Nacre

thumb|upright=1.2|The iridescent nacre inside a [[nautilus shell]]

thumb|upright=1.2|Nacreous shell worked into a decorative object

Nacre ( , ), also known as mother-of-pearl, is an organicinorganic composite material produced by some molluscs as an inner shell layer. It is also the material of which pearls are composed. It is strong, resilient, and iridescent.

Nacre is found in some of the most ancient lineages of bivalves, gastropods, and cephalopods. However, the inner layer in the great majority of mollusc shells is porcellaneous, not nacreous, and this usually results in a non-iridescent shine, or more rarely in non-nacreous iridescence such as flame structure as is found in conch pearls.

The outer layer of cultured pearls and the inside layer of pearl oyster and freshwater pearl mussel shells are made of nacre. Other mollusc families that have a nacreous inner shell layer include marine gastropods such as the Haliotidae, the Trochidae and the Turbinidae.

Physical characteristics

Structure and appearance

thumb|upright=1.2|Schematic of the microscopic structure of nacre layers

thumb|upright=1.2|Electron microscopy image of a fractured surface of nacre

Nacre is composed of hexagonal platelets, called tablets, of aragonite (a form of calcium carbonate) 10–20 μm wide and 0.5 μm thick arranged in continuous parallel lamina. These layers are separated by sheets of organic matrix (interfaces) composed of elastic biopolymers (such as chitin, lustrin and silk-like proteins).

Nacre appears iridescent because the thickness of the aragonite platelets is close to the wavelength of visible light. These structures interfere constructively and destructively with different wavelengths of light at different viewing angles, creating structural colours.

The crystallographic c-axis points approximately perpendicular to the shell wall, but the direction of the other axes varies between groups. Adjacent tablets have been shown to have dramatically different c-axis orientation, generally randomly oriented within ~20° of vertical. Theoretical and computational methods have also been developed to explain the experimental observations of nacre's mechanical behavior. Nacre is stronger under compressive loads than tensile ones when the force is applied parallel or perpendicular to the platelets. This results in the ductile protein phase deforming such that the crack changes directions and avoids the brittle ceramic phase. Based on experiments done on nacre-like synthetic materials, it is hypothesized that the compliant matrix needs to have a larger fracture energy than the elastic energy at fracture of the hard phase. Developing synthetic composites that exhibit similar mechanical properties as nacre is of interest to scientists working on developing stronger materials. To achieve these effects, researchers take inspiration from nacre and use synthetic ceramics and polymers to mimic the "brick-and-mortar" structure, mineral bridges, and other hierarchical features.

When dehydrated, nacre loses much of its strength and acts as a brittle material, like pure aragonite.

Function

thumb|upright=1.2|Fossil [[nautiloid shell with original iridescent nacre in fossiliferous asphaltic limestone, Oklahoma. Dated to the late Middle Pennsylvanian, which makes it by far the oldest deposit in the world with aragonitic nacreous shelly fossils.]]

Nacre is secreted by the epithelial cells of the mantle tissue of various molluscs. The nacre is continuously deposited onto the inner surface of the shell, the iridescent nacreous layer, commonly known as mother-of-pearl. The layers of nacre smooth the shell surface and help defend the soft tissues against parasites and damaging debris by entombing them in successive layers of nacre, forming either a blister pearl attached to the interior of the shell, or a free pearl within the mantle tissues. The process is called encystation and it continues as long as the mollusc lives.

In different mollusc groups

The form of nacre varies from group to group. In bivalves, the nacre layer is formed of single crystals in a hexagonal close packing. In gastropods, crystals are twinned, and in cephalopods, they are pseudohexagonal monocrystals, which are often twinned.

Uses

Decorative

Architecture

Both black and white nacre are used for architectural purposes. The natural nacre may be artificially tinted to almost any color. Nacre tesserae may be cut into shapes and laminated to a ceramic tile or marble base. The tesserae are hand-placed and closely sandwiched together, creating an irregular mosaic or pattern (such as a weave). The laminated material is typically about thick. The tesserae are then lacquered and polished creating a durable and glossy surface. Instead of using a marble or tile base, the nacre tesserae can be glued to fiberglass. The result is a lightweight material that offers a seamless installation and there is no limit to the sheet size. Nacre sheets may be used on interior floors, exterior and interior walls, countertops, doors and ceilings. Insertion into architectural elements, such as columns or furniture is easily accomplished.

Jewelry

Mother of pearl is commonly used in jewelry due to its smooth texture and iridescent appearance. It is sourced from the inner layer of mollusk shells, such as oysters and abalones.

Mother of pearl is frequently crafted into earrings, pendants, rings, bracelets, and brooches. It can be carved into various shapes or inlaid into metal settings, often combined with gold, silver, or gemstones. The material is valued for its natural luster and the subtle color variations it displays, which can include white, cream, pink, and green.

Musical instruments

Nacre inlay is often used for music keys and other decorative motifs on musical instruments. Many accordion and concertina bodies are completely covered in nacre, and some guitars have fingerboard or headstock inlays made of nacre (or imitation pearloid plastic inlays). The bouzouki and baglamas (Greek plucked string instruments of the lute family) typically feature nacre decorations, as does the related Middle Eastern oud (typically around the sound holes and on the back of the instrument). Bows of stringed instruments such as the violin and cello often have mother-of-pearl inlay at the frog. It is traditionally used on saxophone keytouches, as well as the valve buttons of trumpets and other brass instruments. The Middle Eastern goblet drum (darbuka) is commonly decorated by mother-of-pearl.

Indian mother-of-pearl art

At the end of 19th century, Anukul Munsi was the first artist who successfully carved the shells of oysters into the shape of a human being. For the British Empire Exhibition in 1924, he received a gold medal. His eldest son Annada Munshi is credited with drawing Indian Swadesi Movement in the form of Indian advertising. Anukul Charan Munshi's third son Manu Munshi was felt to be one of the finest mother-of-pearl artists in the middle of the 20th century. As the best example of "Charu and Karu art of Bengal," the former Chief Minister of West Bengal, Dr. Bidhan Chandra Roy, sent Manu's artwork, "Gandhiji's Noakhali Abhiyan", to the United States. Numerous figures, such as Satyajit Ray, Bidhan Chandra Roy, Barrister Subodh Chandra Roy, Subho Tagore, Humayun Kabir, Jehangir Kabir, as well as his elder brother Annada Munshi, were among the patrons of his works of art. "Indira Gandhi" was one of his famous mother of pearl works of art. He is credited with portraying Tagore in various creative stances that were skillfully carved into metallic plates. His cousin Pratip Munshi was also a mother-of-pearl artist.

Other

Mother-of-pearl buttons are used in clothing either for functional or decorative purposes. The Pearly Kings and Queens are an elaborate example of this.

Mother-of-pearl is sometimes used to make spoon-like utensils for caviar (i.e. caviar servers) so as to not spoil the taste with metallic spoons.

File:Flügelretabel Perlmutt Augsburg um 1520.jpg | Altarpiece, , with extensive use of carved nacre

File:Powder flask img 2091.jpg | Nacre gunpowder flask, , mostly made of Turbo marmoratus shell

File:Istanbul.Topkapi046.jpg | Inlay with nacre tesserae, Topkapı Palace, Istanbul

File:Ile Salomon Pendentif MHNT ETH AC SL 23 Roquemaurel.jpg | Engraved nacre pendant, Solomon Islands 1838

File:CaviarSpoonSilverEngravedHandle.jpg|Mother-of-pearl caviar spoon with sterling silver handle

File:Train photo enhanced with mother of pearl.jpg|Train photo enhanced with mother of pearl. Made for display in the office of E. J. Gorman, president of the Chicago, Rock Island & Pacific RR. Now at the Western Trails Museum, Knott's Berry Farm, Buena Park, California.

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Manufactured nacre

In 2012, researchers created calcium-based nacre in the laboratory by mimicking its natural growth process.

In 2014, researchers used lasers to create an analogue of nacre by engraving networks of wavy 3D "micro-cracks" in glass. When the slides were subjected to an impact, the micro-cracks absorbed and dispersed the energy, keeping the glass from shattering. Altogether, treated glass was reportedly 200 times tougher than untreated glass.

References

External links

Objects with mother-of-pearl in the Staten Island Historical Society Online Collections Database