In the olden days, back before we could buy glow powder on the internet, people made it themselves. Sometimes they took a scientific approach, mixing chemicals to see if anything cool happened. More often than not, glowing materials were discovered by accident. For example, the discovery that rotting corpses and meat emit bioluminescence was likely a creepy and unpleasant accident (although useful to coal miners). The discovery that oyster shell powder can be made to glow in the dark, on the other hand, seems to have been a result of experimentation. The project is described in Works of the Cavendish Society: Handbook of Chemistry, by Leopold Gmelin, translated in 1848 and available as a free download online. The resulting powder is phosphorescent, which means it glows brightly under ultraviolet light (black light) and continues to glow for some time after the light is turned off.
If you’re the type who loves to play Mad Scientist, this is the project for you. It’s not difficult, plus there are a lot of changes you can make to “see what happens”.
Glow in the Dark Powder Materials
The recommended materials for this project are:
- Oyster shells – Oyster shells may be obtained from a beach, pet store, or feed store. If you absolutely can’t find them, substitute egg shells with the inner membrane removed.
- Sulfur powder – Sulfur is found in chemistry kits and online.
- Heat-safe dish or borosilicate tube – I recommend using a piece of aluminum foil on a heat-safe surface.
- Sifter or strainer – You can use any kind of mesh to separate large and small particles.
- Mortar and pestle – Or you can use a hammer and a bag.
- Butane or MAPP torch – Find this at a hardware store.
The Chemistry of the Project
Oyster shells, scallop shells, and egg shells all consist of calcium carbonate (CaCO3). Applying heat (848 °C) to calcium carbonate calcines it to form calcium oxide or lime (CaO):
CaCO3 → CaO + CO2(g)
When heat is applied to a mixture of sulfur and calcium oxide, calcium sulfide (CaS) forms. Unfortunately, pure calcium sulfide is not particularly phosphorescent. However, trace impurities found in oyster shells act as doping agents, making it phosphorescent.
Make Oyster Shell Powder Glow
The 1848 text states the glow from oyster shells is best if the shells are not ground, but the researchers likely had a furnace to heat their materials. Unless you have a furnace, it’s more practical to grind the materials together.
- If the oyster shells are not broken, crush them. One easy method is to place them in a paper bag and beat them with a hammer. Eye protection is probably a good idea.
- Pour the crushed shells into a strainer or sifter. Stir them around so the dust and small particles fall through to the bottom. Collect this dust for the project.
- Place the dust into a mortar. Add twice the amount of sulfur as you have dust. You can measure to have twice the mass of sulfur as calcium carbonate, but it’s not necessary. The goal is to have sulfur in excess. Grind the shell and sulfur together with the mortar and pestle until you get an even mixture.
- Place the shell and sulfur mixture in a borosilicate tube (not completely full) attached to a ring stand or else pour it onto a sheet of aluminum foil. You could use a cookie sheet, but it’s not recommended (assuming you ever want to eat cookies from it again).
- Heat your materials under a fume hood or else outdoors. Sulfur compounds smell horrible and aren’t particularly healthy to inhale. Using the torch, evenly heat the mixture until it’s red hot. Continue heating about 10 more minutes. During this time, the sulfur will melt (blood red) and eventually burn (blue flame). The red of sulfur is not the same as “red-hot”. Make sure you see a cherry red glow.
- Allow the mixture to cool. Energize it with an ultraviolet lamp and examine the phosphorescent glow in the dark. You may find the glow is more intense if you scrape away the top layer that is exposed to air.
Tips for Success
Phosphorescent calcium sulfide often glows red. However, the color of the glow, its intensity, and how long it lasts depend on the particle size (larger glows brighter and longer) and the dopant. If you add a small amount of copper chloride (CuCl2) dissolved in water before you heat the materials, you may get a blue glow. The original reference and other sources state adding a couple of drops of bismuth subnitrate (Bi5O(OH)9(NO3)4) helps form a phosphor. Some researchers also recommend adding a bit of corn starch or rice starch.
If you don’t see a glow, the two most likely causes are insufficient heating or inadequate ultraviolet light. A certain minimum temperature must be reached to supply the activation energy to make calcium sulfide. There are many “flavors” of black lights. Some do a better job offering excitation for a phosphor than others. If you have access to a true ultraviolet lamp, use it.
See How It Works
This video does a great job explaining the procedure, although the author’s video camera didn’t do well showing the afterglow when the black light was turned off. For utterly stunning examples of what is possible with this technique, explore Cran Cowan’s Flickr gallery of synthetic fluorescent minerals. Notice he offers rough recipes for his creations that you can use as a starting point for further experimentation.
A related project is described in another free e-book:
The Journal of the Royal Institution of Great Britain, vol 1, January 1831.