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PHOTO: SHAJU JOHN

The coloured components must contain certain unsaturated groups, which are called chromophores (colour bearing groups). A compound containing a chromophore is called a chromogen.

Certain groups called auxochromes (colour intensifying groups) are present in the chromogen. This auxochrom by itself does not produce colour but deepens the colour of a chromogen. Colours can fade mainly by the following three ways:

1) By simple washing with water. By washing the concentration of the colour may get reduced because of its poor affinity to an object. Here no chemical reaction takes place but only reduction in concentration of the colour takes place and thus the colour looks faded.

2) By chemical oxidation or reduction. When the colours are exposed to oxidative chemicals like bleaching agents (e.g., hypochlorite) the colours undergo chemical oxidation and thus get faded. When the colours are exposed to reductive chemicals like hydros (e.g., sodium hydrosulphite) the colours undergo chemical reduction and thus get faded.

3) By exposure to light (sunlight or artificial light). When the colours are exposed to light they undergo either photo-oxidation or photo-reduction and thus get faded.

The colour bearing group chromophore contains a double bond in its structure which is responsible for the depth and chroma of the colour and these double bonds are attacked and cleaved by the oxidative and reductive chemical reactions and thus the strength of the colour goes down and the colour looks faded.

Finally, the colour fades when exposed to perspiration as sweat contains l-histidine monohydrochloride monohydrate and this chemical also attacks the double bond of the chromophore.

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