Question:
I'd just finished installing a in-line water filter when I discovered a 2nd spool of solder mixed in my work area. I know that I had one set that was lead-free, but don't know about the 2nd. Now I am unsure which I used on my pipes, the lead-free or the unknown. Unfortunately neither were labeled. I only soldered 2 joints in the pipe, would it be worth the effort to undo my work just to redo it with known lead-free solder? I don't know the danger level of potentially having solder with lead in my incoming water lines.

I tried looking up if there was any way to differentiate between lead-free solder & solder with lead, but couldn't find anything.

Answer: I am curious about that too. I am thinking that there is a very small amount of surface area of solder exposed to the water, and that prior to 1990 or whenever they banned leaded solder, every copper water pipe in the country was soldered with tin-lead solder, without much in the way of bad effects. Also if your water is like ours, it tends to line the inside of the pipes with minerals anyway. But, then why did they ban leaded solder?

Because lead is bad for you and some does get sucked inside soldered joints when they're sweated. Enough small pieces adds up to enough to be a concentration of potential for concern, apparently...

I think solder has composition on the roll somewhere. Any plumbers' solder in the US (other than, of course, something imported that might not meet US guidelines) will currently be lead-free. Electronics solders and other special-purpose solders may still contain varying amounts of lead...