The "exact science" behind the CFC: CFCs use a few simple principles... surface area, surface suitability for precipitating on to, and variable solubility by temperature. It works like this. During the initial, slow cooling of the solvent, the solute wants to precipitate out a bit at a time, as the solution becomes more supersaturated only slowly. In order to crystallize, the dissolved solute has to "bump into" an appropriate surface... lots of things need to be right, the angle of the molecule and the shape and polarity of the surface. This is called an effective collision. The point is, the better the surface, the more likely a random bump into it by a solute molecule will result in it "sticking". Now, I'm not positive the absolute best surface for DMT to crystallize on is already crystallized DMT, but it's very good, much better than glass or floating dust. So, since the dissolved solute molecules stay more or less equally spaced throughout the solution as things precipitate, all the surfaces get "bumped" at about the same rate, but a growing DMT crystal will have more "bumps" result in the molecule precipitating out of solution. The crystals will gain more molecules per surface area than the side of the jar. Next, we bring the next fact into play, by moving the jar from the freezer to the fridge. This slowly warms the solution, and now instead of precipitating, the solvent is unsaturated and the equilibrium reaction wants to move solid-state molecules of solvent to dissolved state... some of your crystallized DMT will redissolve. Except this time, instead of looking for an effective collision between surface and solute, you're looking for effective collision between solvent and solute... all the naphtha in the jar is the same (well, technically it's not since naphtha usually contains more than one type of hydrocarbon, but the solubility is the same everywhere in the jar). This means, everywhere naphtha is in contact with DMT crystals, the molecules of those crystals have an equal chance of being dissolved. Since small crystals have more surface area exposed, they are subject to more collisions per molecule and will dissolve completely before the large ones. The larger ones will lose mass, but more slowly by percentage. Ideally, you will take advantage of this by allowing the solvent to warm gradually until all of the small crystals have dissolved, then immediately return the solvent to the freezer as not to lose any more mass on the large crystals. Now, in the freezer, the initial effect takes place again. Your existing larger crystals will be more likely to gain molecules of undissolved DMT than the glass. As the solution cools and molecules of DMT must precipitate out, the crystals will remove molecules of DMT from solution more quickly per surface area than the glass until no more needs to come out of solution for the equilibrium reaction to be stable. So, as long as the amount of mass gained by the large crystals during the cooling process exceeds the amount of mass lost during the warming process, the crystals will grow while the small crystals formed on the glass are continuously redissolved and do not. Thus, as long as you time your warming and cooling cycles correctly, each cycle will deposit X moles per surface area on the large crystals, Y moles per surface area on the glass, then dissolve Y moles per surface area, leaving none on the glass and X-Y moles on the crystals. If you write this into a proper equation you'll quickly discover that by adjusting the length of the cooling and warming periods, you're in effect setting a minimum weight/surface area ratio on any crystals for them to remain undissolved at the end of each warming period. Crystals exactly at this ratio will remain the same size and all with a better ratio will grow. Since larger crystals "enclose" more molecules, they have less molecules "touching" the solvent per molecule that doesn't, and their ratio is better. The Merck Index is probably your best bet on turning this knowledge into something useful via math. If that doesn't mean much to you, then try this: next time you freeze precip, cool the jar as slowly as possible to encourage the first effect to give you at least some larger-size crystals... say, room temp, fridge to as cold as it gets things plus a bit, freezer on low, then medium, then high. Then, transfer the jar to the fridge and check every little while until all the smaller crystals are gone, note how long it took, and use this amount of time in the fridge. For the freezer stage, you can't take too long (after a while DMT will just stop coming out of solution and you're wasting time, but no ill effects on the CFC), but you CAN not take enough time (you didn't let it cool enough for all the DMT you redissolved to reprecipitate, which means you're dissolving more than you're precipitating and all your crystals lose mass). My advise would be no matter what, make sure the final freezer stage lasts extra long so even if you fucked up your CFC, you still have everything precipitated out, just in smaller pieces than it would be if you'd done it perfectly.
