[loulac]

I always enjoy the kind of exchange of ideas we find in Tomatoville. Everybody is courteous and attentive, even if some ideas (mine) may look far-fetched. I’ll add some details to clarify some points.
First the French system is different from yours. We first place a seal-coated lid on the jar. Then we screw on it a second lid with a central bump that will press on the first lid but let hot air escape during sterilization. Then you remove the second lid to store the jar. If sterilization didn’t work you can take off the first lid easily when the jar is cold.

Of course I use tin without lead. Its composition is written on the wrapping. I think I bought mine in an electronics store.

A flux should be used, either paste or liquid. Personally I use a liquid flux intended for stainless steel, just use the flux you have at home.

Increasing occurrence of jars not sealing : First check the condition of the seal, it must be perfect. Then check the rim of the jar. If a child hits it with a spoon it too often causes tiny dents. Run a finger on the surface, it should be perfectly smooth. This explains the source of most accidents when sterilizing with new seals. Checking these two points makes sterilizing safe.

Safety first : never do it with meat ! Re-using lids is mostly a pastime if you enjoy experimenting, saving money can't be your first aim !.