So, I'm working on a flintlock fantasy series, and my protagonist is what's called an Arcane Engineer. This is someone who makes magical devices. Among those she has made there is a mask that let her breathe both underwater and in environments where there's smoke or other gases that would be bad to inhale.

My thinking is that the masks works by converting the CO2 she exhales into O2, similar to the way plants do when they perform photosynthesis. However, this leaves the issue of what to do with the carbon in the carbon dioxide. The temperature of atoms would not be changed, only how they are arranged into molecules.

Any other gases she exhaled are not affected and simply pass through the mask. When she inhales, any CO2 in the air is likewise converted, benign gases are permitted through, and toxic gases are blocked.

So: What should the mask do with the carbon? That's what I'm trying to figure out right now. The answer will have some effect on how the mask is designed. (Like if it has to have some kind of container where the carbon will go after it is separated from the O2.) Suggestions?

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    $\begingroup$ Do you have criteria for what answer would be best, and what the limitations of your system are? Otherwise this strikes me as primarily opinion-based which answer is best. $\endgroup$ – Cadence Aug 28 at 1:47
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    $\begingroup$ Diamonds. Go big or go home. $\endgroup$ – Cephalopod Aug 28 at 11:06
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    $\begingroup$ @RancidCrab definitely no, common air is 21% oxygen and 0.04% co2. $\endgroup$ – Peteris Aug 28 at 11:19
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    $\begingroup$ great minds @Cephalopod, or fools seldom differ? $\endgroup$ – JCRM Aug 28 at 14:11
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    $\begingroup$ If you want to keep magic as something that just does things that are very unlikely in the real world rather than outright breaking some very foundational laws of physics, you should actually make the products of your splitter considerably colder than the incoming carbon dioxide. This way you don't create a perpetual motion machine - just an improbably efficient chemical reactor using ambient heat to supply the reaction which would normally proceed imperceptibly slow. The cooling could even be a plot point, and lead to many technologies exploiting this effect :) $\endgroup$ – Luaan Aug 28 at 18:21

12 Answers 12


You don't need to expel any carbon

There is actually a set of non-magical chemicals and reactions that can do exactly what you want, the magical part would be regenerating the "waste" chemicals back to the reactive form, keeping the temperature stable (some reactions are exothermic), and facilitating the reaction like a catalyst.

For converting carbon dioxide to oxygen, you can use a lithium peroxide reaction, which produces lithium carbonate and oxygen. Exhaled air is about 5% carbon dioxide, with a similar drop in oxygen content. Since the reaction of lithium peroxide is 2:1, this can enable you to breathe underwater if you provide half the oxygen, remove the exhaled water vapor and toxins, and regulate the pressure with inert gas such as nitrogen or helium.

Generation of nitrogen gas can be done in a separate partition by mixing liquid ammonia with hydrazine (using a magical catalyst), not a lot is needed since it is only there to replenish small amounts of removed impurities to balance the pressure and volume of air. This produces pure nitrogen and hydrogen gas, the hydrogen is expelled externally into the water or stored to produce pure water from air later.

For gas exchange, you would need to convert about 20ml of carbon dioxide per breath to oxygen at standard air pressure, at 2kg per cubic meter, that is around 40mg of carbon dioxide, requiring full utilization of 32mg of lithium peroxide. If you store say, 80 grams in the mask (about the weight of a respirator cartridge), that gives you 2500 breaths or 2 hours at normal exertion. If you are swimming or running you would have substantially less, maybe only 30 minutes, this is assuming there is no magical autoregeneration happening while you are breathing. You also need enough material and surface area for complete reaction, but this should not be a problem at the given utilization rates.

In other environments, where the mask filters out particulates and toxins, you would be more interested in getting rid of carbon monoxide from smoke. This is done by adding oxygen to produce carbon dioxide, at levels that are not toxic or could be removed by reaction.

In low oxygen environments or underwater where you need it, sodium chlorate can decompose into salt and release pure oxygen gas in the process. Generally this is a very high temperature reaction, magic would be needed to keep this safe in a mask. 160g of chlorate produces enough oxygen for around 1 hour of normal breathing in a no air environment, or 2 hours when combined with CO2 conversion. Magic could then be used to regenerate the salt into chlorate with a fresh oxygen or water source later, or it could just be replaced with new chlorate if salt is impervious to magic in your world. You can make it with electricity and hot salt water.

Designing a mask

Because of the different functions of both filtration, waste removal, and oxygen generation, the mask would need to be layered and partitioned. The outer layer (filtration) and inner layer (conversion) would need to be separated if needed, and an air bladder in the middle for breathing.

Underwater, the outer layer is isolated from the outside which is now water. The exhalation valve is shut off, and the nitrogen gas generator takes its place. The CO2 conversion layer is simply powder in a filter media that exhaled air passes through, it would normally leave the exhalation valve, but now underwater it goes through the filter and into the expanding air bladder, converting the CO2 to O2 as it passes through twice.

The outer layer is then routed back to the inner layer before the bladder, to slowly remove toxins and water vapor from the breath. The oxygen generator would be the closest to the mouth and work on a feedback mechanism, if O2 is low it makes more. The O2 generator also needs to be hooked to the outer layer to convert carbon monoxide to dioxide.

Magical components should keep the mask similar in size and shape to a standard half-face cartridge respirator mask like a 3M 7500 series with a little more bulk where the cartridges would attach, and an air bladder that expands out the side or bottom like a frog croaking. Depending on how much magic you want, the air bladder, exhalation valve, and separator valves could be replaced with force shields, further reducing the size and weight of the mask. The reactant weight would be around 240g in powder and maybe 50 in liquid. Removing CO2 increases the weight, but making oxygen reduces the weight, so the weight of the mask will vary a little with use, but not much. Ammonia and hydrazine can be provided by small ampules with rubber plugs, that are easily replaced when expended.

Chemically, all reactions are done without violating the laws of thermodynamics or by using cold fusion or alchemy. They could even be regenerated without magic at all, by adding fresh reactants by hand, and the magic components only acting to catalyze and regulate the reactions. A magical regneration of lithium peroxide at the rate that would be used by heavy breathing in normal air (1g per hour), or higher, would be ideal. That way you do not need manual exchange of the powder, and it adds a plot point if you have used it up underwater and now need it to regenerate. That is 80 hours of no mask use for full regeneration if fully depleted, mask use would pause generation but it would still be able to remove carbon monoxide.

The outer toxin filter would be the same design as those used today, activated carbon combined with reactants and chemical treatments, and a particulate blocking media, something like cellulose or rayon. It could be magically enhanced to continually clean itself or last longer than a non-magical filter.

The mask itself, plus the reactants, magical components, and outer filter would weight maybe 1.5 to 2.5 pounds, depending on mask material. I am assuming you do not have plastics, so probably leather and refined metals like aluminum or titanium, maybe held together with spider silk.

  • $\begingroup$ ps. the note about no alchemy specifically refers to elemental transmutation, your definition may vary. Also size and weight calculations are done for an average size human, and may need to be changed for other species $\endgroup$ – Richie Frame Aug 29 at 2:33
  • $\begingroup$ Brilliant! And you even provided me with an example of how the mask could look! Maybe make a tad more "steampunk" in design, and we're in business. Also, you're right on the money with the materials. My character uses transmuted Wyvern leather for a lot of her gear, transmuted silk from Spiderings (basically Driders) in her garments, and transmuted metals for her weapons and guns, so using that stuff to make a breathing mask makes sense. Now it's just a question of the aesthetics, since she always has to make things stylish. (Hey, functional can still be fashionable.) $\endgroup$ – Patrick-Leigh Aug 29 at 3:03
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    $\begingroup$ @Patrick-Leigh I looked at your requirements in both the question and in other answer comments, and decided to go full Watney and science the shit out of this.. which was easy since this is how it is done in space travel for CO2 removal and emergency oxygen generation, just without the magic component. $\endgroup$ – Richie Frame Aug 29 at 3:13
  • $\begingroup$ Makes sense. And, since lithium was discovered in 1800 by José Bonifácio de Andrada (what a name!) it's still something that would be known at a time when flintlock weapons were in use. With Alchemists in my setting, I could see them at least figuring out that it can be used to get oxygen from CO2. From there, trial and error would eventually lead to breathing masks like the one my protagonist could use. She'd keep tinkering with it using her expertise to get a breathing mask that suits her purposes. Just 'cause she makes magical guns don't mean everything is destructive. $\endgroup$ – Patrick-Leigh Aug 29 at 3:21
  • $\begingroup$ When diving very deeply, the air mixture used is oxygen and hydrogen, as opposed to oxygen and nitrogen: en.wikipedia.org/wiki/Decompression_sickness#Inert_gases $\endgroup$ – Ferrybig Aug 29 at 11:57

Use the carbon to add new activated carbon to the mask filter. Though ineffective against toxic gases, active carbon is good at capturing other contaminants.

Activated carbon, also called activated charcoal, is a form of carbon processed to have small, low-volume pores that increase the surface area available for adsorption or chemical reactions. Activated is sometimes substituted with active.

Due to its high degree of microporosity, one gram of activated carbon has a surface area in excess of 3,000 m2 (32,000 sq ft) as determined by gas adsorption.

Filters with activated carbon are usually used in compressed air and gas purification to remove oil vapors, odor, and other hydrocarbons from the air. The most common designs use a 1-stage or 2 stage filtration principle in which activated carbon is embedded inside the filter media.

Activated carbon filters are used to retain radioactive gases within the air vacuumed from a nuclear boiling water reactor turbine condenser. The large charcoal beds adsorb these gases and retain them while they rapidly decay to non-radioactive solid species. The solids are trapped in the charcoal particles, while the filtered air passes through.

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    $\begingroup$ Oh, that's a really good idea! It would make the breathing mask more efficient at filtering things without having to expend additional magic energy! $\endgroup$ – Patrick-Leigh Aug 28 at 3:13
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    $\begingroup$ The amount of activated carbon would quickly fill up the mask, but after making a stupid mistake in my earlier calculations I don't feel like giving exact numbers. It's a simple back-of-the-napkin calculation if someone feels like it. So you still need another way to get rid of the left over. $\endgroup$ – Nobody Aug 28 at 16:00
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    $\begingroup$ @Nobody then whenever a new N+1th inner layer of activated carbon is complete, get rid of the outermost activated carbon layer along with whatever impurities it has adsorbed, returning the mask to its nominal configuration.. $\endgroup$ – Monty Harder Aug 28 at 21:50
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    $\begingroup$ Filter media needs changing pretty regularly. Most carbon masks that are thin layers within fabric need to be changed after 6 hours of use (not 6 hours of wearing them but of active filtering). In cases where the air is really toxic, it could be less. Thicker carbon can go 6 months (of intermittent use, maybe 20 hours active use?), but not if used on really bad air. All the character needs is a way to expel the carbon. $\endgroup$ – Cyn says make Monica whole Aug 28 at 23:47

Honestly, I'd just have it release as elemental carbon or carbon black. I think it would be an awesome visual if every exhale came out as a puff of smoke or soot

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    $\begingroup$ That's an interesting idea. Maybe the mask can be used with or without an attachment to collect the carbon soot, depending on the circumstances. (She doesn't need to be clouding up the water with soot while she's diving under the ocean, for example.) $\endgroup$ – Patrick-Leigh Aug 28 at 3:10
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    $\begingroup$ This would be in theme with "flintlock fantasy". $\endgroup$ – Schwern Aug 28 at 4:14
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    $\begingroup$ I thought this, too - you could take a bit of license with the amount and form of carbon produced: After wearing the mask for a while, taking it off reveals that her skin is covered with shiny black graphite underneath, or worse, micro-stalactites of carbon nanotubes growing from every tiny hair on her face. $\endgroup$ – Beejamin Aug 28 at 10:37
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    $\begingroup$ Flintlock fantasy should not have perfect technology yet. Little flaws add to the charm. While turning the carbon into something useful in the mask is ideal, it is not as interesting for the theme. Iff for plot reasons you want her to make a diamond or activated charcoal, make that be an external step that calls back to the mask later. $\endgroup$ – kaine Aug 28 at 14:00
  • $\begingroup$ @kaine Excellent point! I have decided that her equipment evolves along with her, since she's constantly making upgrades in response to new challenges, so the Mark 1 version of the mask would probably just dump carbon soot into a tank or expel it as smoke. The Mark 2 version might arrange the carbon in a particular way, the Mark 3 might combine it with hydrogen and oxygen to make triglycerides, etc. I don't know if she'll ever have it making diamonds, as that would probably be highly illegal. (See Fullmetal Alchemist and the laws against making gold with alchemy to protect the economy.) $\endgroup$ – Patrick-Leigh Aug 28 at 22:31

She is exhaling 2 molecules containing water: CO2 and H2O. Use both.

Our metabolism: CHO + O2 -> CO2 + H2O

CHO here is carbohydrate; food.

Plants run it backwards: CO2 + H2O + light -> CHO + O2.

If you are getting biochemical, go all the way. Using both gases she exhales provides her more oxygen and also yields a bunch of hydrogen. Carbon likes hydrogen: put them together without oxygen and you have methane.

Thus: her magic machine makes methane. On land she could light it and have a flame on top of her head which is a good look for a magic user.

Using H2O as a O2 source might come in handy underwater where there is plentiful H2O. Her machine might then produce an excess of hydrogen which will bubble away.

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    $\begingroup$ You make a good point about getting oxygen while underwater. Of course, if she collects the hydrogen in some kind of container, could she use it as fuel for some kind of flamethrower? ;-) $\endgroup$ – Patrick-Leigh Aug 28 at 23:50

As a side effect your mask makes diamonds, about 1 carat a minute.

3 hours of use gives a diamond sphere just over an inch in diameter, or something the size of the Daria i Noor

These are all rough figures, but a human produces about 1kg of CO2 per day, and CO2 is about a quarter carbon by weight, so allowing for the extra exertion (~15%) of swimming or moving through water that's about 0.2g per minute

0.2g per minute is 36g in three hours.

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    $\begingroup$ I'm not sure that math checks out. $\endgroup$ – Draco18s Aug 28 at 13:07
  • $\begingroup$ Which part @Draco18s $\endgroup$ – JCRM Aug 28 at 13:37
  • $\begingroup$ The amount of carbon pulled off CO2 and converted to diamond. I don't think a "carat a minute" is correct. $\endgroup$ – Draco18s Aug 28 at 13:47
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    $\begingroup$ Now that you've included the math, I don't see a problem with it. Thanks. $\endgroup$ – Draco18s Aug 28 at 14:22
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    $\begingroup$ Great minds, of course. I mean, creating a magic works-under-all-circumstances breathing device that doesn't create thousands of dollars worth of diamonds while you use it would just be a waste. $\endgroup$ – Cephalopod Aug 29 at 9:34

Why not just use the magic to transmute the carbon to more oxygen? Four carbon atoms contain exactly what you need to make three oxygen atoms.

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    $\begingroup$ There is magic and there is MAGIC. Any technology sufficiently advanced is indistinguishable from magic. Having technology that could separate O from CO2 is theoretically possible - where transmuting elements in a matter-of-fact way is a step way beyond known technology. So even though in this context it truly is "magic", no need to overdo it. $\endgroup$ – manassehkatz-Reinstate Monica Aug 28 at 16:24

How powerful is your magic? Can it teleport the atoms? I am thinking a huuuge diamond slowly building itself on a shelf in your Arcane Engineer home.

  • $\begingroup$ While that would be possible, it wouldn’t be “energy efficient” in most situations. Magic is power by an energy called Arthyr. If you don’t have Aethyr, you cannot do magic. Thus, conserving Arthyr is important. You have to make the most of it, and teleportation is a magic energy hog, no matter the scale. $\endgroup$ – Patrick-Leigh Aug 28 at 21:00

My thinking is that the masks works by converting the CO2 she exhales into O2, similar to the way plants do when they perform photosynthesis.

Do it like plants do and keep it in the body. But make lipids rather than sugars, because and because that way you are more oxygen-efficient.

Here are some lipids she can do. Triglycerides in particular are an amazing source of energy. They store more than twice as much energy per mass than sugars or proteins.


And if you think this precious energy storage would be undesirable because in real life this usually gets stored in the abdomen, remember that:

  • Some people like that
  • If she doesn't, she may use her to make it go somewhere else as she sings "because you know I'm all about that bass, 'bout that bass, no treble".

Edit: OP commented:

Well, if you check my Deviant Art profile (under Patrick-Leigh,) you'll find my journals on my character's physiology, and, yes, she is "all about the bass." (She's a Half-Orc, so she has to keep a certain amount of body fat on her or she'll burn through it too quickly and start losing muscle mass.) That, actually, makes the idea of the mask also proving her with sustenance all the more viable. It's restricted by the whole conservation of mass thing, so she'd need to get the other atoms to form the triglyceride molecules from somewhere. Any ideas on that?

The formula for a triglyceride such as a molecule of unsaturated fat is C55H98O6.

So, she needs 55 carbons. That would release 110 oxygen atoms. If she sacrifices of those 6 to make the triglyceride, she would still be recycling over 90% of the oxygen she uses. Makes for very long dives, but she would have to resurface at some point.

There is a better solution. She will also need 98 hydrogen atoms, so she will have to break water. She either breaks 49 water molecules from her surroundings (with help of the mask), or she takes it from her own body. In this way, she gets all the hydrogen she needs, and 43 extra oxygen atoms on top of the ones she recycled (remember, six of those oxygen atoms are to form the triglyceride).

If taken from her own body, she will need to break approximately 1.3 liters of water for every 1.5 kilograms of fat she makes.

So now you have recycled all of your oxygen AND got almost 50% extra. What do you do with that excess oxygen? Don't breath that, gas mixtures with more oxygen than we need are poisonous. Instead, you can form ozone (O3) in the mask. Ozone is a powerful disinfectant, it can destroy a lot of microbes and many viruses. It also has a pleasant smells in low concentrations. You can just cycle a little ozone at the front of the mass and release most of it to the surrounding water. Don't breath the ozone, though, as it can damage the lungs.

  • $\begingroup$ Well, if you check my Deviant Art profile (under Patrick-Leigh,) you'll find my journals on my character's physiology, and, yes, she is "all about the bass." (She's a Half-Orc, so she has to keep a certain amount of body fat on her or she'll burn through it too quickly and start losing muscle mass.) That, actually, makes the idea of the mask also proving her with sustenance all the more viable. It's restricted by the whole conservation of mass thing, so she'd need to get the other atoms to form the triglyceride molecules from somewhere. Any ideas on that? $\endgroup$ – Patrick-Leigh Aug 28 at 15:01
  • $\begingroup$ @Patrick-Leigh I have a good response for that, but it is too long for a comment, so I'll edit the post :) $\endgroup$ – Renan Aug 28 at 15:14
  • $\begingroup$ Sounds good! I'll look forward to it! $\endgroup$ – Patrick-Leigh Aug 28 at 15:24
  • $\begingroup$ @Patrick-Leigh I just finished it, please check. $\endgroup$ – Renan Aug 28 at 15:25
  • $\begingroup$ Oooh, this is GOOD. This would enable her to go on long dives and keep herself sustained with a form of "food" so she doesn't have to stop to eat. The mask can use the O3 to disinfect itself and filter out any pathogens she might otherwise inhale. Yeah, this is definitely going to be one of its features. $\endgroup$ – Patrick-Leigh Aug 28 at 15:31

What about turning the carbon into plant material? That's what happens in photosynthesis. Almost all the mass of a plant comes from carbon extracted from the air.

A nice magical effect would be if small flowers collect at the bottom of the mask. This will limit the time a mask can be used, though it takes a LOT of carbon dioxide to make even a single flower. A perhaps more steampunky, but less colorful, effect would be to have algae accumulate on the inside of the mask, slowly obscuring sight.

  • $\begingroup$ She will need some water to pull this off. $\endgroup$ – Willk Aug 28 at 13:15
  • $\begingroup$ When you have magic, you need only what your plot demands. $\endgroup$ – WGroleau Aug 28 at 13:53

So I know this is supposed to be a fantasy universe, but if you are producing excess carbon with nowhere to go, why not use it for production of materials made purely out of carbon. Someone earlier mentioned diamond, but alternatively modern material science has given us carbon nano-tubes and graphene, both of which could be used to incredible ends.

Carbon nano-tubes are insanely strong and it has been recently discovered that there were naturally occurring carbon nano-tubes in Damascus Steel, which already has a legendary reputation as a lost technology. We of course have stronger steels today thanks to modern refining capabilities, but it was generally considered the strongest steel of the time.

Graphene on the other hand is still incredibly experimental, but there have been experiments in using sheets of graphene to produce cloaking materials (like serious scifi cloaking). Alternatively, graphene has incredible potential for computer science, which I imagine isn't likely to be super prevalent in a fantasy world, but there's potentially to come up with something interesting.

So that gives you three strong candidates for potentially fantastic items. Invisibility cloak, check. Insanely strong sword, check. While you're at it go ahead and make some diamonds for diamond head tools, jewelry, currency exchange, whatever.

  • $\begingroup$ Actually, my character is partnered with an Alchemist. In my setting, Alchemy is a combination of chemistry and magic, where you use magic to augment chemical reactions or give them magical properties. So, she could also collect the carbon and give it to him to use for a variety of purposes. Using it for making metals is something she could do as well, since, being an Arcane Engineer, she has to make metal items all the time. $\endgroup$ – Patrick-Leigh Aug 28 at 15:04

Use it as a nutritional supplement or internal deodorizer.

  • $\begingroup$ Deodorizer is definitely one thing to do with it. (My character is a Half-Orc, so she has a sensitive nose.) $\endgroup$ – Patrick-Leigh Aug 28 at 14:48
  • $\begingroup$ The second part of this answer is a duplicate to the earlier answer suggesting activated carbon (which is used for deodorizing among other things). The first part doesn't make much sense, while it's probably not overly unhealthy to eat carbon, it's of no use either. On top of that this site is looking for longer answers justifying themselves and elaborating their ideas. $\endgroup$ – Nobody Aug 28 at 15:44

Plants use the excess carbon to build themselves. You could have your mask regenerate itself with the carbon it pulls out.

  • $\begingroup$ Self-repairing. I like that. $\endgroup$ – Patrick-Leigh Aug 30 at 18:50

protected by L.Dutch - Reinstate Monica Aug 29 at 8:39

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