His superiors excluded him from the manufactory, and in the end actually imprisoned him for his indiscipline and suspected heresies; his whirling mind was confined to a cell for thirteen years. When released, he was exhausted; his final works remained incomplete.

Ferron listened to this soberly. ‘But after Bacon, for two hundred years, underground, all invisible, the monks of his great manufactory have toiled at weapons of war. Yes? Quite remarkable. You know, I am told Colon has used Bacon’s writings in trying to construct his own case for the monarchs. In his Opus Majus Bacon surveys geographical understanding, and argues, for example, against the existence of a Torrid Zone below the equator.’

Grace said, ‘Perhaps we can use that to persuade Colon and his brother to accept these, the fruits of Bacon’s genius.’

‘It’s possible.’

They moved through a low passage into another, smaller chamber. Here only enclosed oil lamps burned, and the murky air stank of dung and piss. Ferron recoiled, and with an impatient snap Grace summoned forward a novice, who presented each of them with a scented napkin to hold over their noses.

Here gunpowder was manufactured, according to Bacon’s carefully researched recipes. It was kept separate for safety, and for the foul air; the brothers assigned to this work didn’t last long.

James said, ‘We mix the ingredients with mortars and pestles, or with these wooden stamps.’ He showed Ferron a clunky device, all levers of iron and mallets of wood. ‘We need to combine the powder into granules of varying sizes, depending on the application. Granule size determines burning rate; you don’t want your powder to burn so rapidly it shatters your bombard’s casing. So we mix up the powder with a binding agent. Sometimes it’s water and wine, but in fact urine is best.’

‘As I can smell,’ Ferron said drily. ‘And the ingredients?’

‘The best charcoal is free of knots, and made of coppiced wood - hazel or ash, gathered in the spring and so full of sap. We import our sulphur from the volcanoes of Iceland, the purest in the world. The saltpetre is more difficult, and needs manufacture.’ He showed Ferron a series of vats, from which a murky water was poured one to the next. ‘Saltpetre is made from dung.’

The monks filled a pit with layers of quicklime, cow manure, wood ash and vegetable waste. They turned this regularly, moistening its surface. It was important that the matter was not allowed to get too wet, or too dry. After some months of this a whitish efflorescence would appear on the surface of the heap, which the monks scraped off and collected. The powder was dissolved in water, which was then passed through the series of vats.

‘This is saltpetre,’ James said. ‘The Arabs call it “Chinese snow”. The saltpetre stays dissolved where other salts precipitate out. It’s an intricate technique, worked out over centuries by the Chinese among others—’