Bleaching, the process of removing colors from fabrics and raw materials and leaving them white. The principal substances to which bleaching is applied are wool and silk, in the animal, and cotton, flax, and straw, in the vegetable kingdom. The coloring matter in these bodies is not essential to their texture, and fortunately can be removed by chemical agents without injury to the structure of the rest of the material. Steeping cloths in lyes extracted from the ashes of plants, and afterward repeatedly washing and exposing them to the action of sunlight, was practised by the ancient Egyptians; but nothing more than this is known of their process. There was scarcely any progress in the art for thousands of years, or until the 18th century, when some improvements were made in Holland. The Dutch process consisted in pouring the alkaline solution over the goods in a boiling condition, and steeping them in it for about a week, and, after washing, again steeping them for another week in buttermilk. After this they were thoroughly washed and exposed to the action of the air and sunlight for several months. These apparently simple processes obtained for the Dutch a high reputation for bleaching, and gave them almost a monopoly of the business for very many years.
For a long period the brown linens manufactured in Scotland were regularly sent to Holland to be bleached. A whole summer was required for the operation; and if the cloths were sent in the fall of the year, they were not returned for 12 months. It was this practice which caused the name of hollands to be given to these linens. The Scotch introduced the business of bleaching for themselves about the year 1749; but it was long believed that the peculiar properties of the water about the bleaching grounds of Haarlem gave to this neighborhood advantages which no other region could possess. - The precise chemical action that takes place in the process of bleaching is not known with certainty, but it is probably due to the action of oxygen when it is in a nascent state, or in that peculiar and active one called ozone. The investigations of Schonbein have proved that atmospheric oxygen, under the influence of sunlight and moisture, passes into an active state, thus explaining the rationale of the old bleaching process. Bleaching by chlorine involves the abstraction of hydrogen from the coloring matter, and the momentary freeing of a portion of oxygen, which enters into a new combination by which it is thought the bleaching is effected.
The action of sulphurous acid, which is usually a deoxidizing agent, does, however, according to Schonbein's investigations, on exposure to the air and light, bring a portion of atmospheric oxygen into an active condition. Chemists, therefore, attribute the action of all bleaching agents to the power they possess of causing oxygen to pass into its active state. The art of bleaching was conducted by alternate steeping in alkaline liquors called buckings, followed by thorough washing and boiling and long continued exposure upon grass, with frequent sprinklings of water, which process was called crofting; and this was followed by the souring process, or keeping the articles soaked for weeks in sour milk, to be afterward washed and crofted several times. By substituting dilute sulphuric acid for sour milk to dissolve out the alkaline matters, as suggested by Dr. Hope, the time required for this part of the process was reduced to a few hours in place of a few months. But the other operations still involved long time, particularly the crofting; and frequent losses moreover were incurred by the exposure of the goods in large establishments upon the great extent of grass lands they required.
Of cotton goods one twentieth to one tenth of the weight is lost by bleaching; but linens often lose as much as one third, by which their strength also is considerably impaired: the finer linens lose only from 12 to 25 per cent. In Silesia and Bohemia, where the chlorine process is not adopted, the linens are exposed to a fermenting process, then washed, and steeped in alkaline liquors, with alternate exposures upon grass, which processes are repeated a great number of times for 60 to 70 days; but to render them properly white, they are afterward passed through a bath acidulated with sulphuric acid, then treated again with the potash lye several times and alternately exposed on the grass, and finally thoroughly cleansed by washing in a revolving cylinder called a dash-wheel. This machine is also employed in the English and Scotch processes for washing the goods without subjecting them to unnecessary wear. The frequent repetition of the different processes is rendered necessary by the complete diffusion of the coloring matters through the flax fibres, and their close union with them; each operation decomposing and removing in succession small portions only. - The discovery of chlorine gas by Scheele in 1774 led to the great improvement in bleaching of applying this gas to the removal of the colors.
The use of it was originally suggested by the French chemist Berthollet in 1785, and explained the next year by him to Watt of Glasgow, who was then in Paris. By Watt the process was soon introduced into Britain, the gas being used in solution in water. Its preparation was found to be highly injurious to the health of the workmen, and the fibre of the cloth was weakened by the action of the chlorine. Berthollet improved the process by diluting the aqueous solution with water, and also by saturating with potash a portion of the acid. This was the first step toward the preparation of the chloride of lime, which was originally prepared after long continued experimenting by Tennant of Glasgow in 1798. Its first employment was in the form of a saturated liquid solution; but in 1799 he patented the use of the dry chloride of lime. (See Bleaching Powder.) Bleaching by chlorine, as now practised, varies somewhat as applied to the ditlerent fabrics; but a succession of different processes is still adopted, as in the old methods.
Thus, in bleaching cotton, there are the preparatory operations of singeing off the loose fibres by passing the cloth over heated cylinders; then soaking some hours in water, followed by the dash-wheel; then boiling in lime water, which acts upon the grease, and prepares it for easy removal by the next operation of boiling in water. This is followed by the souring process, which dissolves out the adhering lime, and a succeeding washing prepares the cloth for bleaching, This consists in steeping the cloth in a dilute solution of the chloride of lime, which is called the chemicking process. The liquor consists, for every pound of cloth, of about half a pound of chloride of lime and three gallons of water. Souring and washing succeed this, and these processes are repeated, it may be, several times; altogether they amount, including calendering, to about 25 in number. Though still very complicated, the time of the operation is greatly reduced from that of the old method. In two days is now accomplished what formerly required a whole summer, and the cost of the process amounts to only about 20 cents per piece of cotton cloth of 24 yards.
Bleaching linens with chlorine, though somewhat more expeditious than the process already referred to in Bohemia and Silesia, is still a tedious operation, and probably is susceptible of great improvements. It involves from 8 to 20 different processes of steeping, boiling, washing, souring, etc, with exposure upon the grass for from 30 to 60 days. Without this exposure a longer time is required for the bleaching action of the solution of chloride of lime. Bags are bleached for the paper-makers, after being thoroughly washed in the engine and reduced to what is called half-stuff, by soaking them from 6 to 12 hours in a solution of chloride of lime; from 2 to 4 lbs. of the dry chloride being used for every cwt. of rags. When the rags are strongly dyed, it is often necessary to add some sulphuric acid (half the weight of the bleaching powder), and cause the mixture, with the rags placed in it, to revolve for some time in a tight cylindrical vessel, till the chlorine evolved has removed the colors. This process is followed by thorough washing. - Wool requires a thorough preparation called scouring, to free it from the soapy and waxy matters exhaled from the skin of the sheep.
Weak ammoniacal lye is found efficient for this purpose, and this is obtained by boiling putrefied urine with four to eight times its quantity of soft water. The wool is steeped and well washed in a warm bath of this liquor, until all the impurities are converted into soapy matters and removed by rinsing in clean water. Caustic soda is sometimes used instead of ammoniacal liquors. Chlorine cannot be employed to bleach animal fibre, because the nitrogen they contain causes them to become yellow, and sulphurous acid is the agent which is generally used instead. Bleaching by sulphurous acid depends upon the production of colorless sulphites, the decomposition of which, however, by alkalies or by prolonged exposure, will allow the color to reappear unless they are removed. This is accomplished by thoroughly washing the goods after the application of the acid. Woollen materials are generally bleached by hanging them in a moistened state in close chambers and passing the vapor of burning sulphur over them; sometimes, however, a solution of the acid in water is used. After sulphuring they are washed and exposed to the air.
The process may be briefly described as follows: 1. They are immersed three times in a bath composed of 24 lbs. of carbonate of soda, 6 lbs. of soap, and 130 gallons of water, at a temperature of 105° F. The bath is removed after each immersion by the addition of three fourths of a pound of soap. The goods are immersed by passing them over a roller, and this bath answers for about 2,000 yards of material. 2. They are then washed twice in clean water at 105° F. 3. Passed three times through a soda solution of the strength of the first solution, adding half a pound of carbonate of soda after each passage. 4. Exposed for 12 hours to the vapor of burning sulphur, using of this about 24 lbs. to 2,000 yards. 5. Passed three times through a bath containing 30 lbs. of carbonate of soda to 130 gallons of water, at a temperature of 124°, adding three fourths of a pound of soda after each immersion. 6. The cloth is again subjected to the sulphur vapor, as in the previous operation. 7. A repetition of the fifth process. 8. Washed twice in water at a temperature of 105° F.
9. Subjected to sulphur vapors for 12 hours.
10. Washed in tepid, and then in cold water.
11. Tinged blue by passing through a bath containing indigo and carmine. - For the bleaching of silk sulphurous acid is also used, but previous to its application the raw silk must, as in the case of wool, be freed of matter which would interfere with the process. Silk contains, according to its quality, from 25 to 35 per cent, of extraneous matter, which was formerly considered to be a kind of gum, and is still called by that name. The investigations by M. Roard, however, have shown this substance to consist of albumen, wax, fat, resin, and coloring matter, and to have the properties of a varnish. After numerous experiments it has been found that nothing removes this varnish so well as a hot soap bath kept somewhat below the boiling point. From 30 to 40 lbs. of very fine soap are used for every 100 lbs. of silk; but the proportions vary according to the uses that are to be made of the articles. After steeping, the silks are well washed, put into linen bags, and boiled for an hour and a half in a weaker solution of soap. Different shades of white are given to the silk, without further bleaching, by the use of very weak dyes of litmus or indigo. A pure white is obtained by the sulphuring process. The Chinese are said not to use soap in cleaning their silks.
One Michel de Grub-bens, who lived in Canton a long time and practised the Chinese method, published in the memoirs of the academy of Stockholm an account of it, according to which they use a small white bean, and also wheat flour and common salt. It is probable that the fineness of Chinese silk is owing much to the superiority of the raw material. The process of bleaching silk proposed by Baume would be an important improvement if it were not too expensive. It consists in macerating the raw silk in 32 parts of alcohol and 1 part of muriatic acid for about 48 hours, when the silk is quite white. - Wheat straw is grown in Tuscany without reference to the grain. The seeds are sown broadcast, and the straw is cut when the grain is in the milk. It is thin and short, but of fine texture. On being cut, it is dried for a few days in the sun, then stacked in bundles, and dried in the mow for a month. After this, it is partially bleached by exposure upon the meadows to the dews and sun; and the process is completed by steaming and sulphuring. In England, a boiling solution of caustic soda is employed to dissolve the hard natural varnish upon the outside of the straw; after which the usual bleaching process, with sulphurous acid or chlorine, is applied.
This hard coating, it is said, may also be removed with economy by several steepings in dilute alkaline solutions, alternating with others of chloride of lime and the vapor of sulphurous acid. - Chlorine is the most common agent employed for bleaching a variety of other substances besides those already named; as, for example, wax, and articles of paper, as maps, prints, books, etc. But frequently, colors imparted to cloth by strong dyes require for their removal different chemical reagents, as chromic acid, or the combination of this with potassa. Protochloride of tin is also employed for the same purpose. These are called discharges, and are principally made use of in calico print works. - The whitening of candles, paraffine, sugar, etc, will be described in treating of those articles. Wax was formerly bleached merely by exposing it to sunlight and moisture; but since the discovery of chlorine that gas has been the agent generally used. The wax is scraped into very fine shreds and put into a tub of water having a tight cover; chlorine gas is then introduced at the bottom of the tub, while an agitator stirs the water. The bleaching is effected in about two hours, when the wax is melted into cakes.
A process has been introduced in France of bleaching wax, which is also applicable to oils, by melting it in hot steam, and subjecting it to its action in passing through a kind of worm. It is also washed with hot water alternately with the steaming. - Hydrate of alumina, prepared by decomposing alum by carbonate of soda, has recently been substituted for animal charcoal, for decoloring liquids. Experiments made by M. Ch. Meric, chemist of the metallurgical works at Creuzot, show that 15 grammes of alumina may replace 250 grammes of animal charcoal, in decoloring a quart of water colored by 10 grammes of litmus; or for sirup colored by molasses, 7 grammes of alumina were equivalent to 125 of animal charcoal. The alumina is, moreover, restored with less expense than the charcoal. - We pass to the consideration of the process for bleaching cotton, which has long been extensively known as the "American bleaching.'1 Before the year 1836 Dr. Samuel L. Dana, acting as consulting chemist to the Merrimack manufacturing company of Lowell, Mass., had completed an investigation on the adhering and coloring matters of the cotton fibres, which led him to devise and carry into practice the application of chemical agents in such order as to insure uniform results in bleaching.
The resino-waxy envelopes of the fibres, as well as the accidental starchy, alburuinous, and oily bodies present in the manufactured goods, are by this method resolved into soluble compounds and removed; and when in 1887 the process as practised became known to the scientific bleachers and printers of Miihl-hanscn, it drew forth their expressions of admiration for its completeness. This method is founded on the two following principles: 1. The conversion of the fatty and waxy matters into soaps; and for security and economy, it is preferable that these soaps should have alkali-no-earthy bases; caustic lime becomes, therefore, a most effectual agent. 2. The decomposition of the basic soaps formed, so as to convert them into soluble soaps, which is effected by the action of an alkaline carbonate. These are the cardinal principles on which this almost perfect process is founded, but there are practical points of interest. After the principles were published, M. Auguste Scheurer of Muhl-hausen suggested the passing of the goods from the lime into diluted acid.
This step, by no means essential, increases the certainty of an easy decomposition of the lime soap, as the acid seizing the base enters into combination witli it, leaving the fatty acid free to combine with the base of the alkaline carbonate, and form soluble soap. In describing the process as almost perfect, a point was in view which called for this qualifying phrase. Dr. Dana found that after the new process had been applied, and modified applications had been made, there still remained adhering to the fibre a substance which has many of the characters of wax. This substance he studied at great length, separating it from bleached cotton by means of boiling alcohol, which deposits it on cooling. Its few affinities do not allow of the application of any special agent for removing it wholly; while the solution of rosin in alkali, combining with it, dissolves a portion. This body, unlike wax in its relation to coloring matter, becomes tinted in ordinary madder printing at the points where it is desirable that white ground only should appear, and no modification of bleaching methods has yet met or overcome this difficulty.
The steps of the process are as follows: 1. Steep the cloth in water at a temperature of about 90° F. for 24 hours. 2. Pass through a bath of milky caustic lime, containing 60 lbs. for 2,500 lbs. of cloth. 3 Boil the cloth as it passes from the second operation six hours, counting from the moment ebullition actually occurs, under a pressure of 40 to 50 lbs. to the square inch. 4. Wash through the washing machine. 5. Pass through a hath of sulphuric acid, diluted till it marks 2° B. 6. Wash in machine. 7. Boil six hours, under a pressure of 40 to 50 lbs. to the square inch, in a solution of carbonate of soda, containing 100 lbs. for 2,500 lbs. of cloth, and in which 40 lbs. of common rosin have been previously dissolved. 8. Wash in machine. 9. Pass in washing machine through a clear solution of chloride of lime, marking 1° B. 10. Expose the cloth, as it is folded from the machine into pits with open sides, to the action of the air and carbonic acid, still saturated with the solution of chloride of lime. 11. Pass in washing machine through sulphuric acid and diluted to 2° B. 12 and 13. Wash twice in machine.
The boiling is done in Barlow's kiers, which are especially adapted to this process, which has come to be regarded both in this country and Europe as the simplest and best in use.