What type of rag should be used to remove excess flux from an unsoldered copper tube joint?

When working with soldering, particularly in copper tube joints, the removal of excess flux is crucial to ensure a clean and effective bond. Using a hot rag is the best choice because it helps to soften the flux, making it easier to wipe away. Flux residues can be sticky and challenging to remove when cooled, so a hot rag allows for better cleanup efficiency.

A hot rag can assist in melting the flux slightly, breaking down its adherence to the surface, which is essential for achieving a neat and tidy joint. Additionally, it ensures that any residual flux is properly removed before the joint cools and potentially hardens onto the metal.

In contrast, using a dry rag may not be effective at removing flux, especially if it has had time to set or cool. A wet rag could introduce moisture to the joint, potentially causing issues with the solder when it is applied. Cold rags would be ineffective for the cleanup process, as they wouldn’t provide the necessary heat to aid in softening the flux. Therefore, the choice of a hot rag stands out as the most effective method for cleaning the copper tube joint before completing the soldering process.