Illustrated Architecture Dictionary
Clinker / Clinker brick
1. An overfired brick with textured surfaces.
2. The incombustible residue, fused into an irregular lump, formed in a grate or furnace where anthracite coal is used
In early brick firing kilns, the surface of the bricks that were too close to the fire changed into the volcanic textures and darker/purplish colors, and were called "clinkers". They were originally discarded, but around 1900, these bricks were discovered by architects to be usable, distinctive and charming in architectural detailing, adding the earthy quality favored by Arts & Crafts style designers....
In the United States, clinker bricks were made famous by the Pasadena, California architecture firm Greene and Greene who used them (often in combination with native rocks) in walls, foundations, and chimneys.
The name "clinker brick" is said to come from the sound that they would make when banged together, being heavier than regular bricks; however, this term is also used for the hardened residue of coal fires, that can have a similar texture.
Clinker bricks were the result of wet bricks being placed too close to the fire in the kiln, resulting in bricks that were darker-coloured and in either ‘melted’ or ‘exploded’ shapes. The surface texture of the bricks could range from glassy to pock-marked from the uneven heat. Richer, darker colours of the ‘clinker’ bricks were another welcome result from the extra heat....
Sometimes clinker bricks were just warped, and added an irregular thickness to the mortar joins in a brick wall....
During Victorian times, these uneven bricks were tossed on the garbage heap, but with the advent of Arts & Crafts construction, they became prized for their organic shapes and colours. Used as accent bricks in a larger wall, they add a roughness and texture, which was perfect for Arts & Crafts sensibilities.
- Stuart Stark, Clinker Bricks and Arts & Crafts Houses
Examples from Buffalo architecture: