What is the effect of overheating the joint during silver brazing on the flux?

Overheating the joint during silver brazing can cause the flux to turn black due to the degradation of its chemical composition. When the flux is subjected to excessive heat, it can decompose, leading to a color change that typically appears as a black residue. This change indicates that the flux is no longer effective in performing its primary functions, such as preventing oxidation and promoting good metal flow during the brazing process.

The transformation of flux into a black substance can significantly impair joint quality. If the flux does not perform adequately, it can lead to poor wetting and reduced bonding between the base metals and the filler material, ultimately compromising the integrity of the brazed joint.

In contrast, the other options suggest effects that do not align with the behavior of flux when subjected to overheating. For instance, while it is true that excessive heat can impact the consistency or functional properties of the flux, the specific effect of turning black more directly reflects the degradation nature of the flux when exposed to high temperatures.