I mix a 20% solution. I start with 75ml of distilled water at around 65 Celcius. Add 20g ferric oxalate and stir in. I mix in a brown bottle by shaking violently. Then I add more hot distilled water to make 100ml and give a good shake. Bostick and Sullivan's ferric oxalate goes in easily. The stuff from JDphotochem takes a very long time to go in, but it will eventually.