Finishing After washing it is very important to rinse the article in clean water till it is absolutely free of soap or detergent. Very often in the last rinse some other reagents are added which can help to restore the freshness or brightness of the fabric. Other reagents may add to the body of the fabric and improve its stiffness or crispness. (i) Blues and Optical Brighteners: You must have observed that with repeated use and washings white cotton articles tend to loose their whiteness and become yellow in colour.
In case of synthetic or manufactured fabrics and their blends the discolouration is more towards gray. The use of blues is recommended to counteract yellowness and to restore the whiteness. They cannot remedy the grayness. Blue is available in the market as ultramarine blue (in the form of a fine powdered pigment) and as liquid chemical dye.
Right amount of blueing should be used in the last rinse. The powder blue is pasted with a little amount of water and then added to more water. This solution is used immediately, as on standing this powder tends to settle at the bottom and would give patchy results. Liquid blues are easier to use and give more even results.
Care should be taken to see that blue is applied to the fabric in a thoroughly wet (but not dripping) condition, which is free of folds of wringing. Move the article in the blue solution for a short duration, remove the excess moisture and place it for drying. Optical Brightening Agents or Fluorescent brightening agents are compounds with low grade or weak dyes that possess the property of fluorescence. These compounds can absorb light at a shorter wavelength and re-emit them at a longer wavelength.
Treating a fabric with an optical fluorescent brightener can give it an intense bright whiteness, which can counteract both yellowness and grayness. They can also be used on coloured printed fabrics. Optical brighteners are sometimes referred to as whiteners. However, they cannot destroy a colour and therefore should not be confused with bleaching.
(ii) Starches and stiffening