Are found in barks, nuts, seedpods and leaves. Some plant and trees contain a better quality and concentrated amounts of tannins. Tannins are weak acids and are somewhat astringent. They dry out the skin. This unpleasant astringency helps protect the plant from insects and predators. This quality of tannins has long been used by the leather tanning industry, where they change the nature of the animal protein making the leather resistant to bacteria and rot and softening the hide.
In Natural dyeing they play an important role as a mordant. They are easy to extract, but simply adding them to warm water. Tannins have an affinity to cellulose as well has protein fiber. When tannins are combined with mordants they form mineral tannates, which are insoluble compounds. Formation of the tannin/ mordant compound is important to the mordanting of cellulose. Tannins with iron will give you a grey or black color.
It is difficult to mordant cellulose fibers as the mordants to bond easily, tannins when applied as a first step in the immersion mordanting process. Once the mordant is applied the tannin losses its solubility and forms a strong bond with the mordant inside the textile.
All tannins when used as a dye source are resistant to fading, in fact they darken after a lengthy exposure to light. When used in combination with other dyes tannins can improve light fastness of other dyes. Tannins do this by protecting natural colors from UV light as they do in the plant where they protect the plant from harmful UV rays.
They are colorless and they aid in assisting the attachment of mordant to cellulose fibers and as a UV protectant on both protein and cellulose fibers. This group of tannins react strongly to iron producing gray and black. In ancient times these were used to make ink. Sources of gallic tannins are Oak gulls, sumac and tara. Oak gulls are the richest source of tannins. ** Oak gulls are a growth of the plant tissue produced by the oak tree reacts with the insect (wasp) released chemical. There are a variety of oak gulls found in around the world. The Aleppo oak produces some of the most concentrated tannin galls Native to western Asia and eastern Europe they are a source of medicines and commercial tannins.
Staghorn sumac and smooth sumac which grows throughout North America have tannin present in the leaves, barks and roots. Leaves can be stored in fall to be used as tannin throughout the year.
These tannins are rich in Flavonoids and are used frequently as a source of yellow dye. Most common tannins in this category are Myrobalan and pomegranate rind.
In India Myrobalan is usually applied on to cotton fabrics before dyeing and printing.
These are tannins that are reddish or brownish. They are a weaker lot of the tannins and do not react as strongly to iron as the Gallic and ellagic. They can be used directly on to protein fibers or as a mordant dye. Cutch and Querbracho are the most commonly used dye/mordant in this category. Black teas and chestnut bark belong to this category. Condensed tannins oxidize when expose to light and deepen their tones. Alkali treatment post dyeing brightens the red tones, but these colors tend to be less resistant to light and fade faster when alkaline is used.