Experiments

Making crystals from bismuth

bismuth crystals

Wouldn’t it be cool if you could make bismuth crystals right in your own kitchen? Guess what? You can! Bismuth is a safe, non-toxic element that can be melted on your kitchen stove to form beautiful, rainbow crystals. 

Some of the science behind bismuth crystals

Bismuth has a low melting point of 271°C (520°F). As bismuth solidifies, it expands to form crystals. Artificially grown bismuth crystals form a spiral staircase structure. This shape is due to the higher growth rate on the edges compared to the inside. Bismuth metal is naturally a white-silver metal with a reddish-pinkish hue.

However, upon oxidation, bismuth crystals turn into a fascinating rainbow color. The rainbow colors are due to variations of thickness of the oxide layer that forms on the crystal surface, causing different wavelengths of light to interfere with one another. Learn more about bismuth here!

Are Bismuth Crystals Toxic?

Bismuth crystals are generally considered to have a very low toxicity level. They are generally considered safe to handle, making this a great project for kids science programs. To put it in perspective, even table salt, sodium chloride, is said to have a “very low toxicity level”, the threshold to call something non-toxic is a very high threshold, as very few compounds are truly “non-toxic” in large quantities.

Important notes about making bismuth crystals

The quality of your crystals will depend on the purity of your bismuth metal. It is important to use pure bismuth and not an alloy if you want to obtain high-quality crystals. 

Materials and equipment
  • Bismuth (Amazon, eBay, and specialty websites)
  • Two metal containers (ex: stainless steel measuring cups; aluminum cans; aluminum foil shaped into a bowl)
  • Heat source, like your kitchen stove
Steps
  1. Put your bismuth in one of your metal containers. Turn your heat source on and let the bismuth melt. If you see a gray skin (slag) form on the surface, don’t panic! That is normal. Meanwhile, preheat the second metal container. 
  2.  Once the bismuth is melted and the second metal container is heated up, pour the liquid bismuth from the first container into the second one. Try to pour just the bismuth underneath the skin and prevent the slag from going into the second container. This slag contains impurities! Alternatively, you can skim off the slag with a spoon before pouring.
  3. Try to cool the melted bismuth slowly. Cooling it slower will produce larger crystals. This can be achieved by leaving the metal container on the heat source (turned off), or by slowly turning down the heat. Do not cool the bismuth until it is solid. 
  4. About 30 seconds after cooling, the bismuth should start to solidify. When this happens, pour the liquid away from the forming solid crystals. The solid crystal should seem set but jiggly. 
  5. Be patient and let the crystals completely cool before removing them from the metal container. If you can not get the crystal out of the container, you can stick it in the freezer for a brief moment. 

Your crystal is not going to be rainbow colored at first. Instead, it will be more silver-colored. Don’t worry! It will quickly oxidize and you will have your rainbow crystal soon. Be sure to show off your homemade bismuth crystal!

Making bismuth crystals

Tips for making great bismuth crystals

  1. Don’t use crucibles or containers that aren’t stainless steel, or any non-stainless steel tools. It will impurify the bismuth and you wont get colors and good crystallization.
  2. Ideally the steel container you pour into will have slanted sides, like b owl, not sides that are straight and vertical like a measuring cup has. Otherwise it could et stuck in the container, as bismuth expands as it cools.
  3. After cooling, slightly smack it upside down on a towel, so the whole geode pops out. With a parallel sided container, this is more difficult as the bismuth can get stuck in there from expansion pressure.

Bismuth-Specific Tips 

Evan McCormick from the bismuth business also offered a few insider tips, or “common wisdom,” from the bismuth crystallization community. 

  1. Always use safety equipment. For Evan, this equipment includes safety goggles, a dust mask (when working with ceramic fiber or cutting the bismuth), forge gloves and a fire extinguisher.
  2. To make bismuth crystals of a significant size, you’ll need to purchase at least 5 pounds of bulk bismuth solid.
  3. Two popular crystal-growing methods exist: the “single crystal” method, which involves plucking grown crystals from the top of an open pot of molten bismuth, and the “geode” method (described above). In Evan’s experience, the geode method produces more crystals–and larger crystals–because it allows for greater growth surface area.
  4. To heat bismuth, use some kind of flame–a stovetop burner or a camp-stove will work. (Evan’s attempts to melt bismuth with an electric hotplate did not heat the material sufficiently, and failed to prompt crystal formation).
  5. Cool the bismuth slowly in order to ensure large crystal size. The best way to do so involves utilizing ceramic fiber insulation (commonly known as forge insulation). If that’s not an option for you, find a burner setting that will allow for this slow cooling process.
  6. After pouring, position your container in the upside-down position.
  7. Use a Dremel Drill, fitted with a diamond tipped circular blade, to cut out crystals. Regular saws get damaged easily and do not facilitate efficient cutting. Using two pliers can also be an effective cutting method, as bismuth is brittle material and tends to break between crystal formations. 

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