Dental Porcelain: Methods, Firing Stages, And Key Properties Explained

Dental Porcelain: Methods, Firing Stages, And Key Properties Explained

Give methods of condensation, stages in firing, and properties of dental porcelain.
Answer: 

Methods Of Condensation:

Condensation can be achieved by several methods:

• Vibration: It consists of applying the wet porcelain to the platinum matrix and then vibrating the die in which the matrix rests. As the particles condense, the water rises to the surface. The excess water is then blotted with a clean tissue paper or an absorbent medium.
• Spatulation: The wet porcelain is applied with a spatula and then the surface is smoothened with the instrument. This will distribute the particles and cause them to become more closely packed. The water rises to the surface and it is removed with a lined cloth or blotting paper.
• Brush technique or capillary action: It consists of adding paste to the matrix and dry powder is sprinkled onto the wet surfaces. The dry powder removes the excess water by capillary action from the mixture already applied. The particles move close together as the water is withdrawn.
• Whipping: After the paste has been applied to the matrix, it may be whipped with a brush. The water is thus brought to the surface and it is removed.

Stages of Firing:

• The process of firing is carried out for fusing the porcelain and is known as sintering.
• Stages of firing: During firing the porcelain moves through different stages.
• The temperature at which each stage occurs depends on the type of porcelain used.

The following are the stages:

• Drying/Green Stage: Initially build-up of the ceramic should be placed at the open entrance of the furnace and the temperature should be raised slowly to 100°C.This leads to the remaining water binder evaporating and prevents the formation of steam in the mass which may lead to compact powder particles cracking. There is a slight contraction reported in this stage.
• Low Bisque Stage: During this stage, the temperature increases gradually and the surface of the particles begins to soften and loose particles just begin to join. The material becomes rigid and is very porous. Particles lack cohesion and they do not have any translucency and glaze. Shrinkage is very minimal.
• Medium Bisque Stage: In this stage, the time of exposure to high temperature still is going on and the fusion of particles increases more and more which brings them closer. This leads to the maximum shrinkage at this stage and now porcelain is non-porous.
• High Bisque Stage: In this, the surface of porcelain is fully sealed and presents a smooth surface. Any of the corrections can be made during this stage before glazing.
• Cooling Stage: Cooling should be carried out at a very slow rate which is very necessary in order to avoid crazing or cracking of ceramic.

Properties of Porcelain:

• Strength: Porcelain is a material having good strength. However, it is brittle and tends to fracture. The strength of dental porcelain is usually measured in terms of its flexure, strength, or modulus of rupture.
  • Flexure strength: It is a combination of compressive, tensile as well as shear strength.
  • Glazed porcelain
  • Compressive strength: Porcelains have a good compressive strength of 331 MPa.
  • Diametral tensile strength is 34MPa which is low because of surface defects.
  • Shear strength is 110 MPa which is low due to its brittle nature or lack of ductility
  • Transverse strength is 62 to 90 MPa.
• Surface hardness: Porcelain is much harder than natural teeth (460 KHN).
• Modulous of elasticity: Porcelain has a high modulus of elasticity of around 69 GPa.
• Wear resistance: Porcelain is more resistant to wear than natural teeth. So it cannot be placed opposite to natural teeth
• Thermal properties: Porcelain has low thermal conductivity. Coeffient of thermal expansion is 12×10–6/°C
• Thermal insulation: It is a good thermal insulator. Its thermal conductivity is 0.0030 and its thermal diffusivity i 0.64 mm2/Sec.
• Dimensional stability: Porcelain becomes dimensionally stable after firing but before firing it is not stable.
• Specifi gravity: Specifi gravity of porcelain is 2.2 to 2.3 gm/cm3. The specific gravity of fired porcelain is usually less because of the presence of air voids.
• Chemical stability: It is insoluble and impermeable to oral fluids
• Biocompatibility: It is quite compatible with the oral tissues.
• Aesthetic properties: The aesthetic qualities of porcelain are excellent. It is able to match adjacent tooth structures in color, intensity, and translucency.