How Glow in the Dark Stars Work 9


Modern glow in the dark stars are much brighter than the old glowing stars. (Carol VanHook)

Modern glow in the dark stars are much brighter than the old glowing stars. (Carol VanHook)

Did you have glow in the dark stars on your ceiling as a kid? The old stars would glow green (sometimes orange or yellow) and faded fairly quickly. If you were to get glowing stars to put on your ceiling today, chances are they would glow much more brightly, last longer, and be available in more colors. The reason is because the chemical composition of stars and other glow in the dark materials has changed.

What Glow in the Dark Stars Are Made Of — Phosphorescent Materials

Zinc sulfide glow in the dark stars only glow this bright under a black light. Once you turn the lights out, the phosphorescence becomes much more dim. (Kat, Flickr)

Zinc sulfide glow in the dark stars only glow this bright under a black light. Once you turn the lights out, the phosphorescence becomes much more dim. (Kat, Flickr)

Glow in the dark stars consist of plastic containing a phosphorescent powder. Back in the 1970s, the only glowing pigment was zinc sulfide, which emitted green light. Zinc sulfide remains the most commonly used phosphorescent pigment today because:

  • It’s inexpensive.
  • It’s approved for use in cosmetics.
  • Very little dissolves in water, so it’s easy to mix into most compositions.

In 2000, improvements were made in zinc sulfide pigments, including combining it with more effective phosphors to get a pink-red and orange glow. Zinc sulfide still produces some of the best red and orange glow, but none of its colors glow very brightly or for very long.

Best Modern Phosphorescent Materials

Modern glowing pigments glow brightly enough to cast a shadow.

Modern glowing pigments glow brightly enough to cast a shadow.

In the article, “How Glow in the Dark Stuff Works” on About.com Chemistry, I explained the best and brightest glow in the dark pigment is a strontium aluminate doped with europium. That’s an oversimplification. The best phosphorescent pigment is an alkaline earth metal aluminate, typically doped with europium, dysprosium or another rare earth. The specific composition depends on the phosphorescent color. The alkaline earths may include strontium, calcium, barium, or magnesium. Silicon and titanium may also be found in some formulations. Uncharged pigments appear as a white powder. These pigments usually are damaged by acids and water, so they need to be coated to protect them for certain applications.

Phosphorescence by Color

Some example chemical composition of glow in the dark materials include:

  • Green – Al2O3, SrCO3, Eu2O3, Dy2O3, TiO2
  • Aqua Blue – Al2O3, SiO2, CaO, Eu2O3, Dy2O3
  • True Blue – Ca2O, SiO2, Eu2O3, Dy2O3
  • Purple – Ca2O, SiO2, Eu2O3, Dy2O3
  • White (fades from white to pale blue glow) – Al2O3, SiO2, CaO, Eu2O3, Dy2O3

Reference: MSDS Sheets found at Glow, Inc.

As you might expect, it’s extremely helpful to charge these pigments using a black light. Purple and blue are so close to the ultraviolet, you need the energy to get a good glow. The green and aqua pigments can get enough energy from sunlight or bright indoor lighting (or your cell phone flashlight).

If you’re familiar with the old zinc sulfide stars, you can see modern phosphorescent pigments are way brighter. You can see in the picture I took of a necklace I made that the glow is bright enough to illuminate the area and cast a shadow! Also, these oxides phosphoresce for a long time… several hours. So, if you paint stars on the ceiling using paint made from these pigments or get stars made from the material, your “night sky” will glow all night long.

Getting a Good Glow Pigment

As with all chemicals, there are different levels of purity available on the market. Also, particle size makes a big difference in how bright the glow will be and how long it will last. Larger particles glow brightly and for a long time, but also may appear granular (because they are). Suppliers grade the pigment, with 0 being the best quality, 1 good, and higher numbers less-good. The best pigments are expensive, so if a price seems to good to be true, it probably is. Trying to save money by ordering the pigment and mixing your own formulation is possible, but if, for example, you want to make a water-based paint, you’ll save yourself a lot of disappointment if you order coated particles or pre-mixed paint (made with a formula known to work).

If you’re interested in what I used for the necklace, it was this sample pack of glow in the dark acrylic paints, available at Amazon or directly from the manufacturer. I painted 2 coats onto clear acrylic beads. If you coat a surface with phosphorescent paint or powder, try to use one that is white or mirrored for the best glow. Transparent objects look cool too, don’t you think? I sealed the beads with acrylic to waterproof them with no noticeable loss of brightness.

Got questions? Comments? I’m happy to try to answer them. I love things that glow…

 


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9 thoughts on “How Glow in the Dark Stars Work

  • Shruthi Sreejith

    So how do the stars glow without being charged? Do they require some sort of em radiation or electron beam which they convert to visible light, or are the phosphorescent materials combined with a radioactive isotope such as tritium to recharge it?

  • youssef

    Is there is way to make transparent liquid out of some phosphorescence material ?
    Like what is the best material to dissolve in phosphorescence material for my above question?

    • Anne Helmenstine Post author

      Yes, you can make a transparent liquid by adding a phosphorescent pigment to a liquid that doesn’t dissolve it. You can suspend coated pigment in water or regular glow-in-the-dark pigment in most organics (like an oil). However, for the liquid to appear clear, the particle size needs to be small and you can’t put in too much or the liquid would seem cloudy. If the particle size is large or the liquid isn’t viscous, the phosphorescent part will sink. Not good, right? What you might try is coating the bottom of outside of a transparent container that holds the liquid with phosphorescent paint. This gives the effect the liquid is glowing without actually making it glow (kind of like how you place a light under a mineral specimen to light it up).

      • youssef

        Thank you very much for your reply!
        But would you kindly elaborate what do you mean by these parts :
        ” coating the bottom of outside of a transparent container that holds the liquid with phosphorescent paint”

        • Anne Helmenstine Post author

          I’m saying it’s easier to coat part of a transparent object with phosphorescent paint than it is to crystallize with phosphorescence inside the crystal matrix. Does that make sense? Coat the outside rather than trying to get it inside the crystal.

    • Anne Helmenstine Post author

      Maybe you could use a viscous liquid, like corn syrup. The thing is, you don’t want to dissolve the phosphorescent material. If you do, it won’t glow.