A hard mass of fused stony matter or brick with its shape distorted due to the vitrification process.

Clinker bricks are bricks used in the construction of buildings. 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.