Conversion of wood to paper consists of two fundamental steps — reducing the wood to pulp, and then forming a sheet of paper from the pulp. In principle neither of these tasks is difficult, but the production of large quantities of good quality paper at low cost can only be accomplished with the aid of complicated engineering and control systems. To understand the role of science in papermaking a brief introduction to the processes involved is necessary. Two methods are used at Tasman to produce pulp from wood, one is mechanical, giving rise to so-called groundwood pulp, the other is chemical, yielding kraft pulp.

Kraft pulping consists of releasing the cellulose fibres by dissolving the matrix of lignin which binds them together. The essential processes of a Kraft mill are (a) digestion of wood chips with alkaline chemicals, (b) separation of the fibres from the extractive chemicals by water washing, (c) bleaching and/or drying the pulp, (d) recovery of the extractive chemicals.

Scientific methods are invaluable for gearing a mill to run at optimum conditions of quality and economy. For example, different types of pulp required for different end-uses, can be produced by adjusting digesting conditions. Temperature, pressure, reaction time, types of chemical, chemical concentrations, and finally wood species, can all be varied to give pulps of varying degrees of delignification, with different strength properties, in different yields. The effect of altering one variable may be well known but to achieve optimum conditions complicated adjustment of the parameters is necessary. Thus a knowledge of wood chemistry and of the basic reactions of the cooking chemicals with lignin and cellulose is required. The same type of knowledge is useful in the bleaching process. The choice of oxidising agent and method used to bleach kraft pulp depends on the brightness or whiteness of the end product.

Chlorine, calcium hypochlorite, chlorine dioxide and sodium peroxide are commonly used in various combinations. For example, a medium brightness pulp, such as that used in newsprint, could be bleached by a chlorination stage followed by caustic extraction, then one or two hypochlorite bleaching stages.

Chemical recovery is an integral part of any kraft pulp mill. The spent cooking liquor which contains dissolved organic matter along with the inorganic chemicals, is concentrated, and burnt in a "recovery" boiler, the heat so produced being used to generate steam for general mill use.

The molten inorganic chemicals are drained from the furnace, dissolved in water, and reconstituted by reaction with slaked lime, producing re-activated cooking liquor. Apart from the chemical reactions involved in the burning of the spent liquor at high temperatures, the process is well understood but careful control over the various steps is necessary to produce a uniform cooking liquor.

Groundwood pulp is produced by pressing barked wood billets, four feet long, against a revolving grindstone. The fibres are washed from the grinding zone by large quantities of water which also serves to dissipate the heat produced by the grinding process. The pulp is subjected to screening and refining operations before it is ready for use on the paper machines.

The pulp that is used by the paper machines to make newsprint consists of a mixture of one part of kraft, which is long fibred and thus imparts strength to the sheet, and four parts of groundwood which, apart from being cheaper, contains shorter fibres and "fines." necessary to produce a sheet of high opacity and smooth surface.

The sheet is formed by projecting a continuous jet of dilute pulp onto an endless wire mesh, moving at a high speed.

The wire is supported by revolving rolls, and water drains from the pulp, aided by the action of these rolls and suction boxes placed under the wire surface. The sheet is then squeezed between woollen felts by the press rolls, the water taken up by the felts being removed by sucking it through into the rolls which have a holed surface. The remaining water is removed by passing the sheet through a long series of steam-heated rolls. Finally a smooth finish is imparted to the surface of the sheet by passing it through a stack of heavy precision ground rolls placed one on top of the other.

The operator of a high speed paper machine is controlling a process in which 150 pounds of water are removed from one pound of pulp in less than one minute. A fragile sheet is handled at speeds around 30 miles per hour by a large and complex machine. As paper machine speeds rise operating tolerances are reduced, hence greater skills embracing an understanding of the scientific principles involved are required by operators for satisfactory machine performance.