Spinel pigments

The pigments of spinel structure are one of the first used in ceramic practice . There are two kinds of spinels – with normal (straight) and inverse structure. The spinels of the first type have the structure of natural spinel МgO.Аl₂O₃ or МgАl₂О_4.The general formula is AB₂O₄. Here, A is bivalent cation, e.g. Мg²⁺, Zn²⁺, Со²⁺, Ni²⁺, Fе²⁺, Мn²⁺, Са²⁺, Сu²⁺ etc. while B is trivalent cation, most often Аl³⁺, Сг³⁺, Fе³⁺, V³⁺, Мо³⁺ etc. spinel crystallize in the cubic syngony as octahedrons or rhombododecahedrons, where the oxygen atoms are closest to the highest packing efficiency. By normal spinels, the bivalent cations occupy only the tetrahedron sites while the trivalent ones – the octahedron sites. The atoms A are surrounded by 4 oxygen atoms and atoms B – by 6. By the inverse spinels, half of the trivalent cations occupy the tetrahedron sites and the other half- octahedron sites together with the bivalent cations. Typical example for such arrangement is magnetite Fе²⁺Fе³⁺₂O₄. The spinels with normal structure have higher heat resitance compared to the ones with inverse structure.

The blue pigments of spinel type are most often doped with cobalt. The earliest known blue pigment is СоО.Аl₂O₃ or shortly Со-Аl. To obtain different shades of blue with magnesium and zinc aluminates, МgО and ZnО are gradually substituted by СоО. Green-blue pigments are prepared on the basis of ZnO.Сг₂O₃ where the zinc oxide is substituted by СоО. In nickel spinel NiO.Сг₂O₃, the substitution of Сг₂O₃ with Аl₂O₃ gives dark green, blue green, grey and blue pigments. Green, blue-green and blue pigments are preparwed in the system СоО-Сг₂O₃-Аl₂O₃.

Pink spinel pigments are obtained in the system ZnO-Аl₂O₃-Сг₂O₃. In this case, twp phases are formed: ZnАl₂O₄ and ZnСг₂O₄. Depending om the ratio between the components in the system, an isomorphic or mechanical mixture of the two spinels is formed which stipulates the different color shades.

Major part of the brown and black pigments produced are based on Zn(Сг,Fе)₂O₄, Zn(Сг,Fе,Мm)₂O₄, Zn(Сг,Fе,Аl)2O₄. Saturated black color without cobalt have the pigments of the spinel system (Zn, Мn,Сu)(Fе,Сг,Мn,Аl)₂O₄. Dark green, brown and black pigments can be prepared using the total miscibility of the spinels NiO.Сг₂O₃, NiO.Fе₂O₃ and NiO.Мn₂O.