#Sulphur Bleaching.#--Bleaching wool by sulphur is a comparatively (p. 030) simple process. A sulphur house is built, the usual size being 12 feet high by 12 feet broad, and about 17 feet long. Brick is the most suitable material. The house should have well-fitting windows on two sides, and good tight doors at the ends (see fig. 9). Some houses have a (p. 031) small furnace at each corner for burning the sulphur, two of these furnaces are fitted with hoods, so that the sulphur gases can be conveyed to the upper part of the chamber, but a better plan, and one mostly adopted where the chamber is used for bleaching pieces, is to construct a false perforated bottom above the real bottom of the chamber, the sulphur being burnt in the space between the two floors. If yarn is being bleached the hanks are hung on wooden rods or poles in (p. 032) the chamber, while with pieces an arrangement is constructed so that the pieces which are stitched together are passed in a continuous manner through the chamber. When all is ready the chamber doors are closed, and the furnaces are heated, some sulphur thrown upon them, which burning evolves sulphur dioxide gas, sulphurous acid, and this acting upon the wool bleaches it. The great thing is to cause a thorough circulation of the gas through every part of the chamber, so that the yarn or pieces are entirely exposed in every part to the bleaching action of the gas. This is effected by causing the gas to pass into the chamber at several points, and, seeing that it passes upwards, to the ventilator in the roof of the chamber. Generally speaking, a certain quantity of sulphur depending upon the quantity of goods being treated is placed in the chamber and allowed to burn itself out; the quantity used being about 6 to 8 per cent. of the weight of the goods. After the sulphuring the goods are simply rinsed in water and dried. Sulphur bleaching is not an effective process, the colouring matter is not actually destroyed, having only entered into a chemical combination with the sulphur dioxide to form a colourless compound, and it only requires that the wool be treated with some material which will destroy this combination to bring the colour back again in all its original strength; washing in weak alkalies or in soap and water will do this. Another defect of the process lies in sulphur being volatilised in the free form, and settling upon the wool causes it to turn yellow, and this yellow colour cannot be got rid of. The goods must be thoroughly rinsed with water after the bleaching, the object being to rid the wool of traces of sulphuric acid, which it often contains, and which if left in would in time cause the disintegration of the wool. Sometimes the wool is washed in a little weak ammonia or soda (p. 033) liquor, but this is not advisable, as there is too much tendency for the colour of the wool to come back again, owing to the neutralising of the sulphur dioxide by the alkali.