Pearl natural

The pearlescent effect in natural pearls arises because of the interaction of incident light with alternating layers of high refractive index (CaCOj) and low refractive index (protein), reflected and diffracted light from which leads to the observed iridescent colours. 

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...

In pigments that simulate natural pearl effects, the simplest case is a plateletshaped particle with two phase boundaries P, and P2 at the upper and lower surfaces of the particles, i.e., a single, thin, transparent layer of a material with a higher refractive index than its surroundings. For small flakes with a thickness of approx. 100 nm, the physical laws of thin, solid, optical films apply [5.203]. 

Natural pearl (guanine, hypoxanthine) 1.85 Fe203 (hematite) 2.9... 

Natural pearl essence (Essence d Oriente, Fish Silver) is a pigment suspension derived from fish scales, skin, or bladder. It is the oldest commercial pearl luster pigment, consisting mainly of a mixture of the purines guanine [73-40-5] (75-97%) and hypoxanthine [68-94-0] (25-3%). 

Due to its tendency to agglomerate in dry form, natural pearl essence is handled as a 22-25% dispersion in various media (e.g., nitro-cellulose lacquer for nail... 

Under appropriate reaction conditions, regular hexagonal platelets (ca. 50 nm thick and 20 pm in diameter) can be obtained. The high refractive index (n — 2.0), high aspect ratio (> 200), and the extremely even surface of basic lead carbonate make it an optical match to natural pearl essence. 

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. 

Pearls are little spheres of calcium carbonate (CaC03) that form in mol-lusks (invertebrate shellfish) such as oysters, usually because of some sort of irritation. They are normally white or off-white in color, but they can have bluish or pink tints, and sometimes they are dark gray. Although many pearls form naturally, pearl production has been gready increased by the cultured pearl industry, which raises beds of oysters into which irritants are routinely introduced. The irritants are usually bits of mother-of-pearl, the lining that forms inside oyster shells. 

Cultured and natural pearls can usually be distinguished from each other with the use of a medical x-ray. Cultured pearls will show a core bead of flat layered shell with thin layers of nacre around it, while natural pearls will be concentric throughout. 

Fig. 5.15 Optical properties of absorption pigments, effect pigments and natural pearls.

A) conventional pigment B) metal effect pigment C) natural pearl D) pearl luster pigment... 

Pearl luster pigments simulate the luster of natural pearls. They consist of alternating transparent layers with differing refractive indices. The layers consist of CaCOa (high refractive index) and proteins (low refractive index). 

Substrate-Free Pearl Luster Pigments Natural Pearl Essence... 

Natural pearl essence is isolated as a silky lustrous suspension from fish scales. The organic pigment particles in the suspension are platelet-shaped with a very high aspect ratio (0.05 pm x 1-10 pm x 20-50 pm), and consist of 75-97% guanine and 3-25% hypoxanthine (natural fish silver) [5.122, 5.123, 5.128, 5.130]. 

Natural pearl essence is very expensive but shows some advantages over synthetic pearl luster pigments. For example, it shows a high but soft luster (no = 1.79 (parallel) to 1.91 (perpendicular)). It is not brittle and has a relatively low density of 1.6 g cm which reduces setthng in liquid formulations. It is almost exclusively used in expensive cosmetic applications. 

Pearls with a nacreous sut ce can be divided into three groups natural pearls, nucleated cultured pearls, and non-nudeated cultured pearls. (Pearls without nacre will be dealt with separately at the end of the Spedes secdon.) A further group - blister pearls - are hemispherical pearls that grow attached to the oyster s shell. These can also be cultured. 

Natural pearls form by chance within the oyster, and are not induced by humans. They consist of sequential concentric layers of nacre, with a resultii structure similar to that of an onion. This contrasts with shell nacre (motherof-pearl), which is laid down in parallel layers. 

Non-nucleated cultured pearls have a similar structure to that of natural pearls, but at their core there is an irregular cavity, which is a result of the initial stages of their growth which was induced by humans. 

The La Paz pearl oyster, Pinctada mazadanica, lives in the eastern Pacific, especially around Central America. It gives white to grey pearls and was probably the source of the famous La Peregrina natural pearl (see Past and present uses section). 

The Atlantic pearl oyster, Pinctada imbricata, at between 5 and 7 centimetres across, is a relatively small oyster. It was the source of the first natural pearls that were traded from Venezuela. It is not farmed. 

The Ceylon pearl oyster, Pinctada radiata, is historically an important oyster as it is the species that gave the natural pearls that were fished around the Arabian Gulf, the Red Sea, and the Indian Ocean. 

The European pearl oyster, Margaritifera margaritifera, is of historic interest as it was the source of most of the European freshwater natural pearls. It was native to Europe and parts of North... 

The term liaroque pearl is used to describe an irregularly shaped pearl. It was first applied to irregular natural pearls, but can also be used to describe the irregular shapes seen in cultured pearls. Baroques can be very beautifiil and are highly prized (Fig. 9.6). 

There are many natural and cultured pearls of interesting shapes that come onto the market. The freshwater mussels of the eastern United States produce natural pearls that are usually silvery ute> often have a slighdy silky sheen rather than a pearly lustre and can be an amazing variety of shapes (Fig. 9.7). 

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