Did you know that actual historical alchemy was often banned by various kings and monarchs? They did so not due to superstition, or because alchemy didn't work. Rather, they banned alchemy because it DID work.

We now know that you cannot use chemical reactions, however complex, to turn base metals like lead or copper into silver or gold. However, you can use alchemy to give these base metals the appearance of silver or gold. Alchemists could coat coins in durable coatings that would appear to be like silver or gold. Dip a copper coin in the right solution and it will take on the appearance of gold. And you can then take that coin out of the solution, clean it thoroughly, and the faux-gold treatment will remain. It's not just a layer of paint resting on the surface; the upper layers of copper atoms have actually chemically reacted to produce compounds that give the appearance of gold or silver.

So, even though alchemy didn't work to truly turn lead into gold, from the perspective of a monarch, that didn't actually matter. Because when it comes to currency debasement, making a fake gold coin so good that it fools people is just as good as making real gold. The alchemists couldn't turn create real gold coins, but they could create counterfeit gold coins that could be quite convincing in the right circumstances. They didn't need to create a forgery that could fool a modern PhD chemist with a lab full of equipment; they just needed something that could fool an illiterate 12th century merchant at his shop. The process:

1. Take a mold or press a stamp of one of the king's official gold coins.

2. Use the mold or stamp to cast, press, or forge coins out of cheap metals like copper or tin.

3. Apply an alchemical process to make the copper or tin coin look like gold.

4. Spend the counterfeit coin as a real coin.

Coins were a better target than bulk gold like bars. With a bar, you would notice that the "gold" has an incorrect density. But a counterfeit coin, mixed in with a larger number of legitimate coins? Easy to pass off as the genuine article.

Kings often banned alchemists from their realms. Practicing alchemy was often a capital offense. In terms of true elemental transfiguration, alchemy failed. In terms of the ability to create spendable wealth from nothing, alchemy absolutely did work. From the perspective of a monarch looking to protect their currency from debasement, alchemy was a very real threat.