During recrystallization, some time the crystals stick to the bottom of the
flask or beaker. So how could we collect them.
Add a small amount of solvent and transfer it to the filter paper by gently digging with a spatula.
Try adding a solvent crystals are not soluble into and put the beaker in a mw bath at very low power (without heating). Pay attention: long exposure to mw may result in powdering the crystals.
One of my Ph.D. fellows in England used to functionalize the surface of the glass performing a pre-treatment with a solution called "Sigmacote". I do not know exactly the details of the procedure (I guess is pretty simple), but what you are doing is basically making the surface of the glass extremely hydrophobic, by reaction of the silanol groups at the surface of the glass with aliphatic chlorosilanes. This sometimes prevents the crystals to stick on the surface of the glass.
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