Turning salt water into fresh water

Serf's parma....
I seem to remember a spell in the RAW that makes salt water drinkable. Maybe in "Magi of Hermes", with that Tytalus who challenges the sea?
Would this be a perdo terram spell to remove the salt?
I'm looking for a spell that would aid sailors at sea. So a temporary CrAq spell wouldn't do the trick.

Any help is appreciated.

You probably want Rego. Big discussion here:



So, for example, you have a barrel of brine, and an empty barrel. You cast a ReAq spell, which moves just the water to the other barrel, leaving the salt, (and any little demons that cause sickness) in the first.
I would think that in theory you now have a barrel of pure water, no? Or would someone still need to boil it?

[emphasis mine]

The bolded part is my problem, and the reason that some other method might be helpful. The demons are entities which want to cause harm, and your manage cannot detect. What stops them gripping onto the water and moving with it, as they normally do?

Nothing. But what tells them the water is about to be moved?

There are demons that cause disease, as well as other spirits and fae, but sickness is generally caused by imbalance of humours. Demons are not microbes, and they are not hiding in drops of water.

The demons don't grip the water any harder than the salt. It's a spell that moves water and it's good enough not to move the salt itself. You would need a Vim requisite to move them.

If you can make a spell that teleport a sword out of a grog's hand, "gripping" it has no effect.

Water can also be desalinated using PeAq, per pg. 29 of Through the Aegis. Briefly, "saltiness" is a property of the water, and can thus be destroyed by a Momentary Perdo spell.

Not the only way to do it, but sounds good to me. You could ReTe the salt out of the water too.

Arguable that you need a Part target to either ReTe only the salt or to ReAq only the water. As you are only trying to target a Part of the "salt water".

You could also PeAq the "saltiness" property (this is a stated example of what you can do with the PeAq 15 guideline; ArM5 page 123).

No Part needed if you think of it differently. You're letting it undergo a natural process of evaporation (or boiling), trapping it in the process. This would be just like other nearly-Muto uses of Rego.

I'm convinced by Jonathan's argument that since the Romans knew how to use salt pans to create salt by evaporating sea water, and therefore it should be doable with a ReAq spell.

But I would add an Ignem requisite, as the Romans knew it took heat to make it happen.


actually they didn't, because it doesn't. Unless the water is frozen, heat just accelerates the process, as it does with a great many other things. If you live in a Mediterranean climate, heat is purely optional, and an ignem requisite very redundant since it is not requires to convert (for example) ice to steam.
Yes from a modern understanding we know about boiling points and the differential saturation of water evaporated in air depending on temperature, but these are definitely not part of the medieval comprehension of evaporation or condensation, which is more likely to view it as the water being destroyed and created.

Sure, depends on which Re guideline you are wanting to use. The original implication was using a "teleport" guideline --- which arguably would need a Part target, as you are only teleporting either the water bit or the salt bit.

If you are using a kind of "craft" Re guideline to manipulate salt water into fresh, then I suppose you wouldn't need a Part target.

Not so sure about that. Medieval thought doesn't have the same systematic understanding of psychrometrics and salt solutions that we do. However:

  • Everyone knows that adding salt to fresh water makes salt water.
  • Every cook knows that evaporating off water concentrates the remaining sauce (or whatever) and that the evaporated water ends up condensing on the ceiling/walls (if cooking inside when it's cold outside).
  • Everyone making chemicals that require a distillation process (done since at least 1st century AD, according to wikipedia) has a working knowledge of evaporation and condensation.

I'm dubious about this one. Saltiness isn't a "property" of water. Everyone knew that salt water was water mixed with salt. It doesn't take a modern understanding of chemistry to figure that out. I suppose one could Perdo the salt, but that's probably not a liquid and Aq appropriate.

saltiness is a property of the water, which it acquires from the salt, the same way warmth is a property of the water which it acquires from fire.
you assume that because we now about the connection between boiling and evaporation that those in the middle ages will as well. In any case it is canon that you can change water to steam with ReAq and no Ig requisite, so there would be no need to use an Ig requisite to remove salt. Looked at another way this is craft magic to distill saltwater into regular water and salt using ReAq and no equipment.

I wouldn't let one "destroy the heat property" either, without using Ig.

Actually it was understood, although certainly not in the same terms we'd use today. Any cook knows how to reduce stock.

and yet it states explicitly in the guidelines that one can change liquid water to ice without ignem

yes, but that doesn't mean they see evaporation- which happens without the application of heat, as being the same thing.

Far be it from me to question canon, but that's a very poorly thought out guideline. It's also Rego and therefore not quite the same as the destroy a property guideline, which is also too broad.

If I were being even more pedantic than usual, I'd point out that the guideline merely mentions changing the liquid into a gas or solid, not changing the temperature of the liquid in any way. Creating scalding-hot steam to damage someone is clearly IG.

I'd rather simply argue though that changing temperature is an IG effect and anything that says differently goes against the spirit of the magic system and is therefore in error. In a game with literally thousands of little rules, following the spirit of the system is not a bad approach, at least in my opinion.

The salt pans have already been mentioned. These were in use in Brittany among other places during the period. What more do you need to accept that evaporation techniques were understood and in use?

I'm getting the impression that your argument is that if Piers the Plowman didn't understand evaporation, then evaporation doesn't occur in Mythic Europe. I don't think this is what is implied by following the medieval mindset.

The point of the Aristotelian physics, love it or hate it, is to present a reasonably coherent alternative set of physical rules that were accepted by educated people. That's not the same as taking a random sample of folk beliefs or popular misconceptions and asserting that the world must work that way. In the case of evaporation, educated people in the 13th century certainly understood the process well enough to perform basic operations.

Serf's Parma, but I think an Ig requisite is needed to change the temperature, and just using Rego will result in warm ice. (Obviously from a real-world perspective that isn't possible, but it's probably not outside Mythic paradigm... Though it'd likely melt fast if the Rego spell doesn't have an ongoing duration.)