Opal glass-ceramics

The microstructure of this opal glass-ceramic shows the different phases of liquid-liquid phase separation responsible for producing opalescence. Initial crystallization of leucite is visible in the interface region between the individual glass grain boundaries (Section 3.2.3). 

Properties Fotoform Glass Fotoform Opal Glass-Ceramic Fotoceram Glass-Ceramic... 

Figure 4-33 Schematic diagram of a glass-ceramic crown of IPS EMPRESS leucite glass-ceramic. Pressed glass-ceramic (dentin), sintered ceramic with opal glass-ceramic (incisal), and glaze.

The Transpa material heightens the transmission properties of the dental product. These materials are either applied with the incisal material or layered on the incisal glass-ceramic. The Effect materials of IPS d.SIGN are opal glass-ceramics that produce an opalescent appearance (see Section 2.4.6). 

Figure 4-52 Application of opal glass and opal glass-ceramic (E1-E5) in restorative dentistry (dental crown), a) scheme, b) clinical situation.

Unlike conventional ceramic materials, glass-ceramics are fully densifted with zero porosity. They generally are at least 50% crystalline by volume and often are greater than 90% crystalline Other types of glass-based materials that possess low amounts of crystallinity, such as opals and mby glasses, are classified as glasses and are discussed elsewhere (see Glass). 

An important step in the fabrication of opal glass and glass ceramics is ... 

Sturkey, S.D. (2000) Explorations in Glass, American Ceramics Society, Westerville, OH. A must-read for anyone interested in glass. Particularly nice discussion on opal glass. 

Cryolite. Natural sodium aluminium fluoride, NajAlF m.p. 980 C sp. gr. 2.95. Because of its low m.p. and its fluxing action, it is used in the manufacture of enamels and glass and in the ceramic coatings of welding rods. Opal glass is often made from batches containing about 10% cryolite a similar preparation is sometimes used in white cover-coat enamels. 

Glass-ceramics with a leucite main crystal phase (Section 2.2.9) are also produced according to the mechanism of controlled surface crystallization of the opal base glass since volume nucleation cannot be controlled. In this case, it is important for the crystallites to achieve a high nuclei density and to uniformly precipitate into the glassy matrix. The coast-and-island microstructure has been specifically developed as the transitional stage (Holand et al., 1996b). 

Figure 3-3 SEM overview of the coast-and-island microstructure in opal leucite glass-ceramics.

Figures 3-4, 3-5, and 3-6 show the individual phases and the interface magnified 20,000, 30,000, and 50,000 times. The glass phase (Fig. 3-4) exhibits phase-separation processes in the form of droplet phases less than 200 nm in size. This phase separation creates the opal effect of the glass-ceramic. Although the crystals of the leucite type (Fig. 3-5) in the coastal areas (marked 2 in Fig. 3-3) measure only approximately 1 pm, they produce a highly translucent effect in the glass-ceramic. The crystals provide the material with a very high coefficient of thermal expansion. The crystal-glass interface is shown in Fig. 3-6. Clearly, crystal growth was interrupted at a specific st e of growth once a crystal front of some micrometer thickness had formed.

Important amonnts of cryolite are used in the ceramic indnstry, the greatest consumption occurring in the manufacture of opal glass and enamels. Less widely known is cryolite s extensive nse as a filler for abrasive wheels, especially the resin- and rubber-bonded types. Another apphcation is in flux coatings for welding rods, particularly those used to join aluminum. 

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