176 DYEING 



Carminic acid is the only dye used in microtechnique that is of 

 animal origin. It is obtained from the scale- insect, Dactylopius 

 cacti. The female of this animal is wingless and lives on the succu- 

 lent plant, Nopalea coccinellifera (fig. 22), a native of Central 

 America. Like other scale-insects, she produces a whitish waxy 

 material from the surface of her body.^^ The male is winged and 

 contains only a little of the colouring matter, but the latter is so 

 abundant in the females that it constitutes about 10% of their 

 dry weight. ^^^ The Central American Indians cultivated the 

 animals for the sake of the dye from remote times. The plant on 

 which it lives is nowadays grown in the Canary Islands, North 

 Africa, and elsewhere, as food for the insect. The dye has never 

 been synthesized. It is less used in the textile industry than form- 

 erly, on account of the competition of synthetic products, but it is 

 still used for uniforms and hunting 'pink'.*^^ 



On opening the body-cavity of the fresh female, it can at once 

 be seen that the colour is in the lobulated fat-body.^^^ A red pig- 

 ment is contained in globules situated near the periphery of the 

 cells. These globules give reactions to metals that are characteristic 

 of carminic acid. The male has as much fat-body as the female, but 

 there are only a few red globules scattered here and there in it. The 

 yolk of the ripe egg also contains red droplets, and the colour is 

 carried over into the embryo and then develops anew in the fat- 

 body. No other part of the animal contains the dye. The contents of 

 the alimentary canal are not red. It is clear that the animal syn- 

 thesizes the substance, but for what purpose is not known (con- 

 ceivably to neutralize some poisonous constituent of the sap of the 

 plant, which forms its only food). 



The dried females constitute cochineal. To extract carminic 

 acid from this, the material is powdered and boiled in water, and 

 the fluid then filtered. On the addition of lead acetate, a dark 

 red- violet precipitate is formed. This is the lead salt of the dye. It 

 is dried and ground up with strong ethanol. On the addition of 

 concentrated sulphuric acid, lead sulphate is formed and carminic 

 acid dissolves in the alcohol. The yellowish red solution is evapor- 

 ated at a moderate temperature. (Heat alters carminic acid. It be- 

 becomes amorphous and eventually insoluble in alcohol.) The 

 material is purified by repeated extraction with benzene or other 

 suitable solvents of contaminating substances, and finally allowed 

 to crystallize slowly from ethanol. *^^ The pure acid consists of red, 

 prismatic crystals. 



