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 shades of brown. Catechu mordants were applied with Sticta coronate a lichen that produces dye for coloring silk fabric [42].
6 vi. Aloe vera: Aloe vera leaves contain sticky substance. Researchers are working on exhibited fixing properties of aloe. Fresh leaves of aloe vera can be taken as biomordant for dyeing silk fabric [44].
Other than above mentioned names many sources have been explored for bio-mordants. Adeel et al. explored the fixing properties of acacia (Acacia nilotica), henna (Lawsonia inermis), turmeric (C. longa), pomegranate (P. granatum) and rose (Rosa indica) with natural dye extracted from cinnamon bark. New shades observed on silk fabric with improved fastness properties [45]. Bark of Xylocarpus moluccensis tested to be used as a biomordant, and significant improvement in the percentages of dye absorbed in the silk fabric was observed [46]. Wool yarn dyed with madder roots with gallnut (Quercus infectoria) extract as biomordant [47]. Rani et al. investigated that harda powder, pomegranate peel, orange peel and amla powder can be used as alternative copartner of metal mordants. Dyeing was done on protein fabrics with Carica papaya leaf natural extract [48]. Wool yarn dyeing performed with Adhatoda vasica extract. The effect of various metal salts and extracts of gallnut, pomegranate peel and babool bark as mordants were comparatively evaluated [49]. Aminoddin extracted Berberine from Berberis vulgaris wood and applied on wool fiber using the extract of roots of Rumex hymenosepalus as biomordant [50]. Banana pseudostem sap was applied as a biomordant with Senegalia catechu stem extract on wool [51]. To provide best options of synthetic mordants various scientists are exploring and developing the new and effective bio-mordants.
1.7.5 Method of Application
1 i. Pre Mordanting (Chrome mordant process): This is two bath process in which fabric is first mordanted in a dye bath then squeezed and immersed for dyeing at required temperature.
2 ii. Post Mordanting (After chrome method): The method involves first dyeing the fabric and then treating it with mordants, later on material is taken out for washing, squeezed and dried.
3 iii. Simultaneous Mordanting (Meta-Chrome process): The meta-chrome process involves only one single step because dye and mordant is mixed together to work simultaneously on fabric.
1.8 Fibers/Fabrics Used for Natural Dyeing
Dyeing is a process where dye molecules transport to a substrate surface from the dye solution. The substrates are fabric or fiber. Mainly fibers have two groups i.e. Natural and Man-made (Synthetic) fibers. These fibers/fabrics are mainly used to perform natural dyeing [52].
1.8.1 Cellulosic Fiber
1 a) Cotton: Cotton is a natural fiber obtained from different species of Gossypium plant. The fiber is almost pure cellulose, soft, fluffy, staple that grows in a ball around the seeds of cotton plants.
2 b) Jute: Jute is a long, soft and shiny bast fiber. It is one of the strongest natural fibers.
3 c) Linen: Linen has a very good quality of absorption. Flax (Linum sp.) plants commonly known as linseed are the source of linen fiber.
4 d) Hemp: It is obtained from a variety of Cannabis sativa plant species as bast fiber.
1.8.2 Protein Fiber
1 a) Wool: It is an animal fiber. Wool is obtained from the sheep or other hairy mammals.
2 b) Silk: It is a natural protein fiber and made up of Fibroin. Silk fibre is obtained from the cocoons of silkworms.
1.8.3 Synthetic Fiber
1 a) Nylon: It is produced by reaction of amino acid with itself or between diamines and diacids.
2 b) Acrylic: Polymer acrylonitrile found in acrylic fiber. It is soft, light weight and warm fiber.
3 c) Polyester: They are polymer and contain ester functional groups in their main chain.
1.9 Extraction of Natural Dyes
Extraction of color from natural dye is one of the most important steps of dyeing. Raw materials of natural dyes are leaves, barks, fruits, flowers and roots of many plants as well as some animals and minerals as described earlier. Sources of natural dyes are carbohydrates, proteins, lipids, fats and many other cell inclusions, so only the desired coloring material requires an extraction process. In the extraction process of natural dyes, the cost of extraction and the yield of color affects the cost of dyeing. Following methods generally used for extraction of coloring materials:
1 i. Aqueous extraction: Aqueous extraction method is a traditional and most popular procedure in natural dyeing. Small pieces of fresh or dried dye material are ground in powdered form. That is soaked in water, boiled, filtered to obtain aqueous dye solution. Aqueous extraction depends on many factors such as time and temperature of the boiling, fresh or dried dye material and material to liquor ratio. Maryam et al. gave one such example i.e. extraction of color from Onion (Allium cepa) skin in aqueous condition as 5 g of dye dissolved in 100 ml water at the temperature of 100 °C for 60 min [53].
2 ii. Alkali or acid extraction: Glycosides can be hydrolyzed in acidic or alkaline condition. Mostly, natural dyes constitute glycosides, so extraction in acidic or alkaline medium can improve color yield. Some of the natural dyes are pH sensitive therefore; they destroy their dyeing properties in unwanted pH condition.
3 iii. Microwave and ultrasonic assisted extraction: The extraction of natural dye can be done by microwave and ultrasonic assistance. Microwave energy used in extraction of natural dye with a very minimum amount of solvent. Microwave increases the rate of the processes so the extraction can be completed
