A Riddle in Bronze

I came to a halt and looked around. The cellar was large, the roof supported by numerous brick columns which cast long shadows across the uneven floor, and I could see workbenches and machinery against the walls, along with racks of tools, spare parts and half-built equipment. There was little time to waste gawking as Roberta was working nearby at a lathe, and even as I watched I saw bright swarf curling away from whatever she was turning. The lathe was driven by a pulley, itself turned by a belt running at enormous speed. The belt ran almost to the arched ceiling, where it looped over a second pulley on a shaft driven by steam from the boiler.
The speeding belt was only a foot or two from Roberta's shoulder, and as I watched it flexing and vibrating I feared the grievous injury she might suffer if it should come free. But she seemed oblivious, the long corkscrews of metal gathering at her feet as she worked. Then, Roberta having finished, she turned and nodded her thanks as she saw me standing nearby. "I know this isn't your area of expertise, Mr Jones," she shouted, "but I really need your help."
"What are you doing down here?" I asked her, raising my voice over the sound of machinery.
Roberta reached for a lever on the wall and pulled it firmly, relaxing the pulley near the ceiling and thereby stopping the frantic spinning of the lathe. The noise lessened immediately, and we were able to converse at a more normal volume. "There's no time to explain fully," she told me. "Suffice to say father has been infected by a particular malaise, and I must draw it out of him."
"Is it… a malevolent spirit?"
She looked at me, surprised. "Why, Mr Jones. Just this morning you were scoffing at such notions."
This was true enough, but at breakfast that morning I'd yet to see the mad scribbling in the ledger, nor the blackness which had briefly flooded the professor's eyes. "Certain… phenomena… have convinced me to take your claims a little more seriously."
"Only a little?" She gave me a wry smile. "Well, it's a beginning at least. But come now, my father suffers while we chatter."
"Mrs Fairacre told me he was fast asleep."
"A glass or two of brandy never fails," said Roberta, with a nod. "I find it easier to treat him like this. He complains a good deal less whilst sleeping."
"Are you saying this has happened before?" I asked, scandalised.
"It's a hazard of our work, Mr Jones."
"But—"
"Please! You shall have your explanations later, if you care to listen. For now, you must do as I ask." Roberta led me to a manual arbor press, where I saw a small bucket of objects which looked like misshapen bronze pebbles. She took one of these pebbles, placed it on the die, then reached up with both hands and put all her weight behind the press's lever. The ram came down, slowly, and when Roberta released the lever I saw the pebble now flattened into a perfectly round disc. Roberta tapped it out of the die and handed it to me, and I felt the warmth in the newly-shaped metal. It was a twin to those in the haversacks upstairs, but this one had intricate tracings stamped into each side. "I need a dozen of those," said Roberta. "Hurry, please."
I set to work, and discovered the lever took a lot more effort than I expected. My arms were more used to the turning of pages in a ledger, and I felt my muscles aching after I'd created no more than two or three of the shaped discs. Then I heard a roar, and I turned to see Roberta near the fire, using a bellows to generate even more heat. She'd placed a small crucible in the flames, the metal glowing cherry-red, and as I watched she plucked the crucible from the fire and carried it to a workbench. Here, I saw a small bronze cylinder propped upright, the open end facing down into a thick metal cup.
Very carefully, Roberta poured a small quantity of molten lead into the cup, where it spattered and hissed. She set the crucible aside, and used the tongs to pick up the cylinder, which now had a domed cap of silvery metal sealing the open end. I recognised the shape, for the cylinder was the same design as those in the professor's study. This one was slightly larger, though, and as I recalled the shattered cylinder I'd spied in the haversack, I understood why Roberta had built one of a heavier design.
I had not paused in my work whilst observing Roberta at hers, and by now I had forged half a dozen of the metal discs. My shoulders were protesting at the labour, and my hands felt like they were about to come apart at the wrists. I stopped to dash the sweat from my brow, and as I moved the latest disc from the press, I noticed something odd. The markings stamped into the surface were arranged in an intricate, swirling pattern, but there were no corresponding ridges on the face of the die. In addition, when I compared the latest pressing to the half-dozen I'd already manufactured, I discovered every single one of them exhibited a different design.
Stunned, I picked up one of the large copper-alloy nuggets I'd been stamping, and I turned it over and over in my hand. It was heavy, as smooth as a beach pebble, and there was no sign of any markings that would explain the whorls in the finished discs. At breakfast, Roberta had told me about the ingredients she'd been experimenting with, and I wondered what manner of other-worldly materials might be present in the metal. Then, realising I was falling behind in my work, I placed the large nugget in the die and applied my protesting arms to the lever once more. A new disc emerged from the die, and I was not surprised