Ok.

As now two people suggested a slow evap I think I'll give it a try. Today I warmed it all up again to redissolve the xtals which formed in the test tube which I was using now to get them out of the tube. There was also some gunk at the bottom of the tube, which I left behind. Then I filled everything in another vessel to do a regular freeze perzip.

I think I also used waaay to much heptane. This is why I percip the xtals out first, then safe the heptane and then redissolve the xtals in the smallest ammount of heptane possible.

Or should I use another solvent? I have heptane, naphta and light naphta at hand. I guess light naphta will evap to fast? If not I'd like to use this.