Series editors' preface - Wood Tools

688 Conservation of Furniture
and may fracture if the substrate metal is
flexed. Their appearance is often considered
somewhat plastic, though this may be
modified by the addition of matting agents or
by using a slower evaporating solvent.
Incralac, a proprietary mixture of Paraloid B-
44 and benzotriazole, is widely used for
copper and copper alloys. Acrylic resins may
be used to provide an easily removable isolating
layer over which other coatings, with
different solubility parameters, may be
applied. Solutions of 10–12% w/v of Paraloid
B72 in acetone have been used to spray coat
brass. Successful brush application may be
more problematic and some conservators have
used B72 in xylene or a slower evaporating
solvent mixture such as 3:1 diacetone alcohol:
ethanol.
Many proprietary metal coatings are available.
These may be based on Paraloid (e.g.
Incralac) or cellulose derivatives such as propionate
and butyrate (e.g. Ercalene, Agateen).
Frigilene, a CN-based lacquer, has been
recommended for coating silver. Such lacquers
are often marketed as product lines and are
available in a variety of formulations that may
contain stabilizers, plasticizers, colorants and
other additives. De Witte (1973/74) compared
coatings applied to silver objects and found
that the permeability to hydrogen sulphide
(H 2S) was higher through polymethacrylates
(PMAs) than cellulose nitrate (CN). The
adhesive properties of CN were better than
PMAs (except B72) and the CN exhibited
better elastic properties. The PMAs resisted
abrasion very well and were easily removable
after accelerated ageing, whilst the CN
performed poorly in comparison. Selwitz
(1988) has pointed out that De Witte’s accelerated
ageing conditions were selectively
destructive for cellulose nitrate, which suggest
CN may exhibit better ageing properties than
DeWitte’s results indicate.
15.3.8 Ferrous metals
Iron is one of the most reactive of the
common and useful metals, with rust being the
inevitable consequence of neglect and disuse.
The corrosion products of ferrous metals are
larger in volume than the original metal and
it is common to find corrosion products on the
surface that may have been generated from a
small corrosion pit. Although the corrosion
product may be present over the surface of
the metal component, careful removal of rust
or other corrosion products may reveal a
surface that is relatively undamaged.
Information on the conservation of ferrous
metals may be found in Clarke and Blackshaw
(1982) and Stambolov (1979). The stability
imparted by various treatment methods has
been reviewed by Selwyn and Logan (1993).
The chief electrolyte in iron corrosion is
chloride (Cl – ). Historic iron that is free from
chlorides can be stable even when heavily
rusted and subject to high humidity. Iron that
is contaminated with chlorides will not be
stable even when humidity is very low.
Objects with intact coatings or a stable patina
will be more resistant to corrosion than
uncoated surfaces. The material commonly
called ‘rust’ usually consists of a mixture of
ferric oxy-hydroxide corrosion products,
particularly goethite (yellow/ochre), akaganeite
(dark red-brown/burnt sienna) and
lepidocrocrite (bright orange red).
Patination of iron
In order to treat ferrous artefacts responsibly
it is important to understand what the original
surface treatments and coatings may have
been, and their appearance. Heat or fire bluing
is the blue temper colour sometimes used as
a decorative and somewhat protective layer.
Fire bluing is often found on scientific
instruments and barometers, particularly on
indicator needles. Chemical bluing/browning
describes a range of chemical treatments used
to yield a coloured, protective patina. These
are often generically called bluing even
though they produce a range of browns. Fire
or mill scale is the silvery-grey layer of
magnetite that forms as iron is being forged.
When the layer builds up to a certain thickness,
it spalls off the object in fine flakes.
Smiths would often finish objects by wire
brushing off all of the loose material, and reheating
the object evenly to form a thin adherent
layer of magnetite. This treatment was, in
some cases, followed by rubbing oil onto the
hot surface and allowing it to char and bake
on. This resulted in a brownish-black semiglossy
surface. Stove blackings and grate
polishes were proprietary polishes consisting
of waxes and pigments, usually lamp black