietary
products such as Autosol are not recommended
as they often contain particles that are
large, excessively coarse and may leave deep
scratches.
Fine (0000) wire wool has been recommended
for the removal of corrosion products
from ferrous metals (CCI Notes 9/6, 1995).
Fine wire wool is a comparatively harsh
abrasive but can be effective for removing
rust, particularly if the corrosion is softened up
by soaking in aliphatic hydrocarbon solvent
(e.g. Stoddard solvent, petroleum benzine) or
penetrating oil (e.g. WD-40). Wire wool may
find application for large scale corrosion, for
example the removal of rust from the underside
of an object, but will obliterate any fine
detail, tends to leave a surface covered with
fine scratches and will round over sharp
edges. It is used in combination with a light
mineral oil, residues of which may be
removed with solvent. Some steel wool is
coated with silicone-based lubricants that can
cause subsequent problems with coatings, so
conservators generally use lubricant-free
grades. Stainless steel wool could also leave
chromium and other trace metals on surfaces,
which could confuse instrumental analysis.
Dry brushing will remove loose flakes and
may utilize a watchmaker’s bristle brush or a
medium hard toothbrush. In some cases a
swab and solvent such as petroleum spirits or
alcohol may be successful, though the
presence of coatings that may be damaged by
the solvent should be ruled out first. If solvent
splashes will not damage the surrounding
area, a brush may also be used.
Air-abrasive methods may find occasional
application for ferrous components of furniture.
Various grits are available for different
jobs requiring more or less removal and
delicacy. Organic grits of corn-cobs and
walnut shells are most gentle, but do not
remove tightly adhering corrosion. Glass bead
‘peening’ may be useful for parts such as
screw threads where appearance is not an
issue and complete corrosion removal and
smooth functioning is important.
Rust converters
Rust converters may be used to treat lightly
corroded historic iron. These chemicals
convert unstable corrosion products into more
stable iron compounds. The most common are
phosphoric acid and tannic acid. The
compounds formed are bonded to the surface
of the metal by an irreversible chemical
reaction. Ethical debate on the use of rust
converters has centred on the change in
appearance and the appropriateness of such a
non-original layer chemically bonded to the
surface. The decision whether to use a rust
converter must balance these factors against
the probability of removing all corrosion
products and the environment to which an
object will be returned. Commercial rust
removers usually consist of citric and orthophosphoric
acids with passivating agents such
as amines (hexamine) to prevent them from
aggressively etching uncorroded metal.
Although rust converters can be applied selectively,
the effect can be visually disruptive. The
use of tannic acid on surfaces that have been
painted black is an exception to this – loose
flakes of corrosion can be picked off and the
tannic acid then applied. Binnie et al. (1995)
tested a range of rust converters for ferrous
objects displayed outdoors.
Phosphoric acid can remove corrosion
products and additionally forms a stable
phosphate layer that resists corrosion. The
conversion products from phosphoric acid are
more stable than those produced by tannic
acid. Phosphoric acid is often a component in
metal primers. The light grey appearance of
treated metal is often considered unsightly. It
is often used as the first stage in a treatment,
followed by the application of lacquer or
paint. Acidic rust removers based on hydrochloric
acid or phosphoric acid, thickened
with cellulose ethers, are called naval jellies.
Tannic acid forms a stable black irontannate
layer (Logan, 1989). Tannic acid effectively
converts and passivates light corrosion,
and the black colour is often in harmony with
blacksmithed objects. Tannic acid does not
penetrate or react with an uncorroded surface,