MODERN TECHNIQUES FOR PORCELAIN COLOURING

Any key of colours represent in fact a sum of different colours encountered frequently at the natural teeth. When the doctor is in front of the patient he will choose from the key the colour that is closest to the natural colour of the patient’s teeth. The appreciation of the colour likeness is done at the natural sun light, other sources of light representing an error factor. The mentioned phenomenon which puts the doctors, the technicians and the patients in difficulties, is due to the different report between the reflexion and the refraction of the light at the teeth surface, restaurations performed or the model from the key colours. The reflexion and the refraction depend on the density and the structure of the environment from which is done that tooth, but they depend also on the radiation frequencies range which fall on its surface. That’s why the aspect of teeth is so different from an acrylic restauration and a porcelain restauration and two teeth which look the same in the natural light may have different colours in other sources of light.

In order to obtain an ideal colour which can better imitate the natural colour of teeth, the structure of the prosthetic piece must imitate the granulation, stratification and pigmentation observed from the natural teeth. Porcelain manufacturers consider being essential the next aspects:
1. The internal structure (the granulation) of the porcelain structure.
2. Obtaining some colours at different depths from the external surface of the restauration.
3. Imitating the fluorescence of the natural teeth (light emission captured in ultra-violet light – 450 nm)

The natural teeth surface is similar to the surface of one pearl and it presents the opalescence phenomenom which consists in turning the colour of the surface to blue when that surface is lightened and turning to orange when the the light comes from one source, recrosses the opalescent object and reaches the beholder eye. This interesting visual phenomenon appears only if the granulation of the particles of that environment is of microns order, order which is very close to that of the smalt prisms. Porcelain teeth with this dimension of the particles bear the name of opalescent porcelain.

In what concerns the colour deepness it was remarked that there are colour pigments at the tooth surface or in the smalt fissure. In this situation between the observer eye and the pigment there is no harsh dental structure. There are also colour pigments situated in deepness which may be observed through the smalt transparency and which are bounded to that tooth age. The firms that produce porcelain made colour maps observed at young patients, medium age patients and elder patients. As a general principle, there are areas that tend to be brown (the area where the tooth is in touch with the gum) or orange at the top of the sharp part of the tooth or tend to be blue the angles where the faces of the teeth are one in front of the other. Beside the surface or deepness pigmentation we may observe at the natural teeth a pigmentation associated with opalescence of the smalt.

Corresponding to these aspects, the porcelain cases offer the possibility of external colour (the usual term of make-up used by the dental technicians), the possibility of internal colour and smalt colour with different hues of opalescent porcelain. The problem which is raised by all pigments of the porcelain mass is bound to the difference of the thermic distention coefficients between the porcelain and the pigment. During the burning, the different distention may produce a separation between the pigment stratum and the porcelain stratum resunting either a fracture or a posterior separation, or a “bubble” aspect which represent in fact separation spaces.

These problems appear especially at internal and external dyeing in which the pigment stratum are differently applied onto the porcelain mass and are subsequently burnt.

The external colouring is made by pigment mixing in the frosting stratum which is applied at the last burning on the restored surface already realized. The obtained stratum stays not a long while in the mouth cavity because in one or two years it disappears even in contact with the soft tissues such as the lips. Not all the porcelain masses need the frosting stratum to be applied. There are autofrosting porcelains which at the last burning the ideal aspect may be obtained and in this case the frosting stratum is used only when the external colouring is needed.

The internal colouring is made in the biggest number of cases through stratum by stratum deposition of porcelain mixed with pigment and the covering of the restoration obtained with a transparent porcelain stratum. This classical technique of internal colouring on each stratum supposes a difficult work and a long working time and a big number of burnings which represent a problem for many porcelain masses. From the beginning of years 80 a modern technique of internal colouring was introduced which supposes the application of the pigment on the surface already restored with porcelain – smalt and dentine types and after this operation is performed the last transparent layer is applied, which gives profundity to the pigments. Practically after the application and the burning of the opaque layer, the reconstitution with dentine and smalt is performed and we proceed to a third burning and after that the morphological corrections are performed and the pigment is applied in the horizontal sense (mezio-distal especially in the area between the tooth and the gum) and is burnt. After this process, the pigment is applied in “cervico-incizal” sense and is burnt again. This technique may only be applied to porcelain masses with remarkable physical properties where the body contraction coefficients and the pigment respectively pigment and translucent have almost identical values. There are situations when the accurate imitation of the natural counterpart tooth implies internal colouring and external colouring too.

The colouring with the help of opalescent porcelain hues is a modern option wich offers many advantages. In the first place, the application is easy to perform, the opalescent layer replaces the transparent layer. The aesthetical aspects are remarkable even if they are not so spectacular as in the internal colouring case. In the third rank, the opalescent particles dimension makes that this porcelain has an extremely smooth surface which leads to a less visible abrasion process of the opposed teeth. There is also translucent opalescent porcelain (no pigments) which may be applied as a last layer in the internal colouring technique.

From the fluorescence point of view, the ideal ceramic mass should be closest to the fluorescence of the natural teeth. However, many patients observe that they have serious aesthetical problems in an environment with violet light when porcelain teeth betray their presence through a different fluorescence.