Stonecleaning and stone decay

An investigation of the long-term consequences of stonecleaning

This research aimed to gather objective and statistically valid data to determine the negative, or indeed positive, effect that the cleaning of sandstone has had on the incidence of stone decay affecting a building facade, when compared to stonework which has not been cleaned. The ultimate aim of the research was to provide an indication of the cost of stone repair as a consequence of cleaning.

The research project was sponsored by the Technical Conservation Research and Education Division of Historic Scotland. The research report builds upon and develops an undergraduate honours degree project in the School of Surveying, The Robert Gordon University undertaken by Marion Roberts.

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The following information is an introduction to the research report. It should be referenced as:
Roberts, M. and D.C.M. Urquhart (1995).
Stonecleaning and stone decay. Research Report to Historic Scotland. (210 pages)

The hypothesis

"Unsubstantiated visual evidence would appear to indicate that there has been an increase in the incidence of stone decay in building facades which have been cleaned, in comparison to stone in facades which have not been cleaned".

This is a quotation taken from the research proposal, for Historic Scotland, upon which this study was founded. The quote perhaps reveals an opinion that is held by some professionals in the field of conservation, as to the effect that stone cleaning may have had on sandstone facades. This is, of course, only one side of the stone cleaning debate, and it should be realised that there are those who oppose such generalisations, believing this not to be the case. They may feel instead that the removal of the layers of soiling from buildings, when undertaken correctly, could have benefits to offer to the long term stability of a building's fabric.

It is important that the actuality of this apparent increase in the incidence of stone decay is validated. If it can be ascertained that our historic buildings are weathering to a greater degree as a result of stone cleaning, the cause of this phenomenon should be clarified. The value in successfully making this connection lies in ensuring that future decisions on stone cleaning are made in the full knowledge of the likely future consequences of any action taken to clean a building. This would also encompass the consequences of allowing buildings to remain in a soiled condition.

Aims

This piece of research has the aim of gathering objective and statistically valid data to determine the negative, or indeed positive, effect that the cleaning of sandstone has had on the incidence of stone decay affecting a building facade, when compared to stonework which has not been cleaned. This study does not intend to formulate opinions as to the merits or demerits offered by stone cleaning, or to question the philosophical propriety of the debate.

The ultimate aim of the research is, provided the hypothesis is not proven null, to predict the cost of stone repair as a consequence of cleaning on a national (Scotland) basis. Emerging from this forecast, the next stage may be to quantify the likely future demand for building stone for facade repairs, in order to provide strategic information to inform policy decisions on the supply of building stones from Scottish resources.

Supplementary to this, it is intended to establish a survey pro-forma and methodology which will operate, not only as apparatus for undertaking this study, but also as a template that could be used by professionals in the industry to undertake the survey of building facades. This would allow for the qualification and quantification of decay manifestations present on a building facade, and subsequently aid in the determination of the mechanisms behind the phenomena observed.

Objective

The original objectives set for this study were,

1. To conduct a survey of a selected number of sandstone buildings, both cleaned and uncleaned, to determine the incidence of stone decay.
2. To relate the incidence, extent and nature of stone decay to the period of time since the building was cleaned.
3. To identify, by visual observation, the nature and type of stone decay and to correlate the decay with such factors as the architecture of the building, orientation, exposure, geographical location, atmospheric pollution.
4. To devise an appropriate methodology for the survey of building facades to act as a template for others.
5. To quantify the costs of any possible increase in facade repair and stone replacement as a result of stonecleaning intervention.

However, due to the time required to accurately survey and record the condition of stone on facades, the above objectives were subsequently modified as follows:

1. To conduct a survey of a selected number of sandstone buildings, both cleaned and uncleaned, to determine the incidence of stone decay.
2. To relate the incidence, extent and nature of stone decay to the period of time since the building was cleaned.
3. To identify, by visual observation, the nature and type of stone decay and to correlate the decay with such factors as the architecture of the building, orientation, exposure, geographical location, atmospheric pollution.
4. To devise an appropriate methodology for the survey of building facades to act as a template for others.
5. To provide a general indication of the likely cost of stone replacement for the sample, making comparison between cleaned and uncleaned facades.

This was to be done by carrying out surveys of terraces only, in both Glasgow and Edinburgh.