Welsh roofing and writing slate has been split from the cliffs of North Wales since before 1900 — a riven blue-charcoal stone cleaved along its natural layers at Penrhyn and Cwt-y-Bugail. The face is cool, dark, and matte, its cleavage striations stacked like overlapping tiles on a Snowdonia roof.

This system distills that stone into an interface: a dark blue-grey ground, Payne's-grey and cool-stone supporting tones, and a single chalk-mark white — the scratched line of the slater and the schoolchild alike — as the only accent that lifts the dark.