Did you heat the ferric oxalate solution when you initially mixed it from powder? If not, take the solution you have and warm it up to 120F for a minute or two. If you do it for much longer than that, you run the risk of producing extremely grainy prints. If you have already heated the solution and it still refuses to disolve, you can let it settle for an hour or two, then decant the clear solution off the top and discard the dregs.
This gives me pause to think, though, that you may run into some problems when trying to use this ferric oxalate to print kallitypes. Ferric Oxalate that has a hard time disolving is usually hard and chunky when it's in 'powder' form. This means that there is almost always lots of impurities bound up inside the large crystals. Oxalic acid, one of the main ingredients in powdered ferric oxalate is the death of kallitype prints. It will cause the silver nitrate to almost instantaneously fall out of solution, making your coating solution impotent.
There are two simple tests for the purity of ferric oxalate:
1) 2 drops of 10% Silver Nitrate solution are added to 5 drops of ferric oxalate 20% - 27% on a watch glass. Observe. If after 5 minutes there is no precipitate formed, the ferric oxalate is free of oxalic acid.
2) 2 drops of 10% Potassium Ferricyanide are added to 5 drops of ferric oxalate 20% - 27% on a watch glass. Hold solution to a 40-60 watt bulb and observe the color. The color should be a brilliant green/turquoise. Expose the solution to sunlight or intense UV. If the ferric oxalate is pure, the solution should rapidly change to a dark prussian blue. If no change, or very little change occurs, the ferric oxalate is contaminated with ferric nitrate or ferrous oxalate.
Good luck, and keep us up to date on your progress!
