‘These are the fires that make water turn to wine,’ he explains. ‘That make iron turn to gold, that make earth turn to life. Everything in this world is growing to a state of purity, of perfection. This is where we speed it up, this is where we make the changes to metals and to waters that the world itself does in its deep bowels, over centuries, with heat like hatching an egg into a chicken. We make it hotter, we make it faster. This is where we can test what we know, and see what we learn. This is the heart of my life’s work.’

Outside in the yard someone pulls a red-hot bar of metal from the coals of the furnace, and starts to hammer it flat.

‘Just think if I could make gold,’ he says longingly. ‘If I could take iron and purify it so that the com traces were burned from it, washed away, so that we had gold coin. I could hire soldiers, I could reinforce defences, I could feed Paris. If I had my own mint and my own gold mine I could take all of France for my nephew and keep it forever.’

‘Is it possible?’

‘We know it can be done,’ he says. ‘Indeed, it has been done, many, many times though always in secret. All metal is of the same nature, everything is made of the same thing: “first matter”, they call it in the books, “dark matter”. This is the stuff that the world is made of. We have to remake it, make it again. So we take dark matter, and we refine it, and refine it again. We make it transform into its purest and best nature.’ He pauses, looking at my puzzled face. ‘You know that they make wine with the juice that comes from grapes?’

I nod.

‘Any French peasant can do that. First he takes the grapes, then he crushes them for their juice. He takes a fruit – a solid thing, growing on a vine – and makes it into a liquid. That’s alchemy itself, making that change. Then he stores it and lets the life within it change that juice into wine. Another liquid but one with quite different properties from the juice. Now I can go further. I have done another change, right here. I can make an essence from wine that is a hundred times stronger than wine, which burns at the sight of a flame, which cures a man of melancholy and watery humours. It is a liquid but it is hot and dry. We call it aqua vitae – the water of life. All this I can do already, I can change juice to aqua vitae – the gold is just the next step, gold from iron.’

‘And what am I to do?’ I ask nervously.

‘Nothing today,’ he says. ‘But perhaps tomorrow, or the next day, they will need you to come and pour some liquid from a flask, or stir a bowl, or sieve some dust. Nothing more than that, you could have done as much in your mother’s dairy.’