Bleaching

(graphic arts) An afterprocess in the production of direct positive photographs, in which an oxidizing solution dissolves the negative silver.
(optics) A decrease in the optical absorption of a medium, produced by radiation or by external forces.
(textiles) A process in which natural coloring matter is removed from a fiber, yarn, or fabric to make it white.

The process in which natural coloring matter is removed from a fiber to make it white. The process may be used on fiber, yarn, or fabric. Prior to the bleaching of a fabric, preliminary purification processes should be used. These processes remove applied encrustants (desizing) and natural encrustants (scouring or boil-off) so that the bleaching agent may act uniformly on the material free of impediment.

Bleaching is also classified as a purification process and varies with the content of the substrate or fibrous content of the material. It should not be confused with the stripping process, which is the removal of applied color.

The fabric off the loom is called gray, grey, or greige goods to distinguish it from the partially or completely finished fabric.

The three most prominent commercial bleaching processes are the peroxide, the chlorine, and the chlorite, in that order.

In home bleaching the predominant bleach is the chlorine bleach, followed by the use of peroxygen compounds such as persulfates and perborates. The latter two are recommended for minimum care and permanent-press fabrics to preclude the yellowing of the whites and the stripping of colored fabrics, which are always potential problems when chlorine bleaches are used.

Optical bleaching uses organic compounds which are capable of absorbing waves shorter than visual waves and emitting waves within the visible range. The most notable of the short waves absorbed is the ultraviolet light present in daylight. These compounds absorb ultraviolet light which is less than 400 micrometers (0.02 in.) and emit blue light which is in the visible range of 400–700 micrometers (0.02–0.03 in.). This emitted blue light will counteract the yellow on the fabric surface and by the subtractive theory of color will produce a white. These agents are also characterized as brighteners and are extensively used in household detergents.

There are several bleaching systems which are classified as combination bleaches because two different bleaching agents are used together or in tandem. These have been projected to lower the cost, lower the degradation, or shorten the cycle as well as to preclude equipment damage in certain cases. See also Natural fiber; Oxidizing agent.