The use of aniline sulphate can also be resorted to, and for microscopic work the most useful reagent is a mixture of zinc chloride and iodine. This produces an intense yellow colour with mechanical pulp and a bluish colour with sulphite and other chemical wood pulps.
The Daily Newspaper.
The newspapers of the present day are made almost exclusively of wood pulp. The use of the latter material for paper-making has steadily increased from the date of its introduction about A.D. 1870, when wood pulp was imported into England in considerable quantities.
News and cheap printings consist of mechanical and chemical wood pulps mixed in varying proportions determined chiefly by the price paid for the finished paper. In some cases the proportion of mechanical wood pulp is as much as 85 per cent., though the average composition of a cheap wood paper is represented by the following proportions: Mechanical pulp, 70 per cent.; sulphite pulp, 20 per cent.; loading, 10 per cent.
Some idea of the enormous quantity of material used for the daily press may be judged from one or two examples. A certain popular weekly newspaper having a circulation of one and a quarter million copies per week requires every week 137 tons of paper produced from 170 tons of wood. A popular halfpenny newspaper boasting a circulation of about one-half million copies per day consumes 185 tons of paper manufactured from 230 tons of wood, every week.
It is easy also from these facts to estimate the amount of timber which must be cut down to supply the demand for newspapers and cheap printings.
The manufacture of news calls for considerable skill and able management, owing to the keen competition amongst the paper mills devoted to this class of paper. The process as carried on in England is as follows:—
The mechanical pulp, reaching the mill in the form of thick sheets suitably packed up into bales, is first broken up again into moist pulp. Various machines are used for this, such as Wurster's kneading engine, Cornett's breaker, or some similar contrivance. An old potcher, such as is used for the breaking and washing of rags, makes a good pulp disintegrator. The broken pulp is discharged into beating engines in any suitable or convenient manner and the right proportion of chemical wood pulp added in the form of dry sheets. The beating process only occupies thirty to forty minutes in the case of the common news, a marked contrast to the eight or nine hours required by rags. China clay is added to the contents of the beater, ten to twelve per cent. being the general practice. This is followed by a measured quantity of rosin size, and after thorough incorporation the size is precipitated upon the fibres by means of alum.
In the commoner qualities of these papers the materials are added in the dry state, but for finer grades of newspaper the china clay is mixed with water, and carefully drained through a fine sieve before use. The alum cake is also dissolved and treated in a similar manner in order to keep out dirt and coarse particles likely to produce holes in the paper.