22 You won't do better than a day's tuition in THE NEW PATENTED METHOD OF GENTLE CLEANING the friendly surroundings of THE , LIME ( CENTRE • one day practical course on the use of lime in building held throughout the year • learn to analyse an old mortar, plaster or render and CHEMICAL FREE ENVIRONMENTALLY FRIENDLY prepare a matching material THE SWIRLING VORTEX THAT DOES NO HARM • slake lime, make mortar and build in brick and flint • make, pigment and apply limewash • all materials and tools provided For afull working knowledge of lime and for details of on-site "bespoke" courses contact: Bob Bennett, Course Director, The Lime Centre, Long Barn, Morestead, Winchester, Hants (0962 713636) The original U.K. Lime Centre, STONEHEALTH LIMITED Winner of the 1991Ford Conservation Award For Heritage (C.I.T.B. Approved) 73 London Road• Marlborough • Wilts • SN8 2AN Tel & Fax: 0672 511515 of concern with these systems. It is not the intention here, however, to determine the advantages of one cleaning method over another (it is often the case that a combination of several systems is most effective), but rather to illustrate that air-abrasive cleaning in sensitive hands can be amongst the least potentially damaging of cleaning methods. A major advantage air-abrasive cleaning has over other systems mentioned is its physical rather than chemical nature; the process is halted on switching off the machine. The level of clean and final appearance is controlled directly by the operator. (Micro airabrasive or laser cleaning systems used within a muse um con text to clean statuary have this advantage in common.) If the objectives are clearly defined by the specifier and the operative is sufficiently skilled, it is possible selectively to remove surface deposits layer by layer, avoiding over-cleaning or damage to the stone surface. If the point of departure is one of minimum intervention, and the operative is working within the constraints of maximum care rather than maximum speed, lowest workable pressure and finest workable abrasive, very good results can be obtained even using relatively simple machinery. It is perhaps a sensible precaution to employ trained conservators (ideally as cleaning operatives) or at least to supervise the cleaning of historic buildings, as the approach is likely to be patient and cautious. A recent example of a combined cleaning strategy in which air-abrasive cleaning played a part is at 5 Argyle Street, Bath. The building is of Bath stone ashlar with a paint layer intact on one elevation and exposed stone to others. The general condition of the surface varied between heavy sulphation at cornice level and light soiling on some of the ashlar. The painted surface, on investigation, proved to have been applied on top of a sooty and sulphated surface. The condition of the stonework was difficult to asses prior to cleaning, but it was evident that cracking due to structural faults, open joints, iron fixings and also the presence of exposed external wooden lintels would mean that there would be an element of risk in using water in any volume to clean the building. In this case, the paint layer was effectively removed using a Peel Away poultice, and the surface neutralised. The sulphation layer below was removed with air-abrasive equipment, in this case with the addition of a fine mist spray attachment to minimise airborne dust pollution. The cornice and remaining ashlar were also cleaned by the airabrasive method. Decorative detail to the window heads illustrated in the before and after cleaning photographs were repaired using lime conservation techniques following cleaning. The approval of the Bath City Council Conservation Architects was sought for this project as previous air-abrasive disasters had led to a moratorium on the use of this technique. The trial proved successful, and I would hope the project has helped to promote the idea that, used with care, air-abrasive is a possible alternative to water and chemical cleaning in Bath. The following plant and materials were used: Hodge Clemco Classic I 028 )-Blast 'Finesse' grade abrasive (less than 1% free silica) Dirk Blast grade 1 (silica free) Air pressure 10 - 30 lb/in 2 at variable . distances from the surface Client: Architect: Milburn Restaurants Ltd David Brain Partnership, Bath Contractor: St Blaise Ltd, Evershot CONTEXT 40
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