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Take a glass rod and rub vigorously the wall of the flask, the substance will crystallize out of the solution.

Take a fire polished stirring rod and etch (scratch) the glass of your beaker. The small pieces of glass that are etched off of the beaker serve as nuclei for crystal formation

But reaching back to my halcyon years of organic synthesis and crystallization, recalling vivid memories of scratching the sides of Erlenmeyer flasks with a glass rod to induce crystallization

It's a common lab technique. But does it have a name?

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  • $\begingroup$ I don't remember it having a name, but the idea is that the bits of glass from the scratching will serve as "seeds" (nuclei) from which crystals can form, i.e., heterogeneous nucleation. $\endgroup$
    – user95
    Jul 9, 2012 at 12:09
  • $\begingroup$ Could you be a bit more verbose in your question? It makes it easier to understand.. Provide as many details as you know (do a bit of research yourself to get more details, that makes it a really good question). $\endgroup$
    – ManishEarth
    Jul 9, 2012 at 13:20
  • $\begingroup$ OK, I have extensively googled it but I can only find descriptions, not the name. I'll add more descriptions $\endgroup$
    – Joseph
    Jul 9, 2012 at 16:14
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    $\begingroup$ @Richard Terret, You are absolutely right, and btw. the need of a fire polished glass rod is nonsense. A old and scratched one is sufficient. I see the nucleation from the (ultra)sound or cavitation happening at the rubbed surfaces. Think of the "starters" used in those hand warmer bags with sodium acetate. A name for the lab technik is "rub". :=) $\endgroup$
    – Georg
    Jul 10, 2012 at 8:49
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    $\begingroup$ @Georg (and Richard)- I agree. I'm not so sure that it is the "microscopic flecks of glass" so much as the sonic disruption of a supersaturated solution. I have accomplished something similar by flicking the flask hard with my fingernail. BTW, I've always called (and heard called) this technique "scratching". $\endgroup$
    – Ben Norris
    Jul 19, 2012 at 1:06

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I think the most specific you can get on this phenomenon is nucleation , specifically heterogenous nucleation / surface nucleation.

The rough surface formed creates scope for nucleation to occur, and this creates the crystals.

I don't think there is a specific name for this technique as a lab tenchique .

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In my experience, my senior lecture has called this method 'scratching' and said that this is a widely used term. The term nucleation is used to aid the description of the technique 'seeding' where a couple of grains of the pure substance you are creating is added to act as a nuclei for crystallisation to take place upon.

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I believe the term you are looking for is trituration. The word itself has other meanings, but this method of inducing crystallisation is one.

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  • $\begingroup$ No trituration exploits relative solubilities for purification. It isn't about forming the initial crystals from solution. $\endgroup$
    – MaxW
    Oct 13, 2016 at 5:17
  • $\begingroup$ If you're correct, why would one be attempting to exploit these solubilities in the first place? The terminology applies to both adding solvent to dissolve impurities vs product (or vice versa ) as well as using a solvent mix or volatile solvent to dissolve and subsequently attempt crystallisation $\endgroup$
    – Beerhunter
    Oct 27, 2016 at 23:51

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