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I have a world setting that contains a variety of underwater species, some crustacean, some cephalopodic, and other varieties. Different species are adapted to different depths, and a few are amphibious. Part of their culture is tied to the ability to metalwork. Not a big part but its an important one.

There have been a fair number of posts on similar topics on this site and I've checked over most of them. I'm aware of the concepts of electroplating and electroforming, and obviously I could just leave it up to the amphibians, but frankly I'd rather not. I know cold forging is also feasible, but limited, and that certain metals have properties that protect them from corrosion, such as aluminum and to some degree copper. These are all things that could fit for many forms of common metal craft, but my merfolk need to be somewhat better than common.

First I thought of thermite just because it burns underwater, and could bring heat to the process. But then I found it leaves metal residue. I did some research and found there has been some effort to forge weapons and such by burning thermite into a mold. Backyard scientist on youtube has a video on it. The world is set in a kind of a steampunk industrial revolution that has a different flavor depending on the race. I've been having a hard time with the merfolk's flavor, I don't want to resort to some made up material or fuel, but they need to be good metalworkers.

Specifically, they're chain makers. They use hooks and chains to attack any ship that dare sail their waters, capsizing or tearing them apart. But also, their capitol is something of a world wonder, a buoyant city held down by massive chains, designed so that species of all depths can coexist to a degree. so their chains need to be strong. I'm hoping this leads to a solution for many peoples concerns of underwater forging. I know only select metals can be used to make thermite. However among them are components for corrosion resistant stainless steel. Such as iron (obviously), chromium, and manganese. Lead and copper are on the list too, although I know at least copper is particularly volatile.

So my questions are this (sorry for the long lead up):

  1. Assuming the species has access to electricity in some form (as discussed in other threads) am I correct in thinking thermite forging underwater is plausible

  2. Is thermite metallurgy remotely plausible? (I.E. mixing the oxide powders and melting them together.)

  3. How necessary is it to worry about protecting from corrosion if most of the metalwork never leaves the water, at least not the stuff that has to last.

EDIT: With bioelectricity I would think ll they would need is magnesium at that point to act as a fuse. Since both substances can burn underwater. I also didn't mention manipulators such as hands or tentacles, which would naturally have to occur in a race capable of working metal.

Also as it has been addressed below, despite researching most of the rest of the details, I had the oversight of failing to research if the requisite metals were naturally occurring. That being said, I figure the ability to do this would be something of a cultural goal, a way to make metal in a "pure" way from their perspective. I know its not as practical, but wanted to know if its possible, even if it has to be with considerable effort. However I also believe this has been answered as a wildly impractical potentiality. Which has satisfied my questioning

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    $\begingroup$ Where does the finely powdered rust and aluminum oxide come from? $\endgroup$
    – RonJohn
    Apr 3, 2018 at 3:41
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    $\begingroup$ Why do the smelting in the water, if you do not want to do it at the sea level surface why not construct a dive bell large enough to hold the metalworks and fill it with gas? Then they could have there metal production in their city without the bother of smelting in water. This is similar to us using wats of liquid to do some industrial processes. The gas for the bell could be either transported from the surface or produced chemically. $\endgroup$
    – lijat
    Apr 3, 2018 at 5:50
  • $\begingroup$ Well based on their culture (which would take some time to get into) it would have almost religious significance, finding a way to make metal that has never been "tainted by leaving the sea." That's the idea. But like with most things, especially forging, a strictly underwater setting is the mother of monkey wrenches $\endgroup$
    – SkyBandit
    Apr 3, 2018 at 16:12
  • $\begingroup$ Have them make air pockets in underwater caves, or do the metalwork on the surface $\endgroup$
    – Innovine
    May 11, 2019 at 9:56
  • $\begingroup$ Aluminum used to be a precious metal. That's why, for instance, the Washington Monument was capped with a piece of this rare and expensive substance. $\endgroup$
    – Ray Butterworth
    May 11, 2019 at 13:30

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No, forging metal underwater simply will not work because water is just too good of a thermal conductor.

Forging is using heat to create plastic deformation of iron or other metal; just getting the metal hot is not sufficient; you need to keep it hot while working it. Water heated to near boiling is used for quenching (heat tempering metal); this is the process of rapidly cooling hot metal by dropping it into water. The mere presence of water will cool metal below working temperature. Water also tends to cool rapidly and is only useful for certain alloys. Cooling heated metal too quickly causes the metal to become brittle. That is why heated oils are more commonly used for quenching; it creates a slower temperature drop than water can. You can't mix of liquid water and forging heat; the working temperature of metals is just too high compared to the boiling point of water, even trying will just weaken the metal. Forging works in air because air is a very good temperature insulator and this slows the cooling of heated metal.

Casting techniques will not develop underwater for the same reason; the water will cool metal too rapidly for it to remain liquid, which is required for it to flow into and fill a casting mold, they will never develop metal casting so they can discover more advanced metal methods that might work.

Even trying to forge metal underwater has another problem; whoever is doing the metallurgy will be boiled alive because they will be heating all the water around, including the water they are in. Again, water is too good a thermal conductor to keep the heat isolated.

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  • $\begingroup$ Well the going real world method of thermite forging (which has been only a few attempts really). Is an entirely sealed process for the most part, it has to be otherwise the thermite will just fly everywhere. to put it simply, you put the thermite in a specialized funnel, put that on top of a crucible holding the mold thoroughly sealed, and then just burn it. it melts into the mold and can then be broken out after its cooled. So that could be done underwater I figure, but as you point out, heat treating is a big problem, and then so is getting thermite in the first place underwater. $\endgroup$
    – SkyBandit
    Apr 3, 2018 at 16:20
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    $\begingroup$ that's not forging that is casting, and only works becasue the mold is full of easily displaced insulating air. If the mold was full of water instead it would just plug as soon as the molten metal hit the water and cooled, It would also cause a steam explosion. This also relies on you having already smelted iron into steel $\endgroup$
    – John
    Apr 4, 2018 at 4:38
  • $\begingroup$ The other problem with forging underwater is the movement of your hammer. With the water density, etc it will take more strength to get "up to speed" to provide the correct kinetic energy to actually shape the metal, assuming you've gotten the temperature/etc issues solved. $\endgroup$
    – ivanivan
    May 12, 2019 at 5:16
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It's not going to happen. Trying to make fire underwater isn't worth the effort and you have to skip all the early steps in forging. You can't go from nothing to thermite in a single technological step.

It's much easier to do it above water. You'd have poolside smelters where the workers hop out or use long handled tools from in the water.

Both crustaceans and cephalopods have the ability to stay out of the water for at least short periods. They can mine coal and ore underwater and then smelt above water.

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    $\begingroup$ Well no I wouldn't imagine it being a single tech step. I recognize its not easier by any means, and you're right I suppose it would start as poolside forges. But I could see it being almost like a cultural challenge to find a way to forge something underwater, and it being culturally significant to do so. They definitely wouldn't start that way, and it would be difficult, but would it be possible, given adequate access to bio electricity to light the magnesium and thus the thermite? $\endgroup$
    – SkyBandit
    Apr 3, 2018 at 5:08
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    $\begingroup$ Where's the aluminium, iron oxide and magnesium coming from to make said thermite? Again you're back to poolside smelters. $\endgroup$
    – Thorne
    Apr 3, 2018 at 5:12
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    $\begingroup$ Not just smelting, but purifying, crushing, sifting, more crushing and sifting, etc. All near the water, where you'll get paste instead of powder. $\endgroup$
    – RonJohn
    Apr 3, 2018 at 5:22
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    $\begingroup$ Also, you need lots of electricity to split aluminum from bauxite. $\endgroup$
    – RonJohn
    Apr 3, 2018 at 5:23
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    $\begingroup$ Electricity in salt water would be such fun for all involved....... $\endgroup$
    – Thorne
    Apr 3, 2018 at 6:10
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Chlorine Trifluoride would probably be better for burning underwater - heck, it might even burn the water. But with enough safety equipment, it might be doable. Maybe.

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  • $\begingroup$ Well it'll definitely burn, probably be a lot more controlled than geothermic vents too, but wouldn't you risk corroding the metal during the forging process? and the substance may just explode anyway it sounds like. :( The thermite seems to be the most viable thing I've come across $\endgroup$
    – SkyBandit
    Apr 3, 2018 at 2:56
  • $\begingroup$ Yeah, possibly. Since this isn't commonly done, the only way to check the viability of thermite would be with thermochemistry, or so I think. I'm afraid I can't help you there, sorry. $\endgroup$
    – Simon H.
    Apr 3, 2018 at 3:03
  • $\begingroup$ Where does the ClF3 come from? How do crustaceans make the safety equipment? $\endgroup$
    – RonJohn
    Apr 3, 2018 at 3:43
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    $\begingroup$ I contest the assertion that crustaceans and cephalopods can make thermite. $\endgroup$
    – RonJohn
    Apr 3, 2018 at 5:14
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    $\begingroup$ @SkyBandit it took 150,000 years and thousands of vital intermediary steps for humans to figure out how to make powdered aluminum, and most all of them would be ruined by water. $\endgroup$
    – RonJohn
    Apr 3, 2018 at 5:26
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Is making a underwater watertight furnace, without water inside, feasible? Perhaps you can interact with the furnace trough membranes/compartments and allow the forging process remain waterless, but all your merfolks still in the water.

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The only way you could smelt metals and keep them at the right temperature would be to work at almost contact with a submarine volcano, or a hotspot.

Unfortunately, this also means your seafolk would boil alive pronto.

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Though I am not a expert in any manner I would like to chime in my 2 cents here. While it seems impossible to make a forge or use chemicals to create enough heat to smelt underwater you could in theory use the volcanic vents at the oceans depths to heat metal and possibly shape it from the force of the water as it rapidly heats and shoots up, now other than manipulating the forces as play you wouldn't be able to make precise tools but makeshift spears and swords maybe. You couldn't use a current to generate power as water conducts electricity. Maybe again at the oceans depths where you have other chemicals than just water you could somehow manipulate metals because of pressure or what not but I don't think we know enough about that world to say one way or the other.

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