Straw and esparto pulps are sometimes bleached in large potchers made of brick covered with cement. The circulation of the pulp is produced by the action of paddles made of wood or, preferably, of iron. Many “potchers” are provided with steam pipes, whereby the mass of pulp and liquor is heated. This should be done with great care, so as to avoid superheating in any part, as this is certain to cause destruction of the fibre. The better plan, where it can be adopted, is to uniformly heat the pulp before running in the bleach. In certain cases the action of the calcium hypochlorite is {112} assisted and accelerated by the addition of either sulphuric or hydrochloric acids; these combine with the lime and liberate hypochlorous acid, which has a more rapid action than its calcium salt. If the acid be added in too large quantity, or of too great a strength, it sometimes happens that instead of hypochlorous acid, chlorine gas is given off, part of which escapes into the air, thus causing loss. Moreover, the chlorine is liable to enter into combination with the fibre substance forming the yellow chlorination products described on p. 18. This liability is greater in the case of highly lignified fibres, such as wood or jute: in these cases, therefore, the use of acids should be avoided. When used the acid should be largely diluted with water and added gradually. The best plan is to allow the action of the bleach to continue for some time, only adding the acid when it is nearly exhausted. In this way risk of damage to the pulp is avoided. The diluted acid should be conveyed by a leaden pipe passing down to the floor of the potcher, and perforated at its lower extremity. Other means of accelerating the action of the bleaching powder have been suggested, such, for example, as the use of bicarbonate of soda, which by double decomposition forms with the bleaching powder carbonate of lime and free hypochlorous acid. It is, however, much too expensive an agent. The amount of bleaching powder necessary to produce a good white colour differs materially with the fibre to be bleached, and of course with the degree of resolution of the fibre substance in the preliminary treatment with caustic soda. The following numbers may be taken to be approximations of the amount necessary to bleach well-boiled pulps:— Per cent. Powder calculated on Unboiled Fibre. Esparto 10 to 15 Straw 7 to 10 Wood 15 to 20 {113} A well-boiled pulp should contain from 90 to 95 per cent. of cellulose.