Porcelain is a vitrified paste ceramic product that contains several components that allow for modulating ceramic properties from plasticity, mechanical properties, melting temperature, etc. Within this porcelain if a certain amount of crystalline silica is added in the form of quartz, this does not melt during the preparation of the porcelain so in fact it is a predominantly glassy system but with some crystalline silica inclusions. The kaolin transformation products also remain crystalline following porcelain processing.

The particles that remain crystalline serve partly to strengthen the material and partly to reduce shrinkage because they remain unaltered and do not undergo the shrinkage that ceramics undergo during firing.

By cooling the ceramic there is a decrease in volume and sintering of the particles resulting in the elimination or reduction of porosity with a reduction in the volume of the final artifact. To regulate the behavior of ceramics, one must modulate its ingredients to control dimensional changes, shrinkage, edge rounding, etc.

One of the ingredients that was often present in traditional ceramics was kaolin ( a hydrated silicate of alumina). This compound increases the workability of the material, but it makes the porcelain opaque and, from an aesthetic standpoint, is not acceptable. Currently, dental porcelains contain feldspar, quartz, boron oxide, and other oxides; thus, the aesthetic appearance of the porcelain is improved.

In the porcelain firing process we go from a mixture of powders in an aqueous suspension to a durable porcelain shell which is the dental restoration, then a heat treatment is performed which sinters these powders then the aqueous phase is removed and a solid artifact is produced. As the vitreous particles are brought to a melting temperature, there is a risk of some rounding of the edges that could result in a loss of the geometric details that had been recorded with the impression. Therefore, the elements that make up the porcelain must be properly proportioned in order to control the shrinkage and removal of the material as a result of firing.

Traditional dental porcelain is a chemically very stable material with excellent aesthetics, does not deteriorate over time and has a conductivity and coefficient of thermal expansion similar to those of enamel and tooth.

For these characteristics is a material still widely used in dental reconstructions, also has a good resistance to compression but on the other hand has a very low tensile strength and has a low hardness, and also is extremely sensitive to surface defects (there are other ceramics that have a greater surface resistance than traditional porcelain). This type of material can be used as a glass phase within ceramic matrix ceramics or as a coating for metal substructures.

The main problems with this material are related to shrinkage and the presence of gases before firing that could lead to the breakage of the piece.

This is because if we have gases such as water vapor or carbon dioxide in the interface between metal and ceramic or inside the ceramic, during firing these gases will be removed from the ceramic and could cause pores or cracks.

In order to avoid breakage due to the presence of gas and to prepare the material, a slow drying process is used to remove the watery phase and then a very slow heating process to avoid an abrupt removal of these gaseous phases that could damage the material.

Other tricks are related to the addition of inert materials such as silica to the mixture and coarse-grained fillers that maintain more stable, from the dimensional point of view, the product. Then it is necessary a correct preparation of the metal substructure, in the case of metal-ceramics, in order to eliminate impurities from the metal surface during the preparation phase and avoid that these, moving away during the heat treatment of firing of the ceramic on the metal substructure, can create bubbles and undermine the metal-ceramic interface.

The traditional ceramic capsule is a shell that is produced with extremely artisanal techniques i.e. it is custom made for the patient and everything is done in the laboratory of the dental technician.

The capsule strengthens the structure of a tooth that is too weak or damaged while maintaining the natural tooth underneath, a bridge replaces a missing tooth.

In both cases, there is the construction of a ceramic shell using artisanal methods.

Integral ceramics differ not only in materials used but also in production methods.

The traditional capsule manufacturing process involves three stages:

• 1 - Compaction which is the making of the ceramic layer.

Initially, the layer is not sintered and therefore consists of the material that will be the final material but does not yet have the mechanical strength. It is usually applied with a mixture of ceramic powders in a suspension so that it can be spread almost like a paint on a metal substructure and then it will acquire mechanical resistance only after the heat treatment of sintering the ceramic. This first step involves the application of 3 layers of ceramic: one opaque shade (to cover the color of the metal substructure), one dentin shade and one enamel shade. A multi-layer fabrication allows for mechanical strength and the desired aesthetic appearance.

• 2 - Firing

The second step is the heat treatment called firing. Here there is a slow heating to eliminate the water and any gases trapped inside this compartment of powders and then there is a heat treatment in which there is a sintering in liquid phase, that is, the glass particles go into fusion and then solidify in the form of a glassy phase.

On the other hand, stable particles such as crystalline silica remain inert but are bound by the molten glass flowing between the particles. We have an absolutely non-negligible volume shrinkage that is around 20% and this process allows to obtain, from a structure without any mechanical resistance, a dense ceramic product with its mechanical properties. Since we have a sintering in liquid phase where this molten glass flows between the particles, if we leave it in the furnace too long we risk losing the initial shape, we risk the famous rounding of the edges and therefore the loss of geometric details. Moreover, this vitrification gives an excessively shiny appearance that is no longer analogous to that of the natural tooth. It is also necessary to pay attention to the external part of the capsule, which will cool down faster than the core of the restoration because it will be exposed to the external environment and since the thermal conductivity of ceramics is low, the external part will be more contracted than the internal part, therefore we could have the development of internal tensions that could cause the creation of micro-cracks in the material. The choice of the type of ceramic, cooling and heat treatments that are done on the material are important to avoid the development of tensions that cause the cracking of the material.

• 3 - Glazing

The final step is glazing to obtain a waterproof outer layer with the correct gloss, to avoid surface porosity that could cause a possible fracture graft. Enameling is necessary to obtain a smooth surface useful both from an aesthetic point of view and to avoid porosity that could be the site of bacterial growth or plaque deposition.

The two most widely used solutions in the dental industry to make dental porcelains more resistant are:

• making the ceramic more resistant by modifying its components and therefore also its mechanical properties
• using a metal substructure.

The use of a ceramic substructure or the application of ceramic directly on the tooth are techniques used more marginally.

There are solutions that cover the entire metal substructure or solutions that cover only the chewing surface:

• High gold content alloys ( 87-99% Au; alloying elements Ag, Pd, Ir, Sn, In)
• Medium gold content alloys (85% Au
• High Pd alloys (Pd 60&; alloying elements Au, Ag, Sn, Ga, In, Ru)

These are some characteristics that the metal alloy we will use to form the metal-ceramic interface must have:

• the melting range of the metal should be greater than the sintering temperature of the ceramic to prevent the metal from yielding during the firing of the ceramic. If the metal softens or melts during ceramic processing, dimensional accuracy will be lost.
• there must be compression because ceramics are much more resistant to compression than to traction so it is essential to use a metal with a thermal expansion coefficient higher than the ceramic.
• the composition of the metal must favor, during its preparation or during the firing of the ceramic, the formation of surface oxides that are favorable to a chemical bond with the ceramic, so they must be chemically active oxides with ceramic, thin and well adherent to the metal substructure.

The metal-ceramic bond is generally divided into 3 main bonds: mechanical bond, compressive bond and chemical bond.

For mechanical bonding, microcavities are created on the surface of the metal structure, generally by sandblasting, which increases the roughness of the metal so that the ceramic penetrates these surface micro-roughnesses either through particles before firing or through the glassy phase of the ceramic during sintering in the liquid phase.

The roughnesses are small so that the metal does not lose strength.

The compressive bond is given by the different thermal behavior of the metal and the ceramic, and since the metal has a lower coefficient of thermal expansion than the ceramic upon firing, the ceramic will be subjected to compression.

The chemical bond is formed between the first layer of ceramic applied and the opaque layer and the oxide that is on the surface of the metal.

The pre-treatment of the metal and the choice of metal alloy are crucial to ensure that this oxide layer of the metal is compatible with the ceramic and is also able to create a chemical bond.

The metal structure is obtained through the lost wax technique, it is then finished with tungsten carbide burs, it is sandblasted to obtain the roughness that is needed for the mechanical bond, it is oxidized to have the oxide layer adherent and similar to the ceramic and finally the opaque layer and subsequent layers are applied.

This is the case for the mainly glassy systems used in the production of metal ceramics.

The second type of ceramic materials we can use are glassy systems with ceramic fillers, i.e. we have a glassy matrix similar to those of porcelain described above and we have crystalline phases that we can select appropriately.

The most commonly used are leucite, lithium disilicate and fluorapatite.

• Leucite improves flexural strength. In particular, if the crystals within the system are small, the system is suitable for coatings on alumina or coatings on metal alloy but this type of system does not have sufficient flexural strength to support an all-ceramic.
• Lithium disilicate comes in the form of small, flat crystals so morphologically they are crystals that act well in deflection and crack arrest. In this case, a system reinforced with lithium disilicate has a fairly high flexural strength so these systems can be used for the construction of integral ceramics but the coefficient of thermal expansion is different from that of alumina so this type of ceramic is not suitable as a coating on alumina.
• Fluoroapatite is the third possible filler.

Glassy systems with filler can be used to make the traditional porcelain capsule by increasing the strength of the material and in some compositions can be used in the construction of integral ceramics.

This is an electron microscope image of a glass-ceramic system with leucite crystals after acid attack...

Glass systems are usually attacked by hydrochloric acid and, in a glass-ceramic system, an acid attack allows the crystalline phase to be exposed by selectively consuming the glass phase.

This is also used to cement the ceramic shell onto the remaining portion of the tooth.

In ceramic systems with glass infiltration we have a matrix that is crystalline (alumina, magnesium oxide alumina...), in this case we use a pre-sintered ceramic block that is made a partial sintering of a powder compound (for example alumina) so that the partial sintering causes the realization of a compound that is not completely dense. The particles are not completely compacted and the cavities are not filled by a glass phase because there are only ceramic powders.

This pre-sintered block is then infiltrated with a glass that must be of low viscosity because it must flow between the particles, at high temperature. This glass becomes very fluid and fills the cavities, solidifies as an amorphous phase, and eventually produces a completely dense object.

These types of systems can have significantly higher flexural strength than porcelain.

Example of ceramic shell made of sintered alumina so only alumina without any infiltration of vitreous type on the left.

On the right you can see the change after the vitreous coating, this makes the transition from a porous system to a translucent system that replicates the aesthetic appearance of the tooth.

A sintered ceramic can be dense or porous; in this case if we have a ceramic that has undergone partial sintering, so still porous, the glassy phase is used as an infiltration to fill porosity. 

In case the alumina is completely sintered, then the glassy phase does not fill porosity, we have a compact ceramic base, but the glassy base is used as an aesthetic coating a bit like in the case of traditional porcelain that is a shiny outer layer that mimics the aesthetics of the natural tooth and at the same time creates a smooth and waterproof surface that hinders bacterial penetration and damage to the material.