A black flames Halloween jack o’ lantern produces flames that actually appear black when viewed under the light of a low-pressure sodium lamp. The effect is an unusual demonstration of atomic emission and absorption rather than a special type of combustion.
First, there was the green fire pumpkin. Then, I made a rainbow flames pumpkin. That was cool, but then there was the flamethrower jack o’ lantern. How could I top that? The answer was black flames. This pumpkin is less likely to require a fire extinguisher than the flamethrower and, in my opinion, considerably creepier.
So, how do you make flames look black? The experiment is surprisingly simple, but the science behind it is fascinating.
Key Takeaways: Black Flames Halloween Jack o’ Lantern
- Black flames are a real optical effect, not black-colored combustion.
- The experiment requires sodium in the flame and illumination from a low-pressure sodium (LPS) lamp.
- Sodium strongly emits and absorbs yellow light at wavelengths near 589 nm.
- The sodium-containing flame absorbs some of the light coming from the LPS lamp, making parts of the flame appear dark or black.
- High-pressure sodium lamps do not produce the same effect because their emission spectrum is much broader.
- The flames are still ordinary hot flames and require normal fire safety precautions.
Why Do the Flames Look Black?
The black flames effect works because sodium atoms both emit and absorb light at specific wavelengths.
When sodium is heated in a flame, it produces an intense yellow color dominated by the sodium D lines near 589.0 and 589.6 nanometers. These spectral lines result from electrons changing between specific energy states in sodium atoms.
A low-pressure sodium (LPS) lamp also produces light concentrated very strongly near these same wavelengths. Under LPS illumination, almost everything in the scene reflects the lamp’s yellow-orange light.
The sodium-containing flame complicates the picture. Cooler sodium atoms in parts of the flame can absorb light from the LPS lamp at the same wavelengths that sodium emits. Against a background illuminated strongly by sodium light, these regions transmit less light toward your eyes and appear unusually dark.
The result looks like black fire.
The flame is not actually emitting black light, and the combustion itself has not turned black. You are seeing an absorption effect caused by sodium atoms interacting with nearly monochromatic sodium light.
Black Flames Jack o’ Lantern Materials
You only need a few materials:
- Carved pumpkin
- Salt (sodium chloride from your kitchen, iodized or not)
- Alcohol
- Low pressure sodium lamp
I tested this project with rubbing alcohol (isopropyl), ethanol in hand sanitizer, and methanol. They all work fine. However, your best bet is to use 70% rubbing alcohol because it has enough alcohol to burn with a clean blue flame, but enough water to dissolve the salt. If you use high-proof alcohol, you’ll need to dissolve salt in a small amount of water and mix it with your fuel.
The critical piece of equipment is the low-pressure sodium lamp (LPS lamp). I got mine from Amazon, but some building supply stores carry them. An LPS lamp produces light concentrated very strongly around the sodium D lines near 589 nm. Other sodium lamps, particularly high-pressure sodium (HPS) lamps, have a much broader emission spectrum and do not produce the same dramatic black-flame effect.
How to Make a Black Flames Jack O’ Lantern
- Turn on the low-pressure sodium lamp and allow it to warm up fully. The lamp should reach its characteristic yellow-orange color.
- Position the lamp so its light illuminates the flames and the front or side of the pumpkin.
- Place the sodium-containing alcohol fuel in a small heat-safe container inside the carved pumpkin. Position the container so the flames are visible through the pumpkin or above its opening.
- Move the pumpkin to a suitable fire-safe location and clear the surrounding area of combustible materials.
- Ignite the fuel using a long-handled lighter and observe the flame under the LPS illumination.
- Compare the appearance under ordinary room lighting and predominantly low-pressure sodium lighting.
How Lighting Changes the Black Flames
The black flames effect is easiest to see when the low-pressure sodium lamp is the dominant light source.
Under ordinary room lighting or in dim surroundings, you still see dark regions in the flame, but you can also see the natural blue and yellow colors produced by combustion and sodium emission.
When the LPS lamp provides most of the illumination, the effect becomes much more dramatic. Because the lamp produces such a narrow range of wavelengths, colors largely disappear from the surroundings. The pumpkin and other objects take on yellow-orange, gray, and black tones, while sodium absorption makes portions of the flame appear strikingly dark.
Complete darkness is not necessary. The important factor is making the low-pressure sodium light strong relative to other light sources.
Why Doesn’t a High-Pressure Sodium Lamp Work?
Low-pressure and high-pressure sodium lamps both contain sodium, but their spectra are very different.
A low-pressure sodium lamp produces light concentrated very strongly around the sodium D lines near 589 nm. This nearly monochromatic illumination is ideal for making sodium absorption in the flame visible.
A high-pressure sodium lamp operates under different conditions and produces a much broader spectrum. Because plenty of light at other wavelengths remains available to illuminate the flame, absorption near the sodium D lines does not make the flame appear dramatically black.
For this experiment, low-pressure sodium is the key.
Safety Information
The black flames might look different from ordinary fire, but I assure you they are every bit as hot as you’d expect. Black fire can burn you. Only light a jack o’ lantern in a fire-friendly location. Don’t let kids, pets, or fools play with the lit pumpkin. Most important: Only add more fuel to the pumpkin after the flame is extinguished.
See the Black Flames Pumpkin in Action
The black flames effect is difficult to appreciate from a still photograph because it looks almost impossible. This video shows the actual jack o’ lantern and demonstrates how the effect changes under different lighting conditions. No digital effect is required to make the flames appear black.
Black Flames and Spectroscopy
The black flames pumpkin is more than a Halloween trick. It demonstrates an important principle of spectroscopy: the same atoms that emit particular wavelengths of light can also absorb those wavelengths.
This type of effect is related to flame reversal, in which relatively cooler atoms in a flame absorb characteristic wavelengths supplied by a hotter or brighter background source. Historically, flame-reversal methods have even been used for temperature measurements.
This principle helps explain why spectra can contain either bright emission lines or dark absorption lines. Hot, excited atoms produce characteristic emission lines, while cooler atoms in front of a suitable light source can remove those same wavelengths and produce absorption lines.
The black flames effect is especially dramatic because both the sodium in the flame and the low-pressure sodium lamp involve the strong sodium D lines near 589 nm. Instead of viewing a tiny dark line with a spectroscope, you see dark regions of an entire flame.
Common Misconceptions
The flame is actually black.
Not exactly. Black is the appearance produced when sodium atoms in the flame absorb light from the low-pressure sodium lamp. The combustion itself has not acquired a “black” flame color.
Any yellow or orange lamp works.
No. The effect depends on the close spectral relationship between sodium in the flame and the emission from a low-pressure sodium lamp.
Any sodium lamp works.
High-pressure sodium lamps have a much broader spectrum and do not reproduce the dramatic effect of a low-pressure sodium lamp.
The black parts of the flame are cooler because they look dark.
The darkness is primarily an optical absorption effect, not a temperature map of the flame. Do not interpret a dark region as safe or cool.
The photograph is digitally altered.
No image manipulation is necessary to produce the black flames. Under appropriate sodium illumination, the effect is visible directly.
Frequently Asked Questions
Are black flames real?
Yes, in the sense that a real flame can appear black under the appropriate lighting conditions. The flame itself is not emitting “black light.” Sodium absorption removes light at wavelengths supplied strongly by the low-pressure sodium lamp, making regions of the flame appear dark.
Why is sodium used to make black flames?
Sodium has exceptionally strong spectral lines near 589 nm, and low-pressure sodium lamps emit strongly at nearly the same wavelengths. This combination makes the absorption effect easy to see.
Can you make black flames without a low-pressure sodium lamp?
Not with the same dramatic effect described here. The nearly monochromatic illumination from an LPS lamp is central to the demonstration.
Why does sodium normally make a yellow flame?
Excited sodium atoms emit intense yellow light near 589 nm as electrons transition between particular energy levels.
Is black fire hotter than ordinary fire?
No special temperature is implied by its black appearance. The dark color results from the optical setup rather than an unusually hot or cold type of combustion.
Can you see the black flames with your eyes, or only in photographs?
You can see the effect directly. A camera is not required, although camera exposure and white-balance settings can change how dramatic the effect appears in photographs and video.