Bleaching

“ Bleaching.—The clean disintegrated rag is next bleached by means of ordinary bleaching powder solution. Bleaching powder is a substance prepared by the action of chlorine gas on dry slaked lime, resulting in the formation of a compound which has the property of bleaching or “whitening” vegetable matters. The clear solution obtained by treating the powder with water is utilised by the paper-maker for bleaching the rag pulp. ”

“ The term bleaching is moreover applied to the decolorizing of castor-oil, bees-wax and other fatty materials by exposure to sunlight. Scouring and bleaching are now largely effected by the use of chemicals and volatile liquids, soda ash, resin soaps and more or less caustic alkalis being utilized in place of baths of lime, lye and sulphuric acid. For the bleaching of silk, after scouring, sulphur dioxide is used, or, better still, hydrogen peroxide. As a bleaching agent, chloride of lime or bleaching powder is resorted to for the removal of metallic and other colors in calico-printing. ”

“ Even more convenient are soap chips which come by the barrel, but may be bought at pound rates.
Bleaching and bluing agents.—The sun, as it bleaches white fabrics, may be counted in this group. Chemical bleaches are used to whiten clothes, but should not be resorted to unless clothes are yellow from poor washing, as in the end they weaken the fabric. Commercial laundries sometimes use an excess of acid for this purpose. Cream of tartar is a harmless bleach. Javelle water is another household bleach, chloride of lime being the bleaching substance. ”

“ The reason home bleaching is so seldom satisfactory is that most amateurs fail to realize the necessity of immediate neutralization and rinsing, and allow the fabric to remain too long in the bleaching solution, and allow it to dry with traces of the bleaching substances present in the fibers. ”

“ Excessive blueing, which is frequently resorted to in order to cover the defects of imperfect bleaching, can readily be detected by washing a sample of the material in water, or, better still, in water containing a little ammonia, and then comparing with the original. The formation of oxycellulose during the bleaching process may either take place in boiling under pressure with lime or caustic soda in consequence of the presence of air in the kier, or through excessive action of bleaching powder, which may either result from the latter not being properly dissolved or being used too strong. ”

“ Bleaching is commonly said to be natural when exposure to light, air, and moisture forms the leading part of the process; and to be chemical when chlorine, chloride of lime, sulphurous acid, or other like substances are employed. ”

“ In the case of pulps which are very difficult to bleach, the action can be greatly assisted by washing out the products of the bleaching action, treating the pulp for a short time with a weak alkaline solution, again washing, and then repeating the bleaching process. In this way very refractory pulps can easily be bleached. Even an intermediate washing with water is useful. ”

“ In modern bleach-houses the chain form is gradually giving place to the method of bleaching separate hanks, partly because so many improvements have been made in the hank-bleaching machinery of late years, which enables bleachers to handle the yarn in the form of separate hanks better than they could do formerly; and as bleaching in separate hanks means that the cotton is kept in a more open form, and is thus more easily penetrated by the various liquors which are used, it follows that the bleach will be better and more thorough, which is what the bleacher aims at. ”

“ By giving longer time to the bleaching there is less risk of damage to the fibre, and consequently a greater yield of bleached pulp.
The action of the bleaching solution should, as far as possible, be confined to the non-cellulose portions of the fibre. A certain action on the cellulose itself invariably takes place, but it should be reduced to a minimum. The action of bleach upon cellulose has been indicated in Chapter I., p. 10. For further information on this subject the reader is referred to the ‘Journal of the Society of Chemical Industry,’ 1884, April 29 and May 29. ”

“ Sour Bleaching.—When the bleaching liquor, after acting upon the half-stuff for some time, has become partially exhausted, dilute sulphuric acid—about one part acid to fifteen parts of water—is added, which, by liberating hypochlorous acid, hastens the bleaching considerably, and when the chemical action resulting from this treatment is nearly complete, the spent liquor is allowed to drain away, and fresh bleaching liquor is introduced, the strength being regulated by the progress made in the first case, which will depend upon the character of the fibre treated. ”

“ It is obtainable pure, in large quantities, and cheaply, in solutions of three per cent by weight or ten per cent by volume; and it has come into extensive use as a bleaching agent, for disinfection, household purposes, and the toilet. It is the really operative agent in air-bleaching on the grass, which has been in use from time immemorial, and is well adapted for bleaching substances of animal origin, in which chlorine agents often fail. In using it the substance to be bleached must first be carefully cleansed from dirt and oil. ”

“ As a rule the lower the temperature at which bleaching is done, the more fullers earth is required to get the proper bleach and at the same time as little earth flavor as possible is a point for careful and intelligent consideration. The best way to determine, where there are large quantities of lard, is to take a small sample and make tests in the laboratory. The point to be obtained is to get the bleach with as little of the flavor of the earth as possible. Generally speaking, from three-fourths to one and one-half per cent of fullers earth will get the desired result. ”

“ An extreme in the opposite direction, however, must be avoided, since a very strong bleach will inevitably cause injury to the fibre. Sometimes the potchers are fitted with steam-pipes, in order that the diluted bleaching liquor may be heated, if required, to facilitate the operation. If the temperature be raised too high, however, the effect upon the fibre will be at least as injurious as if too strong a bleach were [91] employed. ”

“ The best bleaching powder contains about 37 per cent. of what is termed “available chlorine.” The substance, on being treated with water, gives a greenish-coloured solution known as bleach liquor, and when raw paper-making material, after having been digested with caustic soda, is treated with this solution, it is gradually bleached to a white colour. The composition of the powder may be represented approximately as follows:—
Since the amount of bleach used for wood pulps varies from 8 per cent. ”

“ In yarn-bleaching the sequence of the operations is about as follows:— (1) Boil in kier with soda ash. (2) Reel in bleaching powder. This operation, which is peculiar to linen bleaching, consists in suspending the hanks from a square roller into bleaching powder solution contained in a shallow stone trough. The roller revolves slowly, so that the hanks, while passing continuously through the bleaching powder, are for the greater part of the time being exposed to the air. (3) Sour in sulphuric acid. ”

“ Suitable agitators may be employed for thoroughly mixing the materials in the chest and in the apparatus shown the bleaching agent and material to be bleached pass through each chest in the same direction—namely from the bottom to the top—although they are carried from one chest to the next in the reverse order, the material to be bleached being primarily introduced into the chest at one end of the series, while the bleaching agent or solution is introduced primarily into the chest at the other end of the series. ”

“ It is well understood by chemists that bleaching agents are usually good disinfectants; that which can so disturb an organic compound as to destroy its color, is capable of either arresting or completing the decompositions that produce vile odors and nourish the organic germs or ferments which usual accompany, or, as some affirm, cause them. Sulphurous acid is, next to hypochlorous acid, one of the most effective bleaching agents within easy reach. ”

“ Scheele discovered the leading characteristics of chlorine, and especially its power of bleaching, which is alone sufficient to place this gas in a high commercial position, when it is considered that all our linen used formerly to be sent to Holland, where they had acquired great dexterity in the ancient mode of bleaching—viz., by exposure of the fabric to atmospheric air or the action of the damps or dews, assisted greatly by the agency of light. ”

“ The vessel is then filled with a weak solution of bleaching liquor, consisting of about one ounce of dry bleaching powder to each gallon of water. "After the vessel has been filled, the liquor is immediately run out, and is replaced by an atmosphere of carbonic acid, which quickly liberates the chlorine on the fibre, and thus decomposes the water, uniting with the hydrogen and [98] liberating the oxygen, the result of which, is to bleach the fibre or fabric. ”

“ The housekeeper should be allowed plenty of help to properly do the work.
Bleaching Linens.
When clothes have become yellow by the use of impure water or any other cause, the snowy whiteness must be restored by a bleaching process. Chloride of lime and oxalic acid are powerful agents, and, if not quickly removed from the fabric, they will corrode and do much injury to the linen. Turpentine has some power as a bleacher as also has borax. Blueing will aid in keeping the clothes white, but do not use too much. There are a variety of blueings to be had. The indigo blue is the best. ”

“ Substances are bleached by alternately dipping them in dilute solutions of bleaching powder and of dilute sulphuric acid. Bleaching powder is also used to purify an offensive or infectious atmosphere. ”

“ Goods properly bleached will stand exposure to air for some considerable time, but those imperfectly bleached exhibit a tendency to regain their yellow colour on exposure to air. One fault which is sometimes met with in sulphur bleaching (p. 034) is a want of softness in the wool, the process seeming to render the fibre harsh.
Washing in a little weak soft soap or in weak soda will remedy this and restore the suppleness of the wool ”

“ In bleaching for whites (“market bleaching”) it is essential that the white should be as perfect as possible, and such goods are consequently invariably blued after bleaching.
For small lots (1-20 pieces) the bleaching can be conducted on very simple machinery. Thus many small piece dyers conduct the whole of their bleaching on the jigger, a simple form of dyeing machine on which most cotton piece goods are dyed (see Dyeing) . ”

“ By solubilizing quebracho with excess of bisulphite an extract is obtained which possesses considerable bleaching powers, and such extracts are also extensively manufactured for the "vatting" or bleaching of heavy leather after tannage. The excess of sulphurous acid not only bleaches the leather, but also swells it up and thus permits a further absorption of strong tan liquor, which is conducive to good weight. These bleaching extracts are usually of 36-38 per cent. strength in tan. ”

“ Manufacturers allow about 20 to 25% of loss in weight of yarn in bleaching from the green to the fully bleached stage; and the intermediate stages of boiled, improved, duck, cream, half bleach and three-quarters bleach, all indicating a certain degree of bleaching, have corresponding degrees of loss in weight. The differences in colour resulting from different degrees of bleaching are taken advantage of for producing patterns in certain classes of linen fabrics. ”

“ Blaz′onry, the art of drawing or of deciphering coats-of-arms: heraldry. [Fr. blason, a coat-of-arms, from root of Blaze.]
Bleach, blēch, v.t. to make pale or white: to whiten, as textile fabrics.—v.i. to grow white.—ns. Bleach′er, one who bleaches, or that which bleaches; Bleach′ery, a place for bleaching; Bleach′-field, a place for bleaching cloth: a bleacher's office or works; Bleach′ing, the process of whitening or decolourising cloth; Bleach′ing-green, a green for bleaching clothes on; Bleach′ing-pow′der, chloride of lime. ”

“ If the production of textile fabrics received such advantages from mechanics, equally was it favoured by chemistry in the discovery of bleaching by chlorine. To bleach a piece of cotton by the action of the air and the sun required from six to eight months, and a large surface of land must be used as a bleach-field. ”

“ The decoloration of textile fabrics and solid bodies generally is called bleaching. See Blanching, Bleaching, Oils, Tallow, Syrups, Sugar, &c.
DECOMPOSI′′TION (-zĭsh′un) . In chemistry, the resolution of compounds into their elements, or the alteration of their chemical constitution in such a manner that new products are formed.
DEFECA′TION. The separation of a liquid from its lees, dregs, or impurities by subsidence and decantation. It is commonly employed for the purification of saline solutions549 and glutinous or unctuous liquids on the large scale in preference to filtration ”

“ The common process of bleaching pulicates, into which permanent colours are woven, is, to wash the dressing or starch well out in cold water; to boil them gently in soap, and, after again washing, to immerse them in a moderately strong solution of the oxymuriate of potash; and this process is followed until the white is good: they are then soured in dilute sulphuric acid. If the goods are attended to in a proper manner, the colours, in place of being impaired, will be found greatly improved, and to have acquired a delicacy of tint which no other process can impart to them. ”