Series editors' preface - Wood Tools

(a) (c)
so if the treatment is to appear even, light
oxide should be present over the entire object.
Mixtures of tannic and phosphoric acids have
been found to be most effective. One recipe
is:
1 part (by weight) 10–40% phosphoric acid
1 part (by weight) tannic acid powder
These ingredients are diluted in a 50/50
solution of ethanol and water to a concentration
of 10–20% w/v. Objects can be soaked in
this solution or it can be swabbed on (Figure
15.10). This is followed by rinsing and drying.
The use of an additional protective coating is
unnecessary for surfaces that have been
treated with tannic acid.
Conserving other materials I 691
Figure 15.10 Rust conversion
(a) The reverse side of the lock cover illustrated in Figure 15.5 being treated with tannic acid to passivate
corrosion. (b) Lock mechanism, iron and steel, of a seventeenth-century strongbox before treatment with tannic acid.
(c) Strongbox lock mechanism after removal of old coatings and passivation of corrosion with tannic acid.
Mechanism has had all working parts greased with a horological grade of grease
(b)
Chemical removal of corrosion products
Citric acid, a weak chelator, forms a watersoluble
colourless complex with iron (see
Table 11.10 for formation constants).
Examples of commercial rust removers based
on citric acid are Modelene (UK) and
Brownell’s Rust Remover (USA). The rate of
removal can be controlled by the strength of
the solution, as directed by the manufacturer’s
instructions. The metal will appear clean and
bright. Rust patinas, fire bluing and temper
colours will be completely removed.
EDTA is a generally effective chelating agent
for a variety of metal ions. Figure 11.22c,
which plots the conditional stability constants
of EDTA and several metal ions, shows that
between a pH of c.4 and 7 EDTA can form