Anyone embarking on cleaning & restoring a historical building has a range of fundamental and serious decisions to make. The most important guiding principle in this is caution & care because, on the one hand, the unique evidence of the past should not be irreparably damaged or ruined due to inappropriate processes and on the other hand, facade cleaning is one of the most challenging cleaning jobs.
A differentiation is made between mineral dirt such as scum, sinters, pigment dirt and corrosion products such as verdigris, pyrolusite and rust film and organic dirt such as soot, algae, moss, lichens and bird droppings. Oily and greasy dirt even sticks to smooth surfaces and forms a strong basis for further dirt deposits.
One of the main problems with facade cleaning is the sheer size of most of the objects that need to be cleaned. This often means that very precise cleaning technology which has a localised effect cannot be used, not only for economic reasons but also for restoration-related and aesthetic reasons because it is almost impossible to achieve even cleaning results across the entire facade (“chessboard effect”).
Despite the huge complexity of the cleaning task involved, historic listed objects should not be damaged in any way and any traces of the original stoneworking should be fully left intact. Even with the most careful cleaning, this isn’t always possible for example, if solid encrusted dirt has formed on crumbly substrata. Dirt particles are often found to have penetrated the historic fabric of a building and developed mixed zones under the surface where it is no longer possible to distinguish between dirt and the original fabric.
Nevertheless, for conservational reasons, in most cases, it is advisable to remove layers of dirt from a building’s surface early on. Deposits on natural stone increase vapour diffusion resistance over time which can lead to a build-up of solutions containing harmful substances and ultimately a weakening of the material’s structure. The removal of layers of dirt is also often required for further restoration work to be carried out. This is because only when the layer of foreign material has been removed from the original fabric can a precise picture of the state of the building’s fabric be formed and further measures be planned. In many cases, the aim of the cleaning is also to make the monument accessible to art lovers and historians in its original state so that they can experience it as the architect or builder intended.
This all means that cleaning work for restoration purposes should never be carried out on your own responsibility but instead, without exception, in close cooperation with monument owners, monument conservationists, restorers, art historians and other experts. As part of this, preliminary investigations are carried out into the state of the facade’s material and level of dirt prior to any cleaning work. Visual inspections are often not sufficient and so further scientific tests are carried out for example using samples. As part of a second step, test areas are then subjected to different cleaning techniques and parameters in order to ascertain the best process.
Since, unlike with everyday cleaning, when it comes to cleaning historically important buildings, it is more complicated than saying “previously: dirty, now: clean.” On top of the scientific and technical problems, there are also very different philosophies on how to deal with cultural monuments. Some people understand restorationto mean the reconstruction of a (supposedly) original state and do not think about the consequences of using unhistorical means on the existing original fabric. Others see the decay as an important part of a building’s history without considering the fact that their views might be conditioned by any nature-related mystical ideas that they might have of German romanticism. There are even tangibly different opinions on how to deal with monuments from country to country.
Cleaning Processes
A variety of cleaning processes have been developed for facade cleaning. The two techniques employed most often also for non-listed buildings are particle blasting and high-pressure cleaning.
With particle blasting or low-pressure blasting, an airstream is used which is dosed with precisely measured spray agent. With the right choice of spray nozzle, quantity and type of spray agent and level of air pressure, the way that the process works can be adapted to almost any facade surface. Currently, approximately 2,000 types of spray agent are available on the market, ranging from maize cob meal and chalk and glass powder to dry ice pearls. The differences are not just down to the output material used but also the size and shape of the grain. By adding water at the nozzle, the development of dust can be largely prevented (“damp blasting”). The water and any accumulated harmful substances are bound in the blasting material and disposed off with it. Disposal of the spray agent is easiest with dry ice cleaning as it literally dissolves into the air.
High-pressure cleaning has many advantages, making it suitable for complex tasks such as cleaning the facades of historic listed monuments. When it comes to flexibility in terms of different substrata, it is in no way inferior to particle blasting: the carrier substance used in both techniques (water for high-pressure cleaners and air for low-pressure blasting) is suitable for all forms of surface. Even when it comes to cleaning with the high-energy water jet, the factors that influence all cleaning results, namely mechanics, time, temperature and chemistry, can be precisely adapted to the requirements at hand.
In terms of the mechanical cleaning power of the high-pressure jet, it is not just the pressure generated in the pump (which is a common misconception) but also the quantity of water that is delivered that is crucial. It is the only decisive factor in removing the dislodged dirt. It is not only the nozzle pressure of the high-pressure cleaner but also the amount of water that flows through the pipe cross-section per unit of time that is responsible for the impact pressure of the water on the facade surface, which primarily tears and loosens the layer of dirt.
The mechanical effect of the highpressure jet depends heavily on its angle of impact and therefore on the choice of high-pressure nozzle. A high area coverage but a lower cleaning effect means using a fan jet nozzle instead of a pencil jet which is better at breaking up dirt. Its disadvantage, however, lies in its small working width which means that it is impossible to cover a large area with it. Alternating nozzles combine pencil jet nozzles with fan jet nozzles in one nozzle holder, meaning that no other nozzle sets need to be carried during operations and the operator can change from pencil jet to fan jet by simply switching between the two.
The rotary nozzle (dirt blaster), which is used for stubborn dirt and pressure-sensitive facades, combines the advantages of the fan jet and the pencil jet. This nozzle uses a pencil jet in rotation (approx. 4,000 revolutions per minute). The overlap of the pencil jet’s rotating movement with the linear movement of the spray lance produces a blanket-wide, consistent spray pattern.
Both heated and non-heated h
ighpressure cleaners are used to clean facades. 80°C hot water can reduce cleaning time by up to 60% and considerably speed up the drying process. With hot-water devices with a steam mode, the steam spray jet can also be used which is particularly suitable for pressure-sensitive and ragged surfaces. The supply of extra heat loosens residues with a high melting point more quickly and easily than the hot-water high-pressure jet, the higher impact pressure of which removes dirt better, e.g. in the case of mineral dirt. The steam jet, however, has high penetrability, no spray back and uses less water and cleaning agent. The better flushing action and low vapour build-up, however, are plus points in favour of the hot-water jet.
Finally, cleaning agents and care products as well as biocides can also be used with high-pressure cleaners. This means that mainly neutral, low-alkaline and low-acidic cleaners are used. The two-step method is usually applied in order to make the most of the cleaning factor of time. For this process, the first step is to apply the cleaning agent and then, after a short or long application time, the second step is to wash away the loosened dirt using high pressure. Foam and gel cleaners require a considerably longer application time on smooth surfaces.
Monuments Cleaned
As part of a cultural sponsorship programme, over the last 30 years the industrial company Kärcher has used the processes described above to clean approximately 100 monuments on every continent. One of the high points recently was cleaning the colonnades in St Peter’s Square, Rome in 1998/1999. Anyone who has ever watched the Papal blessing “urbi et orbi” on the television will recognise them: two magnificent halls of pillars that give St. Peter’s Square its elliptical shape, forming a border around it from the outside.
The colonnades are some of the most important structures from the Baroque era: they were built between 1656-1667, designed by Gianlorenzo Bernini. The sheer size of the two halls are impressive: 284 Tuscan pillars and 88 Tuscan columns four rows deep surround St. Peter’s Square. Each pillar is 12.80 metres high. At their widest point they measure 1.42 metres. The pillars are made of travertine (freshwater calcareous mud), which was used to build thousands of buildings in Rome.
The dirt was typical for any large city: a stubborn, oily layer that had built up on the stone’s surface because of the heavy traffic in the colonnades’ immediate surroundings. Industry and households had also definitely played a smaller role: there is traditionally less industry in the administrative city of Rome and heating is not used as extensively as it is in these parts. Other contamination discovered included pigeon droppings and some graffiti; however not very much, since St. Peter’s Square is closed off and guarded at night.
After several preliminary investigations, the Vatican Museums and the restoration departments decided to use the particle blasting process. The spray agent used was a powder made of calcium carbonate (grain size 40 – 90 μm). It is very soft (Mohs hardness 2.5) and harmless to the environment. The small amount of water used (50°C, 450 l/h for three blasting guns) also helped to bind the dust and pre-soak the dirt. After the pillars had been blasted they were rinsed with clear water in order to remove any spray agent residues. It took the team composed of three experts and six assistants nine months to clean the entire 25,000 m² surface area.
The results were astounding. The layer of dirt was soot black and the stone surface hidden underneath it was very bright which meant that the contrast between the pillars that had been cleaned and those that hadn’t jumped out. The cleaning completely changed the character of the building. A positively dreary, oppressive hall was converted back into a light-filled, airy building, just as its architect had experienced it.
The most complex task undertaken to date was cleaning the Colossi of Memnon in Luxor, Upper Egypt in 2002. The two 3,300-year-old, over 800-tonne stone figures once guarded the entrance to the memorial temple of Amenhotep III, of which very little remains. As part of various different conservational investigations and measures, dirt deposits which had attacked the surface of the stone were also removed from the monoliths, which are sitting statutes.
Both monuments which are over 14 metres high and made of quartzite sandstone, depict the Pharaoh Amenhotep III and once stood in front of the first pylon of perhaps Egypt’s largest ever temple. The architect was Amenhotep, son of Hapu, who had the Colossi transported 700 kilometres upstream by barge from the quarries of the Red Hill north-east of Cairo to Thebes. The technical question of how they were actually achieved is still a matter under discussion amongst scientists today.
Both monuments today show strong signs of damage from various different influencing factors. In the 16th century, at the time of the Mamluks, the face of the southern Colossus was destroyed with catapults. The huge differences in temperature between night and day have led to micro-cracks on the arms and legs. High concentrations of air pollutants in the morning dew accumulate on the surface of the stone and over the years stubborn encrusted dirt had formed under which salt weathering had damaged the stone.
Dirt particles in the stone’s pores were left there in order to prevent a renewed build-up of particles full of aggressive air pollutants. As a precaution, very crumbly sections, in particular around the head and back of the northern Colossus, were not touched; they were supposed to first be pre-stabilised at a later date. The bases of both statues were also not cleaned, as they are the only ancient Egyptian monuments that still have traces of earlier Nile floods.
The Badenweiler thermal springs date back to the second century AD during the Roman occupation of the
country and are situated between the Rhine and the Danube. The source, which is today is a pleasant 26.4°C, with a flow rate of 1 million litres a day, was perhaps used earlier by the local Celtic population for bathing. As part of the last stage of developments, the symmetrical complex, measuring 33 m x 95 m, included four swimming pools, changing rooms, baths and sweating baths. Considerable remains have been conserved and parts of the drainage and wastewater channels, the substructures, the wall rendering and the underfloor heating can still be seen today. The main building material used was the locally cut “Hauptrogenstein”, a limestone with a grainy surface.
Discovered in 1784 and excavated under Margrave Karl Friedrich, the complex was immediately given a protective roof which has been replaced several times over the years. When, in 2001, it was to be replaced with a column-free glass/steel construction, the entire complex was cleaned with a heated high-pressure cleaner as part of the building work. With a water temperature of approximately 60°C and a flow rate of 1,000 litres of water an hour, a rotary nozzle enabled even heavy vegetation to be easily removed. Being able to adjust the water pressure smoothly directly from the manual blasting gun was a huge advantage because this meant that it was possible to adapt to the very different solidity levels of the brickwork. The sensitive rendered surfaces still standing inside were cleaned with superheated steam.
As part of an extensive overhaul and rework of the Roman soldiers’ baths in Baden-Baden, the entirecomplex was also cleaned to restore it in spring 2003. The ancient bathing complex excavated between 1846 and 1900 is one of the best preserved in Germany. In some places, the walls still stand above head height. The outstanding degree of preservation allows visitors to get a glimpse into the construction of Roman wall and underfloor heating (hypocaust) with the very rare “tubuli” (hot air pipes in the wall).
The aim of the cleaning was to expose and preserve the original building material. The walls, pillars and arches of the thermal springs, which are over 1,900 years old, were extremely dusty and sintered in parts. The floors mainly lay under the rubble of subsequent fixtures and fittings. This was initially removed with the help of shovels and industrial vacuum cleaners. Any loose-lying dirt was then carefully washed away with Kärcher’s HDS 995 highpressure cleaner. As the water input of a high-pressure cleaner is low – much less than that of a garden hose – and the water was immediately vacuumed up again with wet and dry vacuum cleaners, the structure underneath a concrete ceiling was not soaked for very long.
As part of the 2004 Olympic Games, Kärcher cleaned a whole range of listed monuments in Athens and Piraeus. The first monument to be cleaned was Costas Varotsos’ 1988 sculpture entitled “The Runner”, a statue made of glass plates. Its surface was covered in a very stubborn, black layer of dirt composed of soot, oily substances and limescale. This was mainly caused by car exhaust fumes and resisted attempts to clean it with conventional techniques, especially since the sharp-edged yet fragile glass plates could only be cleaned without contact and without using brushes or cloths.
After various lab tests at the Kärcher development centre, a process was found within just two weeks that yielded very good cleaning results: the alkaline cleaning agent RM 31 was first applied using the FS 2000 sprayer and left damp for four hours. Then “The Runner” was rinsed with an HDS 1000 BE mobile petrol-powered hotwater high-pressure cleaner with 80 to 100 bar at 60°C. Osmosis water was also used, which was produced by a Kärcher WTC 600 drinking water treatment system. This helped to prevent a build-up of marks on the glass. Over three working days, a total of 20 hours were spent rinsing in order to remove all cleaning agent and dirt residues between the glass plates.
The city and port walls of Piraeus were built under Themistocles after the first Persian invasion of Greece and connected to Athens via the “Long Walls”. The approximately 2.5 kilometres of wall, which still stands to this today, mostly date back to the year 394 BC and are punctuated by 22 towers. The building blocks of the wall are made up of a shell limestone, the consistency of which varies heavily.
The imposing construction had been defaced at various points with graffiti consisting of various different paints. After several preliminary tests, the decision was made to use low-pressure jets for the cleaning process. The best results were obtained with an extremely fine glass powder meal (grain size 40 – 80 μm, Mohs hardness 6). The graffiti was fully removed without damaging the surface of the stone.
Through this unusual type of cultural sponsorship, Kärcher is making a very unique and personal contribution to cultural heritage which goes far beyond simple donations. The industrial company is not only providing money but also providing the services of its people and their experience, commitment and knowledge. The very close relationship between culture and technology has enabled Kärcher to show just what can be achieved with its machines and what is just as important is the fact that this has expanded the company’s expert knowledge in a unique way which will, in turn, benefit the development of its products.
Frank SchadPress Spokesman,
Alfred Kärcher GmbH & Co. KG