Understanding the Weathering Behaviour of Caen Stone

Microscopic study shows that Caen Stone varies in its petrographic structure between pellet-rich and bioclast-rich forms. The former is much more microporous than the latter, and tends to draw water into its structure if wetted. In historic usage, this has apparently led to more severe decay than is seen in the latter variety, which has generally displayed good durability. Examples of Caen Stone that survive on the exterior of medieval buildings have led to the material acquiring a good reputation, but less evident, poorer-quality medieval Caen Stone may also be found internally in remnant blocks, where surface decay was cut out for the insertion of new material in earlier episodes of repair. Both pellet-rich and bioclast-rich varieties are encountered in new stone available in the market, and are likely to be distinguishable in standard tests. The report on Caen Stone usage at the Palace of Westminster by Tim Yates which follows lends weight to this conclusion.

Figure 1a–c Variation in the petrological character of Caen Stone seen in thin section through the optical microscope. Width of view of each specimen = 4mm.

Figure 1a Variety with pellets dominant and scattered, light-coloured debris from the skeletons of invertebrate animals, particularly crinoids. Decayed stone used post-1830 on eastern elevation of Canterbury Cathedral, removed in recent repair work.

Figure 1b Variety with dominant crinoid debris and natural calcite cement forming a rigid three-dimensional matrix within the stone, with scattered well-separated dark pellets. Well-preserved medieval stone on eastern elevation of Canterbury Cathedral; sample from inside putlog hole.

Figure 1c Variety intermediate between A and B shown above. Modern stone sample from suppliers.

Tim Palmer C. Geol., FGS, Honorary Research Associate, University of Aberystwyth, and National Museums and Galleries of Wales.

Tim Palmer is a geologist who carried out research on limestones and taught in UK universities from 1970 to 2001. His current interest is in the use of limestones in medieval buildings and monuments, and the relationship between stone behaviour and petrography.

More information on the Journal of Architectural Conservation.

How to subscribe.

Donhead Publishing 2012