Back in the washroom of the Old Ferry warehouse, the large scrubbed pine table had been converted into a hands-on testing bench. Imran Bell sat on an ash stool, his breathing mask resting around his neck, carefully weighing piles of dark, granular material on Sister Merrow’s brass balance scale.
Before him sat three wooden boxes. One containing crushed hardwood charcoal from the South Ironworks kilns, another containing smooth, pale blue river clay dug from the deep slips behind the South Wharf. A third containing coarse, sharp river sand that smelled of wet stonework and pipe lime.
Pella Rook stood beside him with her father’s brass slide rule, calculating the flow pushback across a four-foot filter bed. Beside her, Eren Tal was using a fine-toothed brass wire sieve—its rim darkened with old green copper green rust from years of sifting foundry sand—to screen the charcoal, separating the fine dust from the pea-sized granules that fell with a pleasant, dry rustle into an empty butter firkin.
‘You can’t use the fine dust, Eren,’ Imran instructed, pointing with the tip of his pencil. ‘Fine charcoal dust cakes the moment damp steam hits it. It turns into an impermeable black sludge that chokes the draft. You want specks between an eighth and a quarter of an inch—eight-to-sixteen mesh in workshop terms—large enough to let the air pass freely without overloading the fan blades, but with enough internal pore volume to trap the sulfur molecules.’
