Clinker is a fused or partly fused byproduct of the combustion of coal or vitrified slag. Use of scoria, red dog, and baked shale to refer to the lithologies associated with natural coal burn are common, but these terms are confusing or poorly described the material. The various lithologies and different degrees of melting associated with the coal burn make it difficult to use one term for the entire sequence. Coal geologists refer to the baked rocks associated with naturally burned coal beds as natural clinker.
Natural clinker deposits are a result of coal combustion often attributed to prairie fires, or lightning strikes. Spontaneous combustion is the most significant cause of coal burns and is a common occurrence in the subbituminous and lignite coal fields of the Western United States. Low-rank coals, because of their high moisture content, have a greater tendency to self heat when exposed in outcrop. Drier air promotes the loss of inherent moisture in coal outcrops, causing the coal to slake, creating increased surface area. The coal oxidizes and self heating accelerates until smoldering or combustion occurs. Some investigators believe the presence of dispersed pyrite in the oxidizing coal significantly increases the potential for spontaneous combustion. Lignite and subbituminous coals can begin self heating at 300C and in bituminous coals at 600C. Natural clinker deposits are often found on the edges of drainage systems, where down cutting of streams exposed coal beds and enhanced the possibility of spontaneous combustion.
Investigations of natural clinker in Wyoming and North Dakota have looked at the origin of these deposits and their potential use as indicators of coal resources. The red, purple, or black colors, caused by iron oxidizing during heating, make natural clinker deposits distinctive. Shale, mudstone, and siltstone closest to the burning coal are altered to a brick-like material and sandstone is hardened by the heat. These rocks often are more resistance to erosion than the unbaked sequences, resulting in prominent topographic features. It is the hardness of this material that makes natural clinker useful in some industrial minerals applications.
Much of the San Juan Basin in northwest New Mexico has outcrops of soft shale, siltstone, coal, and sandstone. Common aggregates such as limestone, sand, and gravel are not readily available. Natural clinker is used as road base on some New Mexico county dirt roads where there is no other suitable material and clinker deposits are nearby. Some coal mines in New Mexico and Arizona use natural clinker for their mine roads, for reclamation purposes, and to stabilize areas. Peabody's Arizona Black Mesa mines have experimented with natural clinker as an aggregate for concrete, but found the product did not have the required strength for permanent structures.
The New Mexico State Highway Department has tested several natural clinker deposits in the San Juan Basin. The different rock types and the degree of heating result in variable quality. LA abrasion test results range from 47.9 to 24.8% and the soundness test from 1.5 to 11.5%. The LA abrasion figures are high, making most of the clinker deposits unacceptable for paved road base, but the soundness values (how material withstands the freeze-thaw cycle) are within the acceptable range. Because of the diversity in quality, the use of natural clinker is dependent on the availability and cost of other aggregates.