Boric Acid Powder
In the jewellery industry, boric acid is often used in combination with denatured alcohol to reduce surface oxidation and fire scale from forming on metals during annealing and soldering operations.
Spoon a little bit of the boric acid powder into a small dish. (It is best to use a dish with a tight fitting lid, such as a baby food jar, as the alcohol will evaporate quickly without.)
Add the alcohol slowly, mixing until a thin paste is made. Add more boric acid powder as needed to thicken. Getting the right consistency can take a bit of trial and error - I tend to prefer my flux a little on the watery side so I can dip small objects.
Place on soldering surface, and light with torch to burn off alcohol. It will produce a lovely green flame. Continue soldering and pickling as usual. Due to the flammability of Denatured Alcohol, it is best to store it in a secure area away from your soldering station