Few natural stones can cope with the high temperatures generated by fire. Volcanic ash, solidified by the pozzolano effect into a lightly cemented soft rock, is one of the few, and it can sustain heat with no significant change. Most other natural stone either cracks due to unequal heating expansion forces or may undergo chemical change should the temperature get high enough.
The big difficulty for the conservation engineer is that key structural elements such as lintels are often the most exposed during a fire, and frequently crack as a result. Cracked lintels frighten engineers, and the pragmatic solution for a reversible ruin would be to cut a new one from the same stone source. This is not always easy or possible. For ruins, where the stone will have obtained a patina of wear or algae discoloration, repair in situ is always possible, even if this requires that the real structural lintel must be concealed behind. Repair of fire-damaged masonry is covered elsewhere in the main text.
Timber can accept a degree of charring, but it is necessary to remember that timber exposed to a temperature of 180°C for any period of time suffers thermal degradation and, in extreme cases, total embrittlement. Even at lower temperatures, where cycling of temperatures has occurred, reduction in timber strength of over half is possible. The charring of timbers does not necessarily render them unsatisfactory, but a rough guide is that a two-hour fire usually results in a charring level of 20 mm, hence it is pointless attempting to conserve timbers less than 140 mm on the lesser axis. The retention of original timbers is frequently of great historic interest, as much can be gleaned from joint detailing, dimensions, saw signatures, etc. However, this is a process for individual selection and will require the expertise of a structural engineer experienced in this field, and alternate structural support must often be provided.
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