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Wednesday, January 21, 2009

The burning of the limestone

The piece which I posted last week from the Transactions of the Bristol & Gloucestershire Archaeological Society avers that there is no adequate explanation for the reddening of burnt oolitic limestone.  But that was published in 1925 and refers to the reddening of the limestone being mentioned in 1818 in the Transactions of the Society of Antiquaries.  An excellent explanation does, of course,  appear in "Crickley Hill: the Hillfort Defences" (Dixon 1994), in the geological chapter by R J Firman:

"An understanding of the mineralogical changes brought induced by heating limestone is thus of crucial importance in interpreting the archaeological evidence.  The most obvious indication of heating is the development of a brick red colour due to the dehydration of rust coloured ferric hydroxides leading to the formation of haematite.  It is important to note that the intensity of the red colour is unrelated to the intensity of the heating, being directly proportional to the concentration of ferric hydroxide, 'limonite', minerals in the original unburnt limestone. Moreover, in a conflagration, conditions may be sufficiently reducing, to preclude the formation of haematite: magnetite or less stable ferrous compounds forming instead.  These, when weathered, impart a grey or brownish yellow colour to the burnt limestones. Given a strong draught, limestone will calcine at temperatures of 800 to 900°C and, in still conditions 1050°C.  Masses of 'meringue-like' material in the prehistoric earthworks show that such calcination did occur, the resulting quicklime having reacted with water to form slaked lime, and this in turn wholly or partly, becoming carbonated as it reacted with atmospheric carbon dioxide.  Reddening and calcination are thus good indicators of burning and destruction by fire.  They must, however, be interpreted cautiously, because some of the red colour, particularly if associated with clays and sands, could be glacially derived from naturally released Triassic sediments."

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