Hi,

In ASTM D6413, the gas used must be Methane.

You can check the ASTM D6413 document if you have it in your library.

Another reference is in this link:

https://marinatextil.com/textile-standard/astm-d6413-protection-fabric-standard#:~:text=REQUIREMENTS%20FOR%20ASTM%20D6413%20STANDARD%20CERTIFICATION&text=The%20flame%20exposure%20time%20is%2012%20seconds.&text=Both%20the%20fabric%20and%20the,no%20more%20than%202%20seconds.