When I did my fire training for CPL at Redhill, the only easy way to get the Jet A1 to light was to pour avgas on the top of it in a tray (lower SpG) and light that. It ignited straight away, and in time raised the temperature of the Jet A1 enough for it to ignite as well after much fizzing and bubbling. The heat given off by the Jet A1 when it got going seemed far more intense than the Avgas did.