with any remnants of investment material. Possible faults at casting: Subsurface nodules – poor investment causing air bubbles which become filled with molten metal Ridges – moving casting cylinder before investment sets Fins – heated too quickly Roughness – overheating causing investment to disintegrate Foreign bodies Distorted casting – faulty impressions or casts, failure to invest wax once immediately removed from the die or insufficient heating of mould Porosity – presence of voids due to uncompensated alloy contraction and absorption of mould gases Slip casting technique This method is only applicable to sintered alumina core porcelain. Firstly, a sub-structure made of alumina powder and modelling fluid is built on the special die. w The die is then sintered by firing with the sub-structure, causing the fluid to be absorbed by the die and the alumina powder become more tightly packed. Following sintering, the outer surface of the sub-structure is painted with lanthanum aluminosilicate glass powder. The sub-structure is porous and therefore allows infiltration of the glass powder when fired again. Further strengthening of the material can be done by applying zirconium oxide. Direct restorative technique By using this technique, an impression of the tooth preparation is not required. Instead, the tooth preparation is coated with a layer of separating material such as glycerin first. Then, a composite restoration is built up directly on the preparation, allowing it to take the shape of the cavity. The restoration is then light-cured in the tooth before being removed from the tooth to be further light-cured. Prior to cementation of the restoration onto the prepared the tooth, the layer of separating materials needs to be removed to ensure effective bonding between the tooth and the restoration. This technique is only applicable when composite is used as the restorative material. Inlays and onlays made from ceramic or metal alloy require laboratory work and therefore can only be fabricated using indirect restorative techniques as mentioned in the previous section. Survival rates A study done by Rippe et al. has shown that ceramic inlays produced by the different methods, via indirect restorative techniques aforementioned, have similar longevity. Materials ] Gold The use of gold as a restorative material for the production of inlays and onlays is fading due to the increase in usage of more aesthetically pleasing tooth coloured materials. Gold has many advantages as a restorative material, including high strength and ductility, making it ideal to