the problem with the paper is that some of that remaining solvent, as it warms up, might redissolve some of your spice, and then it will infuse the paper so you'll have some loss yield
two things you can do instead: Freeze, then pour out naphtha, and then stick in the freezer again, with the lid on, but upside down.. This way, any (or most) remaining solvent will drop off to the lid.. So just leave it there for few hours, then take it out, open carefully letting the solvent drop away and you can scrape the quite dry crystals more easily.
the other thing is to, instead of scraping against paper, scraping against some dish.. that way if solvent warms up and redissolves some spice, it will just stay there in the plate as it evaps...
as for the wash, you should NOT wash crystals... Its a very inneficient way of cleaning it. you should only wash when the spice is dissolved in naphtha pulls, and then mix the naphtha with the sodium carbonate solution. As for how much sodium carbonate, you really do not have to worry about it.. Add a pinch to some water (can be same amount of water as naphtha, or less, doesnt matter, SWIM used half volume of water than naphtha but its all good), and that will be enough for sure, because all that matters is that the solution is basic, and a pinch of sodium carbonate will guarantee that
