The narrower the tube , the easier it is to build up the layer. Five millimeter NMR tubes are excellent vessels to use for this crystal growing technique. This similar to the other diffusion methods except that solutions of the reactants are allowed to diffuse into one another. If the product of the reaction is insoluble, crystals of the product will form where the reactants mix. The first way is to simply seal a sample under vacuum into a glass tube and placing the tube into an oven for a few days or weeks.
Larger crystals tend to grow at the expense of smaller ones.
If it doesn't work raise the temperature of the oven or move to another hotter one. In some cases a tube furnace can be used. The tube can be placed close to one end of the furnace so that there is a mild temperature gradient. The tube is then evacuated and inserted into a length of copper pipe which will serve to produce a temperature gradient. Andrea learned this method from his Ph.D. supervisor M. L. H. Green of Oxford.