Some of the pages of the four-sheets were printed with a grid of pictures, others with a single illustration. Lurid primary colours overspilled the outlines of humans and natives engaged in violent action. Clarity Forrester, a swag-bellied elderly person dressed in the grey tunic and scarlet trousers of his rank, explained that four-sheets like this were printed and distributed clandestinely, to circumvent the disapproval of councils which believed them to be both seditious and blasphemous, and reflected a popular belief that humans were secular gods come to right all wrongs and protect ordinary people from oppression and the consequences of out-of-control techne.

‘This particular series is important because it is one of the first examples and most of its print run was seized and destroyed by lawkeepers. Swift Singletree knows me all too well,’ Clarity said. ‘Helping you is the least I can do for a prize like this, even though it puts me in no small danger. I ask only that you will not mention my name should you be caught.’

‘We have never met or heard of you,’ Goodwill said, stone-faced. ‘But we are grateful, nonetheless.’

Clarity seemed satisfied with that. He rewrapped the stack of four-sheets, handed Goodwill a pass letter for the closed stacks, with the location of Swift’s carving written on the reverse.

‘I will take you to one of the stairs to the basement levels, but after that you will be on your own. If anyone challenges you, show them the letter and tell them that you are looking for a lost relic of great significance to your tribe.’

He led Ysbel and Goodwill through a maze of offices, all of them empty at this late hour, to the Hall of Stones and Bones. Their footsteps echoed from the high ceilings as they passed display cabinets containing specimens of gemstones, ores and crystals, and crossed a chamber where fossils of every era and articulated skeletons of creatures long extinct were displayed, from ammonites and rare thunder lizards to the chickens whose bones could be found everywhere at the Burn Line. Three skeletons of ogres, two adults and a child, stood in a glass case before a painted backdrop showing an erupting volcano; Ysbel would have liked to linger and read the caption, but Clarity was hurrying on, leading her and Goodwill to an atrium where four hallways met under a dome painted with the starry night sky.

They turned down a corridor lined with ladders of narrow drawers labelled according to some arcane cataloguing system, and at the far end Clarity unlocked an unmarked door and handed Goodwill a wooden baton with a small key hung from one end, explaining that it was for the door to the stacks four levels below, where their prize was stored.

‘Return this to me when you are finished, and do not spend too long down there,’ he said, and scurried off.