I've found that intermittent agitation can help a stubborn-to-dissolve batch of FO. I have a magnetic stirrer which I will let run for 10 minutes or so. I then turn it off and let the solution sit for several hours, then repeat as necessary. In the end, if there's some solids left, I simply filter them out and use the solution as is. I agree that too much heat can cause more problems than a solution that is at a slightly lower concentration. Analytical filtration units like this with fine filter paper (0.45 micron) and a vacuum pump make filtering quick and very effective.
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Originally Posted by Kerik
Thanks very much for the link to the analytical filtration units. I have been looking for something just like this to filter the pigmented gelatin solutions I use to make carbon tissue.