Series editors' preface - Wood Tools

etouching and coating of lacquer objects is,
however, beyond the scope of this section.
16.8.5 Cleaning
Potential problems
As with other varnished or decorative surfaces,
different areas on the same lacquer object may
exhibit different responses to an identical
treatment. The cleaning of Oriental lacquer
may be complicated by previous treatments or
by a differing history of exposure to light. The
exterior of a box, for example, may respond
differently to an interior that has been protected
from exposure to UV. Black lacquer
made by a reaction between raw lacquer and
iron hydroxides, and dark brown lacquer are
particularly prone to UV damage as they age.
Black lacquer that has been coloured with pigment
is the least sensitive to such changes.
Exposure to short-wave light and UV changes
the physical and chemical properties of Oriental
lacquer. There is an inverse relationship between
the previous exposure to UV of a lacquer surface
and its resistance to water and polar organic solvents.
The more UV exposure Oriental lacquer
has received, the more polar the surface, the
more polar the dirt adhering to it and the more
polar the solvents needed to separate the two.
Light-damaged lacquer is extremely hydrophilic.
With reference to cleaning, the most important
changes are the development of cracks in the
lacquer layer and an overall increase in sensitivity
to water, polar solvents and elevated pH.
Cracks in Oriental lacquer may extend
through the entire thickness of the lacquer
coating (Figure 16.22). Such cracks can act as
capillaries for solvents or water used to clean
or deliver consolidant, allowing liquid to penetrate
through to the ground layers. The effect
of the capillarity is that liquid may wick in
rapidly but evaporate out very slowly. As a
result, solvents may be retained in both ground
layers and lacquer for an extended period.
Retained water or polar solvents may cause
delamination or discoloration of the lacquer. If
the surface is very light-damaged, the degree of
penetration (and retention) of solvent during
cleaning is best limited by using minimal solvent,
e.g. a barely dampened swab or soft cotton
cloth. Gelled solutions do not sufficiently
limit exposure of the lacquer surface and, as a
general rule, should be avoided.
Conserving other materials II 763
Figure 16.22 Photomicrograph of a thin section of
Oriental lacquer. Long exposure to light has led to
photochemical degradation of the lacquer and the
resultant cracks extend through the decorative layers
(2). In some cases such cracks can extend through to
the ground layers (1). Old restoration varnishes are
visible on the surface of the lacquer (3)
The effect of pH and conductivity on the
water-sensitivity of aged lacquer coatings has not
been quantified as yet. The sap of urushi, when
initially collected, has a pH between 5.5 and 7.
This corresponds to the optimal pH for enzyme
activity as the sap matures. The pH of matured
liquid raw urushi is around 4.5 (Kenjo, 1988). It
seems safe to assume that pH of dried lacquer
films starts similarly low, and may drop even further
as it ages. Certainly, aged urushi coatings can
be completely solubilized by aqueous solutions
with a pH of 8.5–9.5. It is prudent to assume that
the more light damaged and oxidized the surface,
the more sensitive to elevated pH it will be.
Removal of surface dirt and accretions
Aliphatic hydrocarbon solvents are unlikely to
damage Oriental lacquer because degradation
products are not soluble in these solvents.
Swabs slightly dampened with solvent (e.g.
white spirit) should be lightly rolled, not
rubbed, over the surface. The exposure of the
lacquer to solvent can be limited by using a
soft cotton cloth. The cloth can be wrapped
around a (gloved) fingertip, a drop or two of
solvent applied with a pipette, and the cloth
wiped very lightly over the surface.
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