Is there evidence which supports the existence of gunpowder in Middle-earth?

Question

Gunpowder is often referred to as the source of Gandalf's fireworks. But, it is never clearly stated what Gandalf's fireworks were made up of.

The fireworks were by Gandalf: they were not only brought by him, but designed and made by him; and the special effects, set pieces, and flights of rockets were let off by him. But there was also a generous distribution of squibs, crackers, backarappers, sparklers, torches, dwarf-candles, elf-fountains, goblin-barkers and thunder-claps.

A more likely possibility is that the fireworks were made with the power of Narya, the Ring of Fire, but that too is not confirmed.

Another reference to gunpowder is through Saruman's inventions.

Even as they spoke there came a blare of trumpets. Then there was a crash and a flash of flame and smoke. The waters of the Deeping-stream poured out hissing and foaming: they were choked no longer, a gaping hole was blasted in the wall. A host of dark shapes poured in.

‘Devilry of Saruman!’ cried Aragorn.

It is never said that this 'bomb' was made up of gunpowder or Saruman's magical capabilities, but Peter Jackson used the former in the 2nd film.

Of course, this is completely non-canonical and doesn't prove that gunpowder exists in Tolkien's Middle-earth.

Also, when Isengard was attacked by the Ents:

‘Isengard began to fill up with black creeping streams and pools. They glittered in the last light of the Moon, as they spread over the plain. Every now and then the waters found their way down into some shaft or spouthole. Great white steams hissed up. Smoke rose in billows. There were explosions and gusts of fire. One great coil of vapour went whirling up, twisting round and round Orthanc, until it looked like a tall peak of cloud, fiery underneath and moonlit above. And still more water poured in, until at last Isengard looked like a huge flat saucepan, all steaming and bubbling.’

Also, during the Siege of Gondor:

But the engines did not waste shot upon the indomitable wall. It was no brigand or orc-chieftain that ordered the assault upon the Lord of Mordor’s greatest foe. A power and mind of malice guided it. As soon as the great catapults were set, with many yells and the creaking of rope and winch, they began to throw missiles marvellously high, so that they passed right above the battlement and fell thudding within the first circle of the City; and many of them by some secret art burst into flame as they came toppling down.

But again, though speculated, it is never stated in these 2 instances that it was gunpowder,

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Therefore, is it even plausible to say that gunpowder exists in Middle-earth?

Note: This is not a duplicate of: Why wasn't gunpowder more common in Middle Earth? I am asking about the evidence concerning the existence of gunpowder. Whereas the accepted answer in the linked post merely describes the existence of magic and it's different forms in Tolkien's legendarium and does nothing to describe the existence of gunpowder. The answer by Dronz is reasonable, but it seems speculative and there was no evidence given.

Answer 1

It's possible, but also likely is just magical

There's evidence to suggest gunpowder existed in Middle-earth, but it's only mentioned explicitly on a one occasion (within Tolkien's Legendarium¹).

This is in The Hobbit when Gandalf's explosion of light, to distract the orcs and escape, is described as smelling like gunpowder. (Emphasis mine)

“But not Gandalf. Bilbo’s yell had done that much good. It had wakened him up wide in a splintered second, and when goblins came to grab him, there was a terrific flash like lightning in the cave, a smell like gunpowder, and several of them fell dead.
_{The Hobbit - Chapter IV, Over Hill and Under Hill}

However, after this everything is left to subtleties. Tolkien left it vague enough that it could be some form of "Magical Engineering". However, whether he intended it to be gunpowder, or a similar magical or fantastical invention to stray minds away from the early invention, the evidence of the existence of something of the like is extensive.

To begin, in the Battle of Isengard, after the Ents flooded the ring, Saruman's machinery is described as having exploded.

Isengard began to fill up with black creeping streams and pools. They glittered in the last light of the Moon, as they spread over the plain. Every now and then the waters found their way down into some shaft or spouthole. Great white steams hissed up. Smoke rose in billows. There were explosions and gusts of fire. One great coil of vapour went whirling up, twisting round and round Orthanc, until it looked like a tall peak of cloud, fiery underneath and moonlit above. And still more water poured in, until at last Isengard looked like a huge flat saucepan, all steaming and bubbling.
_{The Two Towers: Book Three - Chapter IX, Flotsam and Jetsam}

Continuing on earlier into the same book, when the walls of the Hornburg are breached. The explosion is described as having a blast of flame and smoke.

Even as they spoke there came a blare of trumpets. Then there was a crash and a flash of flame and smoke. The waters of the Deeping-stream poured out hissing and foaming: they were choked no longer, a gaping hole was blasted in the wall. A host of dark shapes poured in.

‘Devilry of Saruman!’ cried Aragorn. ‘They have crept in the culvert again, while we talked, and they have lit the fire of Orthanc beneath our feet.
_{The Two Towers: Book Three - Chapter VII, Helm's Deep}

[...]

They have a blasting fire, and with it they took the Wall.
_ibid.

Again although this could be argued to be something other than gunpowder, if it was magic, it would be strange for the orcs to have been able to set it off themselves.²

Finally, further references to "blasting" are found in The Return of the King

‘They have taken the wall!’ men cried. ‘They are blasting breaches in it. They are coming!’
_{The Return of the King: Book Five - Chapter IV, The Siege of Gondor}

[...]

But the engines did not waste shot upon the indomitable wall. [...] As soon as the great catapults were set, with many yells and the creaking of rope and winch, they began to throw missiles marvellously high, so that they passed right above the battlement and fell thudding within the first circle of the City; and many of them by some secret art burst into flame as they came toppling down.
_ibid.

The Return of the King, again, does nothing to suggest this has to be gunpowder, however this time the orcs are using inventions of Sauron, and unless Sauron and Saruman had shared their magic, it seems more likely that they had a common weapon, gun powder. I am of this opinion as both were very skilled Maiar of the Valar Aulë, known as The Smith of the Valar.

The final argument for the existence of gunpowder is it Gandalf's fireworks. Although entirely plausible these are entirely magical, it is likely, from the above examples that there is some gunpowder in them.

... the special effects, set pieces, and flights of rockets were let off by him.
_{The Fellowship of the Ring: Book One - Chapter I, An Unexpected Party}

The distinguishing of the three parts of the fireworks suggest that they were different. From films, we see that the special effects are likely magical, as a giant Dragon flies around as a firework. However the rockets themselves are likely powered by some form of gunpowder. Gandalf is described as being an expert of pyrotechnics, which suggest possibly "Magical Pyrotechnics" but not necessarily.

In conclusion, I am of the opinion, the Tolkien's inclusion of explosions, likely from his involvement at the Somme, was inspired by gunpowder but left vague enough to be described as the aforementioned "Magical Engineering" to stray doubts of the extremely early existence of gunpowder.

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¹ Fireworks are mention in Farmer Giles of Ham as used in fireworks, this is however not part of the Legendarium, and therefore not canonical.

² From the following we can see that the explosive needed fire to be set off, this however is not necessarily canonical as it comes from one of Jackson's films.