If the flux turns black and does not reflect the flame, what does this indicate?

When the flux turns black and fails to reflect the flame, it is a clear indication that overheating has occurred at the joint. In brazing processes, flux is used to prevent oxidation and improve the flow of filler metal. However, if the temperature exceeds the optimal level, the chemical composition of the flux can break down, leading to carbonization, which manifests as a blackened appearance. This decomposition not only alters the effectiveness of the flux but can also indicate that the joint has been overheated, potentially compromising the integrity of the brazed joint and affecting the quality of the finished product. Recognizing this sign can help the technician adjust the heat application and maintain proper control during the brazing process.