The inherent stiffness of birch wood-fibres makes it an excellent raw material in the manufacture of high bulk pulp. By a combination of chemical impregnation of birch chips and pre-heating of the chips to a high temperature prior to refining, it is possible to manufacture CTMP with very high bulk with a low energy input. The bulky pulp obtained is low in shive content and is a suitable choice for the middle-layer of multi-ply board.