Читать книгу Ceramics in Dentistry - J. Robert Kelly - Страница 7

1

Introduction to Ceramics in Dentistry—Where Did This Stuff Come From?

Craft Art or High Technology

When I have the opportunity to lecture on the history of ceramics in dentistry, I enjoy challenging audiences to commit by a raise of hands as to whether they think dentistry borrowed ceramic technology from craft art or pursued it through innovation and high technology. To further develop the point, I draw a clear distinction between high technology and craft art by providing some defining characteristics of each. Many would agree that high technology should include: (1) dentistry borrowing materials/processes shortly after their development by an unrelated industry, (2) incorporation of new learning from recent scientific literature outside of dental medicine, and (3) the spread of outright new inventions within dentistry. Craft art, on the other hand, brings to mind materials and techniques borrowed from highly skilled artisans involved in jewelry making, the arts, and the manufacture of everyday goods. More than 90% of people vote for an origination through craft art, as I would have done prior to my literature search!

It is useful to review how and why ceramics came to be used in dentistry, and this introduction serves three purposes: (1) to alert practitioners to the fact that the use of ceramics has always represented the adoption of high technology, not borrowed craft art; (2) to reinforce the concept that ceramic technology and improved ceramics were introduced to solve specific problems or to increase restorative versatility; and (3) to provide some background into the nature and science of ceramics. (Astute readers will also find clues about where to watch for the emergence of new ceramic technologies.)

In the Beginning

In the late 1600s and early 1700s, many European rulers and aristocrats were dispensing enormous sums to import porcelain from China and Japan. Schloss Charlottenburg in Berlin has an impressive assortment of porcelain, and Fig 1-1 represents just a small portion of the collection. Augustus II of Saxony (who was the reigning King of Poland and Elector of Saxony at the time) amassed one of the largest collections in Europe; it is now on display at Dresden’s Zwinger Museum housed in his former palace. Such expensive activity led China to be characterized as the “bleeding bowl” of Europe. Between 1604 and 1657 alone, over 3 million pieces of Chinese porcelain reached Europe.¹ In just one day in 1700, East Indiamen ships unloaded 146,748 pieces of porcelain in a European port as the market for porcelain became insatiable.¹

Fig 1-1 A small portion of the china collection from Schloss Charlottenburg in Berlin.

One response to this situation involved state-sponsored research into porcelain discovery. Notable European leaders, including Augustus II the Strong and the Medici family of Florence, were independently sponsoring research into the development of a European porcelain to match the hard, translucent, and sonorous material developed in Eastern Asia nearly 1,100 years earlier. Europeans strived for porcelain discovery without much success for about 200 years, and this activity is credited with the growth of modern analytical chemistry from its roots in alchemy. Figure 1-2 shows the historical timeline of porcelain discovery.

Fig 1-2 Timeline for the development of several related porcelains.

State-sponsored research into porcelain discovery initiated in France and the Germanic state of Saxony in the late 1600s. The efforts of Count Walther Von Tschirnhaus in developing the mineral resources of Saxony on behalf of Augustus II were particularly important for dentistry. He used a series of large “burning lenses” (magnifying glasses up to 1 meter in diameter) to create a solar furnace; these lenses directed to a focal spot, allowing Von Tschirnhaus to subject minerals to extensively high temperatures, easily in excess of 1,436°C.²

Meanwhile in Berlin (in the Germanic state of Prussia), Johann Friedrich Böttger was manipulating metals as a journeyman apothecary. Böttger’s parlor trick involved melting base metals such as silver coins and then adding a dose of the Arcanum of the philosopher’s stone.³ When poured into molds and cooled, the resulting product was analyzed to be pure gold! Böttger inadvisably performed this “transmutation” demonstration at his employer’s house to impress some important guests, resulting in a summons by King Frederick I of Prussia for a command performance. Placing discretion ahead of valor, Böttger fled south to Saxony, where he attempted to study medicine at Wittenberg University. Wanted posters appeared in Berlin, and a price was put on the head of Böttger. For the local representative of Augustus II in Wittenberg, the arrival of a contingent of a dozen troops from King Frederick seemed excessive for the capture and return of a supposedly common criminal. Therefore, Böttger was placed under house arrest for months while Augustus was alerted and the situation explored. With the presence of foreign troops confounding the situation, Böttger was finally spirited away by coach in the dead of night, using back roads to avoid Prussian troops, and delivered to Augustus in Dresden. To further deceive the Prussians, the Saxons continued to bring food to the room of Böttger. For any state needing to support armies, Böttger’s ability to turn base metals into gold was simply too important to let slip away, so Böttger was held as a prisoner under the wing of Von Tschirnhaus to perfect gold production.

Serendipity and a clever intuition prevailed to save Böttger from certain execution following over 3 years of unsuccessful gold making, a project costing Augustus a small fortune. Experimenting with his burning lenses, Von Tschirnhaus had discovered that while neither sand nor lime (calcium oxide) would fuse individually, they would do so when combined; in fact, the resulting white product looked suspiciously like porcelain. What had been discovered was the use of a “flux” to create lower melting intermediate compounds, promoting glass formation and allowing the fusion of the high-silica sand. Because it was known that high-quality clay was a major ingredient in Chinese porcelain, Saxony was secretly scoured for sources of the purest clay. Böttger, whose expertise in chemistry was by then extensive, realized that porcelain had to have a glassy component resulting from very high–temperature reactions. Building on the discovery of Von Tschirnhaus, he reasoned that lime added to clay was worth exploring.

Between 1704 and 1708, research was conducted under extreme secrecy beginning in the Albrechtsburg Castle, which still exists in the city of Meissen, Germany (Fig 1-3), and then in the dungeon basement of the feared Jungfernbastei (Maiden’s Bastion) in Dresden (Fig 1-4a). Böttger used what is known today as the “Edisonian approach,” whereby a wide variety of formulations are systematically tried. Figure 1-4b shows a page from his laboratory notebook memorializing the successful mixture of clay and lime (obtained from calcined alabaster that was pulverized and heated to drive off water and sulfur, leaving fine calcium oxide powder).

Fig 1-3 Recent photograph of Albrechtsburg Castle in Meissen, Germany, the site of an early porcelain discovery laboratory and the first manufacturing site of European porcelain rivaling that from China. Today’s Meissen factory and museum are located nearby.

Fig 1-4 (a) Early depiction of the feared Jungfernbastei (Maiden’s Bastion) in Dresden, the site of Böttger’s secret porcelain discovery research. (b) Page from Böttger’s laboratory notebook memorializing the successful mixture of clay and calcined alabaster in January 1708.

Manufacturing operations were moved back to Albrechtsburg Castle, and by 1708 the first pieces were being demonstrated at the Leipzig Easter Fair, with production for sale beginning in 1710. Around that time, Böttger substituted feldspar for lime as the flux, a move that (1) cleanly put the Meissen formulation within that of the Chinese “triaxial” porcelains (Fig 1-5) and (2) introduced feldspathic glass, which would later become the main ingredient in esthetic porcelain formulations for dentistry. This high-temperature reaction of the kaolin-type clay (heavily weathered granite or feldspathic rock) and feldspar yielded a high-silica glass containing needle-like crystals of mullite.²

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, remaining definitively high tech!

References

1. Plumb JH. In the Light of History. New York: Delta, 1971:59–65.

2. Kingery WD, Vadiver PB. Ceramics Masterpieces: Art, Structure, and Technology. New York: Free Press, 1986.

3. Gleeson J. The Arcanum: The Extraordinary True Story. New York: Warner Books, 2000.

4. Ring ME. Dentistry: An Illustrated History. New York: Harry N. Abrams, 1985:108–211.

5. McLean JW. A higher strength porcelain for crown and bridge work. Br Dent J 1965;119:268–272.

6. McLean JW. The Development of Ceramic Oxide Reinforced Dental Porcelains with an Appraisal of Their Physical and Clinical Properties [thesis]. London: University of London, 1966.

7. Hasselman DPH, Fulrath RM. Proposed fracture theory of a dispersion-strengthened glass matrix. J Am Ceram Soc 1966;49:68–72.

8. Mörmann WH, Brandestini M. Cerec-System: Computerized inlays, onlays and veneers [in German]. Zahnarztl Mitt 1987;77:2400–2405.

Acknowledgment

Parts of this chapter were previously published in the Australian Dental Journal. They are reprinted here with permission.