As shown in Figure 5, the intrinsic viscosity of pulps derived from hemp stalks (Hs3-6) and hemp wood (Hw-nr) bleached according the method described in the methods section (OD0EPD1 sequence) was relatively low (500 - 600 cm3/g) and comparable to that of birch- and pine pulps. The low viscosity of these pulps after bleaching is generally thought to be due to the presence of metal ions, such as iron, manganese and copper in the unbleached pulps. These ions, especially in bleaching processes which involve H2O2 (e.g. oxygen delignification, alkaline extraction reinforced with H2O2), generate hydroxyl radicals (OH·) which strongly depolymerise the cellulose.