Nacre
Encyclopedia
Composite material
Composite materials, often shortened to composites or called composition materials, are engineered or naturally occurring materials made from two or more constituent materials with significantly different physical or chemical properties which remain separate and distinct at the macroscopic or...
produced by some mollusks as an inner shell
Seashell
A seashell or sea shell, also known simply as a shell, is a hard, protective outer layer created by an animal that lives in the sea. The shell is part of the body of the animal. Empty seashells are often found washed up on beaches by beachcombers...
layer; it is also what makes up pearl
Pearl
A pearl is a hard object produced within the soft tissue of a living shelled mollusk. Just like the shell of a mollusk, a pearl is made up of calcium carbonate in minute crystalline form, which has been deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other...
s. It is very strong, resilient, and iridescent
Iridescence
Iridescence is generally known as the property of certain surfaces which appear to change color as the angle of view or the angle of illumination changes...
.
Nacre is found in some of the more ancient lineages
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
of bivalves, gastropods and cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
s. However, the inner layer in the great majority of mollusk shells is porcellaneous
Porcelain
Porcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between and...
, not nacreous, and this usually results in a non-iridescent shine, or more rarely in non-nacreous iridescence such as flame structure
Flame structure
A flame structure is a type of soft-sediment deformation that forms in unlithified sediments. The weight of an overlying bed forces an underlying bed to push up through the overlying bed, generally when both strata are saturated with water...
as is found in conch
Conch
A conch is a common name which is applied to a number of different species of medium-sized to large sea snails or their shells, generally those which are large and have a high spire and a siphonal canal....
pearls.
Pearls and the inside layer of pearl oyster
Pearl oyster
Pearl oysters are saltwater clams, marine bivalve molluscs of the genus Pinctada in the family Pteriidae. They have a strong inner shell layer composed of nacre, also known as "mother of pearl"....
and freshwater pearl mussel
Freshwater pearl mussel
The freshwater pearl mussel, scientific name Margaritifera margaritifera, is an endangered species of freshwater mussel, an aquatic bivalve mollusc in the family Margaritiferidae....
shells are made of nacre. Many other families of mollusk also have a nacreous inner shell layer, including marine gastropods such as the Haliotidae, the Trochidae
Trochidae
The Trochidae, common name top snails, are a taxonomic family of very small to large sea snails, marine gastropod molluscs in the clade Vetigastropoda ....
and the Turbinidae
Turbinidae
Turbinidae, the turban snails, are a family of small to large marine gastropods. Turbinidae have a strong, thick calcareous operculum readily distinguishing them from the somewhat similar Trochidae or top snails, which have a corneous operculum...
.
Appearance
Nacre appears iridescentIridescence
Iridescence is generally known as the property of certain surfaces which appear to change color as the angle of view or the angle of illumination changes...
because the thickness of the aragonite platelets is close 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.
Structure
Nacre is composed of hexagonal platelets of aragoniteAragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...
(a form of calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
) 10-20 µm wide and 0.5 µm thick arranged in a continuous parallel lamina. These layers are separated by sheets of organic matrix composed of elastic
Elasticity (physics)
In physics, elasticity is the physical property of a material that returns to its original shape after the stress that made it deform or distort is removed. The relative amount of deformation is called the strain....
biopolymers (such as chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
, lustrin and silk
Silk
Silk is a natural protein fiber, some forms of which can be woven into textiles. The best-known type of silk is obtained from the cocoons of the larvae of the mulberry silkworm Bombyx mori reared in captivity...
-like protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s). This mixture of brittle platelets and the thin layers of elastic biopolymers makes the material strong and resilient, with a Young's modulus
Young's modulus
Young's modulus is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke's Law holds. In solid mechanics, the slope of the stress-strain...
of 70 GPa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
. Strength and resilience are also likely to be due to adhesion by the "brickwork" arrangement of the platelets, which inhibits transverse crack propagation. This design at multiple length sizes increases its toughness
Toughness
In materials science and metallurgy, toughness is the ability of a material to absorb energy and plastically deform without fracturing; Material toughness is defined as the amount of energy per volume that a material can absorb before rupturing...
enormously, making it almost equivalent to that of silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
.
The crystallographic c-axis points perpendicular to the shell wall, but the direction of the other axes varies between groups. In bivalves and cephalopods, the b-axis points in the direction of shell growth, whereas in the monoplacophora it is the a-axis that is this way inclined. The interlocking of bricks of nacre has large impact on both the deformation mechanism as well as toughness of nacre. In addition, the mineral - organic interface results in enhanced resilience and strength of the organic interlayers.
Formation
Nacre formation is mediated by the organic matrix, which controls the onset, duration and form of crystal growth. Individual aragonite "bricks" quickly grow to the full height of the nacreous layer, and expand until they abut adjacent bricks. Bricks nucleate on randomly-dispersed elements within the organic layer. This produces the hexagonal close-packing characteristic of nacre. Nacre differs from fibrous aragonite – a brittle mineral of the same form – in that the growth in the c-axis (i.e. perpendicular to the shell, in nacre) is slow in nacre, and fast in fibrous aragonite.Function
Nacre is secreted by the epithelial cellsCell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
of the mantle tissue
Mantle (mollusc)
The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...
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
Pearl
A pearl is a hard object produced within the soft tissue of a living shelled mollusk. Just like the shell of a mollusk, a pearl is made up of calcium carbonate in minute crystalline form, which has been deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other...
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 mollusk lives.
Evolution
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.Commercial sources
Currently the chief sources of mother of pearl are the pearl oysterPearl oyster
Pearl oysters are saltwater clams, marine bivalve molluscs of the genus Pinctada in the family Pteriidae. They have a strong inner shell layer composed of nacre, also known as "mother of pearl"....
, freshwater pearl mussels, and to a lesser extent, abalone
Abalone
Abalone , from aulón, are small to very large-sized edible sea snails, marine gastropod molluscs in the family Haliotidae and the genus Haliotis...
. Widely used for pearl buttons especially during the 1900s, were the shells of the great green turban snail Turbo marmoratus
Turbo marmoratus
Turbo marmoratus, the marbled turban or great green turban, is a large species of marine gastropod with a calcareous operculum in the family Turbinidae, the turban snails. These large snails live in tropical reefs in the Indian and western Pacific oceans. They are nocturnal and feed on algae.The...
and the large top snail, Trochus niloticus.
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 ceramicCeramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
tile
Tile
A tile is a manufactured piece of hard-wearing material such as ceramic, stone, metal, or even glass. Tiles are generally used for covering roofs, floors, walls, showers, or other objects such as tabletops...
or marble
Marble
Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or dolomite.Geologists use the term "marble" to refer to metamorphosed limestone; however stonemasons use the term more broadly to encompass unmetamorphosed limestone.Marble is commonly used for...
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 2 mm thick. The tesserae are then lacquer
Lacquer
In a general sense, lacquer is a somewhat imprecise term for a clear or coloured varnish that dries by solvent evaporation and often a curing process as well that produces a hard, durable finish, in any sheen level from ultra matte to high gloss and that can be further polished as required...
ed and polished
Polishing
Polishing is the process of creating a smooth and shiny surface by rubbing it or using a chemical action, leaving a surface with a significant specular reflection In some materials polishing is also able to reduce diffuse reflection to...
creating a durable and glossy surface.
Instead of using a marble or tile base, the nacre tesserae can be glued to fiberglass
Fiberglass
Glass fiber is a material consisting of numerous extremely fine fibers of glass.Glassmakers throughout history have experimented with glass fibers, but mass manufacture of glass fiber was only made possible with the invention of finer machine tooling...
. 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.
Fashion
Mother of pearl buttons are often used in clothing either for functional or decorative purposes. The Pearly Kings and QueensPearly Kings and Queens
Pearly Kings and Queens, known as pearlies, are an organised charitable tradition of working class culture in London, England.The practice of wearing clothes decorated with pearl buttons originated in the 19th century. It is first associated with Henry Croft, an orphan street sweeper who collected...
are an elaborate example of this.
Nacre is also used to decorate watches, knives, guns and jewelry.
Musical instruments
Nacre inlay is often used for key touches and other decorative motifs on musical instruments. Many accordionAccordion
The accordion is a box-shaped musical instrument of the bellows-driven free-reed aerophone family, sometimes referred to as a squeezebox. A person who plays the accordion is called an accordionist....
and concertina
Concertina
A concertina is a free-reed musical instrument, like the various accordions and the harmonica. It has a bellows and buttons typically on both ends of it. When pressed, the buttons travel in the same direction as the bellows, unlike accordion buttons which travel perpendicularly to it...
bodies are completely covered in nacre, and some guitar
Guitar
The guitar is a plucked string instrument, usually played with fingers or a pick. The guitar consists of a body with a rigid neck to which the strings, generally six in number, are attached. Guitars are traditionally constructed of various woods and strung with animal gut or, more recently, with...
s have fingerboard or headstock inlays made of nacre (as well as some guitars having plastic inlays designed to imitate the appearance of nacre). Greek bouzouki
Bouzouki
The bouzouki , is a musical instrument with Greek origin in the lute family. A mainstay of modern Greek music, the front of the body is flat and is usually heavily inlaid with mother-of-pearl. The instrument is played with a plectrum and has a sharp metallic sound, reminiscent of a mandolin but...
and baglamas instruments typically feature nacre decorations. Bows of stringed instruments like the violin
Violin
The violin is a string instrument, usually with four strings tuned in perfect fifths. It is the smallest, highest-pitched member of the violin family of string instruments, which includes the viola and cello....
and cello
Cello
The cello is a bowed string instrument with four strings tuned in perfect fifths. It is a member of the violin family of musical instruments, which also includes the violin, viola, and double bass. Old forms of the instrument in the Baroque era are baryton and viol .A person who plays a cello is...
often have mother of pearl inlay at the frog.
Other
Mother of pearl is used to make spoon-like utensils for caviar, so as to not spoil the taste with metallic spoons.Further reading
- Frýda J., Bandel K. & Frýdová B. (2009). "Crystallographic texture of Late Triassic gastropod nacre: evidence of long-term stability of the mechanism controlling its formation". Bulletin of Geosciences 84(4): 745-754. doi:10.3140/bull.geosci.1169.