Named for the distinctive sound they make when banged together, clinker bricks are the result of wet bricks placed too close to the fire. The intense heat of coal-burning traditional kilns created a hard, durable brick that often twisted into volcanic shapes and textures. Overbaking produced rich, warm colors as well that ran the gamut from reds, yellows, and oranges to deep, flash-burned browns, purples, and blacks. No two clinker bricks were alike, rendering them trash to brick manufacturers who prized uniformity, but treasure to early modern architects, builders, and homeowners seeking uncommon architectural detail.
From The Accidental Charm of Clinker Bricks by Susan VanHecke