How did an Alchemist turn silver into gold?

And how did he do it in front of a crowd that included Austria’s Emperor Leopold?

In centuries past, if a king wanted his face on a gold coin, he had to have some gold in his treasury. In 1677, Emperor Leopold’s gold ran out. He had nothing to pay the troops who were struggling to defend his empire against the Turks.

Then, on a day in November, a stranger arrived with an offer to solve the king’s money problems. He said he would turn silver into gold, and then tell the Emperor the secret, for a price... The cash-strapped Emperor agreed.

The stranger arranged to perform the demonstration on the steps on the palace. According to all accounts, there was no sleight of hand. The stranger held up an ornately embossed silver medallion for the Emperor to see. He dipped it into a cauldron. And then the medallion changed from silver to gold… as if by magic.

You might think this story can’t be true.

But in the coin gallery of the Kunsthishistoriches Museum that same gold medallion is on display. And though curators can’t discern the purity of the gold without destroying the medallion, a non-invasive X-Ray proves that gold is definitely present.

So how did the Alchemist do it?

Answer: Obviously the seemingly transmutation can be explained by the metals’ reaction since silver keeps its colour even in a strong alloy whereas only small additions suffice to change the appearance of gold significantly. The medallion, therefore, had a silvery appearance at first, the high amount of gold notwithstanding. When, in the presence of the Emperor, doused into the liquid, possibly nitric acid, silver and copper dissolved at the surface so that only gold was left. It looks as though the silver object had turned to pure gold. The top part, untouched by the liquid, was left unchanged and still exhibits the original bright colour of silver. 

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The title doesn't seem to have much to do with the story... unless?...

Unless one reads the embossed text of the medallion:

“Visita interiora terrae rectificando invenies occultum lapidem”,

which is an alchemical expression for VITRIOL (take the first letter of each word). It can be translated as:

"Visit the innermost of the Earth and by rectifying you will find the Hidden Stone”

That is some kind of a coincidence, isn't it? The vitriolic liquid turning silver into gold happens to be named after an expression talking about the search of the philosopher's stone.