Series editors' preface - Wood Tools

548 Conservation of Furniture
Figure 11.23 The removal of grime from a nineteenth
century Norwegian painted bowl. The bowl was cleaned
using hydrocarbon solvent (low aromatic white spirit)
followed by the chelating agent triammonium citrate
for example where specific ions (e.g. Fe +3 ,
Cu +3 ) are present and available for reduction.
Sodium dithionite would be inappropriate, and
generally ineffective, for use on varnished and
decorative surfaces.
Chelating agents may assist the action of
detergents if oily material is present as fatty acid
salts of calcium, iron or copper, that would
otherwise be insoluble in water. Chelating
agents can be used to solubilize inorganic ions
forming part of a surface dirt layer (Figure
11.23). They may also help remove dirt layers
bonded to a surface by the presence of metal
ions. It has been noted that citric acid (as the
triammonium salt) is effective in removing both
organic and inorganic components of dirt
(Phenix and Burnstock, 1992).
As well as the chelating agents mentioned
above, other substances may also function as
chelators. Carboxymethyl cellulose, for
example, used in some aqueous gels, may
chelate the transition metal ions, Ca 2+ and
Mg 2+ . Some solvents may also function as
weak chelators, for example acetyl acetone
and triethanolamine (Table 11.12).
11.5.6 Enzymes
Enzymes are used in cleaning to break down
the large molecules of an unwanted material
into smaller water-soluble fragments. Enzymes
are biologically derived catalysts. They are
produced by all living things and have evolved
to act as catalysts for specific reactions.
Table 11.12 Formation constants of acetyl
acetone and triethanolamine with some metal ions
Acetyl acetone Triethanolamine
Metal ion Log K f Metal ion Log K f
Aluminium 15.5 Copper(II) 4.3
Cobalt(II) 9.54 Nickel 2.7
Copper(II) 16.34 Silver 3.64
Iron(II) 8.67 Zinc 2
Iron(III) 26.7
Magnesium 6.27
Manganese 7.35
Nickel(II) 13.09
Zinc 8.81
Source: Dean (1992)
Enzymes catalyse the breakdown of food into
constituents that can be converted by an
organism into the materials needed to live.
Enzymatic reactions may involve oxidation–
reduction, hydrolysis, substitution or addition
reactions. Because enzymes are catalysts, they
are not changed or used up in reactions and
therefore only a small amount is required in
cleaning preparations. The appeal of enzymes
in conservation is that, theoretically, they will
serve as catalysts for reactions in a single class
of materials whilst leaving others unaffected.
Gelling an enzymatic solution may assist in
controlled application to a surface.
Enzymes are primarily proteinaceous but
may also contain carbohydrates, lipids, and
metal ions. Their complex structures become
active in water of an appropriate pH and
temperature, where the hydrophilic side
groups fold outwards and the hydrophobic
groups fold inwards. Several theories have
been proposed that describe how and why
enzymes operate. A simple theory is the ‘lock
and key’ theory. The active site of the enzyme
(the lock) has a very particular shape and can
only accept a molecule (the key) that matches
exactly. Once attached, the chemical bonds of
the molecule can be constrained, lengthened,
twisted or broken and reformed. The reactivity
of the active site will be inhibited outside
of a given pH range or distorted by excessive
temperature. In either case no reaction will
take place.
Enzymes have been utilized in paper
conservation for more than twenty years, and
more recently have been applied to textiles
and paintings conservation (Landi, 1992;