What puts the heat into the joint to melt the filler metal during silver brazing?

The correct answer focuses on the role of the joint in the silver brazing process. During brazing, the joint itself is where the heat is applied, facilitating the melting of the filler metal. The heat is conducted to the joint area from the flame and other heating sources, enabling the filler metal to flow into the joint by capillary action.

The joint's surfaces must be prepared properly for effective heat transfer, as the quality of the joint significantly influences the brazing process's success. The joint's materials should be able to withstand the necessary temperatures and form a good bond with the filler metal, maximizing the effectiveness of the heat applied.

When considering the other options, while the flame is indeed a heat source, it is the joint that ultimately receives and conducts that heat to melt the filler metal. A heat sink refers to a component that absorbs heat, which is contrary to the desired effect in brazing, where heat needs to be concentrated at the joint rather than dispersed. The filler metal is the material being melted and does not generate heat on its own. Therefore, understanding the dynamics of heat transfer in the context of the joint is key to mastering the silver brazing process.