Aesthetics of cleaning on building sandstones
This research was carried out as part of an investigation of the effects of stonecleaning on Scottish building sandstones. The research project was funded by Historic Scotland. Results were published in a research report (Stonecleaning in Scotland: Research Report Volume 3) and in a PhD thesis by Dr. Chris Andrew.
One of the aims of researchers in the field of experimental aesthetics in relation to facade soiling and cleaning has been to develop a model which predicts the change in ascribed aesthetic value of building facades as they progress through cycles of soiling and cleaning.
Building facades pass through cycles of change as soiling accumulates on the surface of stone. The speed of this change varies considerably as building materials vary in their susceptibility to the influence of weathering, but all material, and so every facade, alters in appearance after long exposure to atmospheric pollution, wind and rain. Research shows that many modern buildings, for example those with exposed precast concrete exteriors or harled surfaces, quickly develop patterns of staining through rainwater run-off which are unrelated to any underlying architectural features and may look unkempt after only a few years. Many old buildings that have developed accumulations of soiling over long periods of time may display an aesthetic quality that enhances the appeal of the building. Indeed, the expectation of some buildings is that they will be soiled (e.g. castles). Soiling on buildings which is either consonant with the underlying texture of the building facade (e.g. that producing shadow like effects on rock faced ashlar stone) or enhances other architectural detail and features can, within certain limits, enhance the aesthetic appeal of buildings. Conversely, soiling which is dissonant with the underlying texture of a building (e.g. heavy soiling which obscures colour) or is unrelated to the building's underlying architectural features tends to be viewed as aesthetically displeasing. Many modern buildings are constructed of materials, or have designs, which do not allow for consonant weathering and soiling patterns. When soiled, they are visually less acceptable than buildings which, through the materials used or design features, allow for longer periods of consonant weathering.
Soiling changes the perception of facades and can be thought of as progressing through a sequence, with facade cleaning interrupting progression and returning the building to an earlier stage in the cycle. On surfaces which have an uneven texture (e.g. rock faced and tooled stone), light soiling initially lodges mainly on horizontal and outermost surfaces of the stone. Similarly, light soiling around architectural detail adds to the visual complexity of the building, increasing contrast and shadowing effects. Verhoef (1988) argues that in northerly cities of Europe, soiling can emphasise architectural designs that for much of the year would be lacking definition due to the absence of sharp, well-defined shadows.
Moderate soiling of building facades can result in a change in the visual appearance of buildings, which interacts with the underlying architectural features or stone surfaces. This type of soiling changes the visual complexity of the building by obscuring some detail, colour and texture, while at the same time adding a pattern of soiling which was originally absent. This interactive effect differs with stone type. On rock-faced and tooled surfaces a heavier build up of soiling may be more aesthetically acceptable than it would be on smooth or polished stonework. While soiling may be initially related to the underlying architectural surface (e.g. in bedding planes of sandstones), patterns of soiling eventually arise which are unrelated to the underlying detail. Continued soiling eventually leads to a complete blackening of the surface of the building that reduces the visual information of architectural details and obscures the colour and shadowing effects. In effect, the visual complexity of the building is reduced by the heavy soiling on the facade. Entire buildings may progress through this pattern of heavy soiling in a relatively consistent way. Different facades designs and stone types may soil at different rates.
A theoretical model of soiling aesthetics
Figure 1. A theoretical model of the relationship between soiling, building complexity and aesthetics.
The cycle of sandstone building weathering suggests that soiling affects building complexity and aesthetics in a relationship shown in Figure 1. The shape of this graph may vary considerably due to many factors which include the type and age of building and the materials used in construction. The diagram may be seen hypothetically to represent the weathering patterns of many tooled, rock faced and rubble stone buildings. On some buildings various parts of the facade may be at different points on the graph.
After construction, a building might be thought of as having a certain initial aesthetic quality. After a number of years of weathering, where accumulations of soiling are consistent with the architectural features and the stone texture, complexity is increased and aesthetic value rises to a peak. Thereafter, it begins to decline as soiling increases, becoming unrelated to underlying architectural features. As soiling becomes increasingly heavy, complexity is reduced and aesthetic value decreases to a point where the whole facade is blackened and complexity is at a minimum. In Figure 1, cleaning a building has the effect of returning it to an earlier point on the graph, and the soiling cycle begins again. The point to which a façade returns, and the subsequent soiling effects, may well depend on the method chosen for cleaning and on the success of the cleaning process; an inappropriate cleaning method may, for instance, affect the initial aesthetic value of the façade.
Figure 2. Provans Lordship House, Glasgow. This rubble building has been cleaned. The light soiling level on its facade adds to the visual complexity and aesthetic appeal of the building.
The relationship between soiling, architecture and aesthetics is complex and is subject to differences of opinion between individuals. The diagram in Figure 1 may represent the general position. The main point is that soiling need not always be aesthetically detrimental and can sometimes be aesthetically beneficial.
Andrew, C.A. (1995). An investigation into the aesthetic and psychological effects of the soiling and cleaning of building facades. PhD Thesis. University of St. Andrews, St. Andrews, Scotland.
Verhoef, L.G.W. (1988). Soiling and cleaning of building facades. RILEM Report. New York: Chapman and Hall.
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