(1) The question of bleaching jute without injury has been studied for a long time. All bleachers have boasted of being able to bleach it as well as, or even better, than linen and hemp, but all have found that the bleaching was more apparent than real, and that the goods,' after lying some months in the warehouse, turned from white to yellow, and no longer offered any resistance. Several manufactuers, for want of anything better, have been content with giving the jute a cream shade, and for this purpose they make use of rollers over which the hanks are hung, so that their lower ends dip into a weak chloride bath, very slightly warmed, for 30, 40, or 50 minutes. The hanks are then passed into water, pressed and dried in the air.
The treatment of jute by complete submersion, air being excluded, is the safest. Treatment on the roller with the intervention of air is the most dangerous. The reason of this is that in the former case the bleaching is a slow oxidation, whilst in the latter it is the result of the rapid and energetic action of hypochlorous acid. It has been tried, but without much success, to use silicate of soda and chloride of soda; chloride of lime is preferable. But to get good white it is necessary to steep the jute alternately in a soap-bath, and in a solution of chloride of lime. The following are directions for a so-called cream shade: - Immerse in a weak and lukewarm soap-bath for about 10 minutes; after draining, immerse for 40 minutes at most, in a bath of chloride of lime, not marking more than 1/2° on the chlorimeter.
The duration of the immersion may be variable, the quality of the jute and the shade which it is wished to obtain being the best guides. As for whites more decided than cream shades, they are produced in the same manner, but the duration of the steepings in the soap-lye and the chloride of lime is shortened, and these operations are repeated several times in succession. Whatever may be the shade at which the process is brought to an end, it is well to finish with two washings, the one in lukewarm and the other in cold water. The jute is then drained and dried at as low a temperature as possible. It is recommended that during the steeping process the jute should be regularly but gently agitated, taking care not to bring it above the surface of the liquid.
(2) According to Scheurer, chloride of lime was at one time considered suitable for jute bleaching, but it was soon found that this reagent made the yarn hard and brittle, likewise removing, along with a portion of its solidity, that silky brightness which constitutes one of its principal merits. Hypochlorite of soda, on the contrary, by reason of the more rapid and uniform oxidation which takes place, can be employed at a high degree of concentration without the resistance of the fibre being impaired. Its action has, however, to be regulated with care, on account of its powerful properties as a reagent. Bleached jute would suffer by being plunged into a concentrated solution of hypochlorite of soda, while such is not the case with jute in the unbleached state, in which the cellulose is protected in the earlier stages of the operation by the incrust-ing substance. It is the latter portion of the process (specially affecting the purification of the white) which is usually found to affect the solidity of the textile substance under treatment, even when the solution has not been a strung one.
Therefore, Scheurer considers that, to save the fibre from the corrosive action of the hypochlorite, it is necessary to diminish the force of the reagent, as the operations succeed each other, and to preserve a certain relation between the degree of concentration of the oxidizing liquid, and the quantity of the encrusting substance which remains to be destroyed. In this way a satisfactory white is obtained, without prejudice to the textile substance operated upon.
(3) According to a patent taken out by T. G. Young, the jute is first soaked in a solution of a sulphide of an alkali, or alkaline earth, till sufficiently softened. It is then washed and submitted to a bleaching agent composed of a solution of chlorine and an alkali, other than chloride of lime, such as chlorine and soda, until the desired bleaching results are obtained.