Nacre

Nacre, also known as mother-of-pearl, is an organic–inorganic 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

The biomineralized mother-of-pearl consists of at least 95% (by mass) of the inorganic chemical compound calcium carbonate (CaCO 3) in the modification aragonite. Up to five percent consists of organic substance. The aragonite takes the form of pseudo- hexagonal platelets with a diameter of 5–15 µm and a height of 0.5 µm. These aragonite platelets are arranged horizontally (in the shell plane) to form individual layers. Vertically (perpendicular to the shell plane), the platelets in mussels are arranged in alternating brick-like patterns, while in snails they are arranged more in stacks. The so-called organic matrix extends between the individual platelets both in the shell plane and perpendicular to it. 

Organic matrix

The organic matrix is responsible for the growth and mechanical properties of nacre. The structure and role of the organic matrix during nacre growth is subject to intensive research. It is usually divided into water-insoluble and water-soluble matrix, depending on how the organic components behave after demineralization of the nacre.

The water-insoluble matrix is the material located vertically (interlamellar matrix) and laterally (intertabular matrix) between the platelets. The interlamellar matrix has a thickness of approximately 30–50 nm and has a core of chitin. This chitin is coated on both sides with various proteins, including silk fibroin. The intertabular matrix is thinner than the interlamellar matrix, but also consists of chitin and proteins.

The water-soluble matrix consists of a series (> 10) of proteins, some of which have a strong influence on the crystallization of calcium carbonate.

Structure and appearance

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 a continuous parallel lamina. Depending on the species, the shape of the tablets differs; in Pinna, the tablets are rectangular, with symmetric sectors more or less soluble. Whatever the shape of the tablets, the smallest units they contain are irregular rounded granules. These layers are separated by sheets of organic matrix (interfaces) composed of elastic biopolymers (such as chitin, lustrin and silk-like proteins).

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. 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 inclined this way.

Optical properties

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 layered structures are on the order of the wavelength of visible light. 

Since part of the incident white light is transmitted and part is reflected at each layer, interference occurs: incident and reflected light rays overlap in such a way that certain portions of the white light spectrum are canceled out, leaving different colors depending on the viewing angle (see also Bragg equation). When the mother-of-pearl is moved in the light, it therefore appears to shimmer in bright colors (iridescence).

Mechanical properties

This mixture of brittle platelets and the thin layers of elastic biopolymers makes the material strong and resilient, with a Young's modulus of 70 GPa and a yield stress of roughly 70 MPa (when dry). Strength and resilience are also likely to be due to adhesion by the "brickwork" arrangement of the platelets, which inhibits transverse crack propagation. This structure, spanning multiple length sizes, greatly increases its toughness, making it almost as strong as silicon. The mineral–organic interface results in enhanced resilience and strength of the organic interlayers. 

The interlocking of bricks of nacre has large impact on both the deformation mechanism as well as its toughness. Tensile, shear, and compression tests, Weibull analysis, nanoindentation, and other techniques have all been used to probe the mechanical properties of nacre. 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. As an oriented structure, nacre is highly anisotropic and as such, its mechanical properties are also dependent on the direction.

A variety of toughening mechanisms are responsible for nacre's mechanical behavior. The adhesive force needed to separate the proteinaceous and the aragonite phases is high, indicating that there are molecular interactions between the components. In laminated structures with hard and soft layers, a model system that can be applied to understand nacre, the fracture energy and fracture strength are both larger than those values characteristic of the hard material only. Specifically, this structure facilitates crack deflection, since it is easier for the crack to continue into the viscoelastic and compliant organic matrix than going straight into another aragonite platelet. 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. Fiber pull-out, which occurs in other ceramic composite materials, contributes to this phenomenon. Unlike in traditional synthetic composites, the aragonite in nacre forms bridges between individual tablets, so the structure is not only held together by the strong adhesion of the ceramic phase to the organic one, but also by these connecting nanoscale features. As plastic deformation starts, the mineral bridges may break, creating small asperities that roughen the aragonite-protein interface. The additional friction generated by the asperities helps the material withstand shear stresses. 

In nacre-like composites, the mineral bridges have also been shown to increase the flexural strength of the material because they can transfer stress in the material. 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. The hardness of this material is also negatively impacted by dehydration. Water acts as a plasticizer for the organic matrix, improving its toughness and reducing its shear modulus. Hydrating the protein layer also decreases its Young's modulus, which is expected to improve the fracture energy and strength of a composite with alternating hard and soft layers.

The statistical variation of the platelets has a negative effect on the mechanical performance (stiffness, strength, and energy absorption) because statistical variation precipitates localization of deformation. However, the negative effects of statistical variations can be offset by interfaces with large strain at failure accompanied by strain hardening. On the other hand, the fracture toughness of nacre increases with moderate statistical variations which creates tough regions where the crack gets pinned. But, higher statistical variations generates very weak regions which allows the crack to propagate without much resistance causing the fracture toughness to decrease. Studies have shown that this weak structural defects act as dissipative topological defects coupled by an elastic distortion.

Formation

The process of how nacre is formed is not completely clear. It has been observed in Pinna nobilis, where it starts as tiny particles (~50–80 nm) grouping together inside a natural material. These particles line up in a way that resembles fibers, and they continue to multiply. When there are enough particles, they come together to form early stages of nacre. The growth of nacre is regulated by organic substances that determine how and when the nacre crystals start and develop.

Each crystal, which can be thought of as a "brick", is thought to rapidly grow to match the full height of the layer of nacre. They continue to grow until they meet the surrounding bricks. This produces the hexagonal close-packing characteristic of nacre. The growth of these bricks can be initiated in various ways such as from randomly scattered elements within the organic layer, well-defined arrangements of proteins, or they may expand from mineral bridges coming from the layer underneath.

What sets nacre apart from fibrous aragonite, a similarly formed but brittle mineral, is the speed at which it grows in a certain direction (roughly perpendicular to the shell). This growth is slow in nacre, but fast in fibrous aragonite.

A 2021 paper in Nature Physics examined nacre from Unio pictorum, noting that in each case the initial layers of nacre laid down by the organism contained spiral defects. Defects that spiralled in opposite directions created distortions in the material that drew them towards each other as the layers built up until they merged and cancelled each other out. Later layers of nacre were found to be uniform and ordered in structure.

Function

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.

Commercial sources

The main commercial sources of mother-of-pearl have been the pearl oyster, freshwater pearl mussels, and to a lesser extent the abalone, popular for their sturdiness and beauty in the latter half of the 19th century.

Widely used for pearl buttons especially during the 1900s, were the shells of the great green turban snail Turbo marmoratus and the large top snail, Tectus niloticus. The international trade in mother-of-pearl is governed by the Convention on International Trade in Endangered Species of Wild Fauna and Flora, an agreement signed by more than 170 countries.

Uses

The polished and polished shells of pearl oysters were a common currency (shell money), for example, in the Polynesian world. Even today, they retain similar value in some places. Another form of currency were mother-of-pearl chips in many European casinos until the end of the 19th century.

Nacre is still used today in the construction of musical instruments (particularly in the form of mother-of-pearl eyes). For example, it is used in the construction of guitars and basses as fingerboard inlays for orientation – so-called "inlays" in the form of blocks, dots, or pictorial representations (such as birds). The frog eyes and bows of string instruments are also often made of mother-of-pearl. White or black mother-of-pearl is also used for the inlays of the keys and valves of high-quality saxophones and brass instruments.

In the past, fishing lures made of mother-of-pearl were widely used. The prismatic shimmer successfully deceived many predatory fish species into thinking they were a tasty treat. Anglers also liked these mother-of-pearl lures because mother-of-pearl is heavy enough to be cast (enough) far out into the lake or sea with a rod and line.

Mother of pearl is an advantageous material for making spoons because it is tasteless when in contact with eggs or caviar. It is being investigated whether artificially produced mother-of-pearl, pearloid, is suitable as a corrosion-resistant protective layer on ship hulls.

Decorative

Nacre has long been used in jewelry because of its iridescent optical properties. Mother-of-pearl buttons are often used for high-quality shirts and blouses. In the northern Thuringian town of Bad Frankenhausen, there was a thriving mother-of-pearl button industry in the 19th century. In addition to precious wood veneers, mother-of-pearl flakes were also used to decorate furniture and wooden boxes (inlays). Mother-of-pearl plays a major role in Chinese lacquer art.

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 2 millimetres (0.079 in) 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 accomplished artist who successfully carved the shells of oysters to give a shape of human being which led to the invention of new horizon in Indian contemporary art. 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 one of the finest mother-of-pearl artists in the middle of 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 illustrious 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 famed 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. It is sometimes used in the decorative grips of firearms, and in other gun furniture. 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.

Biomedical use

The biotech company Marine Biomedical, formed by a collaboration between the University of Western Australia Medical School and a Broome pearling business, is as of 2021 developing a product nacre to create "PearlBone", which could be used on patients needing bone grafting and reconstructive surgery. The company is applying for regulatory approval in Australia and several other countries, and is expecting it to be approved for clinical use around 2024–5. It is intended to build a factory in the Kimberley region, where pearl shells are plentiful, which would grind the nacre into a product fit for use in biomedical products. Future applications could include dental fillings and spinal surgery.

Manufactured nacre

The appearance of mother-of-pearl does not come from pigments; the superposition of layers of different refractive index creates interferences, like those that occur in a dichroic filter or in structural colors, so that the color depends on the angle of incidence of the light and the position of the observer, which allows characteristic iridescences to be seen. The possible coloration of mother-of-pearl comes from carotenoids, contained in conchiolin. Their complexation with proteins to form carotenoproteins can modify the initial color of the pigment and give shades ranging from yellow to purple.

Manufacturers have long tried to reproduce the appearance of mother-of-pearl. From the 17th century onwards, formulations based on fish scales were found under the name of Oriental essence. Oriental essence, listed in the Colour Index under the reference NW1, is a mixture of guanine and hypoxanthine, which varies depending on the species used. The plastics industry produced pearl buttons from lead phosphates. These toxic compounds are banned for cosmetics. PW14 is a bismuth oxychloride and is the leading pearl pigment today. Compounds of mica and metal oxides, patented in 1963, provide pearl pigments of all dominant colours. Finally, pearl pigments can be made with silica or aluminium particles, covered with layers of varying refractive index to create the interferences that make up the pearly appearance. These pigments find an outlet in cosmetics and in the automotive industry, where they enrich the range of available aspects of ordinary and metallic paints.

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.

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