Instead of using the gas, the sulphur dioxide may be applied in the form of a solution in water. The goods are then simply steeped for some hours in a solution of the gas in water until they are bleached, then they are rinsed in water and dried. In this method it is important that the solution of the gas be freshly made, otherwise it is liable to contain but little sulphurous acid, and plenty of sulphuric acid which has no bleaching properties, but, on the other hand, is liable to lead to damage of the goods if it be not washed out afterwards.
A better method of utilising the bleaching action of sulphur in a liquid form is to prepare a bath of bisulphite of soda, and acidify it with hydrochloric acid, then to enter the wool, stirring well for some time, and allowing it to steep for some hours, next to expose to the air for a while, and rinse as before.
It is better to allow the wool to steep for about an hour in a simple bath of bisulphite, then enter into a weak hydrochloric acid bath for a few hours. The acid liberates sulphur dioxide in a nascent condition, which then exerts a more powerful bleaching action than if it were already free.
Even with liquid bleaching the bleach is not any more perfect than it is with the gas bleaching; the colour is liable to come back again on being washed with soap or alkali, although there is a freedom from the defect of yellow stains being produced.
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 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; at the same time care must be taken that the alkaline treatment is not too strong, or otherwise the bleaching effect of the sulphur will be neutralised as pointed out above.
Bleaching Wool by Peroxide of Hydrogen.--During recent years there has come into use for bleaching the animal fibres peroxide of hydrogen, or, as the French call it, oxygenated water. This body is a near relation to water, being composed of the same two elements, oxygen and hydrogen; in different proportions in water these elements are combined in the proportion of 1 part of hydrogen to 8 parts of oxygen, while in the peroxide the proportions are 1 of hydrogen to 16 of oxygen. These proportions are by weight, and are expressed by the chemical formulæ for water H2O, and for hydrogen peroxide H2O2. Water, as is well known, is a very stable body, and although it can be decomposed, yet it requires some considerable power to effect it. Now the extra quantity of oxygen which may be considered to have been introduced into water to convert it into peroxide has also introduced an element of instability, the extra quantity of oxygen being ever ready to combine with some other body for which it has a greater affinity than for the water. This property can be utilised in the bleaching industry with great advantage, true bleaching being essentially a process of oxidation. The colouring matter of the fibre, which has to be destroyed so that the fibre shall appear white, is best destroyed by oxidation, but the process must not be carried out too strongly, otherwise the oxidation will not be confined to the colouring matter, but will extend to the fibre itself and disintegrate it, with the result that the fibre will become tendered and be rendered useless.
Peroxide of hydrogen is a weak oxidiser, and therefore, although strong enough to destroy the colouring matter of the fibre is not strong enough to decompose the fibre itself. Hydrogen peroxide is sold as a water-white liquid, without any odour or taste. Its strength is measured by the quantity of oxygen which is evolved when one volume of the liquid is treated with potassium permanganate; the most common strength is 10 volume peroxide, but 30 and 40 volume peroxide is made. On keeping it loses its oxygen, so that it is always advisable to use a supply up as quickly as possible.
Articles of all kinds can be bleached by simply placing them in a weak solution of the peroxide, leaving them there for a short time, then taking out and exposing to the air for some time. The best plan of applying peroxide of hydrogen is the following: Prepare the bleaching bath by mixing 1 part of peroxide with 4 parts of water. The strength can be varied; for those goods that only require a very slight bleach the proportions may be 1 to 12, while for dark goods the proportions first given may be used. This bath must be used in either a wooden or earthenware vessel. Metals of all kinds must be avoided, as they lead to a decomposition of the peroxide, and therefore a loss of material. To the bath so prepared just enough ammonia should be added to make it alkaline, a condition that may be ascertained by using a red litmus paper, which must just turn blue. Into the bath so prepared the well-scoured goods are entered and worked well, so that they become thoroughly saturated. They are then lightly wrung and exposed to the air for some hours, but must not be allowed to get dry, because only so long as they are moist is the bleaching going on; if they get dry the goods should be re-entered into the bath and again exposed to the air.
If one treatment is not sufficient the process should be repeated. The peroxide bath is not exhausted, and only requires new material to be added to it in sufficient quantity to enable the goods to be readily and easily worked in the liquor. Any degree of whiteness may be obtained with a sufficient number of workings. No further treatment is necessary. It is found in practice that an alkaline bath gives the best results.