Another plan of preparing the bleaching bath is to prepare a bath with peroxide and water as before, then add to a sufficient quantity of a solution of silicate of soda 4 parts of water to 1 of silicate of soda at 100° Tw., to make the bath alkaline. Into this bath the goods are entered and are then exposed to the air as before, after which they may be passed through a weak bath of sulphurous acid, being next well washed in water and dried.
The advantage of bleaching with peroxide is that, as it leaves only water in the goods as the result of action, there is no danger of their becoming tendered by an after development of acid due to defective washing, as is the case with the sulphur bleach. The goods never alter in colour afterwards, because there is nothing left in that will change colour. Some bleachers add a little magnesia to the bath, but this is not at all necessary.
Bleaching with Peroxide of Soda.--Peroxide of soda has come to the front of late for bleaching wool. With it a stronger bleaching bath can be made, while the product itself is more stable than peroxide of hydrogen, only it is needful to keep it in tightly closed metal vessels, free from any possibility of coming in contact with water or organic matter of any kind, or accidents may happen. In order to bleach 100 lb. of wool, a bath of water is prepared, and to this is added 6 lb. of sulphuric acid and then slowly 4 lb. of peroxide of sodium in small quantities at a time. Make the bath slightly alkaline by adding ammonia. Heat the bath to 150° F., enter the wool and allow to remain five to six hours, then rinse well and dry. If the colour does not come out sufficiently white repeat the process.
The Chlorination of Wool.
The employment of chlorine in wool dyeing and wool printing has of late years received an impetus in directions previously little thought of. The addition of a little chlorine to the decoction of logwood has been recommended as increasing the dyeing power of the wool. Treating the wool with chlorine has a material influence in increasing its capacity for taking dye-stuffs, and although but little attention has been paid to this circumstance by wool dyers, yet among wool printers it has come largely into use, and enables them to produce fuller and faster shades than would otherwise be possible.
The method involves the treatment of the wool first with an acid, then with a solution of a hypochlorite. The staple becomes soft and supple and assumes a silky character; in dyeing it shows a greater affinity for the dyes than it did previously. Although not deteriorated in strength, it almost entirely loses its felting properties. On account of this feature the process cannot be adopted for wool which has to be fulled, but it is of service where felting of the goods is to be avoided, for worsteds, underwear, woollen and half woollen hosiery, etc., in which the felting property that occurs on washing is rather objectionable.
By the chloring of the wool the intensity of the shade dyed is increased to such a degree that when dyeing with Acid black, Naphthol black, Naphthol green, Nigrosine, Fast blue, Water blue, and some others dyed in an acid bath, but little more than half the dye used on unchlored wool is required, while with Induline, more even and intense shades are obtained than is otherwise possible.
The operation of chlorination can be done either in one or two baths. The choice depends upon circumstances and the judgment of the dyer. The process by the two-bath method, with subsequent dyeing in the second or separate bath is (for 100 lb. of wool), as follows. The first bath contains, for light cloths, yarn, etc., from 3 to 4 lb. sulphuric acid, 168° Tw., and for heavier cloths and felt, where the penetration and equalisation of the colour is difficult, from 8 lb. to 10 lb. of acid. Generally speaking, a temperature of 170° to 175° F. is sufficient, although for heavy wool and for wool with poor dyeing qualities it is well to use the bath at the boil. The treatment lasts for half an hour, in which time the acid is almost completely absorbed.
The second bath contains a clear solution of 10 lb. bleaching powder, which solution is prepared as follows. Dry bleaching powder of the best quality is stirred in a wooden vat with 70 gallons of water, the mass is allowed to stand, the clear, supernatant liquor is run into the vat and the sediment stirred up and again allowed to settle, the clear liquor being run off as before, and 5 gallons more water is run in. The clear liquors of these three treatments are then mixed together to form the chloring bath. Special care should be taken that no undissolved particles of the bleaching powder should be left in, for if these settle on the wool they result in too great a development of chlorine, which injures the wool.
The goods after being in the acid bath are entered in this chlorine bath at a temperature of 70° F., which is then raised to the boil. If the acid bath has been strong, or been used at the boil, it is perhaps best to rinse the goods before entering into the chlorine bath. The hypochlorous acid disappears so completely from this bath that it may at once be used as the dye-bath, for which purpose it is only necessary to lift the goods, add the required amount of dye-stuff, re-enter the goods and work until the bath is exhausted, which generally happens when acid dyes are used. If a separate dye-bath be preferred, this is made and used as is ordinarily done.