I have had the same problem occasionally with B&S FO. I use hot distilled water (about 140 degrees or so) and then shake vigorously and let it sit overnight. Then I shake again for about 2-3 minutes and let it sit another couple of hours. If all else fails, then I add very tiny amounts of water and pinches of oxalic until it all dissolves. Normally the extra water will do the trick. Needless to say, I plan ahead of actual need!

I can also vouch for the artcraft FO. I mixed up a batch just as Carl described, and the resulting prints were indistinguishable from B&S derived prints.

Shake it!