Pigments based on magnetite (Fe.sub.3 O.sub.4) having particle sizes in the range from 0.1 to 2.0 .mu.m are used for a wide range of applications. As black pigments, they are used for coloring building materials and lacquers. Their magnetic properties are utilized in one-component toners for photocopiers. Their heat resistance is generally judged to be adequate. However, where they are used in building materials which harden under hydrothermal conditions, they undergo a change in color to red or brown.
Pigments having the narrowest possible particle size distribution are generally required to obtain optimal properties.
Accordingly, the production of such pigments is an important development target. The general processes by which pigments of the type in question can be produced have been adequately described.
Thus, DE-A 900 257 describes the so-called precipitation process in which iron(II) salts are reacted with atmospheric oxygen in the presence of alkalis to form Fe.sub.3 O.sub.4 pigments.
Another variant is the two-step precipitation process. In this process, which is described in U.S. Pat. No. 2,631,085, an iron(III) oxide or iron(III) hydroxide is reacted with iron(II) salts with addition of alkalis to form magnetite pigments.
However, the heat resistance of these pigments is inadequate, so that hitherto it has only been possible to use expensive mixed-phase pigments, such as for example metal (II)-containing ferrites (for example copper-manganese ferrites).
Fe.sub.3 O.sub.4 pigments having very good color properties can be produced by the two-step process. Pigments such as these are also distinguished by a narrow particle size distribution.
Magnetite (Fe.sub.3 O.sub.4) crystallizes in the cubic system with a spinel structure in the structure type O.sub.h 7. Development of the crystal faces (100) leads to cubic crystals and, of the (111) crystal faces, to the octahedral forms which have hitherto been observed among the magnetite pigments. With irregular shaped pigments particles, there is no development of a defined crystal face.
The object of the present invention is to provide iron oxide black pigments which do not undergo any change of color in the production of steam-hardened building materials such as for example, lime-sand bricks or fiber-reinforced cement.