‘Thought to be?’ Nick echoed. ‘Doesn’t anyone know?’

‘Medieval cathedral builders didn’t necessarily spell out what they meant by their decoration. There are clues in the symbolism, but it’s in the nature of symbols that they’re ambiguous. The kings on the front of Notre-Dame, for example: they’re an unimpeachable biblical theme. But it’s no coincidence that they were put on a building which the kings of France wanted to use as a symbol of their own power. The medieval mind was much more sophisticated than we give it credit for. Semiotics, symbology, whatever you want to call it: they were profoundly alive to the overlapping meanings of the world. If you were a layman walking past Notre-Dame in the fourteenth or fifteenth century, you’d see the statues as the kings of Israel, but you’d also see them as the kings of France. One king becomes another, depending on how you look at it.’

‘It sounds like Gillian you’re describing.’ Nick was surprised he’d said it. ‘The same person, but so different in different contexts.’

‘Everyone’s like that, a bit.’ It could have sounded dismissive, but she said it so gently it sounded like agreement. ‘You mustn’t give up hope.’

Nick wondered if she was thinking of the picture in his wallet. ‘I just want to find her.’

‘Rescue the damsel in distress.’ Again, it might have sounded snide but didn’t. To Nick, it felt almost wistful. He smiled in the darkness.

‘I hadn’t thought of it like that.’ His mind wandered back to all the late nights in Gothic Lair killing monsters and storming castles; and before that, Friday nights in high school, sitting around with his friends rolling dice in the basement, totting up the numbers that would decide whether their fellowship lived or died. Perilous quests had been so safe then, something to look forward to through the dreary week at school. A far cry from the lonely, terrifying reality.

‘What was it you wanted here?’ he asked, changing the subject. ‘Did you find it?’

‘Oh – it was the kings. They reminded me of the kings of Israel, that’s all. I thought perhaps it might trigger some sort of insight.’ She shook her had. ‘But – nothing.’

They finished their tour of the cathedral, Emily pointing out different features as they walked up and down the dark aisles. The way the architecture became more elaborate as you moved from east to west, the shift in style from Romanesque to Gothic which had happened over the centuries of its construction recorded in stone. She showed him the pillar where angels blew the trumpets of the Resurrection, and numerous stone carvings tucked away on buttresses and bosses. At first Nick paid attention out of politeness, but gradually he found himself becoming drawn into the intricacy of the art. By the time he emerged from the darkness into the gloomy day, he had a whole new vocabulary.

‘I’m going to go and buy some new clothes before the shops shut,’ said Emily. Snow was still falling, frosting the ridges of the cathedral. ‘I’ll see you back at the hotel.’

‘Be careful,’ Nick warned.

Strassburg

Twenty-seven kings stared down from their glass thrones: proud and solemn, elevated above the cares of the world. Beneath their vitrified gazes, the world they had left moved apace. The cathedral echoed with the ring of hammers, the shouts of masons, the creak of pulleys and the squall of infants. Somewhere in all the din, the choir was trying to sing a litany. And at the back of the church, two men stood in an alcove whispering furiously.

‘You promised me nothing could go wrong.’ Andreas Dritzehn was neither proud nor solemn. His cheeks were flushed with anger, his fists balled tight as if poised to strike someone. Probably me. That was why I had insisted on meeting in the cathedral.

‘Do you think you are the only one who has put money into this venture?’ I felt sick just thinking of it, though I did not expect Dritzehn to sympathise.

‘We must melt down the mirrors we have made and sell the metal.’

‘No. What we bought was lead and tin and antimony. What we have now is alloy. We cannot unmix it, any more than we could melt those windows to make sand and lime.’

‘Then sell the alloy.’

‘That metal is the key to our enterprise – and our fortunes. If we sell it, other men will realise its power and teach themselves to copy it. If one of them happens to be an Aachen goldsmith, then he will cast the mirrors and take all the profits of our labours.’

‘Let him.’ Dritzhen’s face puffed with anger. ‘I need my money back.’

‘The pilgrimage has been postponed, not cancelled. All we need is to hold our nerve and sit out one extra year. Then we will be as rich as we ever dreamed.’

‘I cannot sit out an extra year! ’ He bellowed it like a gelded colt. I looked to see if anyone had heard, but the sawing of carpenters hid it.

‘I should have listened to my brother Jörg,’ he moaned. ‘He told me you were a vagabond, a conjurer. That you would be the ruin of my family.’

It was then I realised, perhaps for the first time in my life, that I was responsible. I was too old to run. I owed too many men too much to be able to disappear. One-armed Karl would find me, or someone like him, and my crushed body would be dragged off one of the canal weirs, snagged among the scum and branches.

I had to free myself. And like a drunk who finds release in one more draught, I reached for the only cure I knew.

‘There is another art I know. Less advanced than the mirrors, though with rewards that might dwarf them. All it needs is patience.’

He shook his head. ‘I have had enough of your secret arts.’

‘Did you never wonder what Kaspar and I were doing in your basement? The mirrors were never more than a sideshow, sowing the seeds for our greater work.’

Even in his despair I could see he was interested. ‘You never spoke of this.’

‘Of course not. The mirrors are already secret enough. But this new art is ten times greater. Only four men know of its existence.’

‘Can it be carried off before next year?’

‘Difficult to say. As I told you, its progress is less advanced than the mirrors. But once it is ready there will be no delay. No waiting for pilgrimages, no shipping it down the river. Even the plague could not stop it. All it will require is a little investment.’

He grabbed my coat and thrust me against the cathedral wall. ‘Are you deaf? Have you listened to a word I say? I have no more money. How can I spend my way out of bankruptcy?’

With a calmness I did not feel, I pulled his hand off my collar and stepped away. Light sparkled on a gold pin he wore on the shoulder of his coat, Christ on his cross with a verse of scripture scrolled around it.

‘What about that?’

He cupped his hand over it. ‘It was a gift from my wife.’

It was beautifully made. Every sinew in Christ’s body strained against death, as if his flesh tried to fight back the spirit from breaking free. The lettering underneath was perfectly even, punched into the thin metal with impossible finesse. It reminded me of the task at hand.

‘You can borrow the money we need. I will be at my house in St Argobast if you change your mind.’

Sometimes I believed that borrowing money was my true business, and all my work with ink and metal existed merely to provide a pretext for the loans. The mirrors had become a monster devouring itself; when nothing remained, I needed a new idea to borrow against. In those days I no longer thought of the arts in terms of profit, or even if they would work. All that mattered was that they kept the stream of money flowing.

Three days after our meeting in the cathedral, Dritzehn came to my house. I met him in the yard between the barn and the forge. Hens pecked around our feet; my pig rooted for apples fallen from the tree behind the barn.