BLEACHING, the process of decolorizing textile fibres and fabrics, papers, oils, waxes and other substances so as to leave them white. The operations of bleaching are essentially chemical, the whitening being accomplished in two ways: (1) By the oxidation of the color ing matter in the substance to be bleached — in which case the coloring matter may be itself whitened or may only be rendered soluble, to be afterward washed away; and (2) by the hydrolizing of the coloring matter, so that it forms a white or colorless compound, remain ing harmlessly in the substance if it is in soluble. In the first instance nascent (anew born") oxygen is the agent employed; in the second, nascent hydrogen. Speaking in a gen eral way the oxidizing processes are used for vegetable substances, and the hydrolizing proc esses for substances of animal origin.
The chemical agents employed in commercial bleaching fall naturally into these two classes. Foremost of those which liberate active oxygen in the bleaching bath is chlorine, either as such or in combination as hypochlorous acid. Prac tically all the cotton bleaching in the United States is done with this agent. Up to a very recent period the form in which it was applied was as the so-called "bleaching powder," a chloride of lime. The commercial production of liquefied chlorine has led to the preferable use of sodium hypochlorite, formed in the bath by passing into a solution of soda ash or soda caustic the gaseous chlorine which vaporizes from the liquid form when the pressure is re leased. This gives a clear solution free from the very objectionable sediment common to the lime bath. Other oxidizing agents in large use are the peroxides of hydrogen and sodium. Both of these are very unstable, and require to be kept cold and in the dark until used. When warmed they give up their loosely combined oxygen. Sodium perborate is a substance of similar properties. Ozone, formed by passing a silent electric discharge through confined air, is an effective bleaching agent especially for yarns; it is discharged into the bath as it is made. The so-called electrolyticprocess con sists in decomposing a bath of sodium chloride in which the goods to be bleached are immersed, by the passage of an electric current. The chlorine made in this way costs about one-fifth as much as that obtained from bleaching powder. Potash permanganate is another oxidizing agent, used extensively for bleaching straw, jute and even ivory. These substances acquire a brownish color in the process, but this disappears in a bath of sodium bisulphite.
The agents used in bleaching by the action of free hydrogen in the bath are sulphur diox ide, the bisulphites of soda and potash, and the alkali hyposulphites.
Bleaching is now a very different procedure from what it was in the 18th century. At that period several months were required to bleach a piece of cloth. Manufactured linens were then sent by the shipload to Holland in the spring of the year, to be bleached on the grassy plains of that country, not to be returned until fall. The process there consisted in boiling repeatedly at intervals in caustics and lyes, and exposing the fabrics to the sunlight between the boilings. After the discovery of the bleach
ing properties of chlorine gas by Berthollet in 1790, the time required for bleaching was re duced from several months to a few days. To some extent, however, the old processes are still continued in certain localities.
In modern bleacheries the process is two fold: First, a preliminary cleansing of the ma terial to be bleached; second, the actual whiten ing.
In the case of fabrics the procedure in the bleaching of cotton cloth may be cited as a typical example. The steps are nine in number, as follows: (1) Singeing; (2) steeping; (3) boiling with lime; (4) souring; (5) boiling wit soap; (6) boiling with lye; (7) chemicking; (8) souring; (9) washing thoroughly.
• The singeing is to remove the long fibres which protrude from the surface of the cloth and make it woolly. It is done by rapidly pass ing the cloth very close to a gas flame. Steep ing is for the purpose of fermenting certain organic substances in the walls of the fibre, and thus rendering them soluble. It is carried on with warm water (140° F.) for from 18 to 24 hours, and in some bleacheries diastase is added to the bath to hasten the fermentation. The cloth is well washed, and then boiled for 12 hours in milk of lime. This is to break down certain refractory compounds which were not affected by the ferment. Again the cloth is washed, and then placed in the This is a bath containing hydrochloric acid, which dis solves any particles of lime remaining in the fibres. This occupies six to eight hours and then another washing ensues. There still re main, however, substances which the lime did not remove, and these are attacked with a "soap boil" in which the active ingredient is a soda ash and resin soap. The boiling in this bath is continued from six to eight hours, and is fol lowed by another washing. The next boiling is still a part of the cleansing process, and is done with a mixture of caustic soda and potash lye in solution. When the cloth has been well washed again it is ready for the bleaching proper, and goes into the "chemic." In most bleaching plants this chemic will be made of bleaching powder (hypochlorite of lime) with a little acid added. The chemical action which takes place is the setting free of hypochlorous acid in the bath, and this substance in the presence of organic matter (in this case the cotton fibre) parts with its loosely combined oxygen, which combines with the coloring matter remaining in the fibre, and either forms a soluble oxide which is washed out subsequently, or turns it into a white or colorless compound, which is not objectionable. The chemicking requires from six to eight hours. The fabric is then washed, and placed in a °sou& made this time with dilute sulphuric acid. It is left here until every trace of lime has been dissolved out, and then the cloth receives a final and prolonged washing, or rather, series of washings in which one contains a little softening soap, and another a weak bluing. It then goes to the drying house as finished.