Knots are unwanted, lage, dark aggregates of wood fibre

Wood fibre

Wood fibres are usually cellulosic elements that are extracted from trees, straw, bamboo, cotton seed, hemp, sugarcane and other sources.The end paper product dictates the species, or species blend, that is best suited to provide the desirable sheet characteristics, and also dictates the required...

s when making chemical pulp.

Knots are incompletely cooked wood chips coming out of the digester. Their origin is often dense parts of branches, such as compression wood or timber knots – hence the name. Knots can also stem from large/oversized wood chips due to insufficient impregnation with cooking liquors. The content of lignin

Lignin

Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...

 is very high. In kraft pulping

Kraft process

The kraft process describes a technology for conversion of wood into wood pulp consisting of almost pure cellulose fibers...

 knots are typically 0.5–3.0 % of the digester throughput. The knots are screened from the pulp, because if left in the pulp they may damage washing equipment and consume large amounts of bleaching chemicals. They are normally sent back to the digester and re-cooked so that their fibres are not wasted.