“Yes, the full moon phase is seen as a most favourable time to harness spirit frequencies, or as we usually refer to it, the wildspark – so named on account of how hard it is to capture. But there is also the new moon phase; that’s when the moon rises at the same time as the sun and the sun’s bright rays mean you can’t see the moon. The nights are darkest then, and spirits are also near.”

Prue tried to remember which moon phase they were in at the moment; approaching a full moon, she guessed.

“Now, this connection between the moon stages, locations and the ghost world weren’t necessarily new thinking – for centuries people have associated the full moon with the supernatural, and been drawn to certain sites, but what Hannah Woolstenbury did was to pin down the details, and most importantly she found the practical connection with science.”

Master White reached inside her jacket pocket, then held out her hand, clenched fist downwards. Slowly, she turned it over and opened her fingers.

There, in the centre of her palm, about the size of a plum, was something crystal-like – but this was no ordinary mineral. It glowed with kaleidoscopic light in a myriad of colours: gleaming silver, lightning blue, blazing pink, blistering gold, searing red. It radiated shimmering light on to Master White’s hand.

Master White spoke in a hushed tone. “This, young apprentices, is qwortzite.” She let the moment sit with them while they all huddled in, transfixed. “When Hannah Woolstenbury discovered there was a wildspark beyond, she looked for a way of luring it back into our world. She suspected it could be trapped or harnessed inside a material, so she travelled the world searching, never giving up, following every lead, until she discovered qwortzite. Thanks to this, some twenty years ago, the Imperial Personifate Guild of Medlock was formed, or the Ghost Guild as you may have heard it referred to informally.”

Prue couldn’t take her eyes from this amazing crystalline substance. “Where does qwortzite come from?” she asked.

Master White smiled. “Deep in the ground, but that’s all I can reveal. I’m afraid that part is highly secret to both apprentices and craftsmen.”

After a few moments, Master White put the qwortzite back in her pocket.

Prue sat back on her heels but kept looking at the pocket. That material was the key to getting back to Francis. Everything she did with the machines on the farm was so mechanical – she understood the workings, the connections, it all came naturally to her – but here there was a new mystical element which made her feel dizzy to think about.

“Right, time for some lunch. This was prepared by Lavender,” said Master White, placing neatly wrapped parcels of brown paper tied with string on top of the picnic blanket. Inside was a rye loaf, cheeses, red apples and a sweet berry pie.