If stirring negatively affects crystal purity, why is it so? My Intellectual Expectations
Intuitively, I would expect strong stirring to improve crystal purity during the process of crystalization. My rationale is that strong stirring would increase contact between each crystal and the variety of ions present in solution at each given level of solubility, and thus minimize the gap between reality and the equilibrium that would be reached if the conditions at each instant (solvent volume and temperature) would be kept equal for a very long time.
Information I have Read
I've read that as a general rule, the size of the crystals that form correlates well with the purity inside the crystals (i.e. that crystal
size in general is a good indicator of purity).
My Conflicting Observations
My observations are that under strong stirring the crystals that form are much smaller than if the crystalization is carried out in still water (at equal rate of cooling / solvent evaporation). This indicates that either crystal size is not a great indicator of purity, or that my rationale is wrong and by counterintuitive mechanisms which I ignore, crystals of greater purity would be formed without stirring.
My Question
Does anyone know 1) Wether stirring the solution during the crystalization  process positively or negatively affects crystal purity? 2) What is the physical explanation of the answer?
&quot;Strong stirring&quot; is a rather imprecise term.  There is such a thing as too much speed.  Rule of thumb for APIs, if you're working with a magnetic stirrer in a round bottomed flask in a lab (rather than in some industrial vessel or pilot plant setup) 350 rpm is too fast (and no, I can't quote a reference on that, it's one of those things you get told and then confirm by experiece - usually by being a wise-arse and bucking the &quot;established wisdom&quot; and ending up with a product that doesn't make medical grade).
Agitation for different crystallisation processes are generally kept in certain narrow bands to avoid inclusions as wel as a number of other reasons.
An example is described in Chow et.al [1] where when crystallizing paracetamol from water in a 500 mL round-bottom flask with p-acetoxyacetanilide present, increasing the stirring speed from 200 to 320 rpm increased product purity, but increasing further to 400 rpm lead to decline in product purity.
According to Meedara et.al. [2] stirrer-particle and interparticle collisions can facilitate crystal attrition  in suspensions with high energy agitation, leading to attrition-induced inclusions.
Urwin et.al. [3] did a reasonably thorough job testing various methods and reasons for presence of impurities in crystals, and is worth a read.
Another good reference is The Handbook of Continuous Crystallization, start with chapter 1 [4]
That said, in crystallisation especially of APIs, mechanisms can be compex and are still not completely understood. All kinds of workarounds have been developed, such as e.g. ultrasonics to help prevent agglomeration when crystallizing paracetamol from certain solutions.
If you have no literature on the process you are attempting, finding the &quot;sweet spot&quot; where you get the greatest purity of product, is only going to happen with a lot of experimentation.
[1] Chow, A. H. L.; Chow, P. K. K.; Zhongshan, W.; Grant, D. J. W. Modification of Acetaminophen Crystals: Influence of Growth in Aqueous Solutions Containing P-Acetoxyacetanilide on Crystal Properties Int. J. Pharm. 1985, 24 (2–3), 239– 258,  DOI: 10.1016/0378-5173(85)90024-9
[2] Meadhra, R.; Kramer, H. J. M.; Van Rosmalen, G. M. Model for Secondary Nucleation in a Suspension Crystallizer AIChE J. 1996, 42 (4), 973– 982,  DOI: 10.1002/aic.690420410
[3] Urwin, SJ, Levilain, G, Marziano, I, Merritt, JM, Houson, I and Ter Horst, JP
A Structured Approach To Cope with Impurities during Industrial Crystallization Development Organic Process Research &amp; Development 2020 24 (8), 1443-1456,  DOI: 10.1021/acs.oprd.0c00166
[4] McGinty, J, Yazdanpanah, N, Price, C, ter Horst, JH and Sefcik, J  Nucleation and Crystal Growth in Continuous Crystallization , in The Handbook of Continuous Crystallization, 2020, pp. 1-50 DOI: 10.1039/9781788013581-00001