Are Shells Ceramics?

Shells are not ceramics and hence they can not be equated in properties and formation. To explain, ceramics are heated and hardened materials formed from oxides such as Alumina, Zirconia, Quartz (Silicon dioxide), Nitrides such as Silicon Nitride and carbides like SIlicon carbide etc. Essentially, the ancient way of making ceramics was by way of mixing clays and heating them at high temperatures. All the eating plates, pots, ornamental vases etc., have been ceramics and in the modern times, ceramics have found indutrial applications due to their heat resistance, chemical resistance, insulation properties etc. Ceramics are being used as super conductor materials also. Industrial ceramics are fomred in a process called crystallization where the various materials as mentioned ealrier are molten at very high temp. and allowed to form crystals. or in a process called Sol-Gel process. Coming to sea shells, these are carbonates. Carbonates of calcium are the outer bodies also known external skeletons of sea creatures (Mollusks). These are protective layers fromed by the Mollusks. The calcium carbonate exterior is a secretion released by the mollusk and thus formed as a natural secretion model outside the soft mollusk. The shells may vary in compositions. Essentially the shells are calicum carbonates but the colours and patterns in sea shells vary based upon the minerals absorbed by the mollusk or from the digestive release of the waste based on the food eaten by the mollusk. These waste secretions are released into the calcium carbonate layers and thus patterns of various colurs are formed. The secretions being a natural process occur at the surrounding temperature whereas ceramics are formed of crystallization at very high temperatures. Ceramics are man made and not natural. There may be some cases of natural ceramic formation due to volcanic activity but normally it is a process developed by human being app 5000 yrs ago.

There is another school of thought that defines ceramics as anything that is not a metal. From the American Ceramics Society (ACerS), http://www.acers.org/acers/aboutceramics.asp, "Ceramics can be defined as inorganic, nonmetallic materials. They are typically crystalline in nature and are compounds formed between metallic and nonmetallic elements such as aluminum and oxygen (alumina-Al2O3), calcium and oxygen (calcia - CaO), and silicon and nitrogen (silicon nitride-Si3N4)." This definition includes naturally occuring materials - not just man made. Also, ceramics span the entire range between fully crystalline (precise arrangement of atoms in a long range, crystalline structure) to fully amorphous (no long range order). Calcium carbonate includes carbon, which makes it organic ... but there are still the inorganic elements. And it's not a metal. Depending on how strictly one defines both ceramics and organics, it can go either way.

You had seen my comments and I have covered all the materials you had mentioned in your reply. There is nothing new about these materials as these have been known to human kind for centuries. The process is such that metallic oxides such as metnioned earlier, have to be taken to an elevated temperature and reprocessed for the crystal growth or sol-gel process. Then only these materials are termed as ceramics. Else, these metallic oxides will be known in their regular names such as Alumina (Al2O3), Silica Gel, Zeolites etc. Calcium carbonate per se does not fall under the catagory of ceramics unless it is processed with other materials such as Coralline, beta-tricalcium phosphate etc. Once a process is undertaken, it no longer becomes natural but man made. Hence, I have created the distinction between naturally occuring metallic oxides and ceramics. Further, I had also mentioned that ceramics do form in nature due to volcanic activity. I hope this clarifies.

Shells are not ceramics. Ceramics are fired and fused mixtures of glass-forming oxides, plus (usually) aluminum oxide, which does not fuse completely, and which allows the ceramic to maintain its shape while hot. The constituents of the ceramic, including some of the aluminum, actually form new molecular compounds when fused. It is the aluminum oxide that differentiates ceramics from glasses. Shells owe their strength and toughness to their hierarchical structure. On a microscopic level, there are layers of calcium-rich mineral substances and layers of protein-like substances. (This is gross oversimplification. The stucture is very complex, and occurs at the molecular level, the cellular level, the micro level, and the macro level. The number of levels is sometimes called the order.) Wood, bone and tendons are other hierarchical structures. Man has not come close to duplicating them. Fiber-reinforced composites are a crude approximation. The Eiffel tower is a large scale second-order structure. Ceramics also have about one-half order because of the mixture characteristic, and this is largely responsible for their generally being less brittle than pure glasses. End of Rant.

While seashells are not strictly ceramics, their complex chemical structural arrangement is an example of a 'biological design paradigm' successfully adapted to efficiently dissipate applied forces and restrain fracture propagation. In light of the highly efficient way this is done, the design of new ceramic composites are being inspired by these seashell design paradigms. GE's nanotechnology group, looking to develop new ceramics for jet engine components, for instance, has successfully mimicked the abalone shell design paradigm. For readers interested in the growing commercial development of 'biological design paradigms' leading to bio-inspired products, http://www.biomimeticsregistry.net provides examples of developments in material sciences and optical, acoustical, mechanical and intelligent software, systems and control engineering.