dye. Woad plant (Isatis tinctoria) also has indigo as the chief blue dyeing component.
2 Anthraquinone dyes: Most of the red natural dyes from both plant and mineral origin are based on the anthraquinoid structure. Madder, Lacs, Cochineal are some examples of this group. Alizarin and purpurin are the main chromophores in Rubia tinctorum.
3 Alpha naphthoquinones: Lawsone (henna) is a most important member of this class. Another dye is juglone, isolated from the shells of unripe walnuts.
4 Flavonoids: Yellow dyes obtained from this group and can be classified under flavones, isoflavones, aurones and chalcones. These yellows are found in a variety of plants, including Persian berries (Rhamnus spp.), young fustic (Cotinus coggygria), old fustic (Chlorophora tinctoria) and yellow wood (Solidago virgaurea).
5 Di-hydropyrans: In chemical structure, di-hydropyrans are similar to the flavones. These natural dyes give dark shades on cotton, wool and silk. Logwood and Sappan-wood are the most common examples.
6 Anthocyanidins: Orange dye carajurin obtained from leaves of Bignonia chica. Carajurin is a chemical member of this class.
7 Carotenoids: The class name carotene is derived from the orange pigment found in carrots. In these, the color is due to the presence of long conjugated double bonds. Usually, red, orange and yellow colors come in this category and can be obtained from different plants, e.g. yellow, orange color in sunflower [37, 38].
1.6.2 Classification Based on Method of Application/Preparation
1 Direct Dyes: Direct dye soluble in water can be taken up directly by the material. Direct dye also called substantive dyes because of their excellent substantivity for cellulosic material like cotton and viscose rayon. Turmeric, Chebulic myrobalan and Annatto used in direct dyes.
2 Vat Dyes: As the name suggests that the dye is prepared in a large container for storing and mixing liquids or wooden vessels commonly known as ‘Vat’. This is a primitive method of dye preparation.
3 Mordant Dyes: Mordant dyes are attached to textile fibers by a fixing agent “mordant” which can be organic or inorganic substance. Since chromium is used extensively hence, mordant dyes are sometimes called chrome dyes.
4 Acid Dyes: These dyes performed in acidic medium. Sulfonic or Carboxylic groups of dye molecules can form electrovalent bonds with amino groups of wool and silk.
5 Basic Dyes: These dyes form an electrovalent bond with the carboxylic group of wool and silk. Berberine has been classified as basic dye.
6 Disperse Dye: Disperse dye have low aqueous solubility and low molecular weight. These dyes require post mordanting treatment with chromium, copper or tin salt (Figure 1.3) [36].
Figure 1.3 Classification of natural dyes.
1.7 Natural Dyes Need Fixing Agent (Mordants) for Bonding
Mordants (from the Latin verb “modere” meaning “to bite”) are natural salts that can form a stable molecular coordination complex with both dye and fiber. Natural dyes and their use in dyeing is the most ancient art of all times. Most of the natural dyes have very low affinity towards fabric, therefore a fixing agent is required to attach dye on fabric. Mordants are substances that are able to form complexes with molecules of dyes. Mordants can be applied before dyeing, after dyeing or within dyeing mixed in a dye pot. Process of Mordanting improves the color fastness properties of dyed fabric. Mordants are classified in three categories such as Metallic mordants, Tannins and Oil mordants [39]. The fourth category is bio-mordant, which is generally obtained from natural resources.
1.7.1 Metallic Mordants
1 i. Aluminum: Potash alum is the most widely used aluminum mordant for natural dyeing. Alum does not affect the color. The shade of dye depends on the amount of mordant. If deeper shades are required on fabric a greater amount of mordant is needed. Alum forms weak sulfuric acid when dissolved in water during the mordant process. This can result in acidic fumes which are corrosive, and irritating when inhaled.
2 ii. Iron: Iron salts in the form of ferrous sulfate (also known as green vitriol) are extensively used in dyeing and printing. Mordanting with iron salts produces a black or gray color to the fabric and reduces the darkness of other colors. Repeated high exposures may lead to nausea, vomiting, stomach pain, constipation and may affect liver.
3 iii. Copper: In copper mordanting, fabric treatment is done with the help of copper sulfate (blue vitriol). It is known for improving the light fastness of various dyed materials. High temperature operations such as boiling in dyeing generate fumes that have different health effects. Long-term exposure of copper can cause irritation, burn the skin, eyes and throat. It can cause headache, nausea, vomiting, diarrhea and abdominal pain.
4 iv. Tin: Tin mordant brightens the color. Stannous and stannic chloride are used as mordants. Stannic chloride is preferred for cotton. It causes severe skin burns and eye damage. It also causes skin allergies.
5 v. Chromium: Potassium dichromate is used in mordanting procedure and referred as Chrome. It is highly toxic and quite hazardous to health. Small amounts can cause contact dermatitis. It is a known carcinogen meaning it causes cancer.
1.7.2 Tannins and Tannic Acid
Tannic acid or tannins are used as a primary mordant for cotton and cellulosic fibers which do not have much affinity for metallic mordants. A cotton fabric treated with tannic acid can absorb all types of dyes.
1.7.3 Oil Mordants
In the past, castor and til (sesame) oils were used as mordants but they were later replaced by Turkey Red Oil (TRO). Sulfated castor oil is largely used in textile industries.
Many metal salts such as chrome, copper, tin and lead are seldom used now due to research evidence of their extreme toxicity either to human health, ecological health or both. Only a small amount of these metal salts get fixed on the fabric and the rest is discharged as effluent which leads to the contamination of land and water resources. In order to make natural dye sustainable many scientific workers are developing natural mordants that can be replaced with metallic-salt-based mordants.
1.7.4 Bio-Mordants
Bio-mordants are those substances that can be obtained from natural sources. According to many researchers bio-mordants are eco-friendly and effective to use than synthetic mordants [40]. There are some examples of bio-mordants:
1 i. Myrobalan: It is one of the most important and widely used mordants in dyeing processes. It can be considered as dye and mordant both. Myrobalan mordant is obtained from fruits of Terminalia chebula commonly known as ‘Harda’. It gives pale yellow color on fabric.
2 ii. Oak gall: Gallnuts are obtained from the oak tree. It is the earliest and richest source for natural tannin. These are collected and ground for use as a tannin mordant.
3 iii. Sumac: The leaves of sumac contain tannin which can be used in the process of mordanting cotton. Rhus glabra species of sumac also known as “rhubarb”. Leaves of sumac are rich in tannin suitable for dyeing and their use as mordant.
4 iv. Pomegranate rind: Dried pomegranate rind (P. granatum) powder also used as mordant in natural dyeing. Sangeetha et al. applied lemon leaves extract using P. granatum rinds as mordant on Silk fabric [41].
5 v. Catechu: Catechu was used as a natural mordant since the ancient times as myrobalan [42, 43]. Catechu is extracted from the heartwood of Acacia catechu. It produces various