Over the last decade there has been an increasing concern by many throughout Scotland that the physical effects of stone cleaning were having a potentially damaging consequence on both the fabric and the aesthetics of stone buildings. As a result, a 2-stage research programme was devised. Initially, the programme investigated the effects of stone cleaning sandstone structures, sandstone being the predominant building stone used in Scotland. This led to the production of 6 different publications on the topic, and the hosting of an International Conference in Edinburgh during April 1992.

To assist those actively involved in writing specifications in this difficult area a subsequent publication entitled “Stone cleaning: A Guide for Practitioners”, was published by Historic Scotland in April 1994. Although this volume considered such topics as the soiling of building facades, aesthetics, physical and chemical cleaning methods, testing methodology, health and safety, and planning issues, it largely only addressed the sandstone issue. Whilst a number of contemporary publications have also been released, the associated problems of cleaning granite had not been dealt with in any degree of detail so far, and this has had to be considered separately.

Granite is generally perceived as being a very durable stone and less prone to the decay processes more usually associated with sedimentary rocks. To the lay, and to many professionals, it appeared that it might be cleaned using comparatively straightforward techniques, and that the stone would be left undamaged by these processes. To the more initiated, it was realised that there was no detailed research evidence to support this belief.

In recognising that it was unlikely that similar problems would occur to those investigated regarding the cleaning of sandstone, Historic Scotland thought that there was a need to provide more relevant information for practitioners upon which informed decisions to clean, or not to clean, could be made.

In order to obtain this understanding some basic historic data was required. In addition, the physical characteristics of the various granites needed to be identified, together with an awareness of the nature of previous interventions, earlier cleaning works and associated repairs. Against that background, a detailed research project into the cleaning of granite was projected and the brief for a multi-disciplinary scientific and technical commission prepared. This brief qualified the intention that the research work should pay particular attention to the techniques currently being used by industry.

To fully realise the project’s objectives, work was required to put the Scottish problem into a wider context. This called for a literature review, which required an investigation of available sources of national and international information relevant to the cleaning of igneous stone.

The study required an assessment of current and proposed cleaning methods, techniques and materials, and a detailed analysis of the chemicals liable to be used.

The research project was jointly funded by Historic Scotland, Scottish Enterprise and Grampian Enterprise Limited, and commissioned from The Robert Gordon University, Aberdeen in January 1993. The final report was presented, with the Literature Review, to the commissioning clients in December 1995.

The outcome of that study has since been translated into this Technical Advice Note. Building upon the exemplary work already undertaken by The Robert Gordon University into the cleaning of sandstone, this new work on granite will greatly assist with the provision of further advice and guidance to practitioners and others operating in the field. However, it should be borne in mind that stone cleaning of listed buildings and buildings within Conservation Areas is not “permitted development” under the terms of the Town and Country Planning (General Permitted Development) (Scotland) Order 1992. Listed Building Consent or planning permission is required. This advice note, therefore, is also intended to be read in con-junction with Historic Scotland’s Memorandum of Guidance on Listed Buildings and Conservation Areas(1993), to be revised 1997, where that need arises.

INGVAL MAXWELL Director, TCRE