# Scratching the inside of a flask to crystallize aspirin - what is it called and has it been scaled up? [duplicate]

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.

• @andselisk there is no accepted answer there, and all three answers there would not even approach a complete answer to this question; glass-on-glass? on the scratches only, or throughout the solution? Has it ever been scaled up? so no, it is not possibly a duplicate. Not even close. Also, other answers here claim it is particles of glass in solution, not the scratches themselves, so I'm really looking for a more comprehensive understanding of what is happening. – uhoh Sep 18 '17 at 4:11
• Your question is not an exact duplicate, but it's very close to that question. If you seek an accepted answer, have a look at this one: chemistry.stackexchange.com/a/44486/41328 – andselisk Sep 18 '17 at 4:19
• I see no contradiction as nucleation can occur both on the scratched glass surface and/or any particles present in the solution (shards, dust, density fluctuation etc.). – andselisk Sep 18 '17 at 4:30
• @uhoh I believe you are correct that this does not constitute a duplicate because the other similar questions do not have well founded answers. I would just recommend trying to not get frustrated. Don't let a misunderstanding devolve into a heated argument. – Tyberius Sep 18 '17 at 4:50
• @Tyberius I don't see any of that. Insta-closes can snowball, so I've tried to encourage the user to reverse the close vote by explaining that I'm looking for someone who may have some knowledge of what's really happening, and that a quick-close would prevent that person from leaving an answer. It looks like andselisk has tried to make that look like it was something very different from what it is. If you read the comment again you'll see it's quite innocuous. Everyone has different knowledge, and SE lets us share. – uhoh Sep 18 '17 at 5:10