Porcelain is considered by many to be without peer as a material for restoring the crowns of anterior teeth, and also for the portions of bridge pontics which contact the gingival tissue. Highly glazed porcelain is probably more compatible with oral tissue than any other dental material and is also one of the most esthetic in appearance.
Porcelain is a ceramic material made by heating selected and refined materials, often including clay , to high temperatures. The raw materials for porcelain, when mixed with water, form a plastic body that can be worked to a required shape before firing in a kiln at temperatures between 1200°C and 1400°C. Properties associated with porcelain include low permeability, high strength, hardness, high durability, whiteness, translucence, resonance, brittleness, high resistance to the passage of electricity, high resistance to chemical attack, high resistance to thermal shock and high elasticity.
Composition and Characteristics of Porcelain
Dental porcelain is made up of several forms of silica, notably kaolin (aluminum silicate) and feldspar (potasssius aluminum silicate) along with certain binders and pigments or coloring agents. The porcelain powder is combined with distilled water to a puttylike consistency which is applied to the platinum matrix.
When the matrix with its veneer of porcelain is fired in the furnace at a prescribed temperature, a reaction occurs which fuses it into a very hard mass. A notable characteristic of porcelain is the fact that it shrinks as much as 40 per cent of its volume when it is “fired” in the furnace. Unless this shrinkage is compensated for in the technique, the restoration may be short at the margins and there may be air spaces of voids in the fused porcelain.
This is one reason it is built up to contour gradually and fired several times in the process. Good technique requires that the porcelain remains wet during its manipulation, but after it has been applied to the matrix as much water as possible must be removed to insure hard dense porcelain free from voids.
To further remove moisture, the matrix with newly applied porcelain is allowed to set in front of the oven for a few minutes before it is placed in the oven.
Another characteristic of porcelain is that it tends to crack or craze if it is subjected to a sadden temperature change. For this reason it is never removed directly from the oven and cooled to room temperature. Instead the oven is cooled down to approximately 1000 degrees F at which point the crown is removed and cooled under a Pyrex dish. This markedly slows the rate of cooling and minimizes the possibility of crazing.
Types of Dental Porcelain
Porcelain is classified as low, medium, or high fusing based on the temperature required to fuse it. Low fusing (1600-2000 degrees F) is used mainly for building up contacts and modifying the contours of porcelain pontics which have previously been completely fused. Medium fusing porcelain which fuses between 2000 and 2400 F, and high fusing porcelain above 2400 degrees F are both used for the fabrication of crowns, inlays and bridges. Because of the high temperature required to fuse porcelain a special oven with extremely accurate temperature controls is required.
When porcelain is brought up to its fusing temperature it acquires a glazed translucent surface. This highly glazed surface is often lost when a porcelain tooth or facing must be ground. When this happens the original highly glazed surface can be restored with a specially manufactured glaze, which is made for this specific purpose. The glaze is painted on with a brush, vibrated to eliminate the brush marks, and fired at the temperature recommended by the manufacturer.
Special stains are marketed which can be used to produce lifelike stains in porcelain crown. They are available in a wide range of colors, from opaque to black. The powders are blended with liquids provided by the manufacturer to produce the desired effect. This is painted on the crown after its third or final bake, and fired in the furnace at the recommended temperature.
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