Chemistry Reference
In-Depth Information
ments have been prepared by mechanically grinding solid raw materials
esteemed for their specific color; most are derived from minerals, although
some are of vegetable and animal origin, derived from the bark of trees, the
shells of nuts, and the bones and horns of animals (Eastaugh et al. 2004). A
few artificial, human-made (synthetic) pigments, known by the generic
names
frits
and
lakes
, have also been manufactured since antiquity (Barnett
et al. 2006; Koenig and Metz 2003) (see Textbox 20).
TEXTBOX
17
COLOR AND COLORANTS - PIGMENTS AND DYES
The meaning of the word
color
varies in diverse fields of interest. In the
behavioral sciences, for example, color refers to the psychophysiological
perception of the appearance of objects or light sources; in chemistry, to
substances that have certain optical properties (to exhibit color), and in
physics, color refers to a specific property of visible light (Orna 1976). Dif-
ferent colors (i.e., different psychophysiological sensations) are perceived
when light from, say, a blue sky, a red rose, or a green leaf falls on the
retina of human eyes. Color may be perceived directly from a light source,
where it originates, as, with a burning substance that emits colored light,
or reflected from colored objects, as with the blue sky or the red rose.
In chemistry, color is generally associated mainly with
colorants
,
intensely colored substances that may be either inorganic or organic, of
natural origin or artificially made (Billmeyer and Saltzman 1981). Colorants
that are insoluble in most liquids, particularly water, and that generally
(not exclusively) are of inorganic origin, are known as
pigments
(see below,
this chapter) (Lewis 1988). Organic colorants soluble in water and other
solvents are known as
dyes
(see Chapter 15) (Hallas and Waring 1994). The
color of pigments as well as of dyes is determined by the characteristic
range of light wavelengths that they reflect. It is this property to reflect
light of specific wavelengths (colors), that makes the pigments and dyes
suitable for coloring other materials (McLaren 1986).
In physics, color is an optical property of electromagnetic radiation
that is visible to the human eye and has wavelengths ranging between
about 410 and 770 nanometers (see Textbox 22). White, colorless light is a
mixture of light of all wavelengths within this range. If a beam of white
light falls on a transparent glass prism, it is dispersed into a sequence of
colored bands, each band characterized by a narrow range of wavelengths,
as illustrated in Figure 15.
An object that reflects only part of white light (between, say, 540
and 600 nm) appears yellow. Yellow light can also be generated by the
