Wool Dyeing. Lecture II.
Tinting or Dyeing White.

Practical Magazine 23, 1876

Pidempi artikkeli jaettu erillisiin osiin.


ONE or two processes stand over from the last lecture for consideration now.

During the last few years a solution of sulphurous acid has been employed, to a considerable extent, for the bleaching of wool. Liquid bleaching presents many advantages over gas bleaching for loose wool, which is much more easily manipulated in a liquid than it can be in the sulphur chamber. On the other hand, sulphurous acid in solution is not as effective a bleacher as it is when in the gaseous state, and the solutions are trouble some to make, or more expensive to purchase than the sulphur, which is the only article required in the gas-bleaching process.

The solution of sulphurous acid used for bleaching purposes is one of the following: —
1. A solution of the gas in water.
2. A solution containing from 3 to 5 per cent. of bisulphite of soda, to which an equal volume of hydrochloric acid is added.
3. A solution, containing from 3 to 5 per cent. of the bisulphite, from which the sulphurous acid is set free in a subsequent operation.

The wool to be bleached should be well scoured with soap, washed, and steeped in one of the above solutions for some hours. If the first or second solution be employed, it will only require to be washed to free it from the acid; it may then be placed in a coach, and covered up with a sheet for some time, under which circumstances the bleaching action will be continued by the sulphurous acid remaining adhering to the wool.

If the third solution be employed, the wool, after draining, should be passed into water containing from 3 to 5 per cent. of hydrochloric acid, which will liberate the sulphurous acid from the bisulphite of soda with which the wool is soaked, and the sulphurous acid being liberated in contact with the fibre, and probably within the fibre itself, the colouring matter of the wool is acted upon more powerfully by this nascent condition of the bleaching agent than it is by free sulphurous acid. This method resembles the bleaching of cotton by bleaching powder in the liberation of the bleaching agent by an acid.

Solutions Nos. 1 and 2 rapidly lose strength by the escape of the sulphurous acid, or by its conversion into sulphuric acid by oxidation. Some loss is, therefore, experienced when the bleaching is only required to be done occasionally. The sulphurous acid may be preserved in them to a considerable extent by neutralizing it with carbonate of soda; when required again, it can be set free by the addition of hydrochloric acid. No. 3 solution holds its strength much longer than Nos. 1 and 2; and, although it requires a little more labour, the bleaching by it is more effective.

The following equation explains the action of the hydrochloric acid upon the bisulphite:
H Na SO3 (Bisulphite of soda.) + HCl (Hydrochloric acid.) = Na Cl (Common salt.) + H2SO3 (Sulphurous acid.)

The colour of the wool is often improved by tinting it with a little blue; this may be done in the acid bleaching bath, or, better still, in a bath specially made up for the purpose after the bleaching has been done. I have found a solution of indigo carmine to be best adapted for the tinting; but where the wool is very yellow, it is necessary to use a red colour in addition. The colour may be given in the cold.

Bisulphite of soda solution of 45° Tw. is sold at the present time at 9s. per cwt.

When required in large quantities for bleaching purposes, or for the preparation of reduced indigo by Schutzenberger and Lalande's method, it may be made by absorbing in a solution of carbonate of soda the sulphurous acid gas produced by burning sulphur. The absorption may be made in a coffee still, or in a tower made of sanitary pipes filled with coke; or, finally, the gas may be absorbed by aspirating it, or by forcing it through the alkaline solution. The gas should be passed till after all effervescence has ceased, and until it is no longer absorbed.

The solution of bisulphite may also be prepared, if required in smaller quantities, by heating crushed charcoal soaked in strong sulphuric acid in an iron vessel, and conducting the gases evolved into a solution of carbonate of soda till saturated. And for small experimental purposes it may be made by heating in a flask strong sulphuric acid, to which is added either copper turnings or crushed charcoal, and conducting the sulphurous acid gas produced into a bottle containing solution of carbonate of soda, and shaking it up from time to time until no more gas is absorbed.

The following equations explain the reactions involved in the above operations: —
2SO3 (Sulphuroud aid gas.) + OH2 (Water.) + Na2CO3 (Carbonate of soda.) = 2HNaSO3 (Bisulphite of soda.) + CO2 (Carbonic acid gas.)
2H2SO4 (Sulphuric acid.) + C (Charcoal.) = 2SO2 (Sulphurous acid gas.) + CO2 (Carbonic acid gas.) + 2OH2 (Water.)
2H2SO4 (Sulphuric acid.) + Cu (Copper) – CuSO4 (Cupric Sulphate.) + SO2 (Sulphurous acid gas.) + 2OH2 (Water.)

Tinting or Dyeing White.

There are some descriptions of wool and woollen waste, the yellow colour of which is but little affected by the action of sulphurous acid, but which may be very greatly improved in tone by tinting with the complementary colours, blue and red. The wool, waste, or cloth is well scoured in soap; the tinting bath is made by heating up water to about 120°, and adding the tinting colours. Good proportions and colours for 250 lbs. yellow material 1, find to be — three tablespoonsful of Brook, Simpson, and Spiller's Humboldt; two tablespoonsful of Brook, Simpson, and Spiller's No. 1 B blue. Work in the bath for forty minutes.

For some descriptions of goods other colours and proportions may be found to be better adapted; the above directions are merely intended to indicate the course to be adopted.

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