Fig 1-5 Ternary (three-part) phase diagram of quartz (sand), clay, and feldspar. Early dental formulations began in the middle of the diagram (as china) and evolved toward feldsparrich compositions to improve esthetics. In Chinese formulations, feldspar was the flux (as a minor component).
Although Saxony tried to maintain a monopoly on porcelain making, the secret escaped as a result of its role in state prestige, industrial espionage, and greed within the Meissen porcelain works. By 1776, porcelain making was the topic of a review paper given at the Academy of Sciences in Paris. In 1770, Alexis Duchateau, an apothecary tired of his stained and malodorous dentures, sought assistance from Parisian dentist Nicolas Dubois de Chémant. Working with porcelain formulations and high-technology kilns of the Guehard Porcelain Factory, they succeeded in fabricating a complete denture for Duchateau in 1774. Porcelain dentures represented a huge step forward in personal hygiene, leading to public honors for de Chémant from the likes of Edward Jenner (pioneer of the smallpox vaccine), the Academy of Sciences, and the Academy of Medicine of Paris University. Because porcelain was a new invention in Europe and only available in collaboration with a high-technology company, from the very beginning its use in dentistry was certainly not craft art!
To escape the French Revolution, de Chémant fled to England in 1792, where he refined formulations of porcelain in collaboration with Josiah Wedgewood as he began his famous manufacturing company. de Chémant presumably worked to improve translucency, moving from the center of the ternary phase diagram toward a feldspar-rich formulation characteristic of today’s feldspathic materials (see Fig 1-5). He was essentially increasing the glass content of the porcelain, transitioning it into a predominantly glassy ceramic (see chapter 2). His porcelain dentures appear to have been very popular (Fig 1-6) due to their hygienic and esthetic superiority over the alternatives, mainly land and sea mammal ivory or human teeth from the battlefields of Europe and Civil War America.
Fig 1-6 Thomas Rowlandson’s etching satirizing the popularity of de Chémant and his porcelain dentures (1798): “Monsieur de Chémant from Paris agrees to offer from one tooth to a whole set without pain. Monsieur can also offer an artificial palate or a glass eye in a manner particular to himself.”
In 1808, another Parisian dentist, Giuseppangelo Fonzi, significantly improved the versatility of ceramics by firing individual denture teeth, each containing a platinum pin. This invention allowed teeth to be fixed to metal frameworks, enabling (1) partial denture fabrication (Fig 1-7), (2) reparability, and (3) increased esthetics. Platinum had only been known to Europeans since around 1741, and given its extremely high melting point (1,769°C), it was generally only worked into small wires and crucibles by hammering individual red-hot nuggets, like a blacksmith. Platinum was not used in jewelry until 1915.4 In 1808, platinum was used by alchemists in early chemistry experimentation. So it is likely that Fonzi obtained platinum wire from a local university or early “scientific supply house.” It was also the only metal that would not crack the denture tooth upon cooling, given its closely matched coefficient of thermal contraction. Again, this major improvement in our ability to use ceramics in dentistry clearly stands as “high technology.”
Fig 1-7 An early partial denture (terro-metallic incorruptibles) utilizing a platinum pin fused into the back of porcelain teeth, allowing the marriage of metalworking in framework fabrication with more esthetic teeth (eg, real embrasure forms) and reparability.
Modern Advancements
Further important steps in the use of ceramics in dentistry include the development of the first increased-strength core ceramic by Dr John McLean in 1965.5 Dr McLean and his ceramic engineering partner, T. H. Hughes, made a formulation of aluminum oxide particles suspended in a feldspathic glass (aluminous porcelain) utilizing a phenomenon called dispersion strengthening (strengthening due to the dispersion of filler particles).6 Dispersion strengthening of metals had been known and practiced for decades, but not for glasses. The first theory attempting to explain the dispersion strengthening of glasses appeared in the Journal of the American Ceramics Society in 1966.7 In fact, around 1965, General Electric began utilizing alumina fillers for increased strength in large power line insulators. So Dr McLean was applying new research findings and technology from the literature of an industry not related to dentistry—high technology again! Likewise, metal-ceramic systems were developed based on PhD thesis papers published in the engineering ceramics literature a few years ahead of the publication of the pivotal dental patent in 1962 (see chapter 5). The inventors of metal-ceramic systems even hired the PhD thesis mentor as a consultant.
Glass-ceramics (see chapter 2) were incorporated into dental practice not long after their discovery at the Corning Glass Works in Corning, New York (see chapter 5). Transfer molding and pressing of ceramics or pre-ceramic formulations brought advanced ceramics processing into the dental laboratory in the mid 1980s. In 1987, Werner Mörmann and Marco Brandestini8 introduced a revolutionary prototype machine (Fig 1-8) that would capture a three-dimensional (3D) image of a prepared tooth, use 3D design software to iteratively develop a proposed restoration, and then direct the computer-aided milling of inlays and onlays from solid blocks of esthetic, filled-glass ceramics (CEREC I, Sirona). Machining of esthetic glass-based ceramics is relatively straightforward, and special formulations were quickly developed that were much higher quality than what was available from dental laboratory processing based on either strengthened and fine-grained feldspathic ceramics (Mark II, Vita) or the first glass-ceramic introduced for dental use (containing interlocking tetrasilisic fluoromica flakes; DICOR-MGC, Dentsply).
Fig 1-8 Dr Werner Mörmann and engineer Marco Brandestini pose with their prototype CEREC machine, “the lemon,” circa 1985.
Today, computer-aided design/computer-assisted manufacturing (CAD/CAM) fabrication of both simple restorations and complex prostheses is routine. Polycrystalline ceramics (see chapter 2) are milled from lightly sintered blocks of zirconia and alumina to form oversized greenware that will shrink to the desired dimensions when fired (see chapter 5). Fully dense glass-ceramic blocks (see chapter 5) can be machined directly to the desired shapes with tolerances of tens of micrometers. Novel materials such as Enamic (Vita; see chapter 5), containing 3D interpenetrating phases of porcelain and polymer, are being introduced into dentistry specifically for CAD/CAM. Automated technologies are creating new business models within both the dental laboratory industry and dental clinics. The fabrication of unique parts (each one different) from identical blocks of starting materials is termed infinitely flexible manufacturing by our engineering colleagues. Among all the industries, dentistry is leading the way in infinitely flexible manufacturing,