Atomic Colors

On July 4th, 2023, inspired by Robert Fulton’s homemade firework display of 1778, I attempted to make a colorful display of my own. Because I didn’t know how to make fireworks, I mixed some magnesium and copper filings into wax in hopes of making candles that would burn with colored flames, but to my dismay, the flames were orange and indistinguishable from any other candle’s flame.

After this failure, I gave up on the project for several months, until I picked it back up in early 2024. This time, I realized that I needed a fuel that burns with a dim flame and a colorant that readily emits excited metallic ions. Typically, the colorant will not be elemental metal filings, but a metal salt instead.

Before I ever produced any chemicals or performed any experiments, I spent a couple months researching and learning a wide variety of ionic compound names, formulae, properties, safety precautions, and incompatibilities.

While researching which salts would produce the best colored flames, I stumbled upon this video from a channel called Pyrophoria, which taught me about the role of copper in pyrotechnics. From that point on, this colored flame project was greatly overshadowed by my efforts to make blackpowder, pyrotechnic stars, and fireworks. Think about it, a firework is much more entertaining than a gentle, colored flame; it is natural to be drawn toward the more exciting project.

Nevertheless, I still made my colored flame display, which was captivating in its own right. To set up this display, I placed a small scoop of several different salts in small containers and I then filled the containers with fuel.

You may have noticed that each of the samples began with a pure blue flame, which is merely the flame color of methanol; it takes some time for the methanol to burn and heat up before it begins to vaporize the salts, which liberates and excites some metallic ions, which then emit some light in the emission spectrum of the metal in the flame.

The fuel that I chose was methyl alcohol, which is often called methanol; I chose methanol because it is easy to acquire and relatively cheap, but most importantly, it burns with a very dim flame. My choice of fuel, containers, and salts for this project was heavily influenced by this video from the King of Random.

WARNING: I don’t recommend that you try this for yourself, as methanol is extremely dangerous; not only is methanol very poisonous, but it also burns so faintly that its flame is nearly invisible in most lighting, therefore, you could unintentionally set other items or yourself on fire before you realize that the methanol is burning. Do not try this without the proper PPE, ventilation, and understanding of the dangers and safety protocols.

Salts and Colors

Some of the salts that I used were ones that I was told work well during my research, while others were some random salts that I had on hand which I decided to try.

Let’s look at a list of the salts and colors that I used in my display, from left to right.

Magnesium Sulfate (MgSO₄): White/Colorless

I used magnesium sulfate, which is commonly known as epsom salt, in an attempt to produce white fire, but the magnesium sulfate was either unable to produce white fire, or it was drowned out by the blue flame of the methanol. I’ve heard that it’s possible for magnesium salts to produce white fire, and maybe this is true in the correct circumstances, but white flames will typically come from burning elemental magnesium, because the magnesium burns so hot that the flames are white.

Strontium Chloride (SrCl₂): Red/Orange

To produce red flames, I was told by multiple sources that you simply need to place some strontium chloride in methanol, however, in my experience, the strontium chloride only produced orange flames. I also tried some strontium nitrate, and while the nitrate did produce some flashes of red, the flame was still mostly orange.

I even tried both the chloride and nitrate of strontium with a small amount of a chlorine donor, since chlorine is what causes strontium to produce red flames in pyrotechnics. The addition of a chlorine donor unsurprisingly had no effect on the strontium chloride (because the strontium is already bonded to chlorine), and the chlorine donor hardly improved the strontium nitrate.

I still don’t entirely understand why the strontium produced orange flames, but the most logical guess that comes to mind is that strontium salts, including strontium chloride and strontium nitrate, are quite hygroscopic, meaning that they absorb moisture from the air. I’m assuming that the hydrated form of strontium chloride (strontium chloride hexahydrate) produces an orange color, rather than anhydrous strontium chloride, which I hypothesize would produce red flames.

Strangely, I found that my best red flame occurred when I dissolved strontium nitrate in a small amount of water and added the saturated strontium nitrate solution to a large amount of ethanol, which may rule out the anhydrous hypothesis.

Calcium Chloride (CaCl₂): Red-Orange

I’ve always heard that burning calcium salts, especially calcium chloride, will emit a brick red flame. In my experience, this was pretty close to reality. Calcium chloride’s orange flame was more faint than sodium chloride, but it definitely had a reddish tint.

Sodium Chloride (NaCl): Orange

In this instance, the sodium chloride, better known as table salt or salt, I used was pink Himalayan sea salt, which did contain traces of other minerals and salts. It probably would have been better to use a more pure source of sodium chloride, but I was curious to see if the trace minerals would induce a unique color.

Himalayan sea salt is pink primarily because of iron oxide, which usually burns with an orange or golden color, therefore, the sea salt may have burned with an orange color because of the iron oxide contamination.

Sodium Acetate (CH₃COONa): Yellow

I made some sodium acetate for another experiment, and I decided to throw it into this experiment simply because I had it on hand. The sodium acetate burned with a yellow to yellow-orange color. This is expected because most sodium salts burn with an intense yellow flame, however, the slight orange color may be due to the organic acetate ion decomposing and burning.

Boric Acid (H₃BO₃): Green

Boric acid isn’t a salt, but as you can tell by its name, it is an acid. Instead of containing a metal to color the flame, boric acid contains a metalloid called boron which colors the flame. Boron burns with an intense and vibrant green color, which produced the best flame quality of this experiment. The boric acid flame wasn’t just somewhat green, but it colored the flame so well that the entire flame was a solid and consistent green.

I’m not normally an enthusiast of green colors, but boric acid quickly became my favorite chemical in this experiment because of the consistency, intensity, and longevity of its color; even small traces of boric acid that were mostly depleted after prior burns were enough to entirely color a flame.

Sodium tetraborate (borax) could be used in place of boric acid, but the green color will be somewhat contaminated by the yellow of the sodium in borax.

The only drawback of boric acid is its toxicity; boric acid and borax can be harmful or fatal if inhaled or consumed. Boron isn’t exactly an innocuous element.

Copper (II) Sulfate (CuSO₄): Teal

The copper sulfate didn’t burn very brightly, which may be due to the fact that it was a hydrated salt, but it did emit a deep and decently pure teal color. Copper sulfate gave off the most clean color of all the copper salts that I tried in this experiment, likely because the only two atoms that accompany copper in the copper sulfate molecule are sulfur and oxygen, which  seem to emit very faint light of their own, if any at all.

I’m starting to realize that the best anions to accompany metal cations in colorants are inorganic anions that do not contain sodium, such as nitrate, chloride, chlorate, perchlorate, oxide, and sulfate. For example, copper acetate is not a good colorant because the copper is washed out by the carbon in the acetate anion, but copper oxide is a good colorant because there is neither carbon nor sodium in the oxide anion.

WARNING:you shouldn’t haphazardly mix random salts containing the anions I previously listed just because I mentioned them, for instance, if you happened to mix a chlorate salt with sulfur or a sulfate salt, the result could be spontaneous and explosive. Always understand the properties of all chemicals in a reaction and the properties of the byproducts of the reaction before you mix them.

Copper (II) Acetate [(CH₃COO)₂Cu]: Emerald

The copper acetate burned with an emerald colored flame, but it was contaminated with a lot of orange fire. The organic acetate ion likely contributed an orange color just like it did in the case of sodium acetate.

Copper (II) Carbonate (CuCO₃): Orange/Green

I was expecting copper carbonate to burn with a green flame, but for some reason, it mostly burned with an orange flame. It is likely that there was a small amount of sodium acetate contamination in my copper carbonate, which was a byproduct that I produced when making the copper carbonate, and because my purification process wasn’t perfect, I likely left some sodium behind.

It’s also strange how the copper carbonate does emit a small amount of green light after some time. I wonder if this is because the copper carbonate decomposed, releasing CO₂ and leaving behind copper oxide, which does burn with a green flame when it isn’t in the presence of a halogen. Maybe I will try some copper oxide with, and without, a chlorine donor in the future.

Potassium Hydroxide (KOH): Violet/Peach

Potassium salts are well known for emitting a violet or magenta color, which is why some people use potassium chloride to make a violet flame. I didn’t have any potassium chloride on hand, but I did have potassium hydroxide, which is a strong base and extremely corrosive, therefore, it should always be handled with great care.

The result was fascinating, as the potassium hydroxide tended to produce an almost peach color. I’m not quite sure where the orange color came from, but I suspect that it was a result of the hydrogen in the hydroxide anion.

Merry Christmas!

Which flame color is your favorite?

Onward American 🇺🇸