BCD 2019

132 T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 1 9 C AT H E D R A L C O MM U N I C AT I O N S Siliceous sandstones tend have a more open pore structure than other stone types and may darken naturally with exposure to the elements but are also vulnerable to the absorption and retention of cleaning reagents. Iron-rich minerals within the stone are subjected to colour change or mineral loss, causing either colour enrichment, bleaching or efflorescence. Surface residue can normally be removed sympathetically from sandstone but sub- surface discolouration should be interpreted as patination in order to avoid the use of more invasive cleaning methods. Salts (principally sulphates, nitrates and chlorides) may occur naturally within stone and brick or may be introduced inadvertently from mortar, adjacent soil, bird guano, de-icing salt, cleaning chemicals, and airborne pollution. Quaternary ammonium salts are used as algaecides but other salts can be created when neutralising acid and alkali cleaning reagents. We’ve described one reaction where pollutant gases generate calcium sulphate at or close to the surface, however soluble salts may also be transported from deep within masonry, drawn by capillary action to the evaporation front. On reaching the surface these salts can wash away naturally with rainwater, but in sheltered or interior locations form visible efflorescence. Confined and exposed to cyclical changes (principally wetting and drying), salts may crystallise within the pore structure of the stone leading to disaggregation and spalling. Therefore the removal of salts (desalination) and overlying lime and sulphate encrustation is normally beneficial. Soluble efflorescence is perhaps best removed by simple brushing and collection. Close sub-surface salts will require repeated application of a plain poultice or, if the salts are insoluble (including corroded metal staining), a chemically activated poultice may be required. Drawing and exhausting the reservoir of salts from deep within stone is difficult to achieve and rarely successful. White deposits observed on masonry are routinely referred to as salt efflorescence but are more commonly comprised of calcium or magnesium carbonates, the presence of which may be disfiguring but are comparatively benign. If salts are suspected then it is best that samples are tested in the laboratory before deciding a course of action. Organic growths include algae, cyanobacteria, fungi, lichen, moss, liverworts and rooted plants. The first in our list are the first to colonise a clean surface, unlocking nutrients and creating the conditions required by the higher growths. In general, organic films and growths reduce porosity and increase moisture in masonry. The excretions (principally oxalic acid) of some of these growths can dissolve calcareous stone and chelate metallic minerals but paradoxically the calcium oxalate deposited may act as a natural shelter-coat. Algae and cyanobacteria are relatively easy to remove while fungi and lichens are more tenacious. In context, these may significantly add or subtract from the aesthetic appearance of a building or memorial, and rare lichens or plants may be legally protected. CLEANING METHODS There is a very wide variety of methods available, but we can place the majority within three broad categories. We can add a fourth; radiation (principally laser) methods. In practice there is a considerable overlap as most chemicals used are of aqueous composition and rinsed with water. Abrasive methods for example are frequently used with water to gain the advantages of softening and dust suppression. Water based methods • atomiser hand-spray bottle • intermittent nebulous spray • dry steam • cold pressure water • hot pressure washer • superheated water system Chemical based methods • acid, alkali, detergent, algaecide, chelating agent, enzyme or other active chemistry • aqueous or organic solvent composition • liquid, gel, latex or poultice consistency Mechanical methods • hand brushing • hand held fibre scourers • hand held abrasive blocks • hand held scalpel or scraper • dry micro air abrasive • wet/dry swirl air abrasive • venturi wet abrasive • wet/dry air abrasive blasting. Equipment must be safe, portable and readily adjustable. The cleaning operative must have the requisite skill and experience with the A limestone mullion with red algae on the weather face and a thick sulphation crust on the sheltered face: both pose a threat to the integrity of the surface which may justify cleaning. (Photo: Restorative Techniques Ltd) A painted consecration mark is revealed after the removal of paint and graffiti layers.(Photo: SMB Restoration Ltd)