Bleaching is the art of whitening or decolorizing substances. It may be conveniently divided into the following heads: -
Leon Maret is the inventor of a curious process of bleaching blood-albumen by the electric light. By a long exposure to this, the colouring matter of the albumen is said to be destroyed gradually, until a product is obtained which is almost as white as egg-albumen. The usual process for obtaining blood-albumen is followed, and when the albumen is separated from the blood, it is exposed to the electric light, either while still in the liquid state or when dried. By means of lenses, or reflectors, the light is projected from the lamps on the albumen. If the latter is in the liquid state, the exposure to the light is done in the drying stoves, where the albumen is placed in shallow trays, the light being projected on the surface. These trays are made of glass, in order to allow the rays to penetrate into the interior; 24 hours' exposure is said to produce complete decolorization. (See also p. 8.)
The bleaching of cotton goods has become a very large industry, and the processes involved are too numerous and complicated to admit of lucid description here. The reader should refer to the complete article on the subject in Spon's 'Encylopeedia,' pp. 471-508.
Esparto pulp for paper-making is bleached in the "poacher" by means of a solution of bleaching powder. Some manufacturers hasten the process by adding a little hydrochloric or sulphuric acid; others steam-heat the mass to about 90° F. (32° C); others put in a small quantity of soda bicarbonate. The quantity of bleaching-powder necessary depends on the quality of the grass and the degree of boiling: 12 4b. per cwt. of esparto is a fair average. After being about 2 hours in the poacher, the almost white pulp is drained in large chests for 8 hours or longer, exposed to the action of light, and finally pressed to remove the excess of liquor.
Dissolve in 20 times its weight of boiling benzol, add 1/10 part plaster of very good quality, and agitate occasionally. By reposing for 2 days, the plaster is deposited and carries with it all the impurities not soluble in benzol. The clear decanted liquid is introduced by small portions into twice its volume of 90 per cent, alcohol, agitating continually. During this operation the guttapercha is precipitated as a pasty, perfectly white mass. The subsequent desiccation of the guttapercha requires several weeks' exposure to the air, but is accelerated by trituration in a mortar.
(1) Ivo:y that has become yellow by exposure can be whitened by washing in a solution composed of 1 oz. of nitric acid and 10 oz. of soft water; apply with a rough brush; cleanse thoroughly in clean water; (2) or by rubbing the ivory with fine pumice and water, and while damp exposing it to the sun under a glass vessel.
(3) Peroxide of hydrogen is used in Sheffield to bleach the inferior ivory for 2 knife handles. The mode of procedure is as follows: - Place, say, 2 qt. of the liquid in a stone pot, adding 4 oz. liq. ammon. fort (830°), immerse the handles, and put over a common shop stove for 24 to 36 hours; the handles are then taken out and gradually dried in the air, not too quickly, or they would split. The deep colour of the ivory is removed, and a beautiful pearly white ivory results when polished. The ivory is previously treated with a solution of common soda, to get rid of greasy matter, and open the pores.
The same remarks apply here* as to Cotton bleaching. (See Spons' 'Encyclopaedia,' "p. 515.)
The crude paraffin is filtered, and boiled for 2 hours with 5 per cent, of its weight of sodium sulphide and sufficient water. It is allowed to cool, so that the mass swimming on the top may become compact and be removed; it is then washed with river water, pressed, and afterwards dissolved in 20 per cent, amyl-alcohol, the paraffin being left as a pasty and pliable mass. It must remain for a time, and then be strongly pressed after filtering through bone-black. (De Molon.)
For this purpose, the hanks, while still damp and well straightened out, are hung in the sulphur chamber (which is of the same construction as that for woollen bleaching), and are there exposed to the fumes of burning sulphur for 5 to 6 hours, or even over night. Afterwards, the silk is well rinsed in a weak tepid solution of soda crystals, in order to wash out the sulphurous and sulphuric acids absorbed by the fibre. To ensure the thorough expulsion of the former, it is customary to hang the rinsed hanks, after wringing out the water, in a stove heated to 85° to 100° F. (29° to 38° C). With reference to the bleaching of silk by sulphurous acid, Moyret's opinion seems to be that probably it does not act directly in destroying the colouring matter of the fibre; but that along with the formation of sulphuric acid there is also a production of ozone.
This operation is necessary to hide the faint yellow hue which the silk still retains. Unlike the analogous operation in use with cotton-and wool, the question here is not always one of simple blueing; to suit the tastes of the merchants, the silk is actually dyed in various delicate shades, e. g. milk-white, snow-white (pare white), azure-white (blueish-white), Chinese white (orange, yellowish and purplish whites). To obtain pure white, a very weak neutral bath of ammoniacal cochineal and indigo carmine is used, care being taken that the dye should not too rapidly fix itself on the fibre; to prevent this, a little ground chalk is added to the bath. A cold or slightly tepid solution of aniline violet, with addition of a little soap, is also very much used for this shade. To obtain Chinese white, a weak soapy solution of annatto may be employed. After tinting, the silk is rinsed in fresh water and dried in a moderately warm stove, admitting as little light as possible. A passing reference must here be made to the wild or Tussah silk, which it is not possible to bleach in the ordinary way; the process of Tessie du Mothay, however, yields very fair results.