With apologies to coal specialists - this note was prompted by a query about the nature of cannel coal and its utilization in carvings of archaeological significance. The author remembers seeing cannel coal and jet samples in England, and notes that these are terms which have vanished from common view, unlike anthracite and amber...
Coals contain minor to trace inorganic components, but are composed largely of a suite of materials termed macerals. Each maceral, as defined by the Encyclopaedia Britannica (2002) is a `microscopic organic component of coal consisting of an irregular mixture of different chemical compounds' --- `macerals are analogous to minerals in inorganic rocks, but they differ from minerals in that they have no fixed chemical composition and lack a definite crystalline structure'. In his description of carbonaceous sediments, Greensmith (1971, pp.299-330) identifies the principal maceral classes as fusain (dirty, carbonized wood debris), vitrain (brittle, with bright lustre), durain (matte, composed of more-resistant plant debris) and clarain (satiny lustre, composed of alternations of thin bright and dull laminae in a very fine groundmass).
Cannel coal is massive and unlaminated, lending itself to carving, and breaks with a glassy conchoidal fracture. It contains little or no recognizable wood fragments, but rather is rich in miospores, resembling the durain of bituminous coals. Typical cannels have been water-transported and deposited as organic sediments. Transported cannels may have a high clastic component, revealed in high ash analyses, and these rocks grade into carbonaceous shales. The cannel coals are volatile-rich and `burn with a bright, smoky flame like that of a candle' (Greensmith, 1971, p.311).
Moore (1968) reviewed cannel coal and boghead coal (torbanite). Sapropelic coals formed by the degradation of standard coal-peat swamp materials and the addition of other remains, such as algae and wind- or water-borne spores, perhaps in the quiet waters of lakes away from the shallow, root-crowded, swampy shore. The microbiology includes remains of fungal affinity (a saprophyte is any organism that lives on decaying organic matter, such as some fungi and bacteria). The mineral content varies, and includes clay minerals, siderite, iron sulphides and sparse quartz. The macroscopic properties include dull lustre, fine-grained uniform texture, conchoidal fracture and a lack of strong bedding. Sapropelic coals are rarely more than 2 feet (60 cm) in thickness: they form lenticular masses of limited lateral extent, found usually at the top or the base of a coal seam. Cannel coals may grade laterally and vertically into the `mainstream' humic coals. Their progenitor, stagnant lake-bottom sediments may have been rich in spore (miospore) debris. In contrast to spore-rich cannel coals, boghead coals are essentially pure algal remains.
Van Krevelen (1981) confirms that the sapropelic coals (cannel and boghead coals, p.59) are of dull lustre and conchoidal fracture, and a splinter of these coals can be ignited by a match. Cannel is dull black and burns with a long and steady flame, while boghead coal is dull brownish in colour. Cannel coal is composed of fine material (micrinite) with abundant dispersed spores (pp.63,78), and is formed in lakes and pools with input of floating masses of spores transported by wind and water and transported in organic-rich mud. Tasmanite from northern Tasmania is an extreme form of cannel coal, a light brown rock composed almost exclusively of spores. Boghead coal, in transmitted light, displays a dark groundmass with many white globules.
Cannel coal traded at a premium price and was a `small, but prestigious' element of coal production in the state of Kentucky in the 19th and early 20th centuries (Hower, 1996). Cannel tends to occur in small, rapidly-exhausted deposits, and the cannel industry was locally important but generally obscured by the larger production of bituminous coal. Cannel coal was used in the chemical industry, as a liquid fuel and as a gas enricher.
|Dull to waxy to vitreous||Lustre|
|Massive, not laminated||Fabric|
|Feels light, S.G. near 1.3, cf. 2.2 to||Specific gravity|
|2.6 for common glass, shale & slate|
|Destructive (of small fragments)|
|Black||Streak on unglazed porcelain|
|Burns readily, clear flame||Volatility, combustion|
|Petrography of polished sample||Mineralogy, texture|
|Carved, not moulded||Method of fabrication|
ARKELL,WJ and TOMKEIEFF,SI (1953) English Rock Terms. Oxford University Press, 1973 reprint, 139pp.
ENCYCLOPAEDIA BRITANNICA (2002) Encyclopaedia Britannica 2002. Deluxe version, 32 volumes in 3 CD-ROM set, Encyclopaedia Britannica, Inc., London.
GREENSMITH,JT (1971) Petrology of the Sedimentary Rocks. Thomas Murby and Co., London, 5th edition of book by Rastall and Rastall, 502pp.
GUION,PD, FULTON,IM and JONES,NS (1995) Sedimentary facies of the coal-bearing Westphalian A and B north of the Wales-Brabant High. In `European Coal Geology' (Whateley,MKG and Spears,DA editors), Geol.Soc.Spec.Publ. 82, 45-78.
HOWER,JC (1996) The cannel coal industry of Kentucky: a brief history of resource development and depletion. Energeia 7 no.1, 1-3, University of Kentucky, Center for Applied Energy Research.
JORDAN,SL (2000) Jet and its imitations. Antique Jewelry Online, 1p. See: http://www.antiquejewelryonline.com/features/victjet/victjet3.htm
MOORE,LR (1968) Cannel coals, bogheads and oil shales. In `Coal and Coal-Bearing Strata' (Murchison,D and Westoll,TS editors), Oliver & Boyd, Edinburgh, 418pp., 19-29.
TATSCH,JH (1980) Coal Deposits: Origin, Evolution, and Present Characteristics. Tatsch Associates, Sudbury, MA, 590pp.
VAN KREVELEN,DW (1981) Coal: Typology - Chemistry - Physics - Constitution. Elsevier Scientific Publishing Company, Amsterdam, 514pp.
WATTS,S and POLLARD,AM (1996) Identifying archaeological jet and
jet-like artifacts using FTIR. Infrared and Raman Users Group,
Postprints, 37-52. See:
WHITTEN,DGA and BROOKS,JRV (1972) The Penguin Dictionary of Geology. Penguin Books, 516pp.