Borax and boric acid are similar in that both will melt onto your metal blocking oxygen, thus preventing fire stain and fire scale. Boric acid is a bit more active, attacking existing oxides a bit more, though neither is very active at this, which is part if why others form coatings that don’t quickly become ineffective with prolonged heat or a lot of oxides. Handy flux is an example of a much more active mix, where fluoride and other additions more aggressively attack oxides, but in trade, will burn out/become saturated with oxides, etc., and stop doing its job. So great as soldering flux, but not so good as a protectant of your metal. Between boric acid and borax, boric acid melts and becomes active at a lower temperature, protecting your metal before any oxides can form. Its commonly used dissolved/mixed with alcohol, where burning off the alcohol leaves a light coat on the metal, as well as stones, some of which (primarily diamond) it will protect. Borax melts at a higher temperature, and is less active at dissolving oxides, but historically was used more than boric acid when working silver. You get, for protective purposes, the best of both. Prips flux is a mix of 3 parts Borax, 2 parts each of boric acid and sodium phosphate, dissolved in tap water. Use the more common TSP, trisodium phosphate. It acts as a wetting agent to let the other two form a uniform protective coat against fires are and fire stain on silver. Unfortunately, it also seems to make Prips flux unsuited as an actual soldering flux. Use it, but then add a good soldering flux just to the seam.