In simple terms, paper consists of pulp, filler, water and chemicals. The ingredients are combined according to the unique recipe for each particular paper, and the grades are then produced in a way that ensures they have the desired properties.

In school, we used to learn that paper comes from trees. This is of course true, but also an extreme simplification. It would be more correct to say that pulp consists of cellulose fibres that usually come from pulp wood – which in turn comes from trees. Cellulose can also be extracted from cotton or grass, but here we will be focusing on pulp made from wood. The most common wood types used for pulp are aspen, eucalyptus, birch, pine and spruce. Different types of wood are used because the properties of the fibres vary. For example, hardwood fibres are shorter and give the paper good opacity and formation, while fibres from softwood trees are longer and make for a strong paper. There are two main ways of producing cellulose fibres from wood: a chemical method and a mechanical method. With the chemical method, which produces chemical pulp, cellulose fibres are released from the other constituents in the tree trunk by first being ground into chips and then digested using chemical additives. This method produces strong, almost pure cellulose fibres which are highly resistant to ageing. Since this method almost exclusively uses the tree’s cellulose – around 50-55 per cent of the tree’s volume – the resulting product is often called woodfree pulp. The other constituents in the tree are instead used as raw materials or energy, which means that many modern pulp mills are self-sufficient in terms of energy. The production of mechanical pulp, on the other hand, involves grinding the tree in a mill or between grindstones until the cellulose fibres have been separated. Thus the name – mechanical pulp – as the fibres are extracted using a mechanical method. Moreover, unlike the chemical method, mechanical pulp production also utilises other parts of the tree – such as lignin and resins – so that almost 95 per cent of the tree’s volume is exploited. Mechanical pulp is therefore often called wood-containing pulp, as all the wood-containing substances are still there. There are several different variations of chemical and mechanical pulp. Also, combinations of the two methods are used to produce a third type of pulp, semi-chemical pulp, one example being Chemo Thermo-mechanical pulp, known as CTMP. In CTMP production, the tree is turned into chipwood in the same way as in the chemical method, and the chips are then partially treated with heat and chemicals before the cellulose is separated mechanically.

Cellulose fibres therefore represent the most important ingredient in paper, although they alone are not enough. Additives are also required, one being filler. As the name suggests, the task of the filler is to fill in the gaps in the complex fibre network. A paper made with filler is softer and more even. It has better formation, higher opacity, better ink-setting properties, a smoother and more flexible surface – all of which make for better printing characteristics. There are various types of filler, the most common being calcium carbonate, i.e. limestone, and kaolin or china clay. The choice of filler depends on the type of production system – acid or basic. Calcium carbonate is used in a basic system and results in a paper with a high pH value, i.e. a basic /neutral paper. Kaolin filler is used in an acid system and produces a paper with a low pH, i.e. an acid paper. A paper’s life span partly depends on whether it is manufactured in a basic /neutral or an acid system. This is because a paper made in an acid system is broken down from within by the low pH value, while the opposite applies for a basic /neutral paper: the high pH value helps the paper better withstand external acid stress. This is a requirement for an age-resistant paper.

	Chemicals

Paper production also requires other chemicals in addition to filler. These are necessary primarily for paper to be made in the first place, but also to ensure that the finished product has the desired properties, such as extra strength, better water resistance and the right shade. For instance, stock sizing is used to stop the paper absorbing too much moisture, while the manufacture of surface-sized fine paper also uses a starch additive to make the paper more print-friendly and durable. Furthermore, shading dyes are also usually required so that the paper maintains a consistent colour from one production run to the next, as the shade of the pulp frequently varies. In order to achieve higher brightness, an OBA, or Optical Brightening Agent, is also used. Paper production also requires other chemicals in addition to filler. These are necessary primarily for paper to be made in the first place, but also to ensure that the finished product has the desired properties, such as extra strength, better water resistance and the right shade. For instance, stock sizing is used to stop the paper absorbing too much moisture, while the manufacture of surface-sized fine paper also uses a starch additive to make the paper more print-friendly and durable. Furthermore, shading dyes are also usually required so that the paper maintains a consistent colour from one production run to the next, as the shade of the pulp frequently varies. In order to achieve higher brightness, an OBA, or Optical Brightening Agent, is also used.

	Water and moisture content

Paper also contains some moisture in the form of water, commonly between 3.5 and 6.5 per cent. The moisture level depends on the application and printing process the paper is intended to be used in. The water content of paper is expressed in two measurements: absolute moisture or relative moisture. Absolute moisture refers to water content as a proportion of the paper’s weight, e.g. 5.5 per cent. Relative moisture is a quota figure that defines existing water and the paper’s (water) saturation at one and the same temperature, e.g. 50-55 per cent at 20°C. If the temperature or air humidity in the paper’s surroundings changes, the paper will either release or absorb moisture. This in turn means that the paper may change format or become wavy during acclimatisation – resulting in poorer printing properties. It is therefore essential to handle and store paper in the correct way. This is discussed later in this section.