Very excellent results are sometimes obtained by substituting sodium hypochlorite for the calcium compound. This is done by decomposing a solution of bleaching powder with an equivalent quantity of either sodium sulphate or sodium carbonate. The precipitate of calcium sulphate or carbonate is allowed to settle and the clear liquor run off. Such a solution is more sluggish in its action than one of calcium hypochlorite, but it is more easily controlled, and is less liable to injure any material treated with it. The time necessary to produce a perfect bleach depends on the nature and condition of the pulp, on the degree of concentration of the liquor, and on the temperature at which the operation is conducted. Some paper-makers prefer to extend the bleaching over a considerable period of time, but this involves the employment of a larger system of “potchers,” which in many instances is an objection. If this method is adopted, especially in the case of straw or esparto, the action of the paddles or roll should be stopped, as otherwise the fibres are liable to felt together in the form of small grains, which sometimes escape the action of the beating roll, and occasionally find their way into the paper, causing an unevenness of surface. By giving longer time to the bleaching there is less risk of damage to the fibre, and consequently a greater yield of bleached pulp. The action of the bleaching solution should, as far as possible, be confined to the non-cellulose portions of the fibre. A certain action on the cellulose itself invariably takes place, but it should be reduced to a minimum. The action of bleach upon cellulose has been indicated in Chapter I., p. 10. For further information on this subject the reader is referred to the ‘Journal of the Society of Chemical Industry,’ 1884, April 29 and May 29. A very good method of bleaching consists in a preliminary treatment in the ordinary way in the potcher, followed by a prolonged steeping in tanks. {114} In the case of pulps which are very difficult to bleach, the action can be greatly assisted by washing out the products of the bleaching action, treating the pulp for a short time with a weak alkaline solution, again washing, and then repeating the bleaching process. In this way very refractory pulps can easily be bleached. Even an intermediate washing with water is useful. In most cases, where a fairly good colour has been obtained by the use of bleaching powder alone, the colour is greatly improved by a souring or treatment with weak hydrochloric acid, or, better still, with solution of sulphurous acid. Or the bleaching may be conducted in two or more operations with intermediate treatments with acid. All these modifications, however, are more or less troublesome, and should only be resorted to when the ordinary method fails.