Grow Crystals in
and cooling bismuth reveals the secret crystalline life of metal.
By Theodore Gray
hard to tell before you pour the excess bismuth out of the cup whether
youíve waited the correct length of time. Just keep trying until you get
it right [photo 1]. Large crystals, like those seen in photos 2 and 3,
will form only with 99.99 percent pure bismuth, and only if you use clean
Project: Making crystals
Time: 30 minutes
When I was a teenager melting elements in my parentsí basement, I noticed that
cooling lead would sometimes form a snowflake-like pattern on its surface.
Snowflakes are crystals, and I had never thought of metal as crystalline. Metals
are shiny, malleable things. You canít bend a crystal into a coat hanger!
But most metals are crystalline, at least at the microscopic level. For example,
iron and its alloys consist of a mixture of microscopic crystal zones
interlocked with one another. The difference in properties between cast iron and
high-carbon tool steel or the nickel-iron superalloys used in turbine blades are
largely the result of the various sizes and shapes of the constituent
microscopic crystals and whatís in the space between them. Quenching (fast
cooling) and annealing (slow cooling) are nothing more than ways of controlling
the size of the metalís crystals. In general, the slower a material is cooled,
the larger its crystals can grow.
The crystals inside a metal are normally not visible, because they are too small
and all tangled up with one another, forming what appears to be a smooth solid.
But if you melt a pot of metal, let it cool until the edges are solid, and then
pour away the part thatís still molten, you can sometimes see the crystals
sticking out of the solid part. Bismuth is especially good for this because
itís reasonably priced, easy to melt, and especially eager to form large,
Bismuth is an oddball element. Typically, elements in the same area of the
periodic table share similar characteristics. If you go up from bismuth on the
periodic table, you find antimony and arsenic, both notably toxic. If you go
left, you find lead, thallium and mercury, all toxic. (A British serial killer
named Graham Frederick Young killed three people in the 1960s and 1970s by
slowly poisoning them with thallium.) If you go right, you find nothing but
radioactive elements; bismuth is the highest-numbered stable element. But
despite living in this rather nasty neighborhood, bismuth is essentially
nontoxic. In fact, itís a natural antacid and the main active ingredient in
Pepto-Bismol. Bismuth has a lower melting point than even lead, so itís easy
to liquefy on a stove top. I use a pair of brand-new stainless-steel measuring
cups, one to melt it in and another to pour the excess into when the first one
has hardened about halfway. It doesnít work every time, but the process will
usually net you some big crystals like those in photo 3 [above], and once in a
while you get a beautiful nest of square crystals, like a geode. Turn the cup
over and bang it on a hard surface to knock the bismuth out.
These large, colored crystals can be made only with 99.99 percent pure bismuth,
which is available for about
$20 a pound from United Nuclear. Just remember to be careful with the molten metal.
Always wear safety glasses, welding gloves, an apron and sensible shoes.