Why didn't Thorin & Co use Mithril armor in the ⚔️Battle of Five Armies?

[MPetkovic01]

Somehow, I like the idea of mithril being exceedingly rare. So, I'd be happy if only the two guards standing at the door of the citadel had mithril helms while on duty. If one mithril jacket is valued the same as one Shire, how many Shires could you buy with 250 mithril helmets? 😊

I guess that just surface of helmets was made of Mithril.

 

[Glorthanion]

Mithril was exceedingly rare certainly. It was, according to Tolkien, mined only in Moria and in Numenor, although I believe in a letter he says it may have been mined in Aman. So, let's deal with the products of the two sources...

Most of what was mined in Numenor presumably sank when Numenor fell. One Numenorian king was named "silver handed", I believe the silver actually being true-silver, aka Mithril. I speculate that the couple of hundred helmets that were worn by the Minas Tirith guard may have been Mithril helmets that were brought from Numenor by Elendil and his followers, that would certainly fit the description "but the helms gleamed with a flame of silver, for they were indeed wrought of mithril, heirlooms from the glory of old days." It's also possible the helms were only decorated with mithril, if not originally, then perhaps originally solid mithril helms were melted down to make mithril available for other uses (if only to make more helmets with less actual mithril in them) once it became exceedingly rare and valuable.

Most of what was made in Moria was presumably horded by the Dwarves, kept for use in only the finest and rarest of treasures, and only grudgingly traded or gifted. We do know that the Elves valued it and a portion of what was produced in Moria was traded to the Elves. That portion was used for making Ithildin, at least one of the rings of power (Nenya, Galadriel's ring), and of course the famous mithril shirt that Thorin gifts to Bilbo.

Whatever portion of the mithril that remained in Moria was lost when the Balrog arose and Khazad-Dum was abandoned. We are told that Sauron gathered most of the mithril of Middle Earth to himself, although for what reason is never given, but its value is obvious, so greed would be sufficient reason.

The mithril shirt was made for an unknown Elven prince, but I expect it was not Legolas. It was more likely made much earlier, likely for a young prince of Eregion at a time when mithril was in production, and so while rare and valuable, it was not nearly as much so as it was by the time of the late Third Age. I doubt it was ever in Thranduil's possession as he only claims the White Gems of Lasgalen which he had given to the Dwarves to make into a necklace for his wife, but which he never received back due to a payment dispute, and then of course, Smaug setting up shop in Erebor. If he had also commissioned the Mithril shirt for Legolas then surely he would have mentioned that in his claim for treasure from Erebor. I also think that Thranduil just was not rich enough that he could have afforded to buy the mithril shirt, or if it was actually made at Erebor, he couldn't have afforded the price of that much mithril. The Woodland realm just doesn't sound like it is that wealthy a realm. If anything, they were likely in a trade deficit situation - whatever they were paying the Lakemen for the deliveries of wine and fruit, it wasn't being put into barrels that were being returned when emptied. I think it's quite possible that Thranduil was having a hard time keeping himself in style, Tolkien doesn't mention anything that they were trading anyways. They may have been collecting tolls on the forest road at one time and I expect they could do some business in game and hides, and likely shipping wood to Erebor, but that's never actually mentioned, although floating logs down river would have been easily viable if barrels worked.

So, to sum it up, while we don't know how the mithril shirt ended up in Erebor, I expect that it was the only significant mithril artifact in Erebor and almost certainly the only armor - unless there happened to be a matching helmet or the like that was misplaced somewhere. Mithril was not mined in Erebor, Tolkien says most of what was in Middle Earth was gathered up by Sauron over the millenia, and so very little likely made it to Erebor or the other dwarf kingdoms.

 

[Deimos]

1) The dwarves made armour that was second to none in wearability and durability, and also beautiful (works of art).
2) Mithril was scarce, difficult to locate and mine and so astronomically expensive; i.e rare and exceedingly valuable , as others have said.

Making armour from mithril would gain them marginally , very marginally, more protection than their own "run-of the mill" armour.
Admittedly one advantage to it would be a significant reduction in weight.
But dwarves are hardy and they have long experience in making armour that is neither unnecessaily heavy, nor cumbersome, yet still effectively protective.
And the amounts of mithril needed would make it totally unfeasible. They would have to spend way too much time locating it, and then mining it.