l86 DYEING 



In the presence of ammonia and atmospheric oxygen, orcinol 

 becomes transformed into orcein, which precipitates. Stale urine 

 was formerly used as a source of ammonia. Orcein is sold as a fawn- 

 coloured powder, soluble in alkaline or alcoholic solution or in 



HO^c/\ HOAOH UOfyu ^ 



CH 



CH3 



o 



Lecanoric acid Orcinol 



acetic acid. It is used for dyeing wool and silk directly (that is, 

 without the use of mordants). It gives dull magenta shades, but is 

 often used with other dyes to give browns. ^'^^ 



The formula for orcein is probably C28H24N2O7. It has been 

 suggested that the two nitrogen atoms may form the chromophore 

 of an azine dye.^^^ 



Litmus is produced from the same lichens as orcein by a closely 

 similar process, but potassium carbonate is required in addition to 

 atmospheric oxygen and ammonia.^^^' *^^ 



The indigo-dyes are briefly mentioned on p. 307, the acridine 

 dyes on p. 310. 



i 



