First, regarding fuel gases, acetylene is the 'dirtiest' of the fuel gas choices, due to the hydrocarbons in the flame.
Propane is a much cleaner choice.
Some prefer to use hydrogen, as it is the cleanest burning. Firescale is caused by presence of oxygen, creating oxide formations. As an example, copper tubing turns black, due to oxide formation, after joints are brazed. The interior of the copper will look the same way. If the interior of the copper tubing is filled with nitrogen, the appearance of the interior will be bright and clean, as new, even after brazing, due to the lack of oxygen.