I've just watched the Periodic Table of Videos on Aspirin and saw some "tricks" used to induce crystallization after the synthesis. Rapid heating with a heat gun followed by rapid cooling in ice was not sufficient, but scratching the bottom of the flask with a stirrer seems to have triggered rapid crystallization.
Warning: For those who are sensitive to the sound of chalk or fingernails on blackboards, better lower the volume during the actual scratching starting after
07:10, it's quite loud!
There is a little bit of an explanation for how the scratching helps:
When you scratch it you are actually forming a nice surface for the crystals to aggregate on.
but I am not sure I really understand what is really happening.
Question: Is the scratching just glass-on-glass? Is this crystallization happening on the (probably) invisibly small scratches only, or does it happen throughout the solution? Is there an "official" name for this technique? Has it ever been scaled up in some way?
For example, this answer says that the scratching produces a rough surface on the bottom of the glass for nucleation, but this answer says it is shards of glass in solution that are providing the nucleation sites. I'm trying to understand what is actually happening when chemists rub stirrers on the inside of flasks to induce crystallization, and once that's understood, to find out if the phenomenon is scaled up in manufacturing situations.
below: "Before" and "after" screenshots showing the clear liquid becomes thick with crystals after scratching the glass.