[MisterTea]

> Though I have no idea if either can be found locally.

Which is specifically why I mentioned rosin. Though any acid could likely be used so citric acid or something that could work as well.

[jaclaz]

Yep, but I don't think that just acid is good enough.

Choridric acid + Zinc was traditionally used in tin (but lead isn't so much different) soldering/brazing, particularly of copper and brass because the Zinc had some role in the chemical reaction, AFAIK.