"Da Edster" actually raises a fair to Midlands question here- let's all go back to HS Chem 101 and get 'fluxed"--
Acid and rosin based are the two main fluxing elements du jour-- We can eliminate rosin flux, it is for electronic. Acid based fluxes are the stronger flux, most contain H2SO4 acids-- but a paste variant also has traces of zinc chloride. This is good for "wetting' the areas to be soldered. But taces of the acid can often remain, trying to corrode the metal surfaces it was used to join. Flux acts as a "wetting agent" and causes flow of solder under applied heat and pressure.
"Wetting action" is a bit akin to dissolving-- like follows like in both chemistry and also metallurgy. Now to the question our "friend?" Eddie Da Torch raised- I do not know. I know my 90 year old American side-by-sides have their barrels still solid to the ribs and keels-just as I know that hot salts tank rebluing of soldered barrel assemblies is the "kiss of death"--
