Traditional stone cleaning for heritage facades


There are few things more rewarding that delivering a cleaning and restoration package to a historic building in the knowledge that the careful specialist approach will help to ensure the preservation of the building for future generations. Cleaning is an important and essential part of conservation and preservation which must be carried out without damage to the substrate, and this requires experience.

So why bother cleaning a building? The answer might seem obvious, that it improves the aesthetic and general appearance – after all, first impressions count. This is true, of course, but more importantly cleaning should be undertaken as a means of preserving the fabric of the building to ensure its longevity, and to enable vital checks for structural problems or issues to be carried out. Surface soiling often contains sulphates from industrial and traffic pollution, when these mix with water (moisture in the air and rain) they create a sulphuric acid based compound on the surface of the facade that can lead to accelerated decay in materials that are not acid resistant.

The approach to cleaning any historic building or structure should always be one of caution. It is far better to under-clean than to over-clean and potentially damage fragile areas. In the same vein it is  equally important to not over-saturate the stone with water which can cause damage and staining.  A number of years ago there were unfortunate patterns of poor cleaning being undertaken, most often exercised using damaging abrasive cleaning on the wrong surfaces causing extensive damage to the building skin. Although not stone, glazed terracotta (faience) facades have also suffered historically from ill-conceived cleaning methods using chemicals, typically strong acids, and harsh aggregates that resulted in serious damage to the fire skin of the material, leaving it open to water ingress and ongoing associated problems. Before any programme of works are undertaken the building should be properly surveyed and a report drawn up by a specialist, such as Szerelmey, indicating the level of cleaning and repairs that might be required, and also the nature of the building fabric itself. Following on from this, and an essential part of the pre-contract  process is exemplar sample cleaning and test panels.

These exemplars assist in  determining  the level of clean and the cleaning method required. This enables building-specific specifications to be written and a cost plan to be drawn up. With heritage and listed properties it is particularly important to ensure the relevant authorities such as conservation officers have seen and agreed the exemplars. Szerelmey have recently completed a full package of cleaning and restoration works to the exterior and interior of Camden Town Hall, a project which involved a number of traditional specialist cleaning processes.

The imposing neoclassical-style building was designed in 1937 by architect Albert Thomas, who also designed a number of housing schemes for the St Pancras Borough Council in the same neoclassical style, and was a former assistant of EL Lutyens. The building, which is Grade II listed, had an eight storey extension built in the 1970s and a conservatory added to the roof in the 1990s. Over the course of time the Town Hall, which is constructed of Portland stone over a steel frame, has suffered from weathering, dirt accumulation and Regent’s Street disease; the corrosion of the underlying steel causing damage to the stone.

Following extensive surveys and a number of cleaning samples, a substantial package of cleaning, repair and restoration works was agreed. This included cleaning and repair to the ornate interior marble and terrazzo flooring and walls and the highly specialized repair of scagliola plaster work. Externally, the cleaning works principally involved the use of nebulous, Doff and Jos  systems. The exterior was heavily soiled and parts of the façade required a combination of these techniques to remove the more stubborn staining.
Areas of the façade including the decorative composite capitals were cleaned using one of the most traditional methods, nebulous cleaning. This is a simple water clean with a nebulous spray; the water pressure and type of nozzles can be adjusted to suit the condition and fabric of the building. The nozzles are specially designed to vaporise the water creating a fine mist and are normally set on a timer system to prevent over-saturating the building material. The fine misty spray softens the deposits on the building, which are then washed down using a Doff system (low volume ,medium pressure steam cleaning). In areas where the deposits were heavier, a natural bristle or phosphor bronze bristle brush were used, or the area was  rubbed with a soft gritstone as  appropriate. The washing sequence commencing  from the upper level, working vertically so that the action of water run-off from the immediate area of cleaning softens the soiling on the level beneath.The DOFF system is  very effective in removing stubborn dirt, many different paints and coatings including some thermoplastic and bituminous mastic materials. This steam and superheated water system is also effective in removing chewing gum, grease, oil, algae matter and killing off bacteria, as well as removing bird droppings and other unwanted deposits, and kills off spores negating the need for chemical biocide. This is a cost effective, efficient method of cleaning that has no negative environmental effects.

The system works on a variable combination of high temperatures, which can reach up to 150°C, and pressure. The operator can vary the temperature and the pressure quickly and easily according to the type of material that needs to be removed. Generally when the temperature is high the water pressure on the surface being cleaned is gentle and the volume of water low. This prevents the material being cleaned becoming saturated and also keeps the amount of water run off to a minimum. These systems are easily portable units that come with a wide variety of nozzles and lances that can be changed depending on the requirement of the project.

At Camden,in areas containing heavy or stubborn staining the JOS method was necessary. This system was developed to enable gentle, safe and effective cleaning of historic buildings with their inherent problems, such as delicate or friable surfaces. It works by creating a soft, swirling vortex using a mixture of low air pressure, low volume of water and a safe, inert fine granulate, which is typically crushed marble. There are different aggregates that can be used varying from very fine to coarse depending on the type of cleaning required. The most commonly used type is calcite which is a fine grain. This cleaning system is not suitable for use on all surfaces such as  faience as the aggregates are too abrasive for the delicate glazed material.

In some cases the above systems are not adequate and it may be necessary to use poultices. These might be used to remove various types of water-insoluble surface contaminants such as paint and grease, soluble salts, insoluble contaminants and metallic staining from porous masonry surfaces. At Camden Town Hall, in addition to the external cleaning and restoration package, extensive interior cleaning and repair was also delivered. Part of this included the badly soiled marble flooring and wall cladding. This was  initially hand cleaned with detergent and water, before poultices were used on heavily soiled areas. Poultice formats vary but can consist of a medium that contains water or alkaline based cleaning agents. Surfaces are pre-wetted to minimise penetration of the cleaning solution into the masonry surface and covered with plastic film to prevent the poultice drying out. At Camden the poultice was left in situ for between 12-24 hours before the film was removed and the poultice then allowed to dry for 2-4 hours. Following this the poultice was removed, the area thoroughly washed and the marble hand polished with a soft, dry cloth. The process of neutralising with clean water to remove potentially damaging residues is really important follow any chemical cleaning procedure. Traditionally poultices were used on small, localised areas of facades, but spray applied poultices have been developed which are more cost effective and useful for larger areas. Poultices can also be used in conjunction with other water cleaning methods, as described above, to soften deep-seated contaminants first.

When working with any water cleaning system it is vitally  important to ensure the minimum amount of water is used to protect the stone. Despite this there will invariably be water run off and a catchment system should or can be used. Typically the water is captured in a catchment system and removed via a puddle pump to the local water way. Before passing into the local water way, the water is filtered with the use of a fine gauze to remove any resulting sediment. When cleaning with chemicals the process is slightly different. The run-off water is captured in a catchment system and passed from here to a neutralization tank via a puddle pump. Dependent on the chemical that has been used, a suitable neutralizer is then added to reduce the chemical concentration to an acceptable PH level before discharging into the water way. If an acceptable level is not achieved the resulting fluid is removed via a certified hazardous waste carrier.

There really is no greater adage than “less is more” when it comes to cleaning approaches to heritage and fragile buildings, and there is no better example of this than a skilfully cleaned and restored building.


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