Paper

Whether done by hand or with a paper machine, the paper making process has three simple steps:

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Preparation of the fibers

The material to be used for making paper is first converted into pulp, a concentrated mixture of fibers suspended in liquid. The source of fiber is often natural (softwood or hardwood trees or other plants) or recycled, such as old corrugated boxes, newsprint, or mixed paper.

When best sheets materials are used to make paper, it is usually necessary to break down the lignin inside of the plant's cell walls. This is done via a chemical process, such as the Kraft process. These processes are not needed when breaking down recycled fibers, as the lignin has already been removed from the source material. If the lignin is retained in the pulp, the paper will yellow when exposed to air and light.

Pulp that has been broken down mechanically is often known as "groundwood pulp." The mechanical process to break down wood chips into pulp requires no chemicals. Since the lignin is not removed from mechanical pulp, yields are relatively high, approximately 90-98%. However, due to the aging issues indicated above, mechanical pulp is most often used for newspapers and other non-permanent goods.

Pulp that is broken down chemically is known as "chemical pulp." The main purpose of a chemical pulping process is to break down the chemical structure of lignin and render it soluble in a liquid (most often water) so it may be washed from the remaining fibers. Removing the lignin from wood chips also serves to break them apart into the fibers that compose pulp.

Recycled fibres do not need to be pulped in the conventional sense. These fibres have already been treated once, so instead they need a more gentle process to break the fibers apart while preserving their integrity.

Once the fibers have been extracted, they may also be bleached, dyed, or have additional ingredients added to alter the appearance of the final product. For example, Kaolin (or calcium carbonate) is added to produce the glossy papers typically used for magazines. The Kappa number indicates how much bleach is required to obtain a given whiteness.

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Sheet formation

The pulp mixture is further diluted with water resulting in a very thin slurry. This dilute slurry is drained through a fine-mesh moving screen to form a fibrous web. A watermark may be impressed into the paper at this stage of the process. This mark is used on paper currency and other things.