pigmented colorants: dependence on media and time - Cornell ...

A
V = O(r2 )
O(r3 )
= 1
O(r)
24
(2.1)
illustrates that the particle size is inversely related to the surface area. As
particles decrease in size, the surface area increases. This affects the tinting strength
of pigments, the colorant power in relation to its mass. As particles get smaller,
the tinting strength is increased and the quantity of pigment needed to produce
a required color intensity is reduced. Tinting strength also indicates how much
a pigment will dominate the color of a mixture with other pigments. Tinting
strength is an important characteristic when determining relative costs of different
<strong>colorants</strong>. For example, an artist purchasing a tube of a particular colorant may
wish to know which brand is the best value. If an expensive brand contains more
pigment per unit volume, it has a higher tinting strength, and hence may be a
better bargain.
A larger surface area demands a higher ratio of liquid to pigment. There-
fore, more finely divided pigments need more binder to maintain a similar paint
consistency to that of larger particles.
As the particle size goes below about 10 times the wavelength of light (particle
diameter ¡ (4000-7000nm)), scattering effects begin to become significant. Incident
light is reflected multiple times before exiting the medium. At this level, the
increase in tinting strength is sometimes offset by the increase in total surface
scattering. High scattering is desirable in white paints, because more scattering
means increased hiding power and opacity; so fewer coats are necessary to cover
the surface.
Figure 2.10 illustrates how differently light behaves in response to varying par-
ticle size, given the same material. Finer ground pigments typically result in more