Metal (2): From molten metal

Some metals have low melting point and melting them at home is possible. When this is frozen, crystals should form.

Pb

MP: 600.61 K ​(327.46 °C, ​621.43 °F)

Pb is very similar to Bi in most properties so we used the same procedure as the one for Bi.

Difference: higher melting point, higher toxicity, no radioactivity, higher chemical reactivity, higher density, solid density larger than liquid, lower hardness and strength, different crystal structure.

We must remove asbestos to allow melting. We made some feather-like crystals on a hemisphere, but in almost all cases it freezes into hemisphere, not well-formed crystals. Maybe it freezes much faster than Bi due to the higher melting point. Also it quickly loses its silvery appearance and becomes dull upon exposure to air.

Bi

MP: 544.7 K ​(271.5 °C, ​520.7 °F)

Bi can be molten easily with alcohol lamp in a stainless steel container. An asbestos mesh is put under it to reduce heating speed and make temperature uniform.

When heated Bi is quickly oxidized into rainbow-colored Bi2O3 which prevents further oxidation. Bi has special properties that it's volume shrinks when melting, as such Bi(s) floats on Bi(l).

When it all melts, use stainless steel scissors to remove most Bi2O3, and stop heating.

A typical process uses a small Bi grain as seed, but when we tried this, it either melts or forms a very small crystal.

We finally decided to use scissors as seed. Our process is analogous to Czochralski process: move scissors a little, and if the crystal is already very close to the bottom, pull up a bit, repeat until you have to pull all up to prevent interlocking. After that, wait for all Bi(l) on the crystal to fall back into the container, and put scissors somewhere to cool down, then you can get it. Very large, hopper-like crystals can be obtained this way. The surface is not rainbow-like, but dark blue, which might be due to prolonged heating or something.

It should be mentioned that, during one experiment we heated the container again to evacuate the crystal out. One face was molten and became smooth. This face did not became colored, but stayed silvery. Maybe the wrong direction of the surface caused Bi2O3 layer to grow in a different way.