**2. Historical background of dyes and pigments**

In the ancient age usually used all the dyes were natural. Some of the natural dyes used in ancient age were alizarin and indigo. Indigo is probably the oldest known dye obtained from the leaves of dyers woad herb *Isatis tinctoria*, and the indigo plant *Indigofera tinctoria* [16]. Early dyes were obtained from animal, vegetable or mineral sources, with no to very little processing. The first synthetic dye, mauve, was discovered serendipitously by William Henry Perkin in 1856. The discovery of mauveine started a surge in synthetic dyes and inorganic chemistry in general.

Artists invented the first pigments—a combination of soil, animal fat, burnt charcoal, and chalk—as early as 40,000 years ago, creating a basic palette of five colours: red, yellow, brown, black, and white. In the early age man used earth pigments on cave walls such as yellow earth (ochre), red earth (ochre) and white chalk. Ochres are probably the oldest known pigments, which are coloured clays found as soft deposits within the earth [17–21].

#### **2.1 Reason for the colour of a dye**

Many theories have been given to correlate the colour of dyes with their molecular structure. In 1876, Otto Witt, a German Chemist observed that the colour of a dye is due the certain groups containing multiple bonds known as chromophores. Some examples of the chromophores are nitro group(-NO2), nitroso group(-NO), carbonyl group(-CO-), ethylenic bond(-C=C), acetylenic bond etc. As the number of chromophores increases for a dye, the colour of the dye also deepens [17–21].

He also observed that not only the chromophores are responsible for the deepening of colour but also there are certain groups which itself does not act as the chromophore but the presence of which deepens the colour of the dye. These groups are known as Auxochromes. Some examples of the auxochromes are -OH, -NH2, -NHR, -NR2, X (Cl, Br or I), COOH. 1,3-Dinitronapthalene (**Figure 1**) is pale yellow but the dye Martius Yellow (2,4-Dinitro-1-naphthol) is orange-red (**Figure 2**) [17–21]. Here group -OH is acting as the autochrome as the presence of it has deepened the colour of 1,3-Dinitronapthalene.

To explain the origin of colour, Valence Bond Theory (VBT) and Molecular Orbital Theory (MOT) are proposed in modern time. The vital difference between the VBT and MOT is that in VBT electrons are treated in pairs while in MOT electrons are treated singly [17–22].

**Figure 1.** *1, 3-Dinitronaphthalene.*

fabric from these dyes the acidic solution of these dyes is used. These can be used to colour wool, silk, nylon and polyurethane fibres. The affinity of acid dyes for nylon is very high due to the higher protonation of the free amino group present in polycaprolactam fibres. Orange-I (**Figure 3**), orange-II,

methyl orange and congo red are some examples of it [22, 23].

sulphonic acid with α and β- Naphthol respectively.

*Structure and Properties of Dyes and Pigments DOI: http://dx.doi.org/10.5772/intechopen.97104*

Diazotisedsulphonic β-Naphthol Orange-I acid sodium salt

(**Figure 7**) are two examples of these dyes.

**Figure 3.** *Orange-I.*

**Figure 4.** *Aniline yellow.*

**Figure 5.** *Butter yellow.*

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Celliton fast pink B and Celliton fast blue B [22, 23].

The dye orange I and orange II can be obtained by coupling diazotised

II.**Basic dyes:** Those dyes which are salts of coloured bases containing amino or dialkylamino group as autochrome is known as Basic dyes. These include triphenylmethane and azo dyes. Modified nylons and polyesters can be dyed with the help of these dyes [22, 23]. Some examples of basic dyes are aniline yellow (**Figure 4**), butter yellow (**Figure 5**) and chrysodine G (**Figure 6**).

III.**Direct or substantive dyes:** These are water-soluble dyes hence they can be applied to the fabric directly [22, 23]. Congo red and Martius yellow

IV.**Disperse dyes:** These are water-insoluble dyes and applied to the fabric in the form of a dispersion in presence of some stabilising agent such as phenol, cresol or benzoic acid. The two examples of disperse dyes are

**Figure 2.** *2,4-Dinitro-1 naphthol.*

