Hematite pigment

Plate 19.111 Outerop of natural Fe oxide (goethite, hematite) pigments at Luberon, France (Courtesy H. Stanjek)... 

PZC/IEP of Hematite Pigment from Bayer Electrolyte T Method Instrument... 

Quantitative spatially resolved analysis of hematite pigments from prehistoric rock art paintings for fingerprinting purposes... 

The recognition of hematite on works of art is common however, in many cases, the hematite is identified as a component of an ochre, umber or sienna (qq.v.). Hameau et al. (2001) hst hematite (as distinct from ochre) as a pigment from post-glacial French PalaeoUthic sites. A neoHthic pot containing unused hematite pigment has been found in Greek Macedonia (Maniatis and Tsirtsoni, 2002). Hematite adulterated with chalk, carbon and... 

Iron blacks, 79 401-402 Iron blast furnace, 76 141-143 Iron Blue, pigment for plastics, 7 370t Iron blue pigments, 79 407 Iron(II) bromide, 74 539 Iron(III) bromide, 74 539-540 Iron brown hematite, formula and DCMA number, 7 347t Iron browns, 79 402 Iron carbide, 4 649t, 690—692 lattice, 4 652... 

As a pigment, each iron oxide has an optimum particle size which is that with the maximum scattering cross section. This optimum particle size is lower, the higher the refractive index of the mineral. For hematite, the size corresponding to the maximum in scattering/absorption cross section is ca. 1 pm. As the particle size decreases, the relative scattering cross section drops to zero and the relative absorption cross section levels out. As a result, very small particles of hematite are transparent. 

A characteristic of the iron oxide system is the variety of possible interconversions between the different phases. Under the appropriate conditions, almost every iron oxide can be converted into at least two others. Under oxic conditions, goethite and hematite are thermodynamically the most stable compounds in this system and are, therefore, the end members of many transformation routes. The transformations which take place between the iron oxides are summarized in Table 14.1. These interconversions have an important role in corrosion processes and in processes occurring in various natural environments including rocks, soils, lakes and biota. In the latter environments, they often modify the availability and environmental impact of adsorbed or occluded elements, for example, heavy metals. Interconversions are also utilized in industry, e.g. in the blast furnace and in pigment production, and in laboratory syntheses. 

The natural iron oxide pigments are termed the ochres which are yellow and contain goethite (10-50%) as the Fe oxide constituent, the reds, with a high content of hematite, the medium to dark yellow siennas, the umbers and the blacks, which consist of magnetite (Benbow, 1989 Buxbaum Printzen, 1993). 

At the completion of the reaction, the aniline is separated from the iron oxides by steam distillation and the umeacted iron removed. The pigment is washed, filtered and dried, or calcined in rotary kilns to hematite (Plate 20.1, see p. XXXIX). Considerable control over pigment properties can be achieved in this process by varying the nature and concentration of the additives and the reaction rate the latter depends on pH, the rate of addition of iron and nitrobenzene and the type and particle size of the iron particles. Two advantages of this method are that a saleable byproduct, aniline, is produced and that there are no environmentally, harmful waste products. 

Thermal decomposition of iron pentacarbonyl. Very finely divided red iron oxide is obtained by atomizing iron pentacarbonyl, Fe(CO)5, and burning it in excess of air. The size of the particles depends on the temperature (580-800 °C) and the residence time in the reactor. The smallest particles are transparent and consist of 2-line ferri-hydrite, whereas the larger, semi-transparent particles consist of hematite (see Chap. 19). The only byproduct of the reaction is carbon dioxide, hence, the process has no undesirable environmental side effects. Magnetite can be produced by the same process if it is carried out at 100-400 °C. Thermal decomposition of iron pentacarbonyl is also used to coat aluminium powder (in a fluidized bed) and also mica platelets with iron oxides to produce interference or nacreous pigments. 

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