Had a dream that I was the size of an ant and I was being chased by spiders and other small horrors. I created a campfire and was able to scare off the insects. When I woke up I came up with a TTRPG campaign that is basically 'Honey I shrunk the Kids'. But when I thought about it I was ever wondering if a fire at that level would even be possible.
So that led to my question, could you create a small campfire if you were that small? Thanks for my random question.
More than a campfire, it might be possible to create a spark: considering the size of wood chip a creature the size of an ant can handle, it won't last much more than a spark.
Additional problem, the combustion heat might be sufficient to lift it in the air, which is not exactly handy if it has to be used to fend off attacking animals.
Wood at that scale would not be manageable, but an easy alternative would be oil lamps:
At this scale, the small flame from the oil lamp is more than enough to warm dozens of ant scale humans. It's also much easier to set up and control for long periods of time, unlike a small pile of wood which will flame out quickly.
I just realized that I wrote a verbose version of L.Dutch's answer. I upvoted his answer. You should, too.
It's amazing how often I've used the following quote on this site....
Can you launch an ICBM horizontally?
Sure. Why would you want to? (The Hunt for Red October)
Can you shave a piece of wood so small that it could be used for a campfire by an ant-sized human? Sure! Why would you do it?
Here's the problem: rate of combustion. It doesn't matter if you're burning wood or oil or anything else. The simple truth is that the smaller the combustible item is (i.e., the less you have of it), the faster it will be consumed.
As in... fractions of a second.
I'm fond of another quote by Harold Ramis about the original Ghostbusters. During the scene when the giant Stay Puft Marshmallow Man is walking down the street, kicking over (among other things) fire hydrants, the special effects experts used very finely ground sand for water. Why? He explained...
You can't miniaturize water.
It's true. You can see every time a movie puts a model of something (like a ship) in water for filming. The water is the wrong size. It's motions out of proportion with the object of our focus. It can't be miniaturized.
Neither can chemical reactions.
And that's the problem. Yes, they could have a campfire. For a fraction of a second. If they added fuel fast enough to keep the reaction going, they'd burn themselves up (or ignite whatever was around them).
So, from a practical perspective, no... ants can't have campfires.
But that shouldn't stop you. This stack is dedicated to building imaginary worlds that need not conform to Real World physics. If you want your ant-sized humans to have campfires, let them have campfires. It's not as if there's a secret cabal of fanatic bibliophiles out there ready to unleash their zombie hordes when they discover (gasp!) that you've written a story that doesn't conform to real-world physics.
P.S., it's worth noting that there may be some chemical compound that could ignite easily and burn slowly enough that mere grains could be used by the ants as a campfire. Such a combination of chemicals won't be available just anywhere, suggesting that they can't be used as a campfire, so I ignored that possibility. But that could, itself, be an interesting idea for a story. Is there a compound that is easy to ignite and yet mere grains could burn for a specified time (you'd need to specify that)? Hmm...
Probably not.
Try burning a matchstick with the head cut off. In my experience, wood this small will only continue to burn if the stick is angled so the flame is creeping upward. Hold a match with the flame at the top, it'll go out. Smaller wood, more so -- you'd have to feed the fire so rapidly you'd go through a pile of fuel much bigger than your tiny self in minutes. Pile up several such sticks (proportionally as large as a good sized log) and they might continue to burn (as they hold the heat and ignite each other), but you'd go through your fuel even faster.
It would take, at a minimum, the full resources of at least a good sized colony of ants to find and bring tiny fuel pieces fast enough to keep a fire fed -- which would largely defeat the purpose of keeping the fire going to drive off equally tiny predators.
As several other answers have pointed out, a wood campfire would not be practical if you were of a very small size.
So, what you would need instead is for everything else to be very large.
Instead of having your campaign set in the normal world with very small people, have it take place on a very large world with overgrown trees (see - Giant Redwoods) and equally overgrown animal life - including insects.
Large insects of course present their own problem - exoskeletons do not scale up very well, and become less effective the larger the creature.
But assuming you are willing to handwave this (or inquire about it in a second Worldbuilding question), you'd no longer need to worry about the rate of combustion, and can happily have your campfire and insectoid foes.
One thing to note is that the human body couldn't really exist at that scale either, for the same sorts of reason that a campfire couldn't.
But that's not a bad thing necessarily. Since you're already bending the laws of physics to have humans running around, why not bend them a bit further to have campfires as well?
There's a few ways you could do that, assuming you don't want to just handwave it without explanation. One is to say that if a fire is lit with a miniature lighter then you get miniature fire, which behaves like full-size fire only smaller, instead of like a real small fire. This is kind of hard to justify in terms of real physics, but it might work for a story.
Another option is to say that although a small sliver of wood can't burn like a campfire, wood that's been shrunk behaves as if it were full size wood. (Just as shrunken human bodies behave like full-size human bodies.) For example, you could have a wooden shed get shrunk along with your humans for some reason, who then find that they can build a perfectly nice campfire out of the wood the shed was made from, even though the non-shrunken wood that they find in the environment just goes "poof" instantly.
While it won't get you an ant-scale campfire there is a way you can get a considerably smaller fire: Put it inside a IIRC glass cylinder. You need material that will pass visible light but is opaque to most infrared, thus holding a lot of heat around the fire. You'll still need a liquid or gaseous fuel, solid fuel can't be adequately controlled or fed.
Japanese "incense" sparklers are small and burn for quite a long time. But they tend to give off a lot of sparks. https://www.youtube.com/watch?v=063sevb2_tU
You can have a campfire that size, you are essentially using wood shaving and saw dust, to your humans it is more bonfire than campfire. (assuming your humans are as strong as ants. the problem is they will have to feed it constantly, non stop as soon as it gets lit. But the real problem is a mild breeze will extinguish it, and they can't protest it or it will go out as soon as there is no wind. Not to mention thy will have a hard time getting close to put the wood in. they might have to burn bigger wood and let it die down before cooking on coals.
If they can carefully control some kind of forced air, they might be able to get something manageable, on the up side at that size food will cook very quickly.
There is a book about miniaturizing people to enter into the blood vessels with a ship. It was something like condensing matter and needed careful support from an energy source to keep items miniaturized.
When some matter entered their field, it also got miniaturized. So maybe you can use this effect in your story. Or maybe surprise - everybody jumps on the spider and it becomes small.
