Hi RogerH,
I used to be involved in Aluminium welder and I do recall the material was pre-heated prior to striking the arc.
The temperature was attained using a soft/ fluvy flame, the edges of the weld would have soap, yes, a bar of white soap was rubbed along the weld when the soap went light brown it had achieved its pre-heat temperature. Much easier than temple sticks and crayons etc.
Hope it may help?
Regards
Nigel