in packaging materials the present invention is described in more detail in the following example with an intended limitation of the invention a 30 sodium hydroxide solution is added with stirring to an aqueous solution of iron sulphate obtained from tanneries containing 150 g l feso 4 until 40 of the iron is precipitated as iron iii hydroxide the precipitated iron is then oxidized to the trivalent state by gassing with air at temperatures of from 30 u00b0 to 35 u00b0 c in this process the ph drops from the initial value of 6 to 3 5 at this stage formation of nuclei is completed additional iron sulphate is then added and the suspension is heated to 80 u00b0 c gassing with air being continued with stirring while the ph is maintained at a constant value of 4 by the addition of sodium hydroxide solution this procedure is continued until 14 times the quantity of u03b1 feooh obtained from nuclear formation has been reached after filtration and washing on a rotary filter the filter cake is spray dried the goethite obtained which has a bet surface area of 12 m 2 g a needle width of 0 15 u03bcm and a needle through of 0 7 u03bcm is reduced by hydrogen with the addition of 20 vol of steam in an indirectly heated rotary furnace at reaction temperatures of from 425 u00b0 to 450 u00b0 c to yield fe 3 o 4 having an feo content of 30 5 by weight after intermediate cooling treatment with nitrogen containing 10 air is carried out in counter current in another rotary furnace at from 100.