Series editors' preface - Wood Tools

530 Conservation of Furniture
agent. It is highly polar, freely available, nonflammable
and non-toxic. Like other solvents,
the chemical and physical properties of water
are defined by its molecular structure. Water is
characterized by hydrogen bonding, a type of
strongly polar secondary bonding that occurs
due to the electro-negative properties of
oxygen and the spatial arrangement, or stereochemistry,
of the molecule. Water molecules
are strongly attracted to each other, resulting
in slow evaporation and poor wetting of nonpolar
surfaces. Wetting refers to the ability of
a liquid to displace the air on a surface, to
spread out on it and achieve intimate molecular
contact with it. The ability of a liquid to
wet is a result of the comparative surface
tension or energy of liquid and solid. The
cleaning properties of water can be enhanced
by the addition of a surfactant, which will
overcome wetting problems and can enable
water to remove non-polar greasy or waxy dirt.
Water has a strong affinity for materials that
are partially or wholly hydrogen-bonded, such
as wood and proteins, and some affinity for
materials that are polar, such as natural resins.
It has little affinity for materials that are
primarily bonded with van der Waals forces,
such as waxes and oils. Most transparent
coating materials used on furniture are not
primarily hydrogen-bonded. Water, carefully
applied and thoroughly removed, is therefore
a good choice for cleaning them. Water will,
however, soften and swell a wide range of
organic materials that are often components of
the substrate. Any disruption of the substrate
will, in turn, disrupt and potentially damage a
decorated surface. Further, the small water
molecules may diffuse into and through a
coating without dissolving it. This may result
in blooming of coatings or disruption of
the cohesive bonds within layers, and the
adhesive bonds between layers or with
the substrate. The diffusion rate of water will
be slowest through non-polar materials but, in
the context of varnished and decorated
surfaces, cannot be completely prevented. The
diffusion rate will be affected by the presence
of additives such as pigments in the coating.
Water should not be considered completely
benign for cleaning decorated surfaces consisting
of oil paint, natural resins or wax, but is
utilized because of its comparatively low affinity
for these materials.
Water is often very effective for removing
surface dirt. There are several parameters that
may be manipulated when working with
aqueous solutions. These include pH, conductivity,
the addition of surfactants or detergents,
the addition of chelators, the addition of
enzymes and raising the viscosity of the cleaning
solution. Saliva is one of the most
commonly used and most effective traditional
water-based cleaning materials. The efficacy of
saliva for cleaning decorative surfaces has
been attributed primarily to the presence of amylase,
an enzyme that breaks down carbohydrates
(Romão et al., 1990). Saliva is a
thickened aqueous cleaning solution that
contains a range of ingredients including pH
buffers, a surfactant, a chelating agent,
enzymes, a thickening agent, an antibacterial
preservative, and has an ionic strength of
about 1000 micro-Siemens/cm (μS/cm)
(Wolbers, 2000). These ingredients give an
indication of the ways in which a conservator
can manipulate aqueous cleaning solutions to
extend solubility, to bring a wider range of
materials into solution, thereby removing them
from a varnished or decorated surface.
Pure water is composed entirely of hydrogen
and oxygen. Water is seldom pure,
however, and may be acidic, alkaline, or
contain dissolved minerals. Tap water contains
a range of trace materials, depending on the
catchment area, such as dissolved salts, acids,
alkalis, fertilizers, domestic sewage, bacteria
and fungal spores, industrial and agricultural
waste as well as compounds from the water
pipes themselves. Distilled or purified water
has been treated to remove all contaminants
and therefore contains only water molecules.
Deionized water has been treated to remove
ions (atoms or molecules with an overall
negative or positive charge), such as calcium
(Ca ++ ), magnesium (Mg ++ ) and chlorides (Cl – ).
Both deionized and distilled water have the
potential to dissolve carbon dioxide from the
ambient atmosphere, which may make them
slightly acidic (pH c.5.5). This can be
minimized by storing them in closed containers,
but it may be desirable to check pH
before commencing cleaning.
Although varnished and painted surfaces
can be cleaned with water based systems they
may be damaged by prolonged contact. It is
important to work in a controllable area and