I've been using ferric oxalate powder from ArtCraft Chemical for a couple years now. It is very pure and hard to dissolve unless doctored slightly. I measure 15 grams of the pure FO, then add two grams of oxalic acid, then one gram of EDTA tetra. Funnel into a 100ml capacity brown bottle. Add 55ml of distilled water, then alternate ten second blasts in a microwave oven followed by vigorous shaking. In tests, I found I could never get it to dissolve without the additives, no matter how much it was heated, but with the additives it suddenly goes into clear state at a hot but well below boiling temperature. I suspect that the purity and amount of either impurities or intentional additives will change the ease with which FO powder dissolves. Using the above technique, I get liquid FO that has excellent shelf life (at least 4-6 months, though I seldom have it around that long) and complete consistency from one mixed batch to another--one time when I'd mixed up a new batch I made comparison prints with the dregs of a months-old bottle, and the prints were identical.