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so my question was VTC because it was considered as setting so i guess i need to remove the reason part (i dont think it harm much to the already answered part), if some of you curious about the reason just check the previous edit section to know it.

just imagine the location is all dirt/sand or concrete.

i want to know is there any other good material or chemical that can make a fire wall that is big and stable while also hot enough to harm/burn a person.

  • outside of common oil/gas (such as alcohol,gasoline, and plant/animals fat/oil) and dry/wood plant (including coal/peat,potato,paper,or any plantbase) which is boring and easy to be recognise by common people, and oil/gas smell.
  • greek fire is out i dont want it to lit forever in there.
  • jet fuel also out.
  • easy to ignite.
  • no explosion reaction just big fire.
  • not natural or location dependence such as swamp gas or lava.
  • as been mention prefferably no smell (so carcass or dead animals including dung/poop also out).
  • the fire will gone after the fuel or material to burn it gone too.
  • can work in any environment such as dirt/sand and concrete (if theres some side effect to this type of floor because of the fuel/material either before or after ignition, please also include it).
  • i ask about fuel, not from purely mechanism/engineering to achieve the same thing.
  • prefferably no need to conceal the fuel in public or the field but still undetected/unrecognisable or raise suspicions by common person (such as a people thats not a scientist or knowledgeable about such unique/uncommon fuel to know the difference between the field and the fuel) basically easy to blend with the field or look like different common nonfuel item.
  • the fuel not made or come from outer space.
  • no timeline or technology restriction so modern/future type of fuel is fine as long it make sense or possible.
  • for how long it has to burn, prefferably not surpassing a time limit for a person to get overheat or cooked alive just from the sheer heat alone (they dont wear any armor just normal fabric cloth), for minimum, i dont know the specific, but at least not last in just a second or five minute.
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    $\begingroup$ If such violence in society is permitted, such cheating is allowed, and the characters have enough money to buy the secret-super-fire substance, then it's probably simpler to just hire the professional assassins lurking around in dank bars and shabby temples. Less risk of being caught by the judge, and less risk of being killed or maimed before you can spring the fire trap. $\endgroup$
    – user535733
    Sep 29 '19 at 23:53
  • $\begingroup$ @user535733 assassin actually will be out since its a third person not even cheat but considered as disturbing the duel, and even if it allowed that willl harm the duelist honor, and at least such cheat is base of his own work or ability since tricking the referee require skill, so it can maintain his honor by showing such ability to the society. rather than using third person which even more insulting for the duelist itself outside of the society judgment that the duelist is a coward and have no skill.. $\endgroup$
    – Li Jun
    Sep 30 '19 at 0:18
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    $\begingroup$ @user535733 and if you mean sending assassin before duel occure it actually happen alot (most not to death just incapacitate) to make their oponent fail to attend the duel in time to make them win by default, infact my two character send assassin for each other but duelist has sharper sense and fighting experience and thats not include bodyguard or servant to taste the food, so assassination is not that easy. also most duel is not to the death you can forfeit/surrender or win from first blood. the fire for example can be just to scare the oponent or distract him or provoke him to open a gap. $\endgroup$
    – Li Jun
    Sep 30 '19 at 2:28
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    $\begingroup$ without a quantity needed undetectable is going to be up to you to figure out. Anything becomes detectable if you have to carry a lot of it. even something like jet fuel is going to require large quantities to make a significant barrier. $\endgroup$
    – John
    Sep 30 '19 at 3:59
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    $\begingroup$ @LiJun I don't think it's off topic (sometimes the closure reasons aren't quite accurate), but from my answer you should see the scoping challenges here. You're asking for any "non-common" fuel that is undetectable by a "common person." Both of those terms are rather fuzzy in a problematic way. I think they'd need more definition. Also not specified is how long it has to burn to do the job. You may want to try the sandbox to refine the question. I think it can be refined enough to be reopened. $\endgroup$
    – Cort Ammon
    Sep 30 '19 at 14:51
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Thermite.

thermite

https://www.youtube.com/watch?v=AkoRXNlg798

Thermite is usually a mix of iron oxide (rust) and powdered aluminum. The oxygen transfers from the iron to the aluminum releasing enormous amounts of heat. It is a good solution for your character.

1: It has no smell. It would be easy to conceal in the dirt because it looks like dirt.

  1. When it burns out there will be molten droplets of iron and black alumina left, which will look like dirt.

  2. If your world has aluminum refining tech, thermite is cheap.

  3. You do need a special hot magnesium fuse to light it.

  4. It is crazy hot. You can weld with it.

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    $\begingroup$ The one thing thermite is not is easy to light, even with a magnesium ribbon to light it it may not light.THERMATE however produces a similar (actually better for this) effect and IS easily to light. en.wikipedia.org/wiki/Thermate $\endgroup$
    – John
    Sep 30 '19 at 3:43
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    $\begingroup$ Any metal oxide will work, and iron oxide may cause least powered reaction, but it's most common oxide available. $\endgroup$
    – user28434
    Sep 30 '19 at 9:05
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    $\begingroup$ It also carries a risk of potential permanent harm of the eyesight of anybody who looks at it. $\endgroup$
    – VLAZ
    Sep 30 '19 at 11:24
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Magnesium

You can use something like magnesium which combines with oxygen and produces a bright flame. Its metallic appearance should help you disguise it and you should be able to trigger a fire or explosion of sorts by creating a powder (by scraping the sword) or setting it on fire with a spark of some sort.

A Magnesium fire is very bright, so not only will it burn, it will blind. As long as you close your eyes and look away when you trigger it, you will likely blind or essentially flash bang your opponent, giving you plenty of time. The remnants of the magnesium could be easily combined with dirt or float of in the air (no idea what form magnesium oxide takes).

You could hide some Magnesium in the hilt or guard of your sword. Where it will be covered by a fabric for comfort and decoration which you can later remove. Just make sure to be wearing gloves when you handle it yourself.

Sodium

There are also other pure metals like sodium which are highly reactive with water and explode when in contact with water. Coating the edge of your blade in sodium would create a strong burning sensation for your opponent when ever you manage to cut or stab them as blood is mostly comprised of water.

Dust

The final option is something akin to a dust or flour explosion. You can use anything like sawdust, flour or dust. When kept in a compact powder form, they don't burn, but when spread in the air they can create explosions. Your protagonist could bring some of this in and throw it in the air, or hide some in the ground and kick it up before setting it off with a spark to create a devastating explosion.

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    $\begingroup$ most of those explode more than burn, and the OP specified non-explosive. $\endgroup$
    – John
    Sep 30 '19 at 4:04
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    $\begingroup$ Sodium: what actually happens is that the sodium breaks the hydrogen-oxygen bonds in water, and ignites the hydrogen gas (the reaction is exothermic - it gives off heat, a lot of it). Coating an edged weapon in Na has a number of problems though (1) Na is incredibly soft (can be cut by a knife), (2) a mere coating would probably not contain enough Na to generate enough heat (being dissipated by the underlying steel), (3) Na oxidizes readily in air, sodium oxide is still violently reactive with water, but may not adhere to the weapon as easily. $\endgroup$
    – frIT
    Sep 30 '19 at 8:36
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You explicitly excluded all "normal" fuels, so you're going to end up with exotic ones. How about sand?

Obviously sand isn't a big fan of burning, but it can be encouraged. Enter Chlorine Triflouride. This stuff is nasty. Nasty nasty nasty stuff. The Nazi's tried to use it as a weapon in WWII, and gave up because it was too damn dangerous to have lots of the stuff around.

You want something which will burn on sand and/or concrete? How about something that will burn sand and/or concrete. Sand is a fuel to this stuff!

As John Drury Clark put it:

It is, of course, extremely toxic, but that's the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water—with which it reacts explosively. It can be kept in some of the ordinary structural metals—steel, copper, aluminum, etc.—because of the formation of a thin film of insoluble metal fluoride that protects the bulk of the metal, just as the invisible coat of oxide on aluminum keeps it from burning up in the atmosphere. If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal-fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes.

Now your duelist may have difficulty finding a store to buy the stuff from, but as you stated, I can handwave such menial details. He's just going to apply the stuff.

(And, as an added bonus, this suggests how to spring the trap. Just store it in a steel container, and have something ready to scrub the metal, exposing the ClF3 to the raw un-flourinated metal)

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    $\begingroup$ I like your thinking "no fuel allowed? lets just get the most extreme oxidiser I can then" $\endgroup$
    – Baldrickk
    Sep 30 '19 at 10:05
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I'm not sure if this is going to be useful, have you heard of Negative X before? It's comprised of: Ammonium Nitrate, Ammonium Chloride, Zinc powder

It could be relatively easy to hide as a powder on a dirt duel field. It won't provide a wall of fire but it does burn an impressive bright green colour with just a drop of water.

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Really you have a slew of materials to pick from, you just need something that burns fairly vigorously. I have listed a few in roughly increasing combustibility

  1. potato chips(or any fried chip)

  2. paraffin wax

  3. hexamine (used in solid fuel tablets)

  4. potassium nitrate and sugar

  5. potassium nitrate and sulfur

  6. magnesium ribbon and water

  7. zinc and sulfur

  8. thermate (not to be confused with thermite)

  9. or if your guy is crazy enough, chlorine trifluoride, that will make the concrete itself burn. Just fill a flat top (church-key) beer can with it.

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White phosphorus

In very technical terms, white phosporus is rated Not Safe For Life. Needless to say, it has combat applications (warning, the "Effect on people" section of the page has an image of a WP burn, and it ain't pretty).

Used in incendiary munitions, white phosphorus ignites spontaneously when exposed to air, and has been known to burn people, their clothes, and even their weapons. It has been used for its incendiary properties in a variety of conflicts, from WWI to current wars in Yemen or Syria.

It also makes a lot of smoke. WP smoke provides visual cover, and it also masks infrared signatures, which can be useful in higher-tech settings. As if that wasn't enough, white phosphorus is also toxic, making its ingestion or inhalation a very bad idea indeed, in case you had any doubts.

White phosphorus is generally safe to store in water since it's not very soluble and water ain't air. In solid form it is usually coated with some compound to prevent self-ignition. It skirts the line on chemical weapons, although that only matters if you sign them.

It looks like "a translucent waxy solid" and allegedly smells like garlic when burning. If firearms or grenades are allowed to pass inspection, it's trivial to hide as regular ammunition. Otherwise you'll have to get more creative hidding it in water flasks for instance, or it might pass as chewing gum in solid form.

From there, you only need to expose it to air. It might not create a wall of fire in a literal sense, but will certainly be an obstacle you don't want to cross. Or just throw it at your opponent, that will definitely "harm/burn" them.

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