Nacreous effect

Pure BiOCl is available in three grades with different nacreous effects that depend on the aspect ratios and crystal size ... 

Some shells have a porcellaneous inner layer, while others produce a nacreous layer. The nacreous effect is called mother-of-pearl and is caused by tiny, overlapping platelets of calcium carbonate in the form of aragonite, which disperse the light to give a play-of-colour. The platelets are polygonal and lie horizontal to the surface. In gastropods and nautilus they are stacked like coins, but they can also be alternated, like a brick wall, as is the case in most bivalves. There can be more than one nacreous layer, as in, for example, freshwater mussels. 

Against a black background or in a blend with carbon black, the transmitted light is absorbed and the reflected interference color is seen as the mass tone (i.e., overall color) of the material. In blends of nacreous pigments with absorbing colorants, the particle size of the latter must be well below the scattering limit, i.e., they must be transparent. The nacreous effect or iridescent reflection is otherwise... 

Nacreous Pigments. Mica is used as a substrate for coatings (qv) of various metal oxides to obtain a peadescent effect. Mica coated in this fashion is used as filler and as a coloring agent in certain types of plastics. 

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. 

As shown in Figure 71 A, conventional pigments interact with light by absorption and/or diffuse scattering (see Section 1.3). Luster pigments comprise nacreous pigments (Section 5.3.1) and metal effect pigments (Section 5.3.2). 

Synthetic or natural pigments used to achieve lustrous, brilliant, or iridescent color effects by interference on thin optical films are called nacreous or pearlescent pigments [5.190]—[5.194]. They were originally used to simulate the appearance of... 

Figure 71. Optical principles of conventional and luster pigments A) Conventional pigment that absorbs and scatters light B) Metal effect pigment with complete regular reflection C) Natural pearl composed of alternating layers of protein and CaC03 D) Nacreous pigment the pearl is simulated by parallel orientation of the pigment platelets...

Nacreous and interference pigments are used as colorants or part of color formulations for all applications where traditional pigments are used, but where additional color depth, brilliance, iridescence, color shift (flop), and other spectacular effects are desired [5.253]. Mica-based pigments dominate their combination of pearl and interference effects, brilliance, stability, and weather resistance is unsurpassed. Furthermore, they are non-toxic [5.221],... 

Using a two-layer coating system in which an upper coat contains the nacreous pigment and the lower coat contains the hiding absorption or metal effect pigments... 

Nacreous Pigments Nacreous, i.e.. pearlescent pigments are used for creating special decorative effects typical of natural pearls. Nacreous pigments are fine, thin, plate-like transparent particles having a high refractive index. Because of these physical characteristics, when dispersed in a transparent film, they produce a silky appearance. 

Also, increase in water temperature favors -conformation. Inasmuch as the conformation of CP probably determines the tertiary structure of MM, slight changes in CP conformation introduced by cellular or environmental effects may alter MM conformation. This in turn is reflected in the mineral form and structural pattern of the inorganic phases. Perhaps, nacreous layers in molluscs represent an almost ideal situation where MM and CP are aligned in a symmetrical way. In fish otoliths, the fibrous organic matrix is a mixture of helices and 0-pleated sheets. It is tentatively concluded that the morphology of shell structures is a macroscopic expression of the molecular interactions between MM and CP which are controlled in part by cellular activities and in part by the environment. 

Despite their monocrystalline habit, the nacreous tablets and the prismatic units of Mollusks shells are composite structures. We do note that these complex structures are rarely visible on simple fractures of the shells, but enzymatic hydrolyzes or/and acidic etchings are effective from this point of view. 

The term luster pigments includes metallic, nacreous and interference pigments. The luster effect is due to directed reflection on planar-shaped and ordered pigment particles. 

Interference pigments are nacreous pigments, which fulfill the interference criteria and hence exhibit color effects... 

Materials developed for esthetic, not stmctural reasons, viz. compositions showing nacreous or wood-grain effect. 

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