In short, with Drach’s help, in a very brief space of time I hauled myself out of the pit I had inhabited and regained my place in the world.

I pulled back the copper lid, angling it so that the liquid did not boil over into the fire. Drach and I both had rags tied over our faces against the foul vapour that billowed out. He dipped the onion into the bubbling broth. The moment it touched the surface a brown scum erupted from the oil, frothing around the onion and racing up the sides of the cauldron.

‘Don’t let it over the rim!’

Drach pulled the onion away and I clamped the lid back down. ‘Too hot,’ he declared.

With a poker and tongs, I spread the coals beneath the trivet to burn cooler. When the boiling oil seemed less vigorous, we attempted the experiment with the onion again. This time the scum rose more slowly, blistering the skin of the vegetable but not threatening to spill over.

‘Perfect,’ Drach declared. While I held the lid open, he took a bowl of resin dust and sprinkled it over the surface of the oil with a ladle. Each time the dust touched the oil it provoked another belch of scum and foam, which needed rapid stirring to keep it from bubbling over into the fire and setting the whole cauldron ablaze. This was the most precarious part of the operation: not just because of the danger (which was considerable) but because of the way Drach wilfully courted it. Each ladleful of resin he added was more than the one before, prompting the foam to climb ever closer to the rim and me to stir with ever more desperation. Drach seemed to enjoy this hugely, like a child baiting a dog with a stick; I hated it. The fumes and exertion and fear all cloyed together to make me feel ill.

Gradually, the mixture thickened. When it was the consistency of soup and the colour of piss, we ladled it out of the cauldron into glass jars. While we waited for it to cool, we damped the fire and went down to the river.

I stripped off my trousers and dived in, kicking back so that I could watch Kaspar undress on the bank. The oil that had coated me drifted away in a foul-smelling slick; my anger went with it. I felt foolish for having allowed myself to become so irritated by his game.

Kaspar waded in and squatted in the shallows. For all his carelessness with fire, he had a strange fear of water. It was the one arena where I could outpace him, and I spent some minutes splashing about in the current, diving down and holding my breath to make him anxious. When I opened my eyes underwater, the sunlight shining through the reeds reminded me of days dredging gold out of the Rhine. I could not believe that had been my life.

I broke the surface and swam back to the bank. Kaspar had waded out so that the water almost reached his hips; he wore a petulant look that made me laugh with delight. Coquette that I was, I delighted in provoking his envy.

I swam round behind him and stood in the mud, sluicing water over his back and scrubbing away the soot and oil. His skin was taut, his shoulders beautifully firm from long hours of work. When he turned around, I sank beneath the water so he would not see my arousal.

We dressed and went back up to the house. We took the oil into the barn, where a pair of wooden tables had replaced the byres and straw, and spooned it out onto a stone slab. By now, the mixture had cooled to a greasy paste. An oyster shell beside it held a small mound of lamp soot, which we gradually stirred in. I watched the black swirl through the varnish, then dissolve into it.

Drach dipped a fingertip in and wiped it on a scrap of paper beside the slab. A black smear appeared on the paper, though as I watched the ink dry it faded to a duller grey. Despite all the effort we had lavished on it, I felt a grain of disappointment.

‘It should be darker. Stronger. Like real ink.’ I thought back to all my weeks in Tristan’s tower in Paris, chasing every hue of the rainbow. ‘Copper powder burns black if the flame is hot enough. If we mixed that with the lampblack, it might be more vivid. Perhaps red massicot too, to add depth.’

Drach looked peeved. He touched a finger to my lips to silence me. ‘This will do. After all, we do not have a press yet.’

Paris

‘Why did he lie?’

The train rattled back towards Paris. Night cloaked the suburbs around them: when Nick looked out the window, all he saw was his own watery reflection and Emily opposite, ghosts in the darkness.

He rephrased his question. ‘Why would he lie? Why pretend he never saw Gillian or the card?’

Emily shivered and pulled her coat closer around her. ‘He was so eager to put the card through his machine. He knew he wouldn’t get a match.’

‘Because he’d already analysed it with Gillian.’

‘But if he didn’t find anything…’

‘… why did he lie?’

The train shook as it crossed a set of points. A station flashed by.

‘I wonder why Gillian took it there.’

Nick looked at her, confused. ‘To analyse the ink.’

‘All Vandevelde’s work has been on printed type – books. But the first book wasn’t printed until about 1455. As far as we know, the cards date from about twenty years before. The cards are printed intaglio – the ink sits inside the grooves cut into a plate and is pressed into the paper. Type is printed in relief, with the ink on the raised surface of the letter. I don’t know for sure, but I’d think they’d use very different kinds of ink.’

‘So she went to a man who couldn’t help her and didn’t find anything – and that’s so secret he has to lie about it?’ The jetlag headache throbbed in his temples.

‘There must have been someone else,’ said Emily quietly. ‘Somebody was after Gillian – maybe they got to Vandevelde. Maybe that’s why he was so frightened.’

They got off at the next station. Nick found a payphone on the empty platform and dialled the second number from Gillian’s phone. His hands were shaking so badly he could barely get the coins in the slot. He told himself it was because of the cold.

The phone answered after three rings. ‘Atheldene.’

Was it a person? A company? A hotel? ‘Is Simon there?’

A guarded pause. ‘This is Simon Atheldene.’

It was a British voice, foreign yet unexpectedly familiar. Nick took a leap in the dark. ‘Do you work for Stevens Mathison? The auction house?’

‘I do.’

‘My name is Nick Ash. I’m a friend of Gillian Lockhart. I think I spoke to you a few days ago.’

Another pause. ‘Are you here in Paris?’

The payphone number must have displayed on Atheldene’s phone. ‘Yes.’

‘Then we should meet.’

Nick and Emily arrived at eight. For Nick, who had never been to Paris, the Auberge Nicolas Flamel was everything he might have imagined from a French restaurant. Stone pillars supported fat oak beams; more stonework framed the leaded windows, and a bull’s head looked down on the room from above a vast fireplace. Most of the tables were full, and a warm hubbub filled the room. Nick was suddenly ravenously hungry.

Simon Atheldene wasn’t hard to find: he was the only man in the restaurant wearing a double-breasted suit. He was sitting on his own at the back of the room, with a bottle of wine open in front of him. He stood as he saw them approaching and shook hands.

‘Nice place,’ said Nick.

Atheldene poured them each a glass of wine. ‘It’s the oldest house in Paris. Built in 1407 by Nicolas Flamel, the renowned alchemist.’

‘I thought he was a fictional character,’ Nick blurted out, then wished he hadn’t.

To his relief, Atheldene laughed. ‘Harry Potter has a lot to answer for.’ He saw Nick’s surprise and gave a modest smile. ‘I have two daughters – when their mother lets me see them. They make sure I’m not completely stranded in the Middle Ages.’