Series editors' preface - Wood Tools

220 Conservation of Furniture
absorbed the object will appear black. If all
light is reflected it will appear white. If all light
is transmitted it will appear transparent or
translucent. If some wavelengths are absorbed
more strongly than others then the object will
appear to have the colour complementary to
the wavelength absorbed. For example, strong
absorption of blue will make the object appear
yellow, strong absorption of red will result in
a bluish green appearance, and so on. If some
light is absorbed at all wavelengths, the object
will appear grey.
Different wavelengths of light have different
amounts of energy. Short wavelength blue
light has more energy than longer wavelength
red light (see Chapter 6). Particular substances
preferentially absorb light of different energies
(i.e. wavelengths) because these amounts of
energy correspond to different excited states
in which molecules can exist (Suppan, 1972).
Each transition from one energy state to
another higher energy state is associated with
a definite amount of energy. If the amount of
energy involved in the transition corresponds
to energy in the visible part of the spectrum
then the substance will appear coloured.
Colour in inorganic molecules is frequently
associated with electron movements in transition
metals. These are the d block elements in
Group 3a of the Periodic Table (Hill and
Holman, 1978), including titanium, vanadium,
manganese, iron, cobalt, nickel, copper and
zinc. Charge transfer, the transfer of a valency
electron from one atom or ion to another,
occurs particularly with oxides, sulphides,
iodides and bromides and is a another
example of how colour arises in inorganic
materials. Colour in organic materials is usually
associated with the presence of chromophores
usually found attached to aromatic rings such
as benzene or other complex ring structures
(Figure 5.19a). These have their colour intensified
by certain other groups called
auxochromes which include amino groups,
halo groups, hydroxyl groups, and methoxy
groups (Figure 5.19b). The colour of alizarin
is a result of the presence within its chemical
structure of chromophores and auxochromes
(Figure 5.19c). In the absence of chromophores,
organic molecules may still appear
coloured due to conjugation, the pattern of
alternating single and double bonds (e.g.
lycopene in Figure 5.19d) or resonance, when
Organic molecules
(a) Chromophores (+ aromatic rings)
O
C O
N
N N
O
(i) Carbonyl (ii) Nitro (iii) Azo
CH N
(iv) Azomethine
(b) Auxochromes – modify intensify
NH 2
(i) Primary amine
R
N
R
(iii) Tertiary amine
(c) Alizarin
O
C
C
O
(d) Conjugation
OH
Lycopene
N N
O
(v) Azoxy
H
N
R
(ii) Secondary amine
OH
(iv) Hydroxyl
OH
C S
(vi) Thio
OCH 3
(v) Methoxy
Figure 5.19
(a) Chromophoric groups, such as aromatic structures
and double bonds, contain delocalized electrons. Some
examples are: (i) carbonyl group; (ii) nitro group; (iii)
azo group; (iv) azomethine group; (v) azoxy group; (vi)
thio group. (b) Auxochromic groups modify or intensify
the colour produced by chromophores. Some examples
are: (i) primary amine; (ii) secondary amine; (iii) tertiary
amine; (iv) hydroxyl group; (v) methoxy group. (c) The
colour of alizarin is a result of the presence of
chromophores (two benzene rings, two carbonyl
groups) and auxochromes (two hydroxyls). (d) The
colour of lycopene (responsible for the deep red colour
seen in tomatoes, pink grapefruit, guava and
watermelon) is due to conjugation, i.e. alternating single
and double bonds between the carbon atoms