Green Fire Halloween Jack-o-Lantern

How to Fill Your Jack-o-Lantern with Green Fire

Green fire jack-o-lantern
You could put a simple candle inside your Halloween jack-o-lantern, but filling it with green fire is much more fun!.

Foap AB / Getty Images

One application of green fire is using it to light your Halloween jack-o-lantern. This is a super-easy effect that produces spectacular results (watch the video). Here's how you do it:

Key Takeaways: Green Fire Jack-o-Lantern

  • The green fire jack-o-lantern is a colorful flame-filled Halloween pumpkin.
  • The project involves dissolving a salt in a flammable solvent and igniting it.
  • The green flame comes from the emission spectrum of the chemical. Either the boron ion or the Copper II (Cu2+) ion will produce a green flame.
  • The project should only be performed by adults. While the salts used in this project are not particularly toxic, they are not safe to eat. Methanol (methyl alcohol) is toxic and should not be handled or inhaled.

Green Fire Jack-o-Lantern Materials

You only need a few simple materials for this project:

  • Carved jack-o-lantern. Traditionally this would be a pumpkin, but you could use a watermelon, cantaloupe, etc.
  • Boric acid (usually found in pharmacy or pest control section of store)
  • Methanol (such as Heet™ fuel treatment, found in the automotive section)
  • Aluminum foil or heat-safe container that fits inside your jack-o-lantern
  • Long-handled lighter

Boric acid is a different chemical from borax (commonly sold as the laundry boost 20 Mule Team Borax). Pure boric acid is used to make a disinfectant solution and as a powder to kill and control cockroaches and other insects in the home. Boric acid dissolves better than borax in methanol, so it produces a better green flame. However, if you can't find boric acid, you can substitute borax and still get green fire.

Green flame test
The green flame color is used in the flame test to help identify sample chemical composition. Nayef Hajjaj / EyeEm / Getty Images

Other Substitutions

Switching out borax for boric acid is not the only substitution you can make. You can use copper sulfate as the salt and a different alcohol for the solvent. Good choices include rubbing alcohol (contains one or more different alcohols), ethyl alcohol (ethanol), or isopropyl alcohol (isopropanol). These alcohols are less toxic than methanol, plus they contain water. The water is important because it dissolves the copper sulfate so it can color the flame.

Copper sulfate is sold as a pure chemical and also as a root killing chemical. It is available online and at some home supply stores. The container should be labeled as copper sulfate, copper sulphate, or copper sulfate pentahydrate.

If you get copper sulfate, you can also use it for other projects, such as growing blue copper sulfate crystals or making a copper sulfate geode.

Fun Fact: The copper ion can emit either green or blue light. Cu2+ is green, while Cu+ is blue. Unfortunately, changing the copper oxidation state is not a simple matter. It requires a series of redox reactions that could can't easily do at home.

Start the Green Fire!

Technically all you need to do is sprinkle boric acid in a heat-safe container, add a little methanol, set the container inside the jack-o-lantern and light the fire. It's important to use a long-handled lighter, since the vapor pressure of methanol is very high and you will hear that "whoof" sound when you light the mixture.

The best result, in my opinion, comes from lining the inside of the jack-o-lantern with aluminum foil and using the pumpkin as the heat-safe container. You can sprinkle boric acid inside the jack-o-lantern, splash a little methanol around, and light the decoration. The aluminum helps contain the liquid, plus it is reflective so it enhances the brightness of the display. Do not add more fuel to the burning fire; wait until it goes out. Safety note: don't do this indoors!

Holiday Clean-Up Tips

The green fire can get very hot, so there is a good chance your pumpkin will get somewhat cooked by lighting it this way. The methanol is burned away by the fire, leaving some boric acid residue with your pumpkin. While boric acid is not particularly toxic, you don't want kids or animals to eat this jack-o-lantern, nor is it ideal for compost since too much boron can be toxic to plants. Be sure to throw your jack-o-lantern away before it rots in place. Just remember the pumpkin contains boric acid, so don't let anyone eat it.