Which two stages must porcelain not be contaminated by handling?

The correct answer identifies the heat treatment and opaque/bake stages as critical phases where porcelain should not be contaminated by handling. During the opaque stage, a layer of porcelain is developed that does not only serves aesthetic purposes but also creates a bond between the core material and the porcelain. Contaminants can interfere with the adhesion and result in an inadequate or flawed bond, leading to compromised structural integrity.

In the heat treatment stage, porcelain is subjected to high temperatures to achieve the needed physical and chemical properties. Any foreign substances introduced through improper handling during this stage can adversely affect the final properties of the porcelain, such as its strength and opacity, and could potentially lead to failure in clinical use.

This explanation helps to highlight the importance of maintaining a sterile environment during these critical stages to ensure the quality and durability of porcelain restorations.