Pearl ash / potash

Wood ash boiled down and purified until almost nothing is left in it but the one salt. Anyone with a fire and enough patience can make a little of it, and that is the whole of the problem, because breaking the curse of Haudh-Gwenn needed a great deal.

Ashanti’s method for destroying the ooze under the keep turned on a reaction that had to run hot enough, fast enough, and through the whole volume of it at once. Her fire-crystal braziers gave a steady flame nowhere near that temperature. She was boiling ash down herself and getting nowhere with it, and she said plainly that pearl ash in bulk was the thing most likely to hold up the entire curse-breaking.

Bulk was the difficulty. Making it means burning enormous quantities of wood, roaring fires rather than quiet ones, and then a slow purification of what the fires leave, none of which a city under siege keeps in a warehouse to be bought. Hinta carried the order out to Leon Yaegar: as much potash as an army can burn. Leon knew the concept from military manuals and said it had come out of Rast, and his army spent its idle weeks felling and burning forest for it. Johanka Materai’s burned estate was raised as a source and dropped again.

Heat was the other half of it, and ash could not supply that. The accelerant taken off a dead arsonist closed most of the gap, and crushed gem flecks out of the keep’s old Mól gem-cutting room, which Mikhail remembered was there, closed the rest.