I rarely sweat copper pipe but did so recently. I bought lead free solder because the solder I had on hand (from long ago) had lead. The flux I found on the web said "lead free". That leads me to ask, Is it ok to use my old flux from lead solder days or do I need to buy the "lead free flux"?

  • Lead was abandoned for the health reason, not for technical reason.
    – Traveler
    May 12, 2023 at 2:19
  • You bought lead free because your supply had lead?? Please clarify.
    – david
    May 12, 2023 at 5:35
  • "Supply" here probably means "solder on hand".
    – isherwood
    May 12, 2023 at 13:43
  • @david perhaps Yehuda meant his existing stock of solder was lead solder so he had to get some lead free solder to sweat a copper pipe. He presumably did not mean his water "supply" contained lead. May 12, 2023 at 16:33
  • It either says "for potable water systems" or it goes in the trash. And it has a MSDS or again it goes in the trash. How about, it just freaking goes in the trash? Lead isn't the only concern. I ain't using no mystery shit in my pipes. It says Oatey on it? pretty sure you're good to go. Can't even read the label? I'll give you three guesses where it belongs....
    – Mazura
    May 12, 2023 at 21:16

2 Answers 2


Silver solder requires / benefits from a more aggressive flux. But since this is plumbing, not electronics, your old flux is probably already an aggressive flux, that works well with exposed metalwork or with the slightly higher temperatures of silver solder.

Your old flux and your lead-free flux are probably the same thing. Read the label.

  • 1
    Good point; flux has always been lead free...
    – keshlam
    May 12, 2023 at 12:33
  • @david why are you bringing up silver solder in connection with plumbing and electronics? AFIK silver solder is never used with water pipes or electronics. There are multiple different lead free soft solders sold and AFIK none contain silver. The last one I got was tin-copper. From what I have seen copper tubing connections for refrigerant lines are soldered wth silver solder. May 12, 2023 at 16:37
  • Didn't some fluxes contain solder? Were these labelled as "tinning flux"? I would get new flux sold as being suitable for whatever lead free solder the OP purchases. May 12, 2023 at 17:03
  • @david I now see there is a family of solders containing tin, silver, and copper which are used for electronics. But AFIK lead free solders for sweating copper water pipes do not contain silver, or do they?! I think my last purchase was 3% copper - 97% tin, but Icannot find it right now. It has been years since I sweated copper tubing. link.springer.com/chapter/10.1007/978-0-387-48433-4_4 May 12, 2023 at 17:14

I would not trust an old container of flux from the lead solder days. Some flux did have enough solder mixed in to "tin" the joint. So it would be possible that the old flux would have lead solder in it.

Get a new container of flux which is formulated to work with the lead free solder that you buy. The last solder I got five or more years ago was 97 % tin - 3 % copper. The link below references a flux which is presumably formulated to work with tin-copper solder.


  • 1
    My old flux worked very poorly with lead free solder but great with lead based solder. The flux designed for lead free worked great with lead free solder.. There is a difference.
    – Gil
    May 13, 2023 at 0:06

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