Nalural Dyes 105 



Natural Dyes 



Cochineal and Carmine 



These are members of a gToup of dyes called "natural" stains. Unlike 

 other natural stains, cochineal and carmine are derived from an animal 

 source — a minute insect, the cochineal insect. Coccus cacti, living on 

 spineless cacti. The dye is present as a purple sap in the females, which 

 are harvested, dried and pulverized to produce cochineal. This dye by 

 itself has little affinity for tissue unless iron, aluminum or some other 

 metal is present. With the salt of one of these metals as a mordant (see 

 page 107), staining will result. Ahmi cochineal, a commonly used form 

 of this dye with mordant, can be an efficient nuclear stain. The dye 

 carmine, is derived from cochineal by boiling the latter with a salt, 

 ustially alum, to produce a precipitate. This precipitate is insoluble in 

 water and before it can be used as a stain must be converted into a 

 soluble compound such as ammoniacal carmine or aceto-carmine, a 

 process that will be described imder Mordants, below\ 



Heuiafoxyliu 



In many respects, hematoxylin can be regarded as most important 

 among the natural dyes. It was one of the first histological dyes, and 

 still remains one of the most widely known and used dyes. Hematoxylin 

 is extracted from the heartwood of logwood trees from Sotuh and 

 Central America and the W^est Indies. The tree is Hematoxylon cain- 

 pecJiianiim, one of the legiunes (Co)ni, 19y3), similar to acacia or cassia 

 trees. The crude material is exported as logs, chips or as dried aqueotis 

 extract of the lieart\\ood. 1 his then is extracted with ether in a con- 

 tinuous extraction apparatus, evaporated to dryness, dissolved in \vater, 

 filtered and crystallized oiu of soliuion. All of these steps are slow and 

 difficult to handle and require costly apparatus, thus making hematoxy- 

 lin one of oia- most expensive dyes. 



In this condition it is not yet a dye, and its color mtist be allowed to 

 de\ elop by oxidation into heniatein (color acid — no relation to hematin, 

 the colored constituent of red blood cells). Oxidation may be accom- 

 plished in either of two ways: "naturally" — a slow process of exposine 

 to air for 3 to 6 weeks, as in Heidenhain's hematoxylin — or artificially 

 by the use of merctiric oxide, hydrogen peroxide, or other oxidizing 

 agent — a more rapid process as in Harris hematoxylin, l^sed alone, 

 hematein is only a weak and diffuse dye with little affinity for tissues. 



