Restoration - Cleaning trials

The photograph on the left was taken in the summer of 1998, looking across Princess Street Gardens, with the Scott Monument in the distance, completely covered in scaffolding during the restoration process.

The proposed cleaning of the Scott Monument provoked much debate, and many opinions were expressed both for and against the cleaning of it. Eventually a scientific /geological investigation was called for and some of the evidence is shown here.

There is a legacy of damage done to historic buildings through ill-informed cleaning. Stone-cleaning is a complex business and these images show to a wider audience the reasoning behind the decision not to clean.

The Scott Monument is built of a stone which contains a residue of shale-oil that attracts and binds with soot and grime from the atmosphere, particularly from the old coal-fired steam trains which used to pass close by. The soot and dirt on the stone was particularly well attached and ingrained into the surface, and to remove the dirt would mean removing the surface, allowing the stone to weather and deteriorate.

This stone foliage detail on the left, is Binny stone, taken from the Scott Monument and used as part of the cleaning trials. This example was subjected to ‘air-abrasion’ with ‘alumina’ abrasive powder, whereby the sand-like material is blasted under pressure at the blackened surface of stone. It is trained onto the surface through nozzles, great care must be used not to etch the surface.

Often this method will remove the blackened surface of the stone, but allow water and weathering to penetrate the stone. This will accelerate the deterioration of the stone and ultimately the monument.

In 1932 D. Tait adddressed the properties of the bituminous presence thus:

“The Binny sandstone is wellknown to be a very durable building material. It might be contended that the small amount of oil in it contributes to this durability by excluding water; but the oil probably has another effect. The Scott Monument and [National] Art Galleries, which are largely built of Binny Sandstone, are disfigured by black patches on the surface of the stone. These patches are generally said to be caused by the smoke of the city, and by the smoke of the locomotives of the railway close at hand. But a close inspection of the distribution and shape of the sooty patches and bands, coupled with the knowledge that the rock does contain oil, suggests the conclusion that the darker markings are oily patches to which cling a specially thick coating of soot.”

This stone detail was cleaned chemically - with a solution of Ammonium Hydrogen Fluoride, which acts like a bleach on dirt attached to the stone. The solution is applied in a gel form and left for up to 24 hours before being neutralised and washed off. However as can be seen from the image the results can be quite patchy and ultimately aesthetically displeasing.

The chemical has turned iron compounds in the stone to a rusty red colour, and ‘cleaned’ the surface quite unevenly.