How to ignite ammonia

This cold liquid in the test tube is liquid ammonia. It becomes liquid at minus 33° C. If I start bubbling as anated oxygen through liquid ammonia, no visible changes will occur. This reaction occurs only at temperatures below approximately -60° C. Therefore, to get the reaction started, I'll cool the test tube from the outside with a stream of liquid nitrogen. As soon as the temperature drops below a certain point, the liquid in the test tube will begin to turn red. And the lower the temperature, the more intense the red color becomes. As soon as I stop cooling the test tube, the color will start to fade. By the way, ammonia doesn't burn in air, but if I try to ignite it while passing that gas through it, it will catch fire very vigorously.