Haematoxylin

Haematoxylin is the colouring material to which logwood owes its importance for dyeing purposes. When first discovered it was known as "haematin," but this name was changed owing to its application to another totally different substance. It is obtained by extracting logwood with ether, allowing the solvent to evaporate and recrystallising the product from its hot aqueous solution. It then forms colourless or slightly yellow crystals easily soluble in alcohol, ether, or hot water. It possesses a sweet taste, but its constitution is not yet well determined. If oxidised, as by exposure to air, it yields a red-brown powder, haematein. Haematoxylin, in the form of extract of logwood, is largely employed for dyeing blacks and blues upon fabrics. Iron or chromium mordants are usually employed, but the colours obtained are unfortunately comparatively fugitive if exposed to light. It is also used for staining microscopic objects and animal tissues.