The nitro dyes belong to the first known group of synthetic organic dyes and were of limited use. The shade range is restricted to brown, yellow, orange and green hues. They are characterised by the presence of nitro (-NO2) groups. Picric acid was originally obtained by treating natural Indigo with nitric acid, resulting in a new dye class. After the invention of this semi-synthetic dye, fully synthetic routes were discovered using products originating from the coal tar industry. In general, nitro dyes are prepared by the action of nitric acid on phenol, as in the case of Picric acid, and naphthols, which are used to prepare Martius yellow and Naphthol yellow S. Despite the fact that the nitro dye class did not find a very wide application, Picric acid, Martius yellow and Naphthol yellow S were often used. The nitro dyes were at one time very largely used by dyers for the purpose of dyeing wool and silk. Their fugitive character led in recent times to their replacement by the yellow azo dyes.