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In a world of my story, which is loosely based on medieval times, fire disappeared. Are there any sources of lighting without fire that would be possible in a medieval setting?
It is a fantasy book and magic does exist and is widely used to create light. But I'm looking for non magical ways to do it.
In terms of what was known before, say, the 18th century, there were very limited sources of what we'd normally consider rather dim light, but fire wasn't the absolutely only light source (not counting the sun, moon -- as it was thought to give its own light in pre-Renaissance times -- and stars).
Glowing insects were the commonest, and brightest. Fireflies can be nearly bright enough to (briefly) read by, if you can get even two or three of them near your page (their tendency to blink in synchronus is both helpful, because they're brighter, and unhelpful, because it'll be bright then completely dark). Firefly larvae are "glow worms" -- and I'm not certain whether they glow steadily or flash like fireflies.
Other sources of phosphorescence, like certain fungi (foxfire), are very faint -- easily bright enough to see in a dark forest, but not really bright enough to see by.
I can't say whether it was known in pre-scientific times or not, but the mineral sphalerite (zinc sulfide) is moderately common in some regions. It's the material that's incorporated into "glow in the dark" paints and plastics -- it will absorb light energy and then release it over time, though it can't be "kept" and only lasts minutes to an hour or so unless there's a source of either UV radiation or something else (this was used in radium clock dial paint, excited by the radiation from the actual radium).
Pure phosphorus was made by alchemists well before the 18th century, and glows from slow oxidation even when stored in water -- but I'm not sure this wouldn't be a form of the lost fire.
Another possibility is electric light. There is (slight, controversial) evidence that electricity had been created during the Bronze Age -- the Baghdad Battery was a wine jar that contained a rolled core consisting of dissimilar metals separated by fabric; it would have functioned as a simple electric cell if filled with wine or vinegar. Speculation is that it was used for electroplating, to allow dishonest jewelers to pass off cheap base metal goods as solid precious metal. However, there are various ways this low voltage electricity could be turned into light without requiring the ability to produce tungsten or platinum alloys (to take the heat).
One possibility is electroluminescence; this requires 100 Volts or so (which is a big bunch of battery jars), but could be made from minerals and metals that were available before the 18th century, if someone knew how (or stumbled on the effect -- say, a jeweler plating a ring that already has a tourmaline mounted).
Proper medieval artificial light sources rely only on fire, or on trapped fireflies.
Though it is not exactly a medieval technology, really dedicated alchemists (and a twist in the story) might come with the invention of the glowing sticks.
A glow stick is a self-contained, short-term light-source. It consists of a translucent plastic tube containing isolated substances that, when combined, make light through chemiluminescence, so it does not require an external energy source. The light cannot be turned off and can only be used once. Glow sticks are often used for recreation, but may also be relied upon for light during military, police, fire, or EMS operations. They are also used by military and police to mark ‘clear’ areas.
During daytime, they can use glass bottles with water, plugged in holes in roof as sort of light bulbs for home illumination.
https://www.instructables.com/id/How-to-build-a-SOLAR-BOTTLE-BULB/
Also with systems of mirrors and lenses its possible to illuminate, for example, cellars and mines using light of Sun.
Unfortunately, at night, you have to use candles, and fires.
Glow in the dark paint.
The brightest glow in the dark paint is supposed to be visible in the dark for 30 hours, which is plenty to get your peasants through the night.
A lamp like this would be clearly visible and would not require fire or electricity. Just charge it up during the day, and the photoluminescence should go all night.
According to Encyclopedia Britannica, glow-in-the-dark was first synthesized in 1603. It technically isn't medieval (5th-15th century), but it's still pretty close.
You might find a way to modify Archimedes' heat ray for your purposes. Essentially, one or more mirrors would be used to reflect sunlight (or light from another source, I suppose) to illuminate a given location. Further, you could use a series of lenses to magnify or diffuse the light as needed.
Hammered iron.
https://www.youtube.com/watch?v=tXF60MOWUeY Cold iron can be hammered until it is glowing red hot. That is a little bit of light. Also the hammering throws off sparks which also produce some light.
Not sure anyone is going to read a book with light like this but it might be a cool thing for a story - someone whaling on an iron rod until it glows then using it to light the way.
There are a few natural sources of light, which could be harnessed if you really need to.
Lightning produce light. If lightning storms are something common in your setting, it's not unthinkable that someone (maybe with some help from magic) has learnt to harness such power to use as light source. Not sure realistic, but hey, it's a magical world...
Sparks. Technically fire (?), but not a real flame. Imagine how much sparks come from a wheeled wetstone when sharpening a knife. Now make it smaller and portable, with a smaller blade attached. Takes a bit of effort, but could be a source of dim light.
Bioluminescent animals and plants. From common fireflies, to rare fishes found on in the deepest trenches on the ocean floor, all the way to some kinds of plankton and algae or shrooms. All sources of dim light. Since it's a living being, it's going to requires some taking care of, or constant replacement.
Moonlights is obviously an option. Maybe your world has multiple moons, making full moons a much more common event.
Magic. I know you said u don't want magic sources, but what about "natural" sources that came to be thanks to magic? Maybe some crystal or other mineral (possibly not radioactive or toxic) that sheds some light? Maybe a glowing small animal, something hamster-sized or larger. Or maybe there's simply some glowing liquid (naturally, or by alchemical means). It's magic, doesn't need to have an evolutionary purpose or even sense!
On medieval tech? It's a very simple answer: no.
Just about every answer above involves tech well beyond the medieval period, or ways of producing light that don't actually produce enough light to be of any use. Sounds like you're reliant on magic, short of handwavium like "Koomatka fruit glow with a bright bluish-green light, and are widely used for lamps."
One could heat a well-insulated mass of metal by using a solar mirror, and then use the black-body glow of the metal when the sun goes down. You could get yellow light at sunset, fading to a deep, soft red as the metal cooled.
The insulation would be layered, with porous pottery at the innermost layer and something more like horsehair on the outside. Insulation is important to reduce the losses, and help the metal heat faster and cool more slowly.
Bioluminescent fungi aka foxfire. It's nowhere near bright enough to be a torchlight, but you may be able to have it cultured brighter. A specific one would the bitter oyster fungus. (Tastes nasty, apparently.)
Magma emits a very beautiful light at night. Might be helpful if you don't mind living near a volcano. You may even scoop some lava from a nearby volcano and hold it in a ceramic or tugsten container, making your very own lava lamp (drumroll).
And while you may not have fire, if you scratch two iron rods really hard you still have sparks. One could crete contraptions which rub iron against iron and give off a little light as well.
Finally, since you are using the magic tag, you can take a page from Terry Pratchett's Discoworld's light dams:
An architectural and engineering feat in the Great Nef desert, designed and built by Goldeneyes Silverhand Dactylos.
While not described in the text, it would appear from context that these are otherwise conventional dams, strategically located in the Nef desert to trap and channel the sluggish Discworld light, so that it can be exported and/or sold on to other parts of the Disc that need it more than a desert does. The further details of the technology/technomancy involved have so far not been disclosed to us. The Light Dams and their devisor make an appearance in The Colour of Magic.
Some ideas for light sources that do not involve lighting something on fire:
Bioluminescence (Fire flies in a jar, jellyfish in a jar)
The moon or sun with a mirror network to redirect light.
Chemical reactions that give off light, for example Zinc Atoms and Nitrous Oxide: When zinc atoms react with nitrous oxide, N2O, the reaction is chemiluminescent.
https://en.wikipedia.org/wiki/Phosphorus was discovered in the 17th century, and by an alchemist trying to produce a philosophers stone so might fit with the time period.
You'd still want to burn it though so depending on your definition of 'fire' it may not count. It burns with a brilliant white light iirc, so would at least look completely different to wood/charcoal fires.
This is not a medieval technology but i can't see a real no-go technological limitation for that era.
Radioluminescent light sources usually consist of a radioactive substance mixed with, or in proximity to, a phosphor.
The only issue is the low light intensity.
Well, it's certainly not historical, but it might be doable with tech that they could have used. A Mercury lamp.
If handed the requirements I can't think of any step that is impossible, though admittedly improbable and no reason for them to invent the tech themselves.
Glass tubing -- Difficult, but at least possible - would be very expensive in comparison to modern manufacturing techniques.
Mercury -- The knew have to produce this.
Electricity - Making batteries does not require advanced chemistry techniques.
You need high voltage. You could put enough batteries in series, but it will take a lot - you need about 250 volts DC to get a mercury lamp started that operates at 120 V.
There are still hard details. E.g., as the mercury heats up and converts to plasma, the resistance of the lamp drops, increasing current and heats in a positive feedback loops that leads to self-destruction. No modern circuitry to rescue you from this problem, but by integrating a variable resistance element in the circuit (hand controlled) until operating temperature is reached you could be OK as the resistance of the batteries in the circuit would provide some negative feedback of the current.
Mercury lights produce a lot of ultraviolet, which are normally filtered out (in one form or another) in lighting applications. You can go blind if you get too much. Can't think of a simple way to block UVA (glass blocks most UVB) using middle age tech off the top of my head.
I should add that producing glass tubes without fire would be rather difficult, but I consider using magic to produce the glass if necessary still qualifies as not using fire to produce light. I am also assuming a low-pressure version of the mercury lamp similar to the original produced in 1860
There are natural rocks that glow in the dark (after charged from other light).
If your world has these rocks readily available, it's possible that fire for light wasn't really a need at all.
With a lot of vinegar (for the batter and pickles), it is theoretically possible to produce enough electricity to make a pickle (or any other material that glows hot from electric resistance, like a non-flammable thread) glow.
https://www.youtube.com/watch?v=aab8VjzuXyM https://www.scienceworld.ca/resources/activities/vinegar-batteries
As some of the comments have hinted, "fire" is more general than you think. If fire doesn't work any more, it's extremely unlikely that life can work either -- fire is based on (exothermic) oxidation, convection (which is based on gas laws and gravity), and radiant energy. Take away any of those things, and you're left with an environment that bears no resemblance to ours.
If you want to hand-wave something that makes fire impossible, you might as well keep waving and invent some other light source that suits the needs of your story.
