Textile bleaching

• 1. Introduction

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  Textile bleaching sits at the crossroads of ancient craft and modern chemistry, yet most people never stop to wonder how a bolt of cloth goes from its raw, yellowish, odor-bearing state to the brilliant white it carries in the store. The raw form of any textile material is called greige, a term that captures something essential: fabric as nature left it, full of natural color, smell, and impurities accumulated during cultivation and manufacture. Those impurities are not just organic. Pesticides, fungicides, worm killers, lubricants, and sizing agents all cling to greige material. Before cloth can be dyed, printed, or worn next to skin, those layers have to come off. The two-stage answer to that problem is scouring followed by bleaching. How did humans arrive at industrial-scale cloth whitening? The path runs from Egyptian linen spread on grass fields to a Scottish chemist patenting powder made from chlorine, and the story involves soured milk, parliamentary acts, and a single bond in the hydrogen peroxide molecule.

• 2. Ancient Grassing

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  An Egyptian list found in the tomb of Rekh-mi-re at Thebes mentions both bleached and unbleached linen, pushing the practice back to at least 1000 BC. Some researchers, like Mulrooney, trace it further still, to around 5000 BC, while Walton argues it arrived in Egypt from Asia. The most plausible origin story requires no invention at all: people noticed that garments left in sunlight and washed repeatedly grew lighter on their own. Wood ash, or impure potassium hydroxide, served as an early soap and was used in bleaching from at least 1 AD. The method built on that chemistry was called Grassing. Linens were boiled with lyes of ashes, rinsed, then laid out on grass for more than seven days. The field where this happened was called a bleachfield, typically sited near a watercourse, and bleachfields were a familiar feature of mill towns during the British Industrial Revolution. Cotton-based and linen fabrics both responded well to Grassing, and linen in particular was treated this way across Europe for centuries.

• 3. Dutch Mastery and Acid

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  By around the 12th century, the Dutch had developed bleaching into a serious trade, and they added a step that softened the harsh effects of caustic lye: soaking the treated cloth in soured milk for five to eight days. The lactic acid neutralised the alkali, and by the 17th century Dutch bleaching skills were so respected that customers from across Europe sent cloth there. The soured-milk soak remained the standard until around 1756, when the Scottish doctor Francis Home proposed replacing it with a weak solution of sulphuric acid. John Roebuck's ability to manufacture sulphuric acid at commercial scale made that substitution viable, and the soaking time collapsed to between 12 and 24 hours. Meanwhile, the English East India Company was importing bleached, painted, and printed calico from India, which cut into domestic silk and wool trades badly enough that Parliament passed the Encouragement of Manufactures Act 1698. That act banned printed calicoes from China, India, and Persia. A second law in 1721 went further, banning the use and wearing of all printed, painted, stained, or dyed calicoes, which pushed demand toward linen and fustian. Both acts were eventually repealed in 1774, when cloth was being made from imported American cotton.

• 4. The Chlorine Revolution

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  Chlorine's discovery in the late 18th century changed the economics of bleaching decisively. The French chemist Claude Louis Berthollet was the first to demonstrate chlorine's bleaching properties, and by around 1789 he had developed liquid bleaches from it. James Watt brought the technology to Britain, and a fellow Scot, Charles Tennant, took it further by patenting a bleaching powder that made chlorine-based bleaching commercially practical. Chemical bleaching overtook Grassing because it was faster and could be carried out indoors, independent of sunlight and open fields.

• 5. Scouring: The First Step

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  Scouring precedes bleaching in the textile manufacturing sequence. Carried out with or without chemicals, at room temperature or at elevated temperatures with the addition of wetting agents and alkali, scouring strips out waxes and pectins. The result is a textile material that is hydrophilic, meaning it absorbs water readily. That water-absorbency matters because the bleaching chemicals that follow need to penetrate the fibre evenly to work.

• 6. Oxidative and Reductive Paths

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  Bleaching agents work by attacking chromophores, the molecular structures responsible for absorbing visible light and giving fabric its yellowish cast. Textile bleaching divides into two chemical families: oxidative and reductive. Oxidative bleaching, which uses agents such as sodium hypochlorite, sodium chlorite, or sulfuric acid, covers most natural fibre types. Cotton, ramie, jute, wool, and regenerated fibres like bamboo are all typically treated this way. The key mechanism involves oxygen breaking apart the conjugated double bonds that make fabric absorb visible light. Hydrogen peroxide is the dominant bleaching chemical globally: around sixty percent of the world's hydrogen peroxide output goes into bleaching textiles and wood pulp. Its active ingredient is a single oxygen-to-oxygen bond that, on breaking, releases a highly reactive oxygen species. Reductive bleaching uses sodium hydrosulphite and handles synthetic fibres including polyamides, polyacrylics, and polyacetates.

• 7. Optical Brighteners and Whiteness

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  Even after scouring and bleaching, some textiles receive one more treatment: optical brightening agents, also called OBAs. These chemical compounds absorb light in the ultraviolet and violet region of the electromagnetic spectrum, roughly in the 340-370 nanometre range, and re-emit it in the blue region, roughly 420-470 nanometres, through fluorescence. The effect is a fabric that appears more brilliantly white than bleaching alone can deliver. OBAs are available in different tints, including blue, violet, and red. The measurement system used to quantify the result is the CIE Whiteness Index, a methodology developed by the Commission on Illumination. That index defines whiteness as the degree to which a surface resembles a perfect reflecting diffuser, an ideal surface that neither absorbs nor transmits light but reflects it evenly in every direction. The continuous bleaching range, the industrial machine set that carries out these treatments, moves fabric through multiple compartments via guide rollers, applying chemicals, heat, rinsing, and squeezing at each stage, and can handle cloth in both open-width and rope form.