Imitation shell

Figure 10.18 TW-tone moulded plastic, imitating shell cameo.

Plastics have been used to copy expensive shell items. In order to imitate shell cameos, a plastic substance can be moulded in layers of different colours. The results tend to be obvious and crude, with mould marks and air bubbles, lack of structure or marks fiom the carvii tools, and a flat reverse side (F 10.18 and 10.19). 

Glass and porcelain have occasionally been used to imitate shell, but they are unconvincing. Both glass and glazed porcelain have a vitreous lustre that is different in appearance to that of the natural material. They also feel cold and hard, and have a complete lack of structure. Both materials are likely to show signs of having been moulded. 

Nanostructured materials are nothing new. Chrysotile fibers are an example (Fig. 16.22), as are bones, teeth and shells. The latter are composite materials made up of proteins and embedded hard, nanocrystalline, inorganic substances like apatite. Just as with the imitated artificial composite materials, the mechanical strength is accomplished by the combination of the components. 

In oxygen, with atoms with 6 outer electrons, stability is achieved by imitating the stable neon atom with 8 outer electrons. This is done by each oxygen atom using 2 electrons from its outer shell, making a total of 4 in the bond. This can be written as ... 

The MEDLA method does not impose any size limitation on the fragments only the feasibilty of traditional ab initio calculations limits the actual size of the fragments and the size of the "coordination shell" around them in the small molecule imitating the actual surroundings within the target molecule. Electron densities of satisfactory accuracy have been obtained in all the test calculations. 

Shell beads are made om the same material as the pearls themselves, so are naturally nacreous. However, the nacre does not cover the entire bead, but appears in layers due to the structure of the shell (Fig. 10.13). When dyed black, shell beads make convincing imitations of black peails, thou they tend to be too regular in shape. 

Cameos can also be moulded in one colour of plastic and painted, to imitate the different coloured layers of the shell. This is usually done in creamy-white plastic with just the background painted an appropriate colour. These imitations are easy to spot (Fig. 10.20). 

Plastic imitations of mother-of-pearl are more tricky to detect. One simulant is made by suspendir mica in a clear synthetic polymer. The effect is very realistic with an apparent play-of-colour, but the shape is often a give-away as the material is moulded. For example, a round napkin ring with no joins in the material could not be made fi m one piece of shell. Small items such as buttons are more difficult to identify. 

Injection moulding is used in the production of inexpensive jewellery in such forms as beads or bangles with patterned surfaces, and items such as brooches and cameos that imitate coral or shell. Other examples of products made this way are the ivory carvings which are often slightly flexible (Fig. 3.11). 

Each symbol in the table, which is an imitation of a musical note, shows the kind of light spurce and is quite useful for planning artistic contrasts and programs of fireworks. The symbol J shows one shell. The position and the form of dots show the kinds of colour and brightness. In this book the symbol is called "firework note". 

Thermal expansion can occur when materials, such as the metal components of a heat exchanger, are heated. For example, in a shell-and-tube heat exchanger, thermal expansion can cause an elongation of both the tube bundle and the shell as the temperature of the imit is increased. Because the tube bundle and... 

A variation of the normal sieve tray is the Turbogrid tray developed by the Shell Development Company. The Turbogrid tray uses slots instead of circular holes and operates as a counterflow plate imit with no downcomers. A typical tray consists of a flat grid of parallel slots. The slots can be stamped perforations in a flat metal sheet or the open spaces between parallel bars. 

Tannic acid is used as a fixative of dyes and as a chemical intermediate and reagent in the manufacture of inks, rubber, and imitation horns and tortoise shells. Tannic acid is also used to clarify beer and wine in photography as a chemical reagent in analytical laboratories and in pharmaceutical preparations. 

Compact (plate or finned-tube) exchangers have 350 ft of surface area/ft of volirme, and about four times the heat transfer per cubic foot of shell-and-tube imits. 

Plate-and-frame exchangers are suited to sanitary service, and, in stainless steel, are 25-50% cheaper than shell-and-tube imits. 

The first man-made polymer was nitrocellulose (NC). The main use of the NC resins was a replacement of the natural and expensive materials, viz., ivory, tortoise shell products, amber, ebony, onyx or alabaster. The use of cellulose acetate (CA) as a thermoplastic began in 1926. Cellulose ethers and esters became commercially available in 1927. Casein crosslinked by formaldehyde gave horn-like materials — Galalith has been used to manufacture shirt buttons, or as imitation of ivory and porcelain [Pontio, 1919]. 

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