Stoltz stood over me. ‘You would be astonished, in my line of work, how forgetful men become. It is as if lending a man gold instantly addles his wits. Fortunately, my memory does not suffer this defect.’

He reached into the bag on his belt and pulled out a small notebook. I remembered the clerk in the Mainz mint with his enormous ledger, the all-knowing book from which my theft could not be hidden. I trembled.

‘Three months ago I loaned you fifty gulden.’

This was worth another blow, this time to my arm. I rolled over on my side. Stoltz stood over me.

‘Some men find money a strange abstraction. It flows from man to man and from country to country and knows no boundaries. In a single day it can go from the hands of a king to the hands of a beggar and back again. But in truth, money is very simple. It is a tool, just as a pair of bellows or a plough are tools. And within that tool lies an inherent utility. This we call value.’

A kick in the ribs. I covered my face with my hands. Nothing destroys a man’s credit so quickly as a mask of bruises on his face.

‘If I lend you a plough, its value is that it can improve your field to make it more fruitful. For that, you pay me. Likewise if I lend you fifty gulden, you pay me for the use of it. For the use of this money, you were to pay me five shillings a month.’

Two swats from the cudgel contorted my back in agony.

‘You have already failed to deliver the last month’s payment. Now I hear that the surety you gave me for the loan – the dowry of the girl Ennelin – turns out to be worthless. You have broken off your engagement to her.’

‘Her mother tricked me,’ I pleaded.

‘Then more fool you. I will not be left with the bill. By breaking the engagement you have forfeited your collateral. Under the terms of our agreement, I am entitled to claim back the entire loan immediately.’

‘I cannot pay.’ The money had barely touched my hands as it passed through them – some to Dunne and various suppliers, but most to pay off other loans that had fallen due.

‘Then I shall ruin you.’ Stoltz nodded to his henchman, who swung the cudgel underarm against the sole of my foot. I screamed. ‘When Karl has finished with you I will set the courts on you.’

‘Please. Please God.’ I scrabbled to get away from the brute. He let me, like a bear handler letting out the leash. I could not go far.

Desperation loosened my tongue. ‘I have invested it in a great labour. One that, God willing, will make me rich. If you ruin me now you will get nothing, pennies for gulden. If you wait I can repay you everything.’

Stoltz said nothing – but there was no movement from Karl. I took this as an invitation to continue.

‘I am devising a new art, one that will make me rich.

‘What is it?

‘You spoke of ploughs and fields. Imagine this is a plough which could make fields give up ten times as much wheat.’

‘Explain.’ Stoltz had no time for riddles. Karl stepped closer and stroked my ribs with the tip of his cudgel.

But I could not say it. Even then – bleeding, bruised and with the promise of worse to come – I could not. It was my secret, incomplete though it was. If every man knew it, there would be no advantage. I had to cling on.

I stared up at the thin, bloodless face looming over me. A wink of light behind him caught my eye: a pilgrim’s mirror that Aeneas had given me by the Rhine. I gazed at it, praying for salvation.

Stoltz swept a glass jar off the table. It shattered on the floor, jolting me back to him. ‘Pay attention.’

He glared down. Karl tapped the club against the side of his leg and licked his puffy lips. And that was when it flashed into my mind.

‘The mirrors,’ I croaked.

‘What?’

I pointed. He stepped back, fearing a trick, and eyed the mirror on the wall. A ring of light played over his face where the mirror reflected it onto him.

‘That will not save you.

‘Not in the way you think. But perhaps…’ I stood. Karl lifted the club to knock me down again, but Stoltz raised a hand to still him. I pulled the mirror off its nail and examined it. My mind raced.

‘This has been cast from an alloy of lead and tin. I have worked with this alloy: it shrinks as it cools and tightens around the mould.’ I ran my finger around the interlocking circles. ‘For a design so intricate, the only way to free it is to shatter the mould. Every mirror requires a new mould to be carved. It is slow and expensive.’

I did not know absolutely that this was true, but just as some physicians can diagnose a man’s sickness by looking at his face, I could read it in the shape and flow of the metal.

I pointed to the figures sculpted on the medallions inside the rings. ‘You see how flat and featureless these faces are? You cannot achieve the quality of detail from casting in this way. I can make them better than that – and cheaper.’

‘How?’

‘A new alloy. One that does not shrink as it cools. I can use the moulds again and again, and each time produce a truer copy than this.’

I pressed the mirror into his hand. He took it, scraping a fingernail over the rough carvings.

‘How many would you make?’

‘A thousand. At twelve shillings each, that would be five hundred gulden. I could repay your loan with double the interest.’

‘Lending money at interest is a venal sin,’ Stoltz admonished me. Karl glanced at him to see if this merited another blow. Thankfully, it did not. ‘You pay me for the use of the money.’

‘Then I would pay you double. For it would be twice as useful.’ I did not know where this extravagance sprang from, or how I would ever honour it; I did not care. My mind glowed hot with the sudden promise of this new idea. All I wanted was to begin it.

Andreas Dritzehn laid the mirror I had given him on the table. ‘And these are to be sold to pilgrims in Aachen?’

‘Do you know the Aachen relics?’

‘I have heard of them.’

‘They are the holiest relics in the empire. The blue dress of the Blessed Virgin Mary. The bands that swaddled Christ in the manger, and the cloth that covered his modesty on the cross. Also a piece of fabric which is said to have wrapped the head of John the Baptist after Herod cut it off.’

‘A complete wardrobe,’ said Kaspar.

‘Once every seven years, they are taken out of their chests and displayed. So great is the number of pilgrims that the whole city can barely contain them. The priests mount a scaffold between the cathedral towers: every street, every square, every rooftop and window becomes an observatory.’

Andreas frowned. ‘It must be hard to see anything.’

‘Exactly.’ I leaned forward, brimming with excitement. ‘The pilgrims carry mirrors – like this – to capture the light of heaven which radiates from the relics.’

‘Is it visible?’

‘Only to God,’ said Kaspar piously.

‘But the holy mirrors capture it. The pilgrims wrap the mirrors in cloths and take them home. Then, when they are in need, they can unveil the mirrors and let the holy light cure their afflictions.’

‘How many do you intend to make?’

The idea had settled since I blurted out the first number that entered my head to Stoltz. I had done some research, ascertained the facts and established a more realistic basis for my estimate.

‘Thirty-two thousand.’

Dritzehn almost dropped the mirror on the floor.

‘There must be over a hundred thousand pilgrims in Aachen when the relics are shown. All of them need mirrors, or the pilgrimage is in vain. Ours will be better quality than our rivals’, and cheaper. As I said, this happens only every seven years. The next pilgrimage will take place in some twenty months. Time enough for our work.’

‘But what of the Aachen goldsmiths? Surely their guild will not allow you to flood their market with your wares, at their expense?’

‘The Aachen goldsmiths forfeited their rights long ago. They cannot make enough of the mirrors to meet the demand. Some years ago there were riots: pilgrims who could not obtain mirrors fought in the streets with those who had. Several died. Since then, the privileges of the Aachen guilds have been suspended for six months each year that the relics are shown.’