You could just use fumaric acid and water, but dose up your product by proportion. IE: if you have 1g of freebase, after salting it weighs 1.4g. the extra weight is due to salting and excess acid to a small extent. You just weigh out 1.4x the dose of fb you would normally take to preserve titration. Purifying is only necessary for aesthetic purposes or for crystal porn, personal preference, etc. IMHO
Excess fumaric acid has no real detriment other than tasting sour when dissolved, and throwing off weight. But i find it useful as it helps make my tap water (slightly basic ~ 8 PH) more readily dissolve the salted alkaloids.
Other than that, if you have acess to clean dry acetone, and consider that foodsafe, i have seen teks on the web where a dmt freebase/fumarate conversion is done with harmala fb. IE- dissolve fb in acetone, take another container, add acetone and fumaric acid, pour small amounts of fasa (fumaric acid saturatd acetone) into your harmala acetone solution and it should cloud as fumarates precipitate. The beauty of this is that excess fumaric acid will stay dissolved in the acetone. And the fumarate salts are completely insoluble in acetone from what i've read. this holds true for other alkaloids so i would trust it.
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