The main aims of the third edition of this reference book are to present research and development highlights since 2012, to update areas of fundamental importance to glass‐ceramics like nucleation and growth, and to add material concerning forming processes, including parent glass forming, novel crystallization processes and ion exchange strengthening.

There have been major advances in products based on glass‐ceramics in dentistry and other biomaterials. In this edition, a new approach has been chosen to present the materials for restorative dentistry: A particular focus is placed on illustrating the immediate benefits of these products to both dentists and patients. New scientific findings have made it possible to increase the processing efficiency of dental restorative materials and significantly extend their application range. Today, patients have a wide array of metal‐free, inorganic biomaterials at their disposal, which offer both excellent function and aesthetics.

In this edition, new products and potential uses involving novel optical, electronic, and mechanical properties are described. These include transparent glass‐ceramics for mobile phones and tablets, and translucent materials for LEDs and phosphorescent displays.

The third edition of this publication, like the first two, is a product of close collaboration between the two authors. They regularly consulted together on their individual sections. They discussed many aspects of composition, phase transformation, microstructure, and useful novel analytic techniques; the latter often used in combination to better understand the sequence of crystallization in various glass systems.

W. Höland would like to give special mention to the following people from Ivoclar Vivadent AG: Markus Rampf, Marc Dittmer, Christian Ritzberger, Marcel Schweiger, and Ronny Watzke for their scientific discussions and R. Ganley, V.M. Rheinberger and T. Hirt for their support to glass‐ceramic research. He would also like to thank the TC 07 Subcommittee of the International Congress on Glass (ICG), and colleagues of this group, especially J. Deubener, M.J. Pascual, T. Komastsu, E.D. Zanotto, I. Mitra. A special Thanks goes to M. Höland (University of Applied Sciences, Buchs, SG, Switzerland) for many scientific discussions. S. Fuchs (South Africa) is thanked for translation work.

Both authors like to thank S. Tanabe, H. Hosono and J. Schmelzer who helped with valuable suggestions and advice.

G. Beall would like to thank Charlene Smith for her helpful comments and advice. He would also credit D.L. Morse, G. Calabrese, C. Heckle and M. Pambianchi for their continuing support of research on glass‐ceramics at Corning Incorporated.

Both authors would also like to thank A. Höland (graphic design, Schaan, Liechtenstein) and M. Höland for graphs in the second and third edition of this textbook, and I. Heidelauf (Ivoclar‐Vivadent AG) for the graphs of the first edition.

Schaan, Principality of Liechtenstein

Corning, NY, USA

April 2019

Glass‐ceramics are ceramic materials formed through the controlled nucleation and crystallization of glass. Glasses are melted, fabricated to shape, and thermally converted to a predominantly crystalline ceramic. The basis of controlled internal crystallization lies in efficient nucleation that allows the development of fine, randomly oriented grains generally without voids, microcracks, or other porosity. The glass‐ceramic process, therefore, is basically a simple thermal process as illustrated in Figure H.1.

It occurred to Reamur (1739) and to many people since that a dense ceramic made via the crystallization of glass objects would be highly desirable. It was not until about 35 years ago, however, that this idea was consummated. The invention of glass‐ceramics took place in the mid‐1950s by the famous glass chemist and inventor, Dr. S.D. Stookey. It is useful to examine the sequence of events leading to the discovery of these materials (Table H.1).

Dr. Stookey at the time was not primarily interested in ceramics. He was preoccupied in precipitating silver particles in glass in order to achieve a permanent photographic image. He was studying as host glasses lithium silicate compositions because he found he could chemically precipitate silver in alkali silicate glasses, and those containing lithium had the best chemical durability. In order to develop the silver particles, he normally heated glasses previously exposed to ultraviolet light just above their glass transition temperature at around 450 °C. One night the furnace accidentally overheated to 850 °C, and, on observation of the thermal recorder, he expected to find a melted pool of glass. Surprisingly, he observed a white material that had not changed shape. He immediately recognized as a ceramic produced showing no distortion from the original glass article. A second serendipitous event then occurred. He dropped the sample accidentally, and it sounded more like metal than glass. He then realized that the ceramic he had produced had unusual strength.

On contemplating the significance of this unplanned experiment, Stookey recalled that lithium aluminosilicate crystals had been reported with very low thermal expansion characteristics; in particular, a phase, β‐spodumene, had been described by Hummel (1951) as having a near‐zero thermal expansion characteristic. He was well aware of the significance of even moderately low expansion crystals in permitting thermal shock in otherwise fragile ceramics. He realized if he could nucleate these and other low coefficient of thermal expansion phases in the same way as he had lithium disilicate, the discovery would be far more meaningful. Unfortunately, he soon found that silver or other colloidal metals are not effective in nucleation of these aluminosilicate crystals. Here he paused and relied on his personal experience with specialty glasses. He had at one point worked on dense thermometer opals. These are the white glasses that compose the dense, opaque stripe in a common thermometer. Historically, this effect had been developed by precipitation of crystals of high refractive index such as zinc sulfide or titania. He, therefore, tried adding titania as a nucleating agent in aluminosilicate glasses and discovered it to be amazingly effective. Strong and thermal shock‐resistant glass‐ceramics were then developed commercially within a year or two of this work with well‐known products such as rocket nose cones and Corning Ware® cookware resulting (Stookey 1959).

In Summary, a broad materials advance had been achieved from a mixture of serendipitous events controlled by chance and good exploratory research related to a practical concept, albeit unrelated to a specific vision of any of the eventual products. Knowledge of the literature, good observation skills, and deductive reasoning were clearly evident in allowing the chance events to bear fruit.

Without the internal nucleation process as a precursor to crystallization, devitrification is initiated at lower‐energy surface sites. As Reamur was painfully aware, the result is an ice‐cube‐like structure (Figure H.2), where the surface‐oriented crystals meet in a plane of weakness. Flow of the uncrystallized core glass in response to changes in bulk density during crystallization commonly forces the original shape to undergo grotesque distortions. On the other hand, because crystallization can occur uniformly and at high viscosities, internally nucleated glasses can undergo the transformation from glass to ceramic with little or no deviation from the original shape.

To consider the advantages of glass‐ceramics over their parent glasses, one must consider the unique features of crystals, beginning with their ordered structure. When crystals meet, structural discontinuities or grain boundaries are produced. Unlike glasses, crystals also have discrete structural plans that may cause deflection, branching, or splintering of cracks. Thus the presence of cleavage planes and grain boundaries serves to act as an impediment for fracture propagation. This accounts for the better mechanical reliability of finely crystallized glasses. In addition, the spectrum of properties in crystals is very broad compared with that of glasses. Thus some crystals may have extremely low or even negative thermal expansion behavior. Others, like sapphire, may be harder than any glass, and crystals like mica might be extremely soft. Certain crystalline families also may have unusual luminescent, dielectric, or magnetic properties. Some are semiconducting or even, as recent advances attest, may be superconducting at liquid nitrogen temperatures. In addition, if crystals can be oriented, polar properties like piezoelectricity or optical polarization may be induced.

Another method of manufacture of glass‐ceramics has proven technically and commercially viable. This involves the sintering and crystallization of powdered glass. This approach has certain advantages over body‐crystallized glass‐ceramics. First, traditional glass‐ceramic processes may be used, e.g. slip casting, pressing, and extruding. Second, because of the high flow rates before crystallization, glass‐ceramic coatings on metals or other ceramics may be applied by using this process. Finally, and most important, is the ability to use surface imperfections in quenched frit as nucleation sites. This process typically involves milling a quenched glass into fine 3–15 μm particle diameter particulate. This powder is then formed by conventional ceramming called forming techniques in viscous sintering to full density just before the crystallization process is completed. Figure H.3 shows transformation of a powdered glass compact (Figure H.3a) to a dense sintered glass with some surface nucleation sites (Figure H.3b) and finally to a highly crystalline frit derived glass‐ceramic (Figure H.3c). Note the similarity in structure between the internally nucleated glass‐ceramic in Figure H.1c. The first commercial exploitation of frit‐derived glass‐ceramics was the devitrifying frit solder glasses for sealing television bulbs. The technology has been applied to cofired, multilayer substrates for electronic packaging and biomaterials for dental restoration.