All papers are unique and have different properties – properties that are crucial to the final result of a piece of printed material, how it is perceived and the feeling it conveys. Choosing the right paper is therefore important, and requires some knowledge. Quite simply, you have to understand the preconditions and properties of the paper in order to make the correct choice. In an ideal world, this takes place in two stages. First of all an objective, quantifiable platform is needed, and an understanding of the paper’s qualities. Once this solid foundation is in place, other more subjective, unquantifiable considerations and feelings come into play based on the paper’s abstract qualities. In this section, we will be building the basic platform by presenting a number of objective, quantifiable properties of paper.

	Wood containing and woodfree paper

Earlier in the section we explained the two principal ways of manufacturing pulp, the chemical method and the mechanical method. The chemical method removes practically all wood-containing substances from the wood, apart from the cellulose.This is why paper that contains over 90 per cent woodfree pulp, i.e. chemical pulp, is called woodfree. A woodfree paper’s properties include high strength and good permanence. The production of mechanical pulp exploits the majority of the tree volume, which means that parts other than the pure cellulose are added to the pulp mixture. A paper with over 10 per cent wood pulp is therefore called wood-containing paper. The properties of a wood-containing paper include good strength, high opacity and a natural feel. Paper grades which mainly comprise semi-chemical pulp change the traditional distribution of woodfree and wood-containing paper because benefits are gained from both types of pulp. By definition these are often referred to as wood-containing paper, although they are often more similar to woodfree grades depending on the desired function.

	Grammage, thickness and bulk

Grammage, thickness and bulk are three important and common paper properties. The three are often mentioned in the same breath, for the simple reason that they are mathematically linked. Thickness and grammage are defined independently of one another, while bulk, sometimes also referred to as volume, is the relationship between grammage and thickness. This section defines each term more closely.

	Grammage=Thickness/bulk

Grammage is the most common measure of a paper’s weight, and is given as weight per square metre, e.g. 130 g/m². The concept is fundamental when choosing a paper, although it is often used somewhat carelessly to refer to a paper’s thickness, which is misleading as thickness actually depends on the paper’s bulk.

	Thickness=Grammage*bulk

A paper’s thickness is measured in micrometres (µm) – thousandths of a millimetre – and is the distance between the paper’s two surfaces. Thickness is in many ways an important consideration in paper selection, as it affects the stability, feel and of course thickness of the final printed product. This is particularly true when printing books, which can contain hundreds of pages. In day-to-day terms, however, grammage is often used to describe a paper’s thickness.

	Bulk=Thickness/Grammage

As the above formula shows, bulk defines the relationship between a paper’s thickness and its grammage. The dimension refers to the volume or compactness of a paper, and the correct definition is in fact ‘the recip-rocal value of density’. BULK The picture here illustrates bulk. The sheet on the left has a high bulk – it is airy, light and thick. The sheet on the right has a low bulk – it is thin and heavy. Both have a grammage of 130 g/m² E.g. Left sheet: Bulk 1.8 x grammage 130 g/m² gives a thickness of 234 micrometres. Right sheet: Bulk 1.1 x grammage 130 g/m² gives a thickness of 143 micrometres. A paper with a low bulk is more compact than one with a high bulk, and contains less air. A low-bulk paper is therefore thin and heavy, while a high-bulk paper is light, airy and thick. Because of the lower proportion of air, a low-bulk paper is often smoother than one with a high bulk, and vice versa. Bulk is a very important factor when producing books, and is crucial to how the final result is perceived. If the aim is to convey an impression of a thick book rich in content, a high-bulk paper should be used. If the idea is to fit in a lot of text but still keep the book thin, a low-bulk paper is more appropriate. Bulk can also be a question of cost in some cases. For example, when constructing a mail campaign and using a paper with a slightly higher bulk, a lower grammage can usually be used without detracting from the feeling. This could save a lot of money in distribution costs.

	Roughness

In some ways, the surface of a sheet of paper is like the topography of a natural landscape, with peaks, troughs and other minor bumps.The term used to define a paper’s deviation from an absolutely smooth surface is roughness. Roughness is expressed in ml/min, referring to the volume of air that passes through the paper and a measuring device in one minute. This measurement is called the Bendtsen value. In general, uncoated paper has a higher roughness – more peaks and troughs on the surface – than coated paper. In order to reduce roughness, the paper is compressed and smoothed in a calender or smoothing machine. This makes for a certain link between bulk and roughness as a smooth, compressed paper will have a lower bulk. So to achieve a higher bulk, the evenness of the paper surface will have to be compromised to some degree, and the paper will be rougher. And this is of course significant to the final result. In some cases the aim is a nice even surface, while in others the printed matter is intended to convey more of a coarse, robust impression. Surface roughness also affects the printing process, especially in high-roughness papers where irregularities can affect the graphics.

	Brightness and Shade

Just like the Eskimos have many different words for the colour and consistency of snow, so paper has a broad spectrum of brightness and shades. All with one thing in common: they enhance the impression of the finished printed product. Because as we know, there is a great difference between white, natural and coloured paper. However, all pulp of the same type basically has a similar brightness and shade. For this reason, different chemicals are added to distinguish the finished papers’ brightness and shade. Adding OBAs (Optical Brightening Agents) to the mixture increases the brightness of the paper, for example, as the chemical reflects white light, which makes the paper look brighter than it actually is. Although shade and brightness are visual properties that may seem hard to distinguish at first sight, they do in fact describe two different things. Brightness is expressed as a percentage of how much light of a particular wavelength is reflected from the paper. Whiteness is a closely related parameter, although this is gauged over several wavelengths and gives a value closer to what the eye actually perceives. However, papers of the same brightness can have different shades or colour tones, which is why the shade of a paper is also measured, e.g. blue, yellow, red or green. Shading dyes are added to most papers. For example, a blue shade is often added to a paper where the aim is to give a whiter impression, while a touch of yellow will make the paper appear more natural. In producing books, for instance, it is of course important to have the same colour and shade in all paper deliveries, and this is another reason why shading dye is used – otherwise the books would look stripy at the side and ends.

	Opacity

Another word for opacity is non-translucency. Therefore, the higher a paper’s opacity, the less translucent it is. A paper of 100 per cent opacity is therefore completely non-translucent, while one with a low opacity, such as greaseproof paper or tracing paper, lets a much higher proportion of light through. Opacity depends on how well the paper surface can distribute and absorb light. The opacity is altered during printing, after which we refer to print opacity. This is an important factor to consider, as printing ink penetrates the paper and reduces its opacity, which could result in the print being visible through the sheet. This is especially important in double-sided printing, particularly on low-grammage paper. It is therefore important to think about where on the page the text and graphics should go.

	Fibre direction

In much the same way as fish in a shoal orientate themselves to swim in the same direction, most fibres point in the same direction as the paper web during production. This direction is called the paper’s fibre direction, and makes the paper stiffer and thereby harder to bend along that axis. It is easy to find out in which direction a paper’s fibres run by laying a sheet over the edge of a table.The axis on which it bends most is against the direction of the fibre. Another way is to drag hard with your thumb-nail and index finger along the paper’s edges. The edge that distorts most runs across the direction of the fibres. The paper supplier’s measurements also reveal the fibre direction, as the figure given first (e.g. the 210 in 210 x 297) refers to the length of the side running across the fibre direction.

	Sheet formats

The sheeting machine cuts the paper into sheets according to the customer’s ordered dimensions or a standard format, and the paper is then either delivered on pallets – usually in a bulk pack where all the sheets are stacked up – or in smaller packs, where sheets are wrapped in reams of 500, for example. As we mentioned before, the sheet dimensions are given depending on fibre direction. The first figure is the length of the side that runs across the direction of the fibres. This means that a sheet of A4 with the dimensions 210 x 297 has the fibres running in the direction of the 297 mm side. There is also a wide assortment of formats in standard use in different countries. Suppliers normally offer these as standard stock at a fixed number of sheets per pallet.

	Format

As the paper comes off the paper machine, it is wound onto a large reel called a tambour reel. If the paper is to be used in a web offset press it is cut down into smaller reels, otherwise it is cut into the required sheet sizes. Sheets and reels are the main paper formats, but there are a great many different formats within each primary group that are either standard or customised depending on the customer and intended use. A paper buyer can be relatively certain that standard formats are in stock with the supplier. However, if a customised format is required there may be a slightly longer delivery time, as it first has to be ordered and then produced at the mill.

	Reel formats

Like sheets, reels are cut into the diameters and widths ordered by the customer, and also according to weight and length. However, standard formats are less common in reels, although they do exist. All paper reels also have a core in the centre so that they can be brought into the printing press. Common standard core diameters are 70, 76 and 153 mm.