This post is based on the work of Theodore Gray in his blog for Popular Science. Mr. Gray, in addition to designing the UI for Mathematica and creating a literal periodic table… table, also appears to be among the few people who can make perfect bismuth crystals the first time he tries it. See his tutorial here
The principle behind metal crystals is relatively simple: the metals found in most everyday objects were formed by pouring molten metal into an appropriate mold and then rapidly cooling the assembly. This means that the atoms that comprise the liquid metal do not have a lot of time to diffuse through the material and find a location that minimizes their electrical potential relative to the other atoms—they might have time to trade places with their neighbors and form small, ordered regions within the metal structure, but overall the metal solidifies before each atom has had time to try out every possible position in the structure and find the one with the least repulsion from the other charged particles. This means that most metal objects lack global order—under a microscope, small crystalline patches will be visible, but overall the structure is a hodgepodge of various crystal structures and orientations.
This rudimentary explanation suggests that cooling a molten metal slowly will allow larger crystals to form, which provides the logic behind Mr. Gary’s approach, in which molten bismuth is slowly cooled on a stovetop. The idea of metal atoms rearranging themselves also suggests why blacksmiths anneal steel knives by raising them to high temperatures for long periods—at high temperatures, it becomes easier for bonds to break and atoms to trade places and find more energetically favorable positions in the material, thus making the knife more crystalline and thus rigid.
Above are some of my efforts to implement Mr. Gray’s approach. The setup is exactly as Mr. Gray and other sources describe–—the bismuth melts at a very low temperature, much like its noxious cousin, lead, and so a steel pot and butane flame are all that are needed to get started. I checked to see how much of the bismuth had re-solidified by blowing on the surface—do not shake the pot, as this will disrupt the formation of larger cubic crystals. Once half or so of the pot has congealed, pour off the remaining liquid bismuth to reveal the crystal structures.
The gorgeous color comes from bismuth oxide, which forms on the surface of the metal almost instantly. I acquired my bismuth from United Nuclear, who have a variety of excellent reagents at reasonable prices. It comes in 5-10 gram pellets like this:
Bismuth can also be found in certain types of game shot (it is often used in lieu of lead)—for more information, see what Scitoys uses bismuth for.