What type of gas should be maintained until the joint is cool to the touch?

Maintaining nitrogen gas until the joint is cool to the touch is advantageous for several reasons. Nitrogen is an inert gas that does not react with the molten metal or the substances involved in soldering or brazing. This property helps to prevent oxidation that can degrade the quality of the joint.

Using nitrogen can create a protective atmosphere around the joint, thereby ensuring that the metal remains pure and free from contamination during the cooling process. By preventing oxidation, nitrogen helps to maintain the integrity and strength of the joint created through soldering or brazing practices.

Other gases, such as oxygen or acetylene, are reactive and can lead to oxidation or undesirable chemical reactions that compromise the quality of the joint. Hydrogen, while it can be used in certain applications, also presents potential hazards and difficulties with controlling the environment around the joint. Therefore, nitrogen is the most appropriate choice for maintaining the joint's quality until it is safely cool.