Comprehensive focus on natural dyes

The first part of the presentation appeared in the May 2009 edition. The chemistry of natural colours cannot fail to fascinate and intrigue. In the past seven years, the availability of natural materials for colour has greatly increased.

Haematoxylin

Haematoxylum campechianum, Logwood The heartwood of Logwood (Haematoxylum campechianum L.) contains approximately 10% of a colourless compound called haematoxylin, which on oxidation transforms to a red to violet-blue substance haematoxein (also known as haematein). An alcoholic solution of haematoxylin (0.2%) is used as an indicator. It has a range yellow to orange in acid solution and purple in alkaline solution. Solutions of haematoxylin are used in histology. Freshly prepared solutions have no staining powers. On keeping, however, the haematoxylin is oxidised to haematein, which is the actual colouring agent. Haematoxylin solutions for this purpose are prepared with ammonia alum, which appears to hasten the “ripening” process. The tissues, after being stained red with haematoxylin, are washed in tap water, to change the colour to blue. The mordants used to demonstrate nuclear and cytoplasmic structures are alum and iron, forming lakes or coloured complexes (dyemordant- tissue complexes), the colour of which will depend on the salt used. Aluminium salt lakes are usually coloured blue white while ferric salt lakes are blue-black. The wood has medicinal properties. The infusion obtained when it is boiled in water has astringent properties. It is used in traditional medicine as a remedy for diarrhoea and dysentery.