Rosin is a natural product, made from special tree resin. It contains organic acids, which, when heated to their activation temperature, are able to remove surface oxides from metal parts. These surface oxides would have hindered proper wetting of the pad and the pin. Therefore, to allow proper wetting to take place, rosins were used as fluxes and activators. Today natural rosins have been replaced by synthetic resins and activators. Note: the vapors emitted by rosins are harmful!