It is a misconception that we can add color to a flame. The flame that we see is the light emitted by various atoms as their electrons are excited by the heat energy of combustion. The wavelength (color) of light that is given off is a characteristic of the elements present in the fuel and surrounding air. There is no one proper color to fire, but since people tend to mainly burn carbon compounds in a mainly nitrogen atmosphere, there is an orange color we are used to seeing and thinking of as the color of fire. If we want to produce other colors, we just have to introduce other elements to the flame, so that they can give off their own characteristic colors. Here’s how:
Metal ions tend to provide the most dramatic colors. A brief (but by no means complete) list follows with the dominant color of each.
Sodium – Yellow
Barium – Green
Copper – Green
Strontium – Red
Lithium – Red
Potassium – Purple
In general, the chloride salt of the metals is the easiest to obtain and work with. Other salts will work to varying degrees, but those that are hydrated will give poorer performances.
The traditional experiment is to place a little of the salt in a wire loop which is then held in the flame of a laboratory burner. This generates the color for observation, but requires continuous effort and usually will only color a portion of the flame.
An alternative method is to dissolve a little of the salt in ethanol in a small watch glass, petri dish, or other short non-flammable container. The mixture is then ignited (carefully). Alcohol flames produce very little visible light, so most of what you see is directly due to the metal.
If you’re looking to color wood for a campfire setting, then your best bet is to soak the wood in a solution of the metal salt you’re interested in, and then allow it to dry thoroughly. If you’ve ever burned driftwood from the ocean (or a saltwater sea) you’ve likely seen wisps of yellow and green from the different salts found in the ocean that saturated the log.
