How to Prevent Discolored Zirconia

Many laboratories experience discolored zirconia restorations after sintering; there are two main causes of this problem:

Coloring liquids contain high concentrations of metal ions. When the liquid is applied to zirconia, it reacts during sintering and is incorporated into the structure, generating the coloring effect. During sintering, the ions evaporate into the furnace atmosphere and are deposited in the combustion chamber and on the sintering tray after cooling. During the next sintering process, these ions are mobilized and react with the zirconium oxide to cause discolorations or color deviations in the restorations (see How Coloring Ions React in a Sintering Oven below). 

Molybdenum disilicide (MoSi2) heating elements have a vitreous protective silicon layer on the surface that prevents oxidation of the molybdenum (see Figure 1).

Over time, crystals...