Natural dyes are dyes or colorants derived from plants, invertebrates or minerals. Most natural dyes are derived from plants, including roots, berries, bark, leaves and wood, as well as other biological sources such as fungi and lichens. In the mid-19th century, the discovery of synthetic dyes led to a decline in the large-scale natural dye market. But because of the pure and subtle color, artists prefer natural dyes. Some commonly-used natural dyes are as follows:
Hematoxylin is a pigment extracted from the dry branches of Haematoxylon campechianum in South America and is one of the most commonly used dyes. Hematoxylin can not be directly stained, it must be exposed to aeration, so that it can be used as oxidized hematoxylin. This is called "mature" process.The “mature” process of hematoxylin takes a long time, and the longer the time after the configuration, the stronger the dyeing ability. The dyed material must be dyed by a metal salt as a vehicle. Therefore, mordant should be used in the preparation of hematoxylin dyes. Commonly used mordants are aluminum sulfate, potassium alum and iron alum. Hematoxylin is a pale yellow or rust purple crystal. It is easily soluble in alcohol, slightly soluble in water and glycerin. It is an excellent material for dyeing the nucleus. It can differentiate different structures in cells into different colors. The color of the tissue dyed during differentiation varies depending on the different treatment condition. The color becomes red after being differentiated with an acidic solution (such as hydrochloric acid-alcohol), and then returns to blue after washing with water, and it is blue after being differentiated with an alkaline solution (such as ammonia).
Carmine is also called coccinellin or carmine red. After drying the tropical female cochineal worm, then grinding it into a powder, extracting carmine, then treating it with alum and removing the impurities, thus producing the carmine. Pure carmine cannot be dyed, and it can be dyed only after being dissolved in an acidic or alkaline solution. Commonly used acidic solutions are glacial acetic acid or picric acid, and commonly used alkaline solutions include ammonia and borax. Carmine is an excellent dye for the nucleus, and specimens stained with it are not easy to fade. It is suitable for the dyeing of a section or tissue block, especially for the overall dyeing of small materials. The color can be maintained for several years after dyeing with a solution prepared from carmine. When the carmine solution appears turbid, it should be filtered before use.
Indigo magenta is made from sodium sulfite and indigo which is purified from Indigofera. Indigo magenta is a blue acid dye, which is mainly used to stain cytoplasm. It is often combined with picric acid to synthesize picro-indigo-carmine, which is green and can be used as a counterstain with basic fuchsin.
Orcein is a dye obtained from lichens. It can be dyed in an acidic or alkaline solution. It is widely used in plant dyeing especially for the dyeing of chromosome of some plants, and the effect is better than that of acetate. The preparation method is similar to that of carmine, and it is usually dye in acetic acid (2.2%) solution. It is commonly used in the artificial synthesis of dyes.
Unlike synthetic dyes, natural dyes soften with age and retain their true color. So the natural dyeing techniques have been preserved by artisans in traditional cultures around the world. And as consumers increasingly focus on the health and environmental impacts of dyes in manufacturing, the demand for natural dye products continues to grow.