7 min readPathway sheetOften a 5-mark answer each, or a 10-mark answer together

Scouring and bleaching

Scouring washes out natural and added impurities so cloth becomes absorbent. Bleaching then removes natural colour to whiten it, using a bleach that suits the fibre.

Cotton scouringBoiling with caustic soda in a kier

Wool scouringWarm soap and soda ash, little agitation

Bleach safe for all natural fibresHydrogen peroxide

Never on wool or silkChlorine (hypochlorite) bleach

Draw it: order of wet processing

Scouring

**Scouring** is thorough washing that removes **natural impurities** (wax, pectin, fats, wool grease, silk gum) and **added impurities** (spinning oils, size, dirt, stains).

1
Before scouring, singeing burns off protruding fibres by passing the cloth quickly over gas flames, and desizing removes the starch size from the warp, usually with the enzyme amylase.
2★
Cotton and linen: boiled for hours with dilute caustic soda, soap or detergent and a wetting agent in a closed vessel (kier boiling); the cloth becomes absorbent and loses some weight.
3
Wool: washed in warm (not hot) water with mild soap and soda ash, with very little agitation so it does not felt; grease is recovered as lanolin.
4
Silk: degummed in hot soap solution to remove the sericin gum.
5
Synthetics: a light scour with detergent removes spinning oils and dirt.
6
Rinse well and neutralise so that no alkali is left in the cloth.

★ one-way, controls the pace

Bleaching agents

Bleach	Type	Used for	Caution
Hydrogen peroxide	Oxidising	Cotton, linen, wool, silk and blends; the most widely used	Needs controlled alkali and temperature
Sodium hypochlorite (chlorine bleach)	Oxidising	Cotton, linen, viscose; household bleach	Damages and dissolves wool and silk; yellows spandex; rinse well
Sodium chlorite	Oxidising	Synthetics such as nylon and polyester	Gives off corrosive chlorine dioxide
Sodium perborate	Oxidising (oxygen bleach)	Mild bleach in laundry detergents	Works best in hot water
Sodium hydrosulphite, sulphur dioxide	Reducing	Wool and silk	Whiteness may slowly turn yellow again in air
Optical brighteners	Not a true bleach	All fibres; added to detergents	Absorb ultraviolet light and give off blue light, so cloth only looks whiter

Scouring and bleaching

ScouringCleans: removes impurities and makes cloth absorbent and ready to dye. Uses alkalis, soap and detergents.

BleachingWhitens: removes natural colour by oxidation or reduction. Done after scouring; the bleach must suit the fibre.

Over-bleaching weakens cloth, so strength is checked after bleaching.

In daily life

Desizing uses amylase, the same starch-digesting enzyme found in saliva. At home, use chlorine bleach only on white cotton (hospital sheets, kitchen dusters), never on wool or silk, and rinse well; hydrogen peroxide or oxygen bleach is safer for coloured and delicate clothes.

Exam angle

2 marks: “What is scouring?” Scouring is the thorough washing of fibre, yarn or fabric to remove natural and added impurities, making it clean and absorbent; e.g. kier boiling of cotton.

5 marks: “Explain the bleaching agents used for textiles.” Oxidising (hydrogen peroxide, hypochlorite, chlorite, perborate) and reducing (hydrosulphite, sulphur dioxide) bleaches, with the fibres each suits; optical brighteners; precautions.

10 marks: “Explain scouring and bleaching of textiles.” Flow chart of wet processing; aim of scouring; methods for cotton, wool, silk and synthetics; bleaching aim and agents table; precautions.

Remember it

Clean first, then whiten: scour before you bleach. Peroxide suits every fibre; chlorine is for white cotton and linen only.