How to make Bismuth crystals

Bismuth Crystals

Bismuth (element #83 on the periodic table) forms beautifully colored and geometrically intricate hopper crystals, shown in the image to the left, as it slowly cools and solidifies from its molten state.

The distinctive, 'hoppered', shape of a Bismuth crystal results from a higher growth rate around its outer edges than on its inside face. The higher rate of growth on the edges forms a crystal which appears to be partially hollowed out in a rectangular-spiral stair step design.

The crystal's eye-catching array of colors results from the formation of a thin oxide layer on its surface. The colors on Bismuth crystals arise in a similar fashion to those on a soap bubble or thin film of oil on water in which light reflecting off the top and bottom of the film produce interference maxima of a particular color depending on the film’s thickness. A thin layer of Bismuth Oxide on the otherwise pure Bismuth crystal causes light of certain wavelengths to interfere constructively upon reflection giving rise to the color seen on the surface. Due to variations in the thickness of the oxide layer, the crystal is not one solid color but rather is a rainbow of colors corresponding to the wavelengths (and colors) of light which interfere constructively at each location.

For a more thorough explanation of this phenomenon research the topic of thin film interference.

Growing Bismuth Crystals

In order to grow high quality crystals, very pure Bismuth metal must be used. A purity of 99.99% Bismuth, or better, is preferred for growing high quality crystals.

Another important factor which affects the quality and size of Bismuth crystals is the cooling time. Larger crystals may be produced by allowing the Bismuth to cool and solidify slowly from its molten state.

The melting point of Bismuth is relatively low compared to other metals at only 271 °C (520 °F), which is low enough to easily melt using a small propane torch or even a laboratory hot plate. Do not forget, however, that you are still dealing with very hot molten metal, which can flow and splatter just like any liquid and can cause painful burns.

Depending on the volume of Bismuth used, a variety of containers are suitable to hold the Bismuth while it melts. A small to medium sized, steel, measuring cup has been show to work well for preparing Bismuth crystals.



Step 1: Melting the Bismuth

Steel cup containing molten Bismuth

Place pieces of Bismuth into a steel measuring cup and melt on a hot plate at high heat. As the Bismuth melts, the exposed surface of the liquid will oxidize readily due to the high temperature and presence of Oxygen in the ambient air resulting in the formation of a gray ‘skin’ on the top surface; this is normal.



Step 2: Pouring the molten Bismuth

Molten Bismuth with a thick layer

of oxidation on it before pouring

After the Bismuth has melted, slowly and carefully pour the liquid Bismuth into another clean, preheated, steel measuring cup. By transferring the Bismuth into a new container one can remove the thick skin of oxide which has formed on the surface of the Bismuth that can negatively affect crystal growth. One can slowly pour out the clean, silvery, liquid Bismuth from underneath the gray oxide layer which remains behind in the original container.



Step 3: Allowing the Bismuth to cool

Bismuth immediately before the first

pour to remove oxidization layer

Set the Bismuth, in its new container, on a well-insulated and heat resistant surface to cool and begin to solidify. Placing the Bismuth back onto the hot plate (with the power turned off) will allow the Bismuth to cool very slowly as it returns to room temperature. Not long after pouring the Bismuth into its new container will a new oxide layer become visible. The new oxide layer will not be as thick as the previous layer. As the new oxide layer growths thicker it will gradually change color as different wavelengths of light constructively interfere after reflecting off its surface (for the same reason why the Bismuth crystals themselves are colored).



Step 4: Pouring out extra Bismuth

Immediately after pouring off excess

liquid Bismuth. Newly formed crystals

exposed. Before the Bismuth fully solidifies, pour out the excess liquid Bismuth into another container. Do not allow the Bismuth to fully solidify; if one does not pour out the excess liquid in time the crystals will become trapped inside the resulting metal lump. The crystals will form and grow progressively larger as the Bismuth freezes. Longer growth times usually result in larger crystals. But if one waits too long, the excess Bismuth which has not yet gone into forming the crystal will solidify and trap the crystals which have formed. There is no set amount of time one must wait before pouring out the excess Bismuth since it will vary depending on the conditions of the experiment. One can test the state of the Bismuth by lighting tapping on the container. If ripples are seen traveling across the surface then most of the Bismuth is still liquid. As more of the Bismuth freezes, the ripples will have less space to travel and the areas where crystals are forming become visible. Take care not to disturb the cooling Bismuth too much since it may trigger new crystal formation; instead of producing just one (or few) large crystals, many much smaller crystals may form. It may take several tries in order to get the hang of when to pour out the excess liquid Bismuth. If one waits too long and the crystals become trapped just re-melt the Bismuth and try again. One can even try to use the excess Bismuth which is poured off to form additional crystals in secondary containers.



Step 5: Removing Crystals

Close up of newly formed crystals

before breaking them free from the

non-crystalline Bismuth which remains

behind in the cup. The colorful and

geometrically interesting portion of the

crystal is facing down toward the

bottom of the cup; the visible portion is

coated with thick gray oxide layer.

After pouring off the excess liquid Bismuth one should be able to see the Bismuth crystals which have grown inside the container. The crystals' color should develop within seconds upon exposure to air as a thin layer of oxidization forms on their surface. The crystals will likely be stuck to the bottom and sides of the container; they are also likely to be partially fused with non-crystalline Bismuth. After they cool one can gently break off the crystals from the non-crystalline Bismuth inside the container. Allowing the crystals to form on the inside surfaces of the container causes inherent imperfections in the crystals' shape, since there will always be a point on the crystal where it was previously attached to that surface. One can avoid this imperfection by using a small seed Bismuth crystal suspended in the molten Bismuth to act as a point of nucleation during crystal growth. Afterward, one can then merely lift the crystal out of the liquid Bismuth rather than having to pour the excess Bismuth out into a separate container (as in Step 4). Take care not to leave the seed crystal suspended too long as the Bismuth may solidify around it and make it impossible to extract. Additionally, the crystal may grow so large it may fuse with other crystals surrounding it, thus also trapping it inside. Like water / ice, Bismuth has the interesting property of expanding as it solidifies, thus causing unattached, solid, Bismuth crystals to float in their more dense, liquid, surroundings.

Pictures of large Bismuth crystals

Bellow is a series of images of three large Bismuth crystals ranging in size from about 1 inch to 3 inches wide. It is certainly possible to grow much larger crystals than these.

Crystal 1

Crystal 2

Crystal 3