Series editors' preface - Wood Tools

efractive index for air is 1.003 (Brill, 1980).
Therefore, in practical terms, refractive index
describes the degree to which a material bends
light in comparison to air. Linseed oil films have
a refractive index of around 1.57, which means
that visible light will travel 1.57 times slower
through them than air.
Where there is an interface between any two
materials (e.g. binding medium and pigment
particle or varnish and binding medium), the
opacity perceived by a viewer depends on
the amount of light that is refracted (bent) at the
interface. The greater the difference between
the refractive indices of two materials, the more
light is reflected (as opposed to transmitted) at
the interface and the more opaque the system
will appear.
The interface between the varnish and the
binding medium will affect the saturation of
colour of painted surfaces. The closer the
indices of varnish and binder, the less light is
reflected at the interface, the more light is transmitted
into the paint layer, the more light is
absorbed by the pigment particles, and the
more saturated the observed colours. Linseed
oil has a refractive index of about 1.57 and is
well saturated by natural resins such as
dammar, mastic and sandarac but comparatively
poorly saturated by synthetic resins such
as Paraloid B72 (see Table 12.2).
Gloss
Colour saturation is dependent on surface topography
as well as the relative indices of binding
medium and varnish. The gloss of a surface is
inversely proportional to its roughness. A surface
that is smooth and reflects light uniformly to the
eye of the observer will be perceived as glossy
whilst an uneven surface that scatters light will
be perceived as matte. Gloss affects colour saturation
because the rougher the surface, the more
light is scattered. Mixing scattered white light
with the coloured light reflected from the substrate
desaturates the appearance of the substrate.
The darker the colour of the substrate, the
greater the desaturating effect of the additional
scattered white light.
In the case of matte paint, pigment particles
are not thoroughly bound (wetted) by a binding
medium. Applying a varnish will result in a
varnish/pigment interface replacing an air/pigment
interface, with a dramatic increase in saturation.
Principles of consolidation, aesthetic reintegration and coatings 589
Wear, abrasion and the accumulation of dirt
and grime will reduce the gloss of a coating
(Figure 12.8d). The final appearance of furniture
may require a balance between the original
intended aesthetic and the effects of the passage
of time. In some cases a high gloss finish
is appropriate, whilst in others a matte varnish,
or a surface with variations in matteness and
gloss, may be a more sympathetic aesthetic.
Molecular weight
The molecular weight of the resin used in a
coating and the evaporation rate of the solvent
in which it is dissolved will affect saturation and
gloss. The higher the molecular weight of the
resin and the faster the diluent evaporates, the
more rapid the onset of an immobile gel phase
and the less opportunity the varnish has to
level. The ability of a varnish to flow out and
level contributes to gloss. If a varnish that cures
by solvent evaporation forms an immobile gel
at a point when considerable solvent remains,
it will tend to form a surface that follows the
irregularities of the paint underneath the varnish.
This increase in roughness produces a
decrease in saturation.
Viscosity is a measure of resistance to flow.
Low molecular weight resins (e.g. dammar,
MS2A, Regalrez 1094) tend to produce low viscosity
solutions, whilst high molecular weight
resins such as Paraloid B72 produce comparatively
high viscosity solutions. This is reflected
in the concentrations of varnish solutions in
which these resins are used. Brush varnish solutions
of low molecular weight resins may be as
high as 30% w/v or more, whilst brush solutions
of high molecular weight synthetics are often
around 10–15% w/v. More viscous varnish solutions
will stop flowing over a surface sooner
and will have less time to level. The rougher
surface that results will result in a slight reduction
in gloss and will appear less saturated.
The evaporation rate of the solvent will also
affect the physical properties of the film. The
use of slow evaporating solvents will allow the
maximum amount of levelling to occur and
therefore produce glossier surfaces (de la Rie,
1987). It should be noted that a slow-drying
coating is likely to pick up more dust particles
than a quick-drying finish. Solvent evaporation
that is rapid enough to inhibit wetting will
reduce the strength of the adhesive bond and
colour saturation of the surface.