Lab technique

In the experiment that I did today, there's a very strange step. After mixing two solutions in a beaker, according to the lab manual, we needed to scratch the bottom of the beaker using a spectula so as to help forming ppt. My question is why does scratching the beaker help forming ppt?
 
There needs to be some irregular surface for the crystals to nucleate. A clean beaker, especially a new one, can be very smooth and thus solutions can be very slow to crystallize without a scratch or seed crystal. The same can be said for boiling, you want a stir bar or boiling ship to prevent bumping. Apparently this is common in cooking to, confectionaries especially have to be careful.
 
There needs to be some irregular surface for the crystals to nucleate. A clean beaker, especially a new one, can be very smooth and thus solutions can be very slow to crystallize without a scratch or seed crystal
Um.. I have cut cardboard using a very sharp cutter on top of a piece of glass before and no scratch could be detected. I doubt whether scratching a beaker with a spectula can create irrigular surface in it.
 

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Nucleation sites can't read, and don't really require that you engrave your initials on a surface deeply enough to be visible to the naked eye from earth or lunar orbit; the nucleation sites you create by rubbing a fire polished glass stirring rod down the side or across the bottom of a "clean" pyrex beaker can be very subtle differences in thickness of layers of adsorbed air (other gases), surfactants, last weeks expt., your fingerprint residue on the end of the stirring rod, pyrolysis (by frictional heating between rod and beaker) of surface functional groups on the glass --- it doesn't ordinarily take much --- there are systems that can absolutely drive Carrie Nation to the bottle (glycerine's a good example).
 

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