Stonecleaning of granite buildings

The following is an extract from Technical Advice Note 9.
"Stonecleaning of granite buildings".
If quoted, it should be referenced as:

Urquhart, D.C.M., M.E. Young and S. Cameron, 1997.
Historic Scotland Technical Advice Note 9.
"Stonecleaning of granite buildings."
ISBN 1 900168 39 1.

Copies of the Technical Advice Note are available from:

Preface

Over the last decade there has been an increasing concern by many throughout Scotland that the physical effects of stonecleaning 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 stonecleaning 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 "Stonecleaning: 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, it was thought necessary by Historic Scotland 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 multidisciplinary 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 stonecleaning 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 conjunction 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
March 1997

Contents

1. Introduction

2. Granite as a building stone

2.1 Formation of granite
2.2 Composition of granite
2.3 Porosity and permeability
2.4 Sources of Scottish granite
2.5 Use of granite in buildings and structures
2.6 Mortar joints and pointing
2.7 Granite in combination with other stone types

3. Soiling and decay of granite in buildings and structures

3.1 Soiling of granite building facades
3.2 Particulate soiling and crust formation
3.3 Biological soiling
3.4 Re-soiling of granite facades
3.5 Decay of granite in buildings and monuments

4. Assessment of the facade

4.1 Preliminary examination
4.2 Records and reports

5. Methods of cleaning granite facades

5.1 Physical cleaning methods
5.2 Chemical cleaning methods

6. Testing methodology

6.1 The decision to clean
6.2 Need for tests
6.3 Identification of test panels
6.4 Physical cleaning test procedures
6.5 Chemical cleaning test procedures
6.6 Special features and finishes
6.7 Reporting results of test cleaning
6.8 Analytical techniques
6.9 Appointment of cleaning contractor for test cleaning

7. Cleaning specification

7.1 Sources of information
7.2 General recommendations
7.3 Content of specification

8. Conduct and supervision of stonecleaning work

8.1 Selection of contractor
8.2 Training of operatives
8.3 On-site supervision
8.4 Assessment of masonry joints
8.5 Dealing with special features
8.6 Maintenance of records

9. Health and safety issues

9.1 Contractual arrangements and legislation
9.2 Hazard information
9.3 Precautionary measures
9.4 Work at heights

10. Statutory consents

Bibliography

Introduction

The cleaning of stone facades to buildings has been the subject of much debate in recent years, and was strengthened by the awareness that the stone cleaning being carried out on the sandstone buildings of Scotland was, in many instances, resulting in irreversible damage to the stone. In the case of granite buildings there is a general perception by both the general public and practitioners that granite is such a hard and durable material that the problems apparent in cleaning sandstone have little relevance to granite, and that any cleaning technique that removes soiling at minimum cost can be employed, without damage to the stone. It is the case that most granites are hard and generally durable, but it is not the case that granite is immune to the processes of decay through natural weathering or through the effects of environmental pollution. Many historic buildings constructed from granite are subject to significant decay such as scaling, flaking or granular disintegration. Pollutants in the form of soluble salts (mainly gypsum) are present on many granite facades. These salts have been linked to decay of granite. Stonecleaning by physical or chemical processes can reduce the level of salts present at the surface of the granite. This is likely to have the effect of reducing the rate of decay of the granite.

There is evidence that the interface between the mortar joint and the granite block maybe a potential source of stone decay. The effect of chemical cleaning materials absorbed and retained in the mortar joints may be a factor contributing to the decay of the mortar and the stone. It is in this context that the cleaning of granite must be viewed.

Stonecleaning is one of the most visually dramatic changes to which buildings can be subjected. It is a process which changes not only the fundamental appearance of buildings but also the architectural and environmental context in which those buildings exist. The colour of stonework and the architectural detailing often become more apparent following cleaning and there may be an important psychological effect on the way in which properties are perceived in urban areas. The significant difference between a typically soiled granite and a cleaned granite is the sparkle produced by the exposed mica grains once the soiling has been removed.

Different cleaning methods can produce variations in colour. If, for example, various properties in a terrace are cleaned at different times or by different methods, this can lead to noticeable colour variations along the terrace. Stonecleaning may also reveal staining beneath the soiling layer which may not have been visible beforehand. Staining may arise from weathering of the granite or from metal fixtures. It is dangerous to attempt the removal of such stains as excessive cleaning can cause damage to the stone.

Although stonecleaning may result in what is considered by some to be a visual improvement, cleaning carried out by unskilled personnel or by the use of inappropriate techniques can lead to permanent damage to buildings. It is therefore important that the potential risks associated with stonecleaning methods are fully understood by the building owner, the architect or agent, the contractor, and the grant awarding body.

This Note is concerned with the cleaning of granite buildings and monuments and will provide guidance on a range of relevant issues. Despite the widely-held perception that granite is virtually indestructible when used as a building stone, it is now clear that granite can be vulnerable to the processes of decay, particularly in polluted urban environments and with certain types of granite. In order to avoid damage it is necessary to ensure that information is available relating to the type and condition of the granite and the nature of the soiling (before cleaning is carried out) as this may influence the selection of a cleaning system.

Consideration should be given to the situation in which stonecleaning is to be carried out. Where the property to be cleaned forms part of a larger block, such as a terrace, cleaning will inevitably produce a patchwork effect, disrupting the architectural integrity of the larger unit. Subsequent cleaning of adjacent properties will generally not restore the uniform appearance of such a facade since the use of different cleaning methods at different times will produce variations in the colour of the stonework.

Despite detailed research and much improved knowledge of the effects of stonecleaning methods, there are still many risks associated with stonecleaning. Proposals to stoneclean, if they are acceptable in principle and accord with stated policy, must therefore be carefully considered. Stonecleaning should be approached on a damage limitation basis; if doubts persist, the option not to clean should be adopted.

Copies of this book are available through the Scottish Conservation Bureau of Historic Scotland at:

Prices, order forms and information on other Historic Scotland publications can be obtained from the above address or on the Historic Scotland web site at: http://www.historic-scotland.gov.uk/

For further information contact Dr. Maureen Young at m.young@rgu.ac.uk