It could be said that the ceramic material known as porcelain holds a special place in dentistry because, notwithstanding the many advances made in composites and glass–ionomers, it is still considered to produce the most aesthetically pleasing result. Its colour, translucency and vitality cannot, as yet, be matched by any material except other ceramics.
Traditionally, its use is in the construction of artificial teeth for dentures, crowns and bridges. From the 1980s onwards, the use of ceramics has been extended to include veneers, inlays/onlays, anterior crowns and short-span anterior bridges. The construction of such restorations is usually undertaken in dental laboratories by technicians skilled in the art of fusing ceramics. In recent years, the advent of computer-aided design–computer-aided manufacture (CAD–CAM) in dentistry has opened up new opportunities to use new materials and the potential to extend the use of ceramics to posterior crowns and bridges.
As people retain their teeth for much longer than in the past, the need for aesthetically acceptable restorations is continuing to increase. This is reflected in the growing use by dentists of restorative procedures using ceramics.
The achievement of making usable pottery was a considerable feat, and involved many trials and tribulations for the early potters. The raw material used for pottery is clay, and this presented the potter with two major problems.
The first problem the primitive potter met was how to transfer the clay into a form that provided the best consistency for manipulation and firing. Clay is usually too sticky to handle when simply mixed with water, and this problem was overcome by the addition of sand and ground seashells. In addition, clay shrinks as it dries out and hardens. If this shrinkage is non-uniform, either in rate or in overall amount, the pots will crack even before they have been fired. Again, the addition of a coarse-grained filler went some way towards overcoming this problem.
It was during the firing of the pots that the problems really began to be serious. Gases present in the mixture, whether air bubbles or gases formed during heating (such as water vapour and CO2), create voids in the clay and may even cause it to fracture during firing. Early potters overcame this problem by beating the clay prior to moulding to get rid of the air. (Wedging is the term often used by craftsmen to describe this process.) Another development was the technique of raising the temperature very gradually during the firing process, as then, the steam and gases could diffuse out of the clay slowly, rather than bursting out and causing the pot to crack.
The most serious obstacle during this phase in the development of ceramic technology was the temperature at which the pottery could be fired. The conversion of clay from a mass of individual particles loosely held together by a water binder to a coherent solid relies on a process known as sintering. In this process, the points at which the individual particles are in contact fuse at sufficiently high temperatures (Figure 3.4.1).
The process relies on diffusion, which is greatly accelerated by elevated temperatures. The demand for high uniform temperatures could not be met by the traditional open fires, and this led to the invention of the kiln. The earliest of these was the up-draught kiln, in which higher temperatures and greater uniformity of temperature were obtained by drawing air through the fire and putting the pots in the rising hot gases.
Early kilns were able to reach temperatures of up to 900°C, and pottery fired at this temperature is called earthenware. The resultant pottery is porous, as the sintering process has only just managed to fuse the particles of clay where they touch. Such pots were suitable for the storage of solid foods but could not hold any liquids. This problem was overcome eventually by fusing a thin layer of a glassy material, i.e. a glaze, over the surface of the pot. This technology was used as far back as 5500 BC in various places, including Turkey.
Gradual progress was made towards higher kiln temperatures, so that many more clays could be partially melted. The liquid phase would invariably solidify as a glass, resulting in impervious pottery that is generally known as stoneware. Stoneware appeared in Europe in the 15–16th centuries AD.
In contrast to what was happening in Europe, stoneware had been produced in China by 100 BC, and, by the 10th century AD, ceramic technology in China had advanced to such a stage that Chinese craftsmen were able to produce:
A ceramic so white that it was comparable only to snow, so strong that vessels needed walls only 2–3 mm thick and consequently light could shine through it. So continuous was the internal structure that a dish, if lightly struck would ring like a bell.
As trade with the Far East grew, this infinitely superior material came to Europe from China during the 17th century. Until then, there had been a distinct lack of interest in tableware. The majority of the population ate off wooden plates and the nobility were satisfied with eating off metal plates. For special occasions, gold and silver tableware would be used.
This all changed with the introduction of Chinese porcelain, which stimulated demand for high-quality ceramic tableware. There was no way in which the trade with the Far East could possibly satisfy this demand, so strenuous efforts were made by the European pottery industry to imitate the Chinese porcelain.
Passable imitations were made by using tin oxide as a glaze (producing the white appearance of porcelain), but it was found impossible to reproduce the translucency of Chinese porcelain. For example, Meissen in Germany in 1708 managed to produce what they called ‘white porcelain’, but their product more closely resembled northern Chinese stoneware. Many other manufacturers, now well-established names, were unable to produce genuine porcelain but still made a name for themselves with high-quality stoneware, such as Majolica from Italy, Wedgwood from England and Delft’s Blue from Holland.
In the up-draught kiln, the technology existed to produce high temperatures, although the Chinese down-draught kiln was somewhat superior at controlling the temperature. The problem of reproducing Chinese porcelain was essentially one of selecting the material and the method of processing. Many, such as John Dwight of Fulham, who was granted a patent by Charles II in 1671, claimed to have discovered the secret of Chinese porcelain, but really only managed to make white stoneware.
In order to produce porcelain, the material has to remain or to become white on firing, and must be so strong that vessels with walls less than 3 mm thick can be produced. If the product needs to be made with walls thicker than 3 mm, even porcelain appears opaque. So, the major differences between stoneware and porcelain are that porcelain is white and can be made in such thin sections that it appears translucent. Stoneware could be made to look white, but had to be used in such a thickness that it was invariably opaque.
This situation prevailed for some time, until, in 1717, the secret was leaked from China by a Jesuit missionary, Father d’Entrecolles. He performed his missionary work in a place called King-te-Tching, which, at that time, was the porcelain centre of China. Going amongst the people in their place of work, he managed to acquire samples of the materials used. He sent the samples to a French friend of his, together with a detailed account of how the porcelain was manufactured. The samples and the description were passed on to M. de Réaumur, a scientist, who was able to identify the components used by the Chinese, such as kaolin, silica and feldspar.
Kaolin, known as china clay, is a hydrated alumino-silicate. The silica is in the form of quartz, and remains as a fine dispersion after firing; the feldspar is a mixture of sodium and potassium–aluminium silicates. These were mixed in proportions of 25–30% feldspar, 20–25% quartz and 50% kaolin. It should be said that, by the early 1700s, the Meissen factory in Dresden was already producing a very passable porcelain based on kaolin, silica and alabaster.
In a way, it is a little surprising that it took so long before the composition of the Chinese porcelain was unravelled. The art of making porcelain involves no complex chemistry, since the process is one of taking three rather common minerals (kaolin, feldspar and flint) and firing them at high temperatures. Once the mystery had been unravelled, however, it did not take long for new porcelains to be developed in Europe. Soon it was possible to make it in any shade or tint, and its translucency gave such a depth of colour that it was not long before the dental potential of this material was recognized.
The dental application of porcelain dates from 1774, when a French apothecary named Alexis Duchateau considered the possibility of replacing his ivory dentures with porcelain. Ivory, being porous, soaks up oral flu/>