How To Make Purple Fire - Science Notes

It’s easy to make purple fire using common household ingredients. Purple is unusual because it’s not a color of the spectrum (unlike violet). Purple and magenta result from a mixture of blue light and red light. For this project, the fire color comes from the emission spectra of safe chemicals. Basically, it’s the practical application of the flame test. You may also wonder whether there is true violet fire and how hot it gets. Here’s what you need to know.

• One method of getting purple fire is using a chemical that burns violet in the flame test, ideally with a colorless or blue-burning fuel.
• Alternatively, use a chemical that yields a pink or red flame test and combine this color with a fuel that has a blue flame. Combining red and blue gives you purple.

Purple Fire Ingredients

There are several metal salts the emit blue, red, or violet light when heated. You combine these salts with a fuel to get the desired purple color. However, only a few of these chemicals are readily available, inexpensive, and non-toxic:

• Salt substitute (check label for potassium chloride)
• Red emergency flare (contains strontium nitrate)

Don’t use “lite salt”, which is a mix of regular table salt (sodium chloride) and salt substitute (potassium chloride). You want just potassium chloride.

The fuel is important. If you toss these chemicals on an ordinary campfire, you don’t get purple flames because the yellow from trace amounts of sodium in wood overpowers violet. Because human eyes are not very sensitive to violet light, you need a fuel that either burns with a nearly invisible flame or else a blue flame. Suitable fuels include:

• Rubbing alcohol
• Hand sanitizer (which contains alcohol)
• Ethanol
• Methanol (Heet fuel treatment)
• Lighter fluid
• Propane
• Natural gas

How to Get Strontium From the Flare

Strictly speaking, you don’t need the strontium from the road flare to make purple fire. However, the red from strontium makes the flame much more purple or magenta than violet, and thus much easier to see.

The emergency flare is a long cardboard tube with a striker at one end. Peel away the cardboard on the end of the flare (the opposite end from the striker) to reveal the powdery material inside the flare. Collect this and store it in a bowl or baggie. You only need a tiny amount, so you can save most of it for other projects.

Make Purple Fire

Once you’ve gathered the materials, making purple fire is easy. Simply sprinkle a bit of salt substitute and flare powder onto the fuel and ignite it with a long-handled lighter.

Other Ways to Get Purple and Violet Fire

You may be wondering about other chemicals that produce purple flames:

• Potassium (Violet) – Potassium nitrate, for example, burns violet in a homemade smoke bomb
• Strontium (Red) – For example, strontium chloride works, not just strontium nitrate
• Rubidium (Reddish violet) – Usually it’s the chloride
• Cesium (Blue-violet) – Usually use the chloride
• Calcium (Brick red, but appears purple with a blue flame)
• Lithium (Crimson red) – Try the chloride
• Zirconium (Pale red)
• Cadmium (Brick red, also toxic)
• Mercury (Red, highly toxic)
• Yttrium (Crimson)

How Hot Is Purple Fire?

When you add chemicals to a fuel to make purple fire, the temperature of the flame is characteristic of the fuel. Here are some typical peak flame temperature for different fuels:

• Methanol: 1870 °C (3398 °F)
• Ethanol: 1920 °C (3488 °F)
• Propane (in air): 1980 °C (3596 °F)

Violet incandescence is theoretically possible. Black-body radiation typically follows the general color scheme (coolest to hottest): red, orange, yellow, white, and blue. But, there may be violet-hot, which would be even hotter than blue. We would perceive violet heat as being white-hot, but a spectrographic analysis could show the true color as violet to ultraviolet.

For incandescence or black-body radiation, temperature relates to color:

• Deep red: 600-800 °C (1112-1800 °F)
• Orange-yellow: 1100 °C (2012 °F)
• White: 1300-1500 °C (2400-2700 °F)
• Blue: 1400-1650 °C (2600-3000 °F)
• Violet: 39400 °C (71000 °F)

To put the extraordinary high temperature of violet heat into perspective, our Sun burns around 8500 and glows yellow and white! The hottest stars we observe are blue-white.

Safety Information

Obviously, fire is hot! This project requires adult supervision. Also, any time you make fire you should have a fire extinguisher or water handy.

Don’t add liquid fuel to a burning fire. Wait until the flame dies out completely before recharging the fire.

References

• Kroemer, Herbert; Kittel, Charles (1980). Thermal Physics (2nd ed.). W. H. Freeman Company. ISBN 0-7167-1088-9.
• Sanger, Michael J.; Phelps, Amy J.; Catherine Banks (2004). “Simple Flame Test Techniques Using Cotton Swabs”. Journal of Chemical Education. 81 (7): 969. doi:10.1021/ed081p969
• Schmidt-Rohr, K (2015). “Why Combustions Are Always Exothermic, Yielding About 418 kJ per Mole of O2“. J. Chem. Educ. 92 (12): 2094–99. doi:10.1021/acs.jchemed.5b00333
• UCSB ScienceLine (2016). “Why is blue hotter than purple?”

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