Do you need to use a flux when brazing copper to copper?

In the process of brazing copper to copper, flux is indeed not strictly required. While flux serves an important role in cleaning the metal surfaces and preventing oxidation during the heating process, it is possible to achieve a successful braze without it, especially if the copper surfaces are thoroughly cleaned and free from contaminants.

Brazing relies on capillary action to draw the filler material into the joint between the two pieces of copper, meaning that the surfaces must be clean and fit together well. If they are adequately prepared, oxidation and impurities can be minimized, allowing the braze to occur effectively without the need for additional flux.

While in many professional applications flux is recommended for safety and reliability, particularly in preventing oxidation at elevated temperatures, it doesn't become an absolute necessity. This is why the option stating that flux is not required can be considered correct in this context.