What are the conditions of a successful precipitation in gravimetric analysis?

In an ideal world, an analytical precipitate for gravimetric analysis should consist of perfect crystals large enough to be easily washed and filtered. The perfect crystal would be free from impurities and be large enough so that it presented a minimum surface area onto which foreign ions could be adsorbed. The precipitate should also be "insoluble" (i.e. be of such slight solubility that loses from dissolution would be minimal).

Without going into detail, it has been shown (Von Weimarn) that the particle size of precipitates is inversely proportional to the relative supersaturation of the solution during precipitation; 

                   relative supersaturation = (Q-S)/S

For the best possible results, conditions need to be adjusted such that Q will be as low as possible and S will be relatively large.