Paper and Board Manufacture and Properties

From wood to separated fibres

Paper and board are made from fibres. Wood is an important source, while used paper supplies fibres for recycled products. Before a sheet can be formed, these fibres must be separated into pulp. A papermill then turns a watery mixture of pulp into a continuous sheet.

For wood pulp, logs are first debarked because the bark is unsuitable for papermaking. The stripped wood is chipped into small pieces. The chips can then follow either a mechanical or a chemical pulping route.

Mechanical pulping grinds the woodchips to separate their fibres. This pulp is used for high-volume printing products such as newsprint and magazine paper.

Chemical pulping cooks the woodchips with chemicals to remove lignin, the substance that binds the wood fibres together. The key distinction is therefore the way the fibres are released: physical grinding in mechanical pulping, or removal of lignin in chemical pulping. Burning by-products from chemical pulping can supply energy for the process.

The separated fibres are washed and screened to remove unwanted material. Prepared pulp can be used directly or bleached where a white paper is required.

Recovering fibres from used paper

Recycled paper also needs to be reduced to pulp. Used paper is mixed with water and broken apart to separate its component fibres. This is repulping, rather than starting again with woodchips.

Printing ink and adhesives need to be removed. De-inking can use flotation to separate these unwanted materials from the pulp. The recovered fibres then enter the sheet-making process. Recycling changes the source and preparation of the fibres, but it does not remove the need to form, press and dry the new sheet.

Making a continuous sheet in a papermill

The pulp is diluted with a large amount of water so that the fibres can spread across the machine. Each stage removes more water or improves the sheet’s surface:

1. Headbox: distributes the water-and-fibre mixture through a narrow horizontal slit onto an endless moving wire mesh.
2. Wire section: water drains through the mesh, leaving fibres that spread and consolidate into a thin mat. This is sheet formation.
3. Press section: rollers squeeze the wet sheet to remove more water. At this stage, the water content can be reduced to about 50%.
4. Drying section: the sheet passes over heated cylinders, at temperatures above $100^\circ\text{C}$, to remove further moisture.
5. Coating and calendering: where required, the surface is coated and passed through rollers to improve its finish.
6. Finishing: the paper is wound into reels or cut into sheets for printing and conversion into products.

The moving sheet is called a web. Notice the difference between pressing and drying: pressing squeezes out water mechanically, whereas heated cylinders dry the sheet using heat.

Preparing pulp for handmade paper

Hand papermaking uses separated fibres and water too, but individual sheets are formed using a small frame rather than a continuous moving machine.

Soaked waste paper can be broken down by beating small handfuls with a pestle and mortar. A bucket and a stout piece of wood can serve the same purpose. Continue until there are no recognisable pieces of newspaper. To check the pulp, shake a small sample with water in a closed, clear jar: remaining lumps show that more pulping is needed.

A liquidiser breaks down the soaked paper more quickly. Process small batches with cold water, using short bursts rather than running the machine continuously. The aim is separated fibres in water, not a mixture containing pieces of intact paper.

Use a liquidiser only with teacher supervision and according to its operating instructions. Fit the lid before switching it on, keep hands away from the blades and disconnect the power before clearing or cleaning it. Keep the electrical base and plug dry, and wipe up water spills promptly. When beating pulp by hand, keep the container stable and fingers clear of the pestle.

The equipment for forming and supporting a handmade sheet

The mould is a flat frame with tightly stretched mesh across it. The deckle is a matching frame without mesh, placed on top to define the edges of the paper. For an A4 sheet, the frame’s internal dimensions are 29.7 cm by 21.0 cm. The mesh provides a surface to hold the fibres while water can pass through it, applying the same drainage principle as the wire section of a papermill.

The frames should lie flat and have smooth, sanded edges. Any fixings must be secure; drawing pins used to attach mesh should be on the sides, not the working face.

After moulding, smooth couching felts support the wet pulp during pressing. Their surface matters: a textured cloth or one with small holes can spoil the paper’s appearance. Cloths measuring 34 cm by 25 cm provide a margin around an A4 sheet.

A simple hand press uses two boards with the supported sheets between them, held together by bolts and wing nuts. The boards distribute pressure across the sheets. Keep fingers clear as the boards are brought together. As in commercial manufacture, pressing removes water by squeezing; it is not the same operation as drying.

Forming, transferring and drying a handmade sheet

The wet fibres cannot initially support themselves as a sheet. The mesh holds them during drainage, then a felt supports them during pressing and the early stages of drying.

1. Form the sheet. Thin the prepared pulp with water in a vat or sink and stir gently to distribute the fibres. Place the deckle on the mesh-covered side of the mould and hold the two firmly together. Dip them into the vat, level them just below the surface, then lift them out steadily. A layer of pulp remains on the mesh, with its edges contained by the deckle. Gently shake from side to side and forwards and backwards to spread the fibres and help them mesh together.
2. Drain the water. Keep the mould level while surplus water drains through the mesh and the fibre layer remains above it. Once the surface water has drained away, the sheet is ready to transfer, although it is still very wet. Allowing the surplus mixture to drain through retains more fibres and gives a thicker sheet; spilling some over the deckle edge carries fibres away and gives a thinner sheet.
3. Remove the deckle. Lift off the open frame and put it aside. The wet sheet stays on the mould’s mesh.

Use a stable work surface and wipe up spills throughout. Air-drying avoids the hazards of heating wet work and reduces the risk of curled edges. Wash the vat, mould and buckets after each batch so that dried pulp does not spoil the next sheets.

Hand and machine papermaking share the same underlying sequence: form the fibre layer, drain, press and dry. The difference is that the hand process makes and transfers one sheet at a time, while the papermill produces a continuous web.

Changing the surface for a purpose

A finish can be aesthetic, changing the appearance or feel, or functional, changing how the material performs. A finish may do both.

At the papermill, a coating containing pigments, binding agents and other additives can be spread over the paper to improve its printing properties. Calendering passes the sheet between rollers, using pressure and heat to give it a smoother, glossier surface. Coating adds material; calendering compresses and smooths the sheet.

Other finishes are chosen for the final product:

A board’s stiffness alone cannot prevent a liquid from soaking into it. Food and drink packaging must combine suitable structure with an appropriate barrier finish.

Understanding physical and working properties

Physical properties describe what a material is like. Working properties describe how it behaves when processed, for example when folded. Material selection means matching these properties to the product’s needs, rather than choosing the strongest or thickest material for every job.

Weight and thickness are related, but they are not interchangeable measurements. Likewise, strength and rigidity are different: a material may resist tearing yet still bend readily.

Choosing paper for drawing and printing

Tracing paper, typically 40–90 gsm, is translucent, smooth, strong and non-absorbent. Its translucency makes it useful for copying drawings and working with overlays.

Layout paper, around 50 gsm, is smooth, translucent and inexpensive. It suits sketching and developing design ideas where many sheets may be used.

Copier paper, commonly 80 gsm, has a smooth, opaque, clean white surface. It is uncoated and suitable for printing and photocopying.

Cartridge paper, typically 80–140 gsm, has a thicker, textured, creamy off-white surface. It works with paints, watercolours, pastels and inks. For an art task, this surface may be more useful than the smooth white finish chosen for a printed document.

Choosing board for structures and packaging

Corrugated cardboard, with a representative thickness of 3,000 microns, combines strength with low weight. Its wavy, fluted structure provides rigidity, making it useful for packaging. Different thicknesses are available. Untreated corrugated cardboard is not water-resistant, so it is not automatically suitable for wet conditions.

Mounting board, around 1,400 microns thick, is rigid and smooth. It suits picture mounts, architectural scale models and concept models where a stable surface and shape are important.

Folding boxboard, approximately 300–1,699 microns thick, is stiff and normally made from recycled paper. It scores and folds well without splitting, making it useful for supermarket food cartons. Scored crease lines allow a flat blank to fold into its intended shape.

These choices involve trade-offs. Rigidity is helpful for a display model, but a carton also needs controlled folding. Corrugated cardboard offers lightweight structural packaging, but a food container may additionally need a coating or laminate to resist absorption. A justified choice connects the material, its properties and any finish to the actual use.

Pulp preparation

• Wood: debark → chip → pulp → wash and screen; bleach if required.
• Mechanical pulping: grind chips to separate fibres; used for newsprint and magazine paper.
• Chemical pulping: cook chips to remove lignin and release fibres.