This is the same procedure used for making rock candy, an edible science experiment popular in elementary school classes. Crystals, of course, are simply highly ordered arrangements of atoms or molecules. Once the molecules have begun assembling in an orderly fashion, other molecules will continue adding to the same scheme, it being energetically favorable to continue the existing pattern rather than to fall apart into randomness. Thus the formation of larger and larger crystals. The cords so cleverly used by Greetje simply provide a scaffolding for this natural process to occur.
You can do this in any sort of container you happen to have lying around, but Greetje uses actual chemical glassware. A set-up like this is mundane to scientists but often seen in popular culture as something sinister...bubble bubble toil and trouble...in which wild-eyed scientists concoct foul brews of death and destruction. I love that here, the lab instruments grow objects of delicate beauty.