That's like the endless English discussion about whether to pour the milk first or the tea! The rationale I was given runs like this...

If you melt the lower MP metal first, even though you get better thermal transfer, you still have to heat that metal way above its melting point (Al 660 - Sn 232) and approach its boiling point (Al 2467 - Sn 2600) in order to start melting the copper (1084). Well before that point (we'd never get there in a small furnace) that metal will begin to evaporate - and that is going significantly to alter the proportion of the two metals. If you heat the copper first, you are making a larger volume of liquid at a temperature that is already hot enough to melt the smaller amount of the lower MP metal. Putting the relatively cold copper into a smaller volume of over-heated Al or Sn will immediately cool it again.

Yes; it is harder to do the initial melt. But you'll lose less tin (or aluminium) if you put it in second - and if (as your video seems to suggest) you are working indoors for at least part of this process, you are less likely to kill yourself with the metal fumes!

Don't ignore the safety! Some more PSE gear would be useful - and do set up extraction facilities if you are pouring indoors. You are worth it!

