Clinker brick

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

Clinkers are burnt under temperatures so high that the pores of the fuel property are closed by the beginning sinter process and are often mis-shapen. Thus they are considerably denser and therefore heavier and more irregular than regular bricks. Clinkers hardly take up water and are very durable, but have higher thermal conductivity than the more porous conventional red bricks, thus lending less insulation to climate-controlled structures. The name comes from the sound they make when banged together.

In early brick firing kilns called brick clamps, the surface of the bricks that were too close to the fire changed into the volcanic textures and darker/purplish colors. They were often 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 and Crafts style designers. Modern brick-making techniques can recreate the appearance of these bricks and produce a more consistent product.

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.

Clinker is sometimes spelled "klinker" which is the original Dutch/Low German word. The onomatopoeic verb "klinken" means "to sound", i.e. a "klinker" is a "sounder". (The Dutch word "klinker" also means "vowel".) These brick stones produce a specifically bright sound when hit with or against something, for instance each other.

Clinker bricks are also known as hard Dutch paving bricks. In 18th century New York, the Dutch interspersed dark clinkers with regular bricks. Some used clinkers to spell out their family initials on brick dwellings. See Jan Van Hoesen House.

Clinkers consist of bits and ends, field-late and white-burning or red-burning clays. Through different mixtures of the raw ingredients, many varied colour nuances can be achieved. For the production of masonry units the source materials—clay and water—are mixed and formed industrially in a string extrusion process. For special purposes, for example the restoration of listed buildings, hand-formed clinkers are used. During the drying process, the water concentration decreases to approximately 3%. Then clinkers are fired at temperatures between 1100 °C and 1300 °C in a tunnel kiln (earlier in ring kilns), in contrast to the 800 °C to 1200 °C temperature range seen with normal bricks.

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Wikipedia