Growing Gallium crystals is easier in a vacuum?

I decided to pour some molten Ga into a vacuum filtering flask, using a solid stopper. I wanted to see the Ga behave physically like Hg; Gallium doesn't stick to glass in a vacuum. When done, I poured the Gallium back into a beaker and was surprised to see a single crystal left behind. I have tried growing one before with no real luck, but as the picture shows, it turned out more than half decent. Does a vacuum support crystal growth? (Technical data: Vacuum was 600 mmHg. Possible contaminants: Iron oxide, Iron (II) Sulfate).