PiersDenny calls for a reappraisal of a very controllable cleaning method RECONSIDERING AIR-ABRASIVECLEANING 0 F LIMESTONE CONTEXT 40 Cleaning of limestone buildings presents technical and aesthetic problems whichever method is chosen. Air-abrasive cleaning has gained an unenviable reputation for unacceptable levels of damage to stone surfaces, the worst cases resulting in the erosion of soft beds, surface pitting, blunting of arrises and loss of decorative detail. Certainly, if badly executed, this method can cause loss of historic fabric, incompatible with conservation principles. However, alternative widely used cleaning methods are not without disadvantages. The dangers of saturation and water penetration through faults in the structure are well known. Although the careful use of intermittent nebula sprays should avoid much of this problem, in practice it is extremely difficult to avoid run-off or localised saturation of masonry. The unpredictable results of such saturation associated with salt and mineral migration, staining, contact of water with iron fixings and timber within the structure, mean that even such apparently harmless activity as water washing must be viewed with some caution. Similar comments could be made regarding chemical methods of cleaning: residues of alkalis or acids, dangers of saturation in extensive rinsing operations again can be problematical. It is perhaps the lack of fine control by the operative, and degree of unpredictability which are Limestonein ArgyleStreet,Bath, before(top)and after air-abrasivecleaning 21
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