CANNEL COAL: BLACK GOLD OF THE 19th CENTURY

Revisiting a forgotten fuel

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...

THE NATURE OF COAL

Coal deposits are relatively limited in stratigraphic distribution, of Devonian to Tertiary age, with a marked peak in Carboniferous time (Whitten and Brooks, 1972). Coals can be divided into humic (low hydrogen content) and sapropelic (high hydrogen content) varieties. Humic coals advance in rank (towards higher carbon content, less volatiles) from progenitor peat through lignite to bituminous and anthracite coals. The latter (cannel and boghead coals) are thought to have formed in lakes rather than peat swamps (mires). They are thin and lenticular, volumetrically unimportant, and often occur at the bases and tops of seams (Guion et al., 1995, p.55).

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

Cannel coal (Arkell and Tomkeieff, 1953, p.18) is `a light bituminous coal which does not soil the fingers'. The term is `supposed to be a corruption of "candle" because it burns without smoke' and was `first mentioned at Wigan by Leland, 1538'. Tatsch (1980, p.33) notes that `cannel coal is a nonbanded, black coal of bituminous rank with a conchoidal fracture' --- `it burns readily with a long smoky flame and is high in volatile constituents'. Cannel coal is very fine-grained, composed in the main of finely comminuted vegetable matter, spores (both miospores and macrospores), algae and fungal material, with some resin globules. The kindred boghead coal varieties are composed of algal and fungal materials: they include torbanite, the oil shale coal of the Midland Valley of Scotland. Cannel coal is widely distributed, occurring for instance in deposits in North America (e.g., Kentucky, Pennsylvania, Texas and Nova Scotia), Europe (Wales and England, U.K.; Poland), South Africa and Australia (New South Wales; Tasmania).

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.

JET

Jordan (2000) gives brief descriptions of dark materials that might be confused with black jet, a favoured semi-precious stone in Victorian England. Jet is believed to be driftwood washed into the sea and replaced by hydrocarbons: a woody structure remains, and the material fluoresces under blue light. Jet has a woody structure, a low specific gravity near 1.3, and leaves a brown streak on a streak plate. Cannel coal differs from jet in producing a black streak. Watts and Pollard (1996) affirm that Fe content can distinguish relatively pure, organic jet from relatively Fe-rich coals and shales. Fourier transform infrared spectroscopy (FTIR) can provide details of the organic geochemistry of carbonaceous matter. Cannel coal displays less aromatic character than jet, and mineral absorptions due to aluminosilicates are sometimes present. Famous jet occurrences reside in the Jurassic strata of western Europe, notably the Upper Lias of Yorkshire and the Posidonia shales of Germany (Greenwood, 1971, p.312).

DIAGNOSTIC FEATURES

The precise determination of the very fine-grained cannel coal requires sophisticated organic geochemical analysis. Petrographic analysis of a polished surface in reflected light is also diagnostic, but the preparation of a polished mount or thin section involves some sample loss in cutting and polishing. Although Jordan (2000) describes some plastics and glasses as materials that may be mistaken for jet, a more likely and less clear-cut problem in diagnosing cannel coal may involve common types of fine-grained rock. With addition of common minerals (quartz, chlorite, clay minerals, etc) coals will grade into other sediments, such as carbonaceous mudstone or (coarser) siltstone. Progressive burial and metamorphism will convert the latter into black shale and slate. Detection of a sedimentary lamination or tectonic overprint (variously known as foliation, parting and cleavage) should indicate one of these common rock types. A number of simple hand-specimen tests are indicated, mostly non-destructive:

Observation Property
Non-destructive
Black Colour
Dull to waxy to vitreous Lustre
Conchoidal Fracture
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
Archaeological context Authenticity
Carved, not moulded Method of fabrication

References

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:
http://www.irug.org/irug2pp/5Watts.pdf

WHITTEN,DGA and BROOKS,JRV (1972) The Penguin Dictionary of Geology. Penguin Books, 516pp.



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Graham Wilson, 17 February 2002, format adjusted 10 July 2011.