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.
