AY Honors/Shells - Advanced/Answer Key

A piece of nacre

A schematic of the microscopic structure of nacre, showing how the layers of aragonite platelets are separated by protein.

Nacre, also known as mother of pearl or sadaf, is a naturally occuring organic-inorganic composite. It is formed of layers of calcium carbonate (CaCO₃) platelets (crystals), in the form of aragonite and conchiolin (a scleroprotein), separated by elastic biopolymers (such as chitin and lustrin). This mixture of hard and elastic domains makes the material strong and resilient. Strength and resilience is also likely to be due to the offset or "brickwork" arrangement of the platelets, which inhibits transverse crack propagation.

The iridescent appearance of the nacre is due to the fact that the thickness of the aragonite platelets are about 0.5 micrometres, which is comparable to the wavelength of visible light. This results in constructive and destructive interference of different wavelengths of light, resulting in different colors of light being reflected at different viewing angles.

Nacre is secreted by the epithelial cells of the mantle tissue of certain species of mollusk. In these mollusks, nacre is continually deposited onto the inner surface of the animal's shell (the iridescent nacreous layer, commonly known as mother of pearl), both as a means to smooth the shell itself and as a defense against parasitic organisms and damaging detritus.

The iridescent inner layer is considered highly attractive by many cultures and is often used in making jewelery or as inlays in wood furniture and guitars.

When a mollusk is invaded by a parasite or is irritated by a foreign object that the animal cannot eject, a process known as encystation entombs the offending entity in successive, concentric layers of nacre. This process eventually forms what we call pearls and continues for as long as the mollusk lives.

Chief sources are the pearl oyster, found in warm and tropical seas, primarily in Asia; freshwater pearl mussels, which live in many rivers of the United States, Europe, and Asia; and the abalone of California, Japan, and other Pacific regions.

Decorative uses

Tiles: Small, hand-cut shapes with straight edges, called 'tesserae', are laminated to either a marble or a ceramic 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 1/16 of an inch thick.

Sheets: Instead of using a marble or tile base, the mother of pearl tesserae are glued to a fiberglass mesh. The result is a lightweight material that offers a seamless installation, and there is no limit to the sheet size. Mother of pearl 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.

Both black and white mother of pearl are used for architectural purposes. The natural mother of pearl may be artificially tinted to almost any color.

Mother of pearl is also used to clad porcelain sinks. Mother of pearl tesserae may be cut into artistic shapes and be laminated to ceramic tile and surrounded by numerous coats of colored lacquer to create an artistic design. The lacquer and the mother of pearl are polished at the same time, creating a durable, glossy hard surface.

It is also available in thin laminated sheet form, allowing easy inlay and cladding work, and modern ceramic tiles.

Shirt buttons were once often made of mother of pearl, though plastic has largely replaced its use. Some of the finer quality clothing companies (e.g. Lacoste) still use it, however.

Key touches and various decorations on musical instruments such as saxophones, trumpets, and guitars are sometimes mother of pearl. Synthetic key touches made of Pyralin or similar imitation materials have been affectionately referred to as mother of toilet seat by musicians.

Reference

• Lin, A., and Meyers, M.A. 2005. Growth and structure in abalone shell, Materials Science and Engineering A 390(Jan. 15):27–41 ([1])
• Mayer, George, "Rigid Biological Systems as Models for Synthetic Composites", Science 310 (18 Nov 2005):1144–1147

([2])

See also

• Ammolite