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.
Hardly any other cleaning task requires the same amount of knowledge and experience or such technical and equipment-related investment. The variety of materials with which building surfaces are made of, their degree of weathering, their different (threedimensional) structures and shapes and the types of dirt that can be found on the outside of buildings mean that a solution must always be based on the specific conditions on site.
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.
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 highpressure 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.