What Color Is the Hottest Flame? The Fascinating Science of Fire
When picturing flames, most people imagine a traditional orange fire. However, there are plenty of instances in which fire doesn’t burn orange. In fact, flames can span the entire color spectrum. So, what causes the difference in flame colors? What chemicals can alter the color of fire? And what role does temperature play? We’re going to shine a light on all of that and more.
What Causes Flames to Be Different Colors?
To understand what causes different-colored flames, it’s important to understand the science behind visible light and fire. First off, fire is a chemical reaction that happens following combustion when chemical and gas molecules interact with oxygen. Additionally, not all fire exists on the visible light scale.
The visible light scale consists of any electromagnetic radiation that the naked human eye can view. Colors, as we know them, appear as a result of light passing through a prism. We’re able to perceive different colors because of their varying wavelengths. For example, red has the longest wavelength, while violet has the shortest.
When flames appear in different colors, they are responding to both heat and chemicals. The ordinary color spectrum of fire spans the range of relatively cool to very hot, which is represented by the acronym ROYGBIV in relation to the color spectrum:
- Red
- Orange
- Yellow
- Green
- Blue
- Indigo
- Violet
Which Color Represents the Hottest Fire?
Despite its icy hue, the hottest color of flame is violet. At over 1,650 degrees Celsius, violet flames’ high temperatures can slice through nearly any metal, glass, or rock with ease. For this reason, you can often spot violet and blue flames at the end of welding torches. (Though welders wear protective goggles to keep from burning their eyes from the light.)
It’s also important to note that violet and blue flames are also produced by the carbon and hydrogen that are present in burning wood. This is why you might spot flickers or blue or violet when you’re gathered around the campfire.
While violet flames can sear the toughest substances, red flames, which are the coolest, are not so powerful. Although red may be a typically fiery color, it’s not the hottest flame, with temps hovering at around 600 to 800 degrees Celsius. This may sound like a high temperature, but compared to the 1650-degree blue flames, red flames are pretty mellow. Still, flames of this temperature can melt aluminum, pure silver, tin, lead, bronze, and brass. Fun fact: Some red flames are so cool that they’re tough to spot with the naked eye, but those 800-degree flames will burn a rosy red color.
As you can tell, there is a direct link between the heat of a flame and the color that a fire burns. Flames that are colder or starting to fizzle away will have a different hue than a raging fire or newly sparked match. The amount of heat and energy released during combustion is tied to the hues of the flames. To review, the hottest flame on the color spectrum is violet and, on the visible spectrum, it’s white.
Here’s a more complete breakdown of the temperatures of different flame colors and what they can burn:
- Red flames burn at approximately 600 to 800 degrees Celsius. The hottest of red flames can melt substances such as magnesium (657° C), glass (700° C), and borax (740° C), while the coolest of these flames can melt lithium (179° C), selenium (220° C), tin (232° C), amber (300° C), and zinc (419° C).
- Orange flames burn at approximately 1100 degrees Celsius. These flames can scorch through bronze (910° C), gold (1063° C), and copper (1083° C).
- Yellow flames burn at approximately 1200 degrees Celsius. These flames will melt arsenic (815° C), calcium (850° C), brass (900° C), silver (960° C), and radium (960° C).
Interestingly, green flames have no link to temperature, but rather to the chemicals that fuel them, which we will discuss below.
- Blue flames burn at approximately 1400 to 1650 degrees Celsius. This makes the coolest blue flames capable of melting asbestos (1300° C), steel (1460° C), and cobalt (1490° C), and the hottest blue flames capable of melting palladium (1552° C), brown iron ore (1570° C), melting clay (160°0 C), and agate (1600° C).
- Indigo flames burn at just under 1650 degrees. These flames can melt through aluminum bronze (1040° C), quartz (1470° C), iron-oxide (1570° C), and sand (1550° C).
- White flames burn at approximately 1300 to 1500 degrees Celsius. These flames can burn through many tough solids, including uranium (1133° C), nickel (1452° C), and cobalt (1490° C).
- Violet flames burn over 1650 degrees. This makes these flames powerful enough to turn hard-to-melt materials into puddles, such as cast iron/forged iron (1200° C), steel (1460° C), porcelain (1650° C), and titanium (1670° C).
What Chemicals Change the Color of Flames?
Temperature isn’t the sole determiner of the color of flames. Different colors of flames can also stem from the types of chemicals that are present in the substance that’s being burned. The type of fuel and its impurities, in addition to the flame temperature, contribute to the color of the flame.
Certain chemicals in wood, candles, or other fuel sources can spark varying colored flames at their source. That is, elemental particles the flame’s illuminating influence its color as much as the temperature the fire burns at. These are the chemicals and materials responsible for fire’s various colors:
- Red: Caused by strontium chloride or strontium nitrate. Spotted in slow-burning fires.
- Orange: Caused by the burning of carbon particles or calcium chloride. Spotted in most campfires, charcoal grills and fireplaces.
- Yellow: Caused by sodium chloride, sodium carbonate, or borax.
- Green: Caused by copper or barium. No link to temperature.
- Blue: Caused by copper chloride or the complete burning of carbon in a fuel source.
- Indigo: Caused by indium.
- White: Caused by magnesium sulfate.
- Violet: Caused by potassium nitrate mixed with potassium sulfate.
Why Are Orange Flames the Most Common?
When it comes to fire, why are orange flames the most (seemingly) common color? Orange flames run at approximately 1100 degrees Celsius, putting them in the middle range of flame temperatures. Most organic items that people burn — paper, wood, charcoal — contain carbon. So, when these items are burned, they release carbon particles into the flame, causing these particles to create deep orange, “clear” orange, and yellow flames.
When all carbon particles are burned out of a fuel source — and there are no remaining traces to be consumed — the flame may then spark blue or violet in response. For example, stovetops and gas grills both feature blue flames because they’re not responding to a carbon-based fuel source.
