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A street performer in the medieval age could make some money. One popular performer is a fire eater. Changing the color of his flames would likely make his popularity rise so that he would make more money.

Is it possible to change the color of a flame?

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    $\begingroup$ This is the exact basis of atomic emission spectroscopy...en.wikipedia.org/wiki/Atomic_emission_spectroscopy $\endgroup$
    – DJohnM
    Jun 1, 2017 at 21:52
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    $\begingroup$ I just hope he has enough money to buy the materials to change the colour of the flames. Also its probably not the greatest idea to eat the poisonous fire, eat the normal one and do other tricks with the different coloured flames. $\endgroup$
    – Necessity
    Jun 2, 2017 at 0:24
  • $\begingroup$ or just don't use poisonous ones $\endgroup$
    – Foxy
    Jun 2, 2017 at 0:25
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    $\begingroup$ This is also how the colors of fireworks are produced. Fireworks in general were invented in ~7th century CE China; I don't know exactly how old colored fireworks are, but knowing that certain substances burn colorfully is totally plausible for "medieval". $\endgroup$
    – zwol
    Jun 2, 2017 at 1:45
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    $\begingroup$ In the process of entertaining people try to not get burned for witchery. $\endgroup$
    – BlueWizard
    Jun 6, 2017 at 21:32

2 Answers 2

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You can't get black

While it is possible to achieve different ranges of colours depending on temperature of the flame as well as combustible material, fire's still light.

Light is bound to the spectrum of visible wavelengths. And there is no visible wavelength for black.

Addition1: It is important to understand that the light of fire is additive. Hence if you'd want to go towards black fire, you'd actually have to find a way to remove light-sources from a) the fire itself and b) its surroundings; thus creating a space that absorbs light.

But what colours of fire can we have?

Looking at pyrotechnics we can achieve the following colours by e.g. burning different metal salts:

Chart of metal ions and flame colours Chart of what substance creates which flame colour - found on http://www.compoundchem.com

These colours are achieved, as already stated, by burning salts. As long as we know where to find metals, we can dig them up and refine most of them with as little tools as a vat of acid and some patience.

The most accessible of these would probably be:

  • Lithium
  • Potassium
  • Calcium
  • Copper
  • Iron
  • Lead
  • Zinc

1Thank you Slipp D. Thompson

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    $\begingroup$ Adding onto the “You can't get black” explanation, it's important to understand that fire's light is additive with the light coming from behind it (with some refraction and other effects). So when you achieve black fire, it just appears clear. To get fire that appears black, you'd need it to not only produce no light, but also block the light of the surrounding environment that would pass through it. $\endgroup$
    – Slipp D. Thompson
    Jun 2, 2017 at 6:38
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    $\begingroup$ Oh, there is a wavelength for black: light that is outside the visual spectrum, or at a wavelength that appears negligibly bright against a stronger light source - for example, alcohol fires can be nigh invisible in bright conditions (happens in car racing, driver's on fire and everyone wonders why he's so agitated...). Add black smoke and you have black... $\endgroup$
    – rackandboneman
    Jun 2, 2017 at 12:23
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    $\begingroup$ "These colours are achieved, as already stated, by burning metals" - no! Re-read you quote, it talks about "metal salts", and they're pretty accessible. Burn table salt, and the flames will be yellow, since it's sodium chloride. $\endgroup$
    – Joker_vD
    Jun 2, 2017 at 12:26
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    $\begingroup$ Sodium shouldn't be hard to come by either. It exists in ordinary table salt (and is kind of required to live: there's a reason why "salary" is what we are paid: back in Roman times a man was paid in salt hence why its called a salary). Although I don't know if table salt really changes the color of the flame: its already yellow. $\endgroup$
    – Draco18s no longer trusts SE
    Jun 2, 2017 at 14:19
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    $\begingroup$ @MiloP you'd literally sprinkle a fine powder into an open flame. This works with really any open flame. $\endgroup$
    – dot_Sp0T
    Jun 2, 2017 at 14:39
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Apart from metal ions as mentioned above, it has to do a lot with flame TEMPERATURE. There is a reason behind the term "color temperature" - the cooler the more reddish (eventually crossing into invisible near-IR), the hotter the more blueish (eventually crossing into invisible ultraviolet. That's why the sun is creating so much UV that we are glad the athmosphere filters it, that's why really hot light sources like arc lamps or halogen incandescents are best enjoyed with UV filters). Actually that behaviour is not specific to flames, any material that is heated will behave that way (so called blackbody radiation) - flames are made out of gas....

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    $\begingroup$ It isn't blackbody radiation that makes some flames appear blue (no normal flames are hot enough to shift the color that far). $\endgroup$
    – Sneftel
    Jun 2, 2017 at 13:44
  • $\begingroup$ The dust of some metals can get surprisingly hot when burning... $\endgroup$
    – rackandboneman
    Apr 11, 2018 at 22:52
  • $\begingroup$ Not that hot. Not anywhere near that hot. Making a flame distinctly blue (as opposed to blue-tinted white) would require temperatures of close to 10000 K. $\endgroup$
    – Sneftel
    Oct 4, 2018 at 14:12

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