Series editors' preface - Wood Tools

Figure 11.24 Old residues of a collagen consolidant
on this painted surface were removed using a protease
gel
Wolbers et al., 1990). Esters in fats and oils
(lipids) are broken down into fatty acids by
lipase; proteins such as gelatins, collagen and
casein are broken down into amino acids by
protease; carbohydrates are broken down into
sugars by carbohydrases such as amylase.
Wolbers has described the use of protease to
remove residues of collagen adhesives from
painted surfaces, and lipase to remove oil
layers applied to paintings in order to increase
the specificity of such treatments (Wolbers et
al., 1990) (Figure 11.24, Plate 4). Whilst
protease and amylase have been used for the
removal of adhesive residues in paper conservation,
the ability of lipase to remove oil
applied to painted surfaces has proven
somewhat more controversial (Wunderlich and
Weser, 1995; Knox, 1995).
Oil has been applied in the past to furniture
to saturate colour and from the erroneous
belief that wooden surfaces need to be ‘fed’.
Linseed oil has also been included in many
traditional recipes for furniture revivers. The
ester linkage between the glycerol backbone
and the fatty acids chains that make up the
triglyceride molecules in linseed oil is not
affected by the polymerization process. Lipases
should in theory be capable of breaking down
crosslinked oil coatings by hydrolysis but may
in fact be hindered by limited access to the
ester linkages in a substantially crosslinked oil
film. The presence of soiling materials on an
oil film, or pigments or a resin component
within an oil film will further inhibit enzymatic
action.
Principles of cleaning 549
Resin varnishes will be unaffected by an
enzyme cleaning treatment but may be
damaged if an excessively alkaline pH solution
is used to deliver the enzyme. Lipases will
only work at the interface between the cleaning
solution and the substrate, which is
comparatively small in the case of decorative
surfaces. In many cases, partial hydrolysis of
a protein or lipid is sufficient to facilitate the
removal of an unwanted oily or proteinaceous
material.
Enzymes vary in the specificity of their
action. Whilst protease will only catalyse
reactions that break down proteins, their activity
is often not as sharply defined as might be
expected. Some proteases, for example, may
hydrolyse a wide range of amino acid bonds
whilst others may be extremely narrow in their
action (e.g. collagenase, which acts only on
the peptide bonds in collagen). Lipases are
unusually non-specific in their action,
catalysing reactions on ester bonds, and can
therefore be expected to act on a range of fats
and oily material.
Aside from generic considerations such as
cost and availability, key factors that influence
the choice of enzyme include the
degree of specificity to materials present on
the object, optimum pH and temperature, and
the presence of activators or inhibitors.
Slightly alkaline pH (7–8.5) and room temperature
are optimal conditions for enzyme
performance in decorative surface conservation
treatments. Trace minerals such as iron,
copper, zinc or calcium are often needed for
enzymatic activity (e.g. calcium ions are
necessary for lipase activity). Activators are
materials that increase the rate of enzyme
activity. Inhibitors reduce or prevent enzymes
from catalysing reactions by occupying an
active site or distorting the enzyme. Both
activators and inhibitors are usually welldefined
for specific enzymes. Whilst surfactants
may be added to an enzymatic solution
to increase wetting, particularly for lipases,
some surfactants may act as enzyme
inhibitors and even a low concentration of
some detergents, e.g. Triton X-100, can
inhibit the activity of lipases.
The use of enzymes for cleaning requires
the enzyme be matched as closely as possible
to the material being removed and that the
enzyme not act on original material. Thus the