Page 73

Grit blasting generally erodes thesurface of the sandstone unevenly. Variabilityintroduced by the operator and variationsin the physical characteristics of the sandstone make it almost impossible to clean a sandstone facade using abrasives without causing some roughening. The amount of roughening which can be caused is greater at higher grit blasting pressures. A compact, well cemented sandstone may be only minimally affected even by high pressure abrasive cleaning. A less dense, lightly compacted sandstone with less cementing material is more vulnerable to abrasion and may be seriously eroded even at relatively low grit blasting pressures. Uneven surface erosion can result where there are differences in the hardness of the minerals which make up the sandstone. This applies both to mineral grains and cements. Calcareous sandstones are especiallyvulnerable to this type of surfaceroughening since calcite cemented areas are more easily eroded than the surrounding sandstone. Variable loss of material from particular bedding layers, resulting from differences in mineralogy or grain size, will result in different degrees of erosion on individual stone blocks. Sandstones in building facades are normally placed with their natural bedding planes horizontal. If cleaning results in raised ridges across the sandstone surface these will trap soiling washed down the facade by rainwater resulting in increased rates of resoiling and increased levels of water uptake. Most physical cleaningmethods are effectivein removing superficial algal growth from sandstone (Plates 4.25 and 4.26), although the potential damage to stone should always be considered before they are used. The method adopted to clean sandstone should be selected so as to produce a level of cleaning consistent with the least damage to the stone.

Plate 4.24 Following cleaning this arris is no longer straight, as the shadow shows.

Profile for Historic Environment Scotland

Guide for Practitioners - Stonecleaning PLU 7545  

The original practitioners guide based on stonecleaning research includes background on sandstone characteristics, the effects of soiling ag...

Guide for Practitioners - Stonecleaning PLU 7545  

The original practitioners guide based on stonecleaning research includes background on sandstone characteristics, the effects of soiling ag...

Profile for hspubs
Advertisement