This is what over half a ton of FIG branded 60/40 solder looks like.
Yeah, I know, there's many more alloys than 60/40, even those less dangerous (lead-free) but we do love to live on the edge.
As to why 60/40, we republish these words of wisdom from Mikey telling you why...
Did you know that...
... the atomic weight of lead is 82, its symbol is Pb, its density at 300k is 11.4? Tin’s symbol is Sn, and its electron configuration is ... oh, never mind, that’s way too much information.
Solder is an alloy of tin and lead, and the proportion of the two affects the melting point, but in a strange way. The most popular alloy for almost all stained glass work, and most definitely copper foil work is 60/40 (60% tin, 40% lead... Helpful Mikey)(a titanic intellect in a world full of icebergs...Ed). Looking at the two metals by themselves we see that tin melts at 450F and lead at 621F. But something really weird happens when you combine the two- the melting point drops down to as low as 361F depending on the mix of the two.
Check this out:
Ratio tin/Lead Melting point
0 / 100 621F
30 / 70 496F
50 / 50 421F
60 / 40 375F
63 / 37 361F
If we continue to raise the amount of tin to lead past 63% the melting point begins to rise again to where it is 450F at 100% tin, therefore 63/37 solder has the absolute lowest melting point. Pretty weird, eh? (what, you forgot about Marv Albert?...Mikey) And you thought that ratio was a gimmick? With respect to cost, the last time Stan (Mike’s metals commodities broker) checked, tin was about ten times more expensive than lead, telling you why solder prices can vary so extremely according to the amount of tin they have.