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Natural dyes can be broken down into two categories: substantive and adjective. Substantive, or direct dyes, become chemically fixed to the fiber without the aid of any other chemicals or additives, such as indigo or certain lichens. Adjective dyes, or mordant dyes, require some sort of substance, (usually a metal salt) to prevent the color from washing or light-bleaching out. Most natural dyes are adjective dyes, and do require the application of a mordant (the metal salt) solution to the fibers at some point in the dyeing process. Aluminum and iron salts were the most common traditional mordants, with copper, tin and chrome coming into use much later. In rural areas where these metals were not widely available, plants were also used as mordants, especially those that have a natural ability to extract such minerals from the earth, such as club moss. Most ancient and medieval dyers mordanted their yarns and fabrics before dyeing them. Alum and Iron were used as mordants in Egypt, India and Assyria from early times, as there are many alum deposits in the Mediterranean region. Medieval dyers used alum, copper and iron as mordants, and cream of tartar and common salt were used as to assist in the dyeing process.
Different fibers also have different tendencies to absorb natural and synthetic dyes. Protein and cellulose fibers (the two main divisions for fibers used historically in spinning and dyeing) need to be mordanted differently because of their structural and chemical composition. Mordants to cellulose fibers such as cotton and linen usually involve the use of washing soda or tannins to create an alkaline dyebath. Tannins (plantstuffs, such as oak galls containing tannic acid) are widely used in dyeing cellulose fibers as they attach well to the plant fibers, thus allowing the dyes to attach themselves to the tannins, whereas they might not be able to adhere to the fibers themselves (Tannins are sometimes classified as mordants in and of themselves, but are usually considered a chemical to assist in the dyeing process.) Mordants for protein fibers, like wool and silk, are usually applied in acidic dyebaths. Alum with the assistance of cream or tartar, is the most common mordant used to assist the dyes in taking to the fibers.
Since the difference in mordanting different fibers has been mentioned, it would be remiss not to spend a moment on the historic nature of the fibers themselves. Wool, a protein-based fiber, has been found in Europe dating back to 2000 BCE. It was a common medieval fabric in both dyed and natural colors, and was processed by both professional manufacturers and housewives. Silk, another protein-based fiber, was imported from China to Persia as early as 400-600 BCE. It became quite popular in the Late Middle Ages, and major silk manufacturing centers were set up in France, Spain and Italy. These silk production centers also became centers of dye technology, as most silk was dyed and required the highest quality dyes available. Cotton was considered a luxury fabric, as it was imported all the way from India and usually dyed or painted before it was shipped. Cotton was also valued because of the brightness and colorfastness of the dyes used to color it, and also for its use in making candle wicks. Samples of cotton fabrics have been found in India and Pakistan dating to 3000 BCE, but it did not appear in Europe until the 4th century. Cotton waving establishments were formed in Italy in the 13th & 14th centuries but they did not make a significant economic impact on the industry as they produced a coarser quality of fabric than the imported fabric, and therefore had difficulty in obtaining a good supply of cotton fibre.
Scientists are almost certain that dyeing was practiced throughout the world, but it is difficult to obtain proof on this for two reasons. First, not all cultures left written records of their practices. Second, because of the wide variance of environmental conditions and degree of geological disturbance, it is not easy to find well-preserved evidence of dyed textiles in many archaeological sites.