At which stage should a metallic framework NOT be contaminated during the fabrication of a porcelain fused to metal crown?

In the fabrication of a porcelain fused to metal crown, maintaining the integrity of various stages is critical. The most crucial phase where contamination must be avoided is between the preheat and opaque stages. This is because the opaque layer serves as a barrier and bonding agent for subsequent porcelain layers to adhere properly to the metal framework. If contamination occurs during this phase, it can lead to compromised bonding strength, resulting in failure of the porcelain to adhere correctly when fired.

Contamination at this stage would affect the final aesthetic and functional properties of the crown. It is essential to ensure that the framework is free from oils, dust, or other particulates to achieve optimal results in the final restoration. Proper handling and environmental control during this phase are vital to the success of the restoration process.

Other stages, although important, do not have the same impact on the bonding process as the preheat to opaque transition.