Series editors' preface - Wood Tools

550 Conservation of Furniture
general character of a layer or adhesive, the
enzyme that will have the most specific activity
on this material, and the composition of
the underlying layers or substrate should be
understood. The conservator should know the
specific material or chemical linkage for which
the enzyme will catalyse reactions, whether
the enzyme is compatible with a desired detergent
or chelating agent, the enzyme’s optimum
temperature and pH range and whether these
are safe for the substrate.
The quality and specifications for enzyme
products can be obtained from the supplier
(e.g. Sigma). Purity of enzyme preparations is
an important consideration – technical preparations
contain only a small percentage
(1–10%) of active enzyme protein. Preparations
often include stabilizers that lengthen
the shelf life of the product and that may also
promote activity due to the presence of
various ions such as calcium. Carbohydrates
and salts are often added to dilute enzyme
concentration to a standard activity (Godfrey
and Reichelt, 1983).
Enzyme preparations are sold on the basis
of weight and ‘unit activity’, which is defined
as the amount of enzyme required to produce
1 microgram (μg) of product per minute of
activity under optimal conditions. A high activity
to weight ratio is usually desirable for
conservation purposes since 10 mg/ml is
usually the upper limit of enzymatic protein
that can be dissolved in an aqueous solution
(Wolbers et al., 1990). One thousand units of
activity per millilitre of solution has been
suggested as a target amount for an enzyme
solution to have an observable effect within a
practical amount of time (Wolbers, 2000).
Enzyme solutions are usually buffered to an
optimal pH that is both safe for the substrate
and ensures that the enzyme is not denatured.
The buffer should match the intended parameters
for a given enzyme solution, i.e. the pKa
of the buffer should equal the target pH for
enzyme activity. As a general rule, chelating
agents should not be added to enzymatic
cleaning solutions as a small amount of metal
ions are often required for enzymatic activity.
The formulation of lipase and protease gels is
described by Wolbers (2000). General purpose
recipes for lipase and protease gels are given
below. Amylase recipes can be found in paper
conservation literature.
General purpose lipase gel
100 ml deionized water
1 g deoxycholic acid (free acid)
as needed triethanolamine to adjust pH to
8.0–8.5
2 g methyl cellulose (4000 cps)
0.5 g lipase (Type VII, Sigma)
Deoxycholic acid is used as part of a buffer
but has a dual role as it will also act as a
surfactant, wetting the aqueous enzyme preparation
onto an oily substrate. One gram of
deoxycholic acid per 100 ml of water will give
a surfactant concentration of five times the
CMC (see under section 11.5.4). TEA is added
to raise the pH to 8.0 or 8.5, bringing the
deoxycholic acid into solution. The type VII
lipase is useful because it has about 800 units
of activity per mg at its optimum temperature
and pH (i.e. 60 °C and pH 8.5) and contains
the calcium required for activity. At the halfgram
concentration it should fully dissolve in
the solution, and will have a concentration of
5 mg/ml of solution. Using the enzyme at
room temperature will result in a loss of
around half the stated activity. A concentration
of 0.5 grams in 100 ml is equivalent to 5
milligrams per millilitre of solution. The stated
activity is 800 units per mg, but the best that
can be hoped for at room temperature is half
that, 400 units per mg. With 5 milligrams per
litre, the effective unit activity of this solution
is still 2000 units per ml. It is essential that
when the enzyme powder is added that it is
gently folded into solution. Brisk stirring will
introduce air bubbles that will denature the
enzyme. The solution should be pale yellow
or straw-coloured when mixed.
The gel may be applied to an area about
25 mm (one inch) square on a surface. The
gel may be moved on the surface with a soft
brush to increase contact. After about a
minute the gel is removed from the surface
with a dry swab. The area may be re-treated
if necessary. It is important to note that the
recipe above uses deoxycholic acid, which
will precipitate onto the surface if the pH
drops below 7.5, so an aqueous rinse buffered
to a pH of 8.0 would be necessary as part of
a clearance procedure. The buffer solution
might then be cleared with a water or solvent
rinse, depending on the counter-ion used in
the buffer.