Knots are unwanted, large, dark aggregates of wood fibres when making chemical pulp.
[1] 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.
In kraft pulping knots are typically 0.5–3.0 % of the digester throughput.