GOB and GOB FIRES. Gob, goal' (pl. Goaves), and waste are synonymous terms in coal mining, designating the worked out and abandoned portion of the underground workings.
In coal mining, as practised prior to the 19th century, when the pillars left for the support of the surface were very small, and only about 45%o of the coal was extracted (see John Buddle on Mining Records 1838), the area of abandoned pillars was named the waste or goaf. Under modern conditions of coal mining all the coal is extracted, except in the case of bad coal or coal which cannot be sold at a profit, so that the goaf or gob contains little or no coal, for which reason "gob fires" are not now, perhaps, as frequent as they used to be. These fires, however, are not neces sarily restricted to the goaf or gob, for they have been known to take place, though rarely, at the coal face, and they have been of frequent occurrence at the edge of shaft pillars (coal left unworked for the purpose of support in the neighbourhood of a shaft where it penetrates a coal seam) .
A gob fire, or what is sometimes termed the "spontaneous heat ing" of coal, is due to the rapid absorption of oxygen by the coal substance which, causing heat to be generated, increases the rate of absorption until the coal bursts into flame. It was for long thought that the iron pyrites which are in coal were the supreme cause of this heating; Dr. Plott gave expression to this view in his Natural History of Staffordshire in 1686. It may be a con tributory cause, but that it is the sole, or indeed the chief, cause was disproved by Dr. E. Richters about the year 1868 (see his paper published in Dinglers Polytechniches Journal, vol. cxc., Dec. 1868) and later by Henri Fayol in 1879 (Etudes sur l' alteration et la combustion spontanee de la houille exposé fair). The two royal commissions on "Ships carrying Coals" (appointed in New South Wales, 1896 to 1900) arrived at similar conclusions.
The report of the Oberschlesische Grubenbrand commission on spontaneous combustion in coal mines, issued in 191o, contains results which may be studied with interest and value. The final report made by the British departmental committee on the sub ject (1921) decided "that some small amount of heat may be developed by the oxidation of pyrites in coal where it occurs as an amorphous form of marcasite ; but that, as pyrites is present in coal in such small proportion as compared with the coal sub stance proper—which is a bad conductor of heat—the effect of this heat is negligible. The chief part played by pyrites when present in an unstable form is that of a disintegrator of the coal, so rendering the latter more permeable by air and exposing a greater area of coal substance to oxidation • . . that the chemical process is mainly one of attachment of oxygen to molecules of high car bon content, but that subsidiary to this, and playing an important part in determining the actual spontaneous ignition of coal, is a chemical interaction between the oxygen thus loosely held by carbon-like molecules and other atoms in these molecules or other portions of the coal conglomerate." (R. R.)