Is there any reason to use tungsten crucibles?

Question

So, in my setting, there are dwarves that make good steel. Shocker, I know. To refine the ore, they rely on the bloomery process like everyone else, using anthracite (when they can get it), charcoal (the normal fuel) or bituminous coal (when they can't get a hold of anthracite or charcoal). From there, most of the more skilled blacksmiths use the crucible process to refine their iron blooms further, although only a minority are able to control the carbon content well enough to make proper steel consistently. For most dwarf-smiths, particularly the younger ones, the process is shrouded in superstition that prevents them from being able to make the highest quality steels (e.g. one must use the knuckle-bone of certain creatures for flux, using the blood of strong beasts, etc)

But the most skilled smiths are also wizards, able to manipulate the heat around them. Typically, they pull heat out of magma flows and geysers underneath the mountain tunnels they inhabit, and direct and concentrate it into the crucibles to get a more consistent, faster, and cheaper melt. These wizards are also capable of refining certain metals out of ores that would otherwise be impossible to produce with their (late medieval) technology, e.g. aluminum or tungsten.

This brings me to my question. Is there a reason that he might make his crucibles out of tungsten, instead of using more traditional clay or clay-graphite crucibles?

For the sake of argument, let's set a few parameters. This master dwarf-smith, although he can use magic to refine, melt, and shape any metal, can only do it a little bit at a time, and doing so drains his magical energy stores for weeks on end. Thus, though he can't use it to refine steel directly, he can produce crucibles with the skill. Further, although they're aware of graphite and have been known to produce graphite crucibles, there aren't any good deposits of it anywhere near their lands, while there are some good seams of tungsten ore. Lastly, dwarves have a lot of experience working tungsten using magic, and due to its extremely high magical conductance (jargon, I know), it's widely used among dwarves for magic wands and staves.

Answer 1

Two downsides to working with tungsten metal crucibles. The first is that tungsten has higher affinity for oxygen than iron, so oxygen in the liquid iron will react with the tungsten. Unfortunately, tungsten oxide is not coherent with tungsten, so the oxide will spall off as it forms, and get into the steel. Because the tungsten oxide isnt controlled in terms of particle size and shape, it is unlikely to benefit the steel properties and will probably embrittle the steel. The crucible will also eventuly fail from corrosion. Note that sapphire, made from aluminum oxide, has higher affinity for oxygen than tungsten, and so sapphire forming is a reasonable application for tungsten crucibles.

The other major downside is that tungsten metal is challenging to shape. Because it has high affinity for oxygen, higher than the carbon in carbon dioxide, and melts at a very high temperature, casting it is practically out of the question. Unless of course the dwarves have a high vacuum or enclosed inert gas melting system and something to hold the liquid tungsten. It is possible to powderize the tungsten, put it in a meltable binder and form the crucible like with clay, in a process called powder processing. Once the tungsten clay is formed, it can be readily shaped, the binder melted off, and the remaining green form, all tungsten, sintered. Sintering can be done at a lower temperature than melting, but without a high pressure, inert gas furnace, I would be concerned about oxide formation and inability to close up porosity where the binder was. Collectively that could lead to a brittle or porous tungsten crucible.

For holding molten iron those of us in the real world would stick to trusty stable oxide materials like alumina, magnesia, and silica. In other words, clay! Of course this is your world, so the choice is yours!

Edit: As an addition, if they can refine most metals, why not produce tungsten carbides, and mix the carbides into cobalt metal to produce advanced machining tools? Controlling carbide particle size is a challenge of course. That would perhaps be a better use of tungsten.

Answer 2

I have a minor hobby interest in this topic, so I can tell you that both clay and tungsten crucibles exist and are in practical use. I am by no means an expert in this topic.

There are likely many benefits, as pointed out by the other answers, but the primary reasons that I know of are:

1. Tungsten will be able to tolerate much higher heat.
2. Tungsten will be much sturdier.

While clay can tolerate very high heats, I have read that you can ruin clay if you get it too hot, and the temperatures that are "too hot" for clay are much lower than tungsten melting point. I have never subjected clay to high enough temperatures for long enough to do this, but I have never cast steel before either.

Even if the clay can tolerate the temperatures that your smiths produce, the clay will not last as long as the tungsten. If they want to keep replacing their clay tools periodically, then they are free to do so. Some people go this route in reality, continually replacing cheap tools. The ones who want a tool that they can rely on for a very long time will likely use tungsten.

People who do this or similar work on a large scale sometimes measure their tools, even their crucibles and molds, in mean number of uses before tool failure. In this sense, the mean use before failure will be lower for the clay, higher for the tungsten. A clay mold is expected to last for X uses, graphite for Y uses, etc..

Now I am going to try to get in character and think what I would do in your situation...

If I am doing my normal day to day work and involves melting iron, personally I would want the tungsten crucible, and if casting it I would prefer tungsten molds for items I cast very often but would use clay molds for items I will not cast many of.

If I am doing experimental work which includes working with different materials or at different temperatures or alloys, I might prefer to use clay crucibles and molds because they are more likely to get ruined (ie: I want cheap stuff if it will get ruined), and I might be trashing the special-use crucibles or molds after.

This assumes I am well off and can just do whatever I want. If I'm a younger and/or poorer smith, then the cost of tungsten materials would likely put me off and I would just do clay for everything. Clay is cheap. Cheap to mine (anyone can extract it from dirt with cheap tools, though it can be very tedious), cheap to form into what you need, and cheap to fire. Tungsten would be the opposite: expensive to mine and expensive to form into the tool.

And there are other cases where I would make one decision or another.

Disclaimer: As I said, I'm not an expert. I have done some hobby work (nothing serious, just playing around with it) with plastics, metals with lower melting points than iron, and glass. I have never used a tungsten crucible, and I would consider them too expensive for my personal use in reality.

So the real, nuanced answer is likely that you will find practitioners in your world using all the different available options. Their choice will be based on their attitude toward throw-away tools vs. lifetime-use tools, and on their budget, the specific project and use case, etc.. Different ones will have different preferences, and some will have multiple crucibles or molds owned at the same time made out of different materials. You will see all the variations, just like I own several different kinds of saws and drills, and my neighbor owns still different ones.

Answer 3

I just did a DuckDuckGo search for tungsten crucibles and found a few sites offering them. One claims:

Tungsten crucible and other parts can be used for sapphire crystal growth and rare-earth melting due to their high temperature resistance, low pollution and other excellent characteristics.

Another supplier says tungsten crucibles are:

Ideal for use in electron beam equipment.

Another supplier offers molybdenum and tungsten crucibles:

for leading sapphire growth furnace OEMs and crystal growers around the world. Corrosion resistance, creep resistance, and high temperature performance make these materials ideal for use in high temperature crystal growth processes.

A fourth gives the advantages:

Advantages of tungsten crucible: low thermal expansion; high density;erosion resistance;high strength and low resistivity

And lists possible uses:

Since the temperature in working environment of tungsten crucible is above 2000℃, it commonly used for sapphire single crystal growth furnace

Applied for quartz glass melting furnace

Used for rare earth smelting furnace

Used for sintering metal mold of high melting point

Tungsten crucibles are also widely used in other industries like ceramics and metallurgical industries, machinery processing and light industries.

In conclusion, I know very little about the chemistry, but real-world evidence proves that there are uses for these. Looks like your smiths might use them for working with metals like tungsten and molybdenum that have high melting points, or for sintering (a technique involving forming metal powders into shape without melting them), or for growing crystals.

Answer 4

I would be in favour of having, and using, tungsten crucibles under these circumstances for at least the following two reasons:

• extremely high temperatures can be used without damaging the crucible, this means that in the event that too much magically concentrated heat is applied you aren't ruining your equipment nearly as often, it also means that you can perform melting operations that would otherwise be impossible without having to expend a lot of magical power.

• uniformity of results, rammed-fused crucibles whether made of clay or more advanced materials are very rarely uniform in the way they turn out because they're made of materials that aren't uniform, a cast crucible is far more uniform and so yields far more uniform results allowing masters with such objects to produce more consistent quality work.

Answer 5

Just as a thought, if they use magic, they might find value in rhenium or tantalum crucibles. They have slightly lower melting points than tungsten, but some other properties are different and might be worth it. Tantalum is corrosion-resistant and rhenium is strong.

Alloys of various sorts might be useful. If magic makes it easier to find sufficient amounts of rare earths, then they could be more practical to use than they are for us.

If your smiths are doing craft-scale work, different smiths may have access to different rare metals. A smith who does a lot of work with copper would have access to copper impurities, while one who collects meteorites would have iridium etc. (Some special-purpose crucibles are made from iridium alloys.)

If it's craft scale work, not unlikely each master has his own special skills he hasn't particularly shared, and having special crucibles with special properties could be a mark of pride. They might have occasional arguments about which is better for some particular arcane purpose, with the agreement that they all have something adequate.

Rather than accept that each smith's knowledge dies with him, the most skilled might possibly share results. Perhaps only with each other, so that to get access a young smith must first share something useful the others don't already have.

A magical smith who wants to exclude oxygen could mostly do that. For example, for any air inputs first burn an excess of something which does not burn to a gas. (My first thought was magnesium, but you'd need to keep magnesium oxide powder away from your molten metal.) Then remaining oxygen would mostly come from the metal being refined. In that case, he could use a diamond crucible. Or even graphene. Or tungsten coated with diamond or graphene.

If you have a giant mob of smiths doing cottage-scale work, then the mass of the work must be done cheap. Most smiths will spend all their time using the cheapest techniques, and there is no need for them to learn anything else. If they can get economy of scale on the cheap stuff, then fewer actual smiths are needed for things like refining metal. Cheap smiths can take ingots of copper, steel, etc (or partly finished products) and shape them. Expensive smiths can do specialty work.

I imagine foundries that melt many tons of copper or iron at once. Much easier to collect rare-earth impurities from tons, than from many batches of a few pounds at a time. So the master smiths in charge of major foundries would have special advantages.

But maybe it turns out that the specialty stuff is mostly good for bragging rights, and what's important is finding cheaper ways to do the bulk work.

Answer 6

Thinking about this, you have magical smiths whose magic is a limiting resource. They can do wonderful things with magic, but only so much. So much gets done without the magic.

They would naturally tend to save the magic for only the most cost-effective uses.

I tend to assume that knowing is cheaper than digging. So smiths who can predict the best places to dig for ore, are more valuable than smiths who do bulk magical digging. On the other hand, smiths who make limited amounts of magical dynamite could be worth a lot. Easier than constantly replacing their steel chisels.

It's generally more cost-effective to use magic to make tools that will be used many times, than to put it into individual products.

The single most cost-effective use for smith-type magic: Growing large, flawless sapphires and rubies. Advanced smiths need to know a lot about crystallization so this is a natural for them. Rubies and sapphires are varieties of corundum which is valuable for other purposes. A cheap magical method to make corundum falls into tool creation.

However, gemstones are only valuable when they are rare. The smith who has the right to do that will not make too many of them. He may have a few hundred stored away, so he could sell them if necessary without having to make them on the spot.... And that's one use for a tungsten etc crucible.

If he has a forge in a volcano, it's very useful for him to predict when the volcano will explode. Early enough to escape. Even better if he has the skill to make a tiny change which lets the volcano adjust itself without exploding, and gently pour lava in a harmless direction. He might pay miners to dig a tunnel that will do most of the work, while he does only the last little bit when it is needed. Still potentially dangerous work for them. And a smith who is not quite as competent as everybody thought, shows it by dying along with the people who most trusted him.

If he is advanced enough then for many purposes he doesn't need a crucible. He can simply no-hands place his work (packed in inert material) into the magma, wait until it is the right temperature, and remove it. For big crystals he heats until the material has liquified, cools it some and waits for the crystals to form (possibly speeding the formation with magic) and then removes them.

Less-advanced smiths who do not have volcanoes handy have the problem of controlling carbon in their steel. Their heat comes from burning carbon. Bits of carbon ash can get into their products, along with CO and CO2 and unburned carbon compounds. They must buy mined coal, or created charcoal etc.

One way to control carbon is to create the heat some other way. For example, thermite. Or magnesium. Of course you have to get the energy to make the thermite or magnesium first, but maybe that step doesn't have to be controlled so much for carbon. I think you can't just get magnesium with electrolysis because water will split first, but maybe you could find something that magnesium oxide would dissolve in, and do electrolysis in that. Ammonia? After you do your work and it cools off, you send workers in with brooms to collect magnesium oxide to reconstitute in the ammonia baths. Yum. smith's employees at work

A dwarf smith might want to hire a lot of big dumb humans for the grunt work. He only does the parts that need his special skills. And if he doesn't have the special skills to manage a big herd of human workers, he might hire a manager who does.