Table of Contents
- 1 The porcelain Jacket crown
- 2 The Aluminous Core crown
- 3 The difference between aluminosilicate glass and aluminous porcelain
- 4 Frits
- 5 The powder condensation technique
- 6 Vacuum firing
- 7 The composition of feldspathic dental porcelain
- 8 The inherent weakness of feldspathic glass structures
- 9 The use of unbreakable cores to support weak feldspathic porcelain
This is the third in a series of pages on dental ceramics. The material presented on each page is designed to stand alone, but a real understanding of this material relies on knowledge presented on the two pages that precede it. Many terms on this page that will be unfamiliar to the casual reader have been defined there. This series represents a mini course in ceramics for the beginner, and persons seriously interested in gaining a basic working knowledge of dental ceramics are advised to take the time to start at the beginning.
This page covers the history of dental porcelain from its very beginnings in 1903 until the invention of aluminous cores in the 1960’s. This page lays the foundation for understanding the advances in the ceramics technology from the 1960’s through the 1990’s which will be the subject of page four.
In 1903, Charles Land (the grandfather of aviator Charles Lindberg) invented the first tooth colored full coverage restoration. In retrospect, it would seem a logical step to make teeth out of porcelain, but he was so far ahead of his time, that mainstream dentists thought he was a quack. Realistically, porcelain was white, fairly translucent, and could be made from easily available and inexpensive porcelain clay. From about 1855 until 1903, the major methods of repairing teeth was to fill or rebuild them with amalgam, or with adhesive gold foil. The gold foil method was the one used by “high class” dentists of the day, but it was a time consuming and expensive project. Amalgam had a bad name due to poor formulations until 1895 when G.V. Black standardized the formula. Amalgam’s poor reputation was also due (undeservedly) to the idea that the mercury in amalgam was poisonous. Neither method resulted in an especially good looking tooth. Land’s idea was to cut the remaining tooth back and then rebuild the stump using a porcelain covering, which he called a “jacket”.
Land’s porcelain jacket was made from feldspathic porcelain clay. It was fabricated by burnishing a piece of thin platinum foil over a die, and brushing layers of dry clay over it using a tiny, wet paint brush. The foil, along with each successive layer was fired in a kiln, and the process was continued until the porcelain overlying the platinum foil resembled a tooth. Since platinum is a noble metal the lack of an oxidized layer meant that the porcelain would not bond to it. After all the firings had been completed, the platinum was removed and the porcelain “jacket” was luted to the tooth using zinc phosphate cement.
Gold and silver casting techniques had been used in dentistry for the fabrication of metal frameworks for partial dentures since the late 1700’s, but were not used to repair decayed teeth until the 1910’s, after the invention of the centrifugal casting machine in 1907 . The term “gold crown” had been popularized by the gold foil technique which was used by expensive dentists to rebuild teeth since about 1855. Since gold was the metal used to make the crowns worn by kings, it suited the mentality of the day to think of being able to afford the services of an expensive dentist as something that brought a royal distinction to the patient. Thus the term “gold crown” was something like an advertising slogan. In the meantime, porcelain jackets became known as porcelain jacket crowns (PJC’s), probably because it helped dentists who fabricated them to compete with those who worked only in gold. The issue of dental materials was further confused in those days by the introduction of amalgam into the respectable side of dentistry in 1895. Dentists who worked in gold foil wanted nothing to do with the newer materials and were constantly engaged in battles with those who used them.
PJC’s eventually became a popular restoration in spite of the fact that they had some serious technical drawbacks. In the first place, the removal of the platinum foil after the crown was fired meant that there was always a substantial gap at the margin (the tooth/crown interface) from which the zinc phosphate cement could leach out and into which leakage of saliva and food debris could take place. Second, the porcelain tended to be too opaque to match the surrounding teeth. Finally, the strength of this old fashioned porcelain crown was not great. Porcelain jacket crowns could not be used for posterior teeth, and they were prone to failure even on anterior teeth. However, they were strong enough to allow Land and his associate, Dr. Edward Bartlett Spaulding to create a shoulder preparation on a spike, bake a porcelain jacket to fit the spike, cement it and drive it through a pine board without fracturing the crown.
If you have read the first two pages in this series, you are already familiar with the diagram above. You can see that domestic porcelain contains about equal amounts of kaolin and quartz, and also a great deal of feldspar when compared with other forms of clay. Feldspar is a naturally occurring glass that contains silica, fluxes and alumina, all neatly bound together. Feldspar in fact binds all three forms of clay bodies together once it has been fused at high temperature. The process of turning the crystalline silica (SiO2), along with associated fluxes and modifiers into glass is called vitrification.
Porcelain, then, can be defined as a highly vitrified ceramic body. “ceramic body” is a term used to describe a piece of pottery from the time it begins as greenware throughout all phases of firing. A ceramic body includes both a refractory skeletal structure which retains it shape through the sintering and fusing processes, as well as the feldspathic glass which is infiltrated between the refractory particles . The key here is that porcelain is not simply a form of glass. It is glass with a refractoryinternal structure. This was why Dr Land’s porcelain crowns were reasonably strong.
This refractory skeleton was composed of about equal parts of kaolin which is an alumino silicate, and fine grains of pure silica in the form of quartz. These materials are present in in the form of crystals which remain unmelted except at their points of contact where they fuse together during the sintering process. While silica is fairly transparent because it has a refractive index very similar to that of the glass matrix, kaolin is opaque due to the fact that much of it it remains in crystalline form throughout the ceramic body, and the crystals have refractive indexes far different from the glass matrix. The opacity is the result of the internal scattering of light by the kaolinite crystals. (Click here for more on why crystals cause opacity.) This was the reason that Land’s porcelain jackets were not very esthetic, and this opacity remained a recurring problem in early dental porcelains.
In light of this problem, ceramic technologists began to formulate feldspathic porcelains with less and less kaolin until by 1938 kaolin was reduced to the amount necessary to produce only an esthetic level of opacity. The glass itself was strengthened by the addition of various stabilizers such as boron and dissolved alumina, but these did not entirely make up for the low level of the crystalline alumina in the substructure. Very fine particles of refractory quartz were left in the formulation in order to give the glass enough structure to resist sagging and to from propagating through the structure, but unfortunately, as the proportion of aluminous kaolin decreased, the strength of the glass declined. (As noted above, quartz crystals have a refractive index close to that of the surrounding glass, and therefore do not have as much of an effect on the optical qualities of the glass as do alumina crystals.) Thus feldspathic dental porcelains slowly began to to be formulated with only a weak refractory skeleton composed mostly of quartz particles and became more prone to failure as a result. Feldspathic porcelain has a flexural strength of approximately 80 MPa, as opposed to modern leucite reinforced porcelain with an MPa of 125-180. On the other hand, they were (and still are), highly esthetic materials for building tooth-like structures.
Fracture in brittle solids is nearly always initiated at a small internal or surface defect such as a scratch, or a micro crack that develops due to uneven shrinkage of the glass structure during cooling. When a tough, crystalline material such as alumina or silicon in particulate form is added to a glass, the entire mass is strengthened because a crack cannot penetrate the alumina particles as easily as it can the glass. When the crack encounters a well fused silica or aluminum oxide crystal, it cannot continue to propagate unless it encounters weaknesses in the glass surrounding the crystal. When crystalline inclusions in a glass structure are used in this capacity, they are known as crack stoppers.
In 1965, dental ceramic technologists were driven by the clinical failures of existing porcelain jacket crowns to address the problem of weakness. They did this by adding large quantities (up to 50%) of finely ground aluminum oxide particles to the raw materials used to make their feldspathic porcelain glasses. Most of it remained in crystalline form. This creates an aluminous porcelain, a material that was already in use in industrial applications, as well is in toilets and sinks. Alumina particles are far stronger than the glass matrix, and are more effective than quartz crystals in preventing crack propagation. The inclusion of tiny alumina crystals into the glass in concentrations of 40-50%increases the flexural strength of the feldspathic porcelain on the order of about 2-3 times. While the added alumina recreated the refractory internal skeleton that had strengthened Land’s original porcelain, it also reintroduced the problem of opacity. The solution was to cut back the facing of the aluminous crown and overlay it with a veneer of the more esthetic feldspathic porcelains that were formulated without the alumina particles. Thus, the aluminous porcelain was used as a substructure,or core, over which a veneer could be applied. This substructure became known as an aluminous core. Newer aluminous porcelains have a flexural strength of between 400-700 MPa depending on the amount of alumina the structure contains. Higher value aluminous porcelain is currently used for fabricating posterior crowns (see In-Ceram Alumina and Procera AllCeram).
The esthetic veneers on aluminous cores, as well as the glass matrix between the aluminum particles are still feldspathic glass. This is true even though many brands of porcelain do not actually contain naturally occurring feldspars, but are mixed up from the raw materials, just like any other form of manufactured glass. Feldspathic glass does not, in fact, have to be manufactured from feldspar. It just has to have the correct general chemical formula.
Dental feldspathic glasses contain the same proportions of chemical constituents as the original feldspar; one flux molecule bonded to one aluminum oxide molecule bonded to a larger number of silica molecules, all fused into an amorphous glass gel. The aluminum oxide in feldspathic glass acts as a stabilizer and is part of its molecular structure. As a stabilizer, it is not in particulate or crystalline form. Without crystalline structures, this form of alumina does not cause internal scattering of light and does not adversely affect the optical qualities of the glass. When aluminum oxide is used as a stabilizer, the glass is called an aluminosilicate glass. Aluminosilicates are much stronger than most other types of clear glasses.
On the other hand, when one speaks of aluminous porcelain, one is speaking of a clear glass to which refractory grains of alumina have been added making it more fracture resistant, but also more opaque.
The porcelain used by dental technicians today is not a simple mixture of ingredients. The porcelain ingredients have already been combined and fired once by the manufacturer. The manufacturer adds metal oxides to adjust the color, opacity, strength, thermal expansion and other characteristics, fires the mixture to the correct temperature and then quenches the molten glass in cold water. The thermal shock of the hot glass hitting the water causes it to shatter into very fine particles which are retrieved and ground even finer into a powder known as a frit.
The frit is mixed with a binder (often starch and sugar), packaged and delivered to the dental technician to veneer the crowns and bridges he creates for the dentist. Thus in making a PFM or a porcelain jacket crown, there are no chemical reactions taking place in the dental lab kilns. That has already been taken care of at the glass factory. Technicians build both aluminous cores and esthetic veneers using powdered frits. This is accomplished with the powder condensation technique and vacuum firing.
The particles in a porcelain frit average about 25 microns with a wide distribution of particle sizes so that smaller particles can fit between larger ones. This allows the technician to create a fairly dense mass of powder while fabricating the porcelain veneer. (“Veneer” as used here refers to the porcelain placed over a metal, aluminous or ceramic substructure.)
The powder condensation technique consists of applying layers of frit to the substructure using a brush dipped in water. The wet brush is dipped into the dry powder which adheres to the brush by capillary action. The wet frit is then “painted” over the substructure (a refractory die or a crown or bridge core), until a fairly thick layer of frit, in approximately the correct shape has been built up.
The brushing action serves to compact the frit particles to create a body of fairly densely packed powder over the die or core. Each stroke of the brush further compacts the particles bringing water to the surface. The excess water is blotted off the greenware body with tissue paper. Each time it is blotted off, the water is drawn toward the tissue, and in the process the surface tension of the water draws the frit particles on the opposite side of the body together. Thus surface tension is ultimately responsible for condensing the powdered frit.
The body is allowed to dry out and then fired inside an automated kiln at a temperature appropriate to the porcelain being applied (Older porcelains were fused at 1350°C, but modern porcelains fuse between about 800 °C and 1100 °C .) This sinters and fuses the frit into a dense porcelain which has shrunk considerably since it was first placed in the kiln.
In order to reduce porosity within the porcelain body, the air pressure inside the kiln chamber is reduced to about a tenth of the ambient atmospheric pressure. This reduction in pressure draws gas out of the unmelted body, leaving less gas to cause porosity when it melts. After fusion temperature has been reached, the automated kiln begins to reduce the temperature. At about 55 °C , below the upper firing temperature, the vacuum inside the kiln is released. As the air pressure approaches normal atmospheric pressure, any air bubbles inside the melted porcelain are reduced in volume to one tenth of their original size.
After the first firing, more layers of powder are added to fill out the ceramic body to the correct shape and size. Porcelain powders of several different shades are used, and it takes considerable time for a technician to develop the art and the skill necessary to create a ceramic structure that looks like a natural tooth.
A typical dental feldspathic glass contains approximately the following proportion of constituents. (The porcelain contains refractory crystalline elements as well.)
Aluminum oxide 15-20%
Boric oxide 5-10%
Potash (K2O) 5-10%
Soda (Na2O) 2-7%
Other Oxides 1-3%
Silica is contained in dental porcelain in two separate forms. The first type is in the form of feldspathic glass in which it is combined with aluminum oxide and a flux and does not have a crystalline structure. In this capacity silica is the major glass former in the porcelain. The concentrations expressed in the table above reflect this type of silica. The second type of silica is in the form of refractory crystalline quartz particles which are dispersed through the glassy phase to act as crack stoppers. Quartz crystalline inclusions have the unique property of remaining nearly invisible within the glassy matrix due to the similarity of their respective refractive indexes.
Aluminum oxide (Al2O3), also exists in dental porcelain in two forms. It, like silica, is a component of feldspathic glass, and in this form aluminum oxide is used as a stabilizer. As a stabilizer, it is combined on a molecular level with the amorphous silica matrix. In this form, it toughens and waterproofs the glass without affecting its optical properties. However, aluminum oxide also exists in the form of tiny crystals dispersed throughout the glass matrix. When it is in crystalline form, it is known as alumina. In this form it strengthens the glass by acting as crack stoppers, but it also diffuses light and causes opacity.
Like silica, boric oxide is another glass former. Used alone, the glass it creates is useless since it is soluble in water, but when combined with silica in concentrations over 5%, it toughens the glass and makes it able to withstand mechanical and thermal shock much better than ordinary silica glass without the boron. This is especially useful in dental porcelains which are subjected to chewing and bruxing forces as well as to extremes of hot and cold (hot coffee followed by ice cream etc.). Borosilicate glass is used to make Pyrex ovenware and laboratory glassware.
Both of these alkaline oxides are used as fluxes to lower the melting temperature of the glass, and both are natural constituents of feldspar. Both sodium and potassium are chemically quite similar, but there are major differences between them when it comes to fluxing glass. The soda does a better job of lowering the melting temperature than potash, but the potash is often used in greater concentration because it increases the viscosity of the molten glass, thus reducing slumping and running in the kiln.
Certain metal oxides are added to the glass in order to produce different colors and opacities thereby causing the porcelain to mimic the correct tooth shade and translucency. For example:
oxides of iron impart a brown color
copper oxide produces a green color
small amounts of titanium oxide produce a yellowish brown color
cobalt oxide imparts a blue color
Manganese oxide produces a lavender color
Zirconium, cerium, titanium and tin oxides, when used as refractory crystals produce opacity
If you calculate the compressive and tensile strength of glass from the bond strengths between the atoms in the glass matrix, theoretically they should be about 100 times stronger than they actually are. The reason that the reality is so different from the theory is that glass and porcelain are brittle materials. In other words, their coefficient of elasticity is extremely low, and tiny surface defects can concentrate stresses at a single point, multiplying the strain in that area to the breaking point. The maximum strain a glass can withstand is 0.1%. (Stress is the force applied to an object, while strain is the actual mechanical movement, or deformation in the object produced by the stress.) The stress in the diagram to the right is applied at the red arrow. All of the micro cracks on the internal surface are preexisting due to tensile stresses during cooling. The stress on the crown in this image concentrates the strain at these pre-existing internal micro cracks causing one of them to fracture.
When a concave/convex porcelain object like a crown cools in the kiln, the outside cools more rapidly than the interior because porcelain is a good insulator. The outside surface contracts and hardens before the inside surface. Thus the inside surface is prevented from contracting as it cools by the harder outer parts of the structure. This places the interior surface under tensile stress. Tensile stresses are stretching forces. Glass has very high compressive strength, but very low tensile strength. Tiny stress cracks develop on the interior of the porcelain shell to relieve the strain. Thus early PJC’s tended to fail due to the micro cracking that occurred on the internal surface. The answer to this problem first appeared in the 1950’s with the invention of the Porcelain Fused to Metal crown (PFM). Since the internal surface of the porcelain is bonded to a metal coping in PFM restorations, the metal-porcelain bond prevents the stress cracks from developing. (The key to the metal/porcelain bond is the formation of metallic oxides on the surface of the metal. This is covered well in my Course on dental alloys.)
In the 1980’s it became possible to bond porcelain crowns directly to tooth structure, effectively turning the tooth itself into an unbreakable core. This brought about a huge improvement the strength of the porcelain without the opaque metal framework. In the 1990’s, more modern substructures made out of very strong opaque ceramics were invented. Although opaque, these ceramic substructures could be made to appear translucent and could be colored the same as natural dentin. They are a major esthetic improvement over metallic frameworks. They will be discussed in detail on the last page in this series.
The most serious drawback with Feldspathic porcelain when used without a core is its inherent lack of strength and toughness. In order to further strengthen crowns made from esthetic feldspathic porcelain so that it can stand up to the stresses encountered in the mouth, it is necessary to reinforce it by layering it over an unbreakable core. There are three ways in which this can be done:
Metal cores–Porcelain fused to metal (PFM)
The technique of bonding feldspathic porcelain to a metal framework was invented in the late1950’s by Dr Abraham Weinstein. The metal alloy could be precisely cast to fit the tooth via the lost wax technique. This effectively solved the dilemma of poor marginal fit, which had always been a problem with traditionally built porcelain jacket crowns Since the metal alloys could form naturally integrated oxide coatings on their surfaces, the feldspathic porcelains formulated to veneer these frameworks could bond intimately with their surfaces. This solved the problem of internal micro cracking which had plagued porcelain jacket crowns as they had been fabricated up until that time. The subject of metal alloy frameworks and the dilemma of forcing porcelain to adhere to them is discussed at length on my pages on Dental Alloys. PFM crowns and bridges are still the most popular and durable tooth colored restorations in the world. Their major drawback is the opacity and color of the metal substructure, a deficit that drove the search for more esthetic forms of porcelain restorations.
Reinforced ceramic cores
The idea of replacing the metal substructure of a PFM restoration with an opaque white porcelain substructure came about in the 1960’s with the invention of the aluminous core. At the time these were invented, they were simply a stronger version of the older feldspathic porcelain jacket crowns. They were cemented to the tooth using zinc phosphate cement, but the cement could form no bond to the porcelain crown. Aluminous cores were an improvement, but they were still weak due to the lack of an underlying supporting structure which could suppress the natural tendency of concave vitreous bodies to form micro cracks on their internal surfaces . It wasn’t until the 1990’s and the 2000’s that truly strong aluminous and zirconia core materials were developed which could approach the strength of a metal framework. These core materials are opaque, but they can be fabricated to be very thin and therefore somewhat translucent. They can also be pigmented with the same colors as the overlying feldspathic veneer. These materials will be discussed on the fifth and last page of this series.
Resin bonded ceramics
It was not until the 1970’s that the concept of “bonding” became accepted by the dental profession. At that time, it became possible to etch tooth enamel and bond resin based restoratives to it. The idea of actually bonding a porcelain jacket crown directly to tooth structure did not become practical until the 1980’s when it became possible to bond both porcelain and dentin with an intervening resin cement. This breakthrough made it possible to strengthen the already existing aluminous core crown by using the tooth structure itself as the unbreakable “framework” to strengthen otherwise weak feldspathic porcelain structures. Even though bonding strengthened the resulting restoration tremendously, and also improved the esthetics, aluminous core crowns were still not strong enough to bond to posterior teeth, or to fabricate fixed bridges. In the 1990’s, new forms of ceramic crowns that could also be bonded directly to tooth structure and which were stronger and better looking than aluminous core crowns were invented. These new glass ceramics are the subject of the next page in this series.