bigger crystals will form slowly. Give a few months.
Rapid crystallization means the crystallization is occuring in the solution many times within close succession. So, basically, before the crystals have time to grow bigger, you've already run low on whatever is still left in solution.
If that makes any sense to you.
So, what you want to do, is slow this process down. You want only one spot in the solution where a crystal can form, and you want this crystal to form very slowly.
So what can you do? A crystal wine glass may already be a good container for you, its smooth and concave, hard for crystals to form on that surface. If its without any scratches, any grooves in the glass will give surface for crystals to grow.
You want the naphtha to evaporate extremely slowly. You can do this by putting the glass in a room where it can remain undisturbed, you can slow down evaporation by putting aluminumn foil with holes poked through over the top of the glass. This also helps keep out dust.
The best way to grow a singular large crystal, is to use a seed crystal. I experimented with methods of suspending the seed in solution, i found nylon to be the best to discourage crystal growth.
Good luck, crystal growing is fun.
