One of the main problems in façade cleaning is the sheer size of most of the buildings that are to be cleaned. The use of high-precision cleaning technology that acts on a small area at a time is often precluded. This is not only for economic reasons, but also from a restoration point of view and for aesthetic reasons because it is hardly possible to achieve uniform result over the entire façade (“chequered effect”). Other factors that must be mentioned here to give a general picture of the complications frequently encountered in façade cleaning are: difficult access to higher parts of the building, the problems involved in collecting, treating and disposing off dirt and the simple fact that cleaning agents often do not have sufficient time to react on vertical surfaces, and the dirt running off can cause secondary soiling of the lower parts of the building. Sometimes dirt particles penetrate the historic fabric of the building and merge with it beneath the surface so that becomes impossible to differentiate between dirt and the original material.
For conservational reasons it is advisable in most cases to remove the layers of dirt from building façades at an early stage. For example, deposits on natural stone surfaces can cause an increase in resistance to vapour diffusion. This, in turn, may result in a build-up of solutions containing pollutants and, ultimately, disintegration of the material structure. Furthermore, the removal of layers of dirt is often a precondition for further restoration work because only after the foreign matter has been removed from the original material can an exact assessment of its condition be made and subsequent measures planned. In many cases, the objective of cleaning is to make the monument accessible and discernible in its original state.
Restorative cleaning operations should be undertaken in close co-operation with the official custodians of monuments, conservationists, restoration experts, art historians and other scientific specialists. In this process the condition of the façade material and the degree of contamination are established in preliminary examinations. A visual inspection is often not sufficient for this purpose and it is necessary to perform further scientific tests with the aid of specimens, for example. In a second phase test areas are cleaned using different cleaning techniques and parameters in order to determine the best possible method. Numerous cleaning methods have been developed for façade cleaning; the two methods that are used most frequently, also on non-listed buildings, are particle blasting and high-pressure cleaning.
In particle or low-pressure blasting, a stream of air is used to which an abrasive is added in precise doses. This method can be adapted to suit almost all façade surfaces by selecting the appropriate blasting nozzle, the type and quantity of abrasive and the required air pressure. There are currently about 2,000 types of blasting abrasives in the market, from cornmeal to lime and glass powder and beads of dry ice. They differ not only in respect of the source material used, but also in the size and shape of the granules. Dust can be substantially suppressed by adding water at the nozzle (“wet blasting”). The water and any pollutants there are bound in the abrasive and disposed off together with it. Disposal of the abrasive is simplest in dry-ice cleaning: It literally disappears into thin air.
High-pressure cleaning offers a number of advantages which make it ideal for complex operations such as cleaning the façades of listed buildings. It is on par with particle blasting as far as flexibility on different surfaces is concerned: The transport medium in both techniques water in high-pressure cleaning, air in low-pressure blasting moulds itself to all surface contours. When cleaning with the high energy jet of water, it is possible to adapt the parameters that affect the result, i.e. mechanical action and time as well as temperature and chemical, to exactly match requirements.
One of the reasons why the pressure washer is not readily associated with the highly delicate materials of historic buildings is probably the very term “high pressure“. With this technique, however, it is possible to work with very low pressures, almost pressureless in fact. Generally speaking, the force of the so-called impact pressure is usually overestimated:
Compared with the pressure measured directly at the high-pressure nozzle, it actually drops exponentially as a function of the cleaning distance from the surface. At a distance of 40cm, for example, not even 1 bar arrives at the surface, even when the nozzle pressure is 200 bar.
As far as the mechanical cleaning efficiency of the high-pressure jet is concerned, it is not only as is often wrongly assumed the pressure produced in the pump, but also the water flow rate that is important. In actual fact it is the sole factor when it comes to flushing away the dissolved dirt. Moreover, it is not only the pressure washer’s nozzle pressure that is responsible for the impact pressure of the water on the surface, causing the layer of dirt to be broken down and dislodged, but also the volume of water that flows through the pipe cross section in a given time.
The mechanical action of the high-pressure jet also depends to a great extent on its impact angle and, consequently, the choice of high-pressure nozzle. Using a fan jet nozzle means a high area coverage but low cleaning power compared to a pencil-jet nozzle which possesses a better dirt busting action. Its disadvantage, however, is its narrow working width which means it is unsuitable for large-area operations. In the case of changeover nozzles, the pencil and fan jet nozzles are mounted on a common nozzle holder so that it is not necessary to carry additional nozzle kits. The changeover from pencil to fan jet is effected by simply twisting the lance.
The rotary nozzle (Dirt Blaster) combines the advantages of the fan and pencil jets. It is used for removing stubborn dirt and stains and for façades that are not pressure-sensitive. In this nozzle a pencil jet is caused to rotate at high speed (about 4,000 revolutions per minute); a uniform spray pattern is achieved by the complementary action of the rotating motion of the pencil jet with the linear motion of the spray lance.
Both hot-water and cold-water pressure washers are used for façade cleaning. Water heated to 80°C can shorten cleaning times by up to 60% and greatly accelerate drying, too. On hot water units with a steam mode, a steam spray jet can be employed in addition, which is particularly suitable for pressure-sensitive and intricately shaped surfaces. The higher heat transfer rate dissolves residue with a high melting point faster and much more easily than with the hot water high-pressure jet, whose higher impact pressure results in better dirt removal on mineral stains, for example. On the other hand, the steam jet has good creeping properties, does not splash back and is more economical in terms of water and cleaning agent consumption. Conversely, the better flushing action and the reduced build-up of vapour are points in favour of the hot water jet.
Finally, the pressure washer can also be used to apply cleaning and care products as well as biocides. Mainly neutral, mildly alkaline and mildly acid cleaners are used. Cleaning is performed according to the two-step method in most cases in order to make full use of “time” as a cleaning factor. With this method, the cleaning agent is applied in the first step and, after a certain reaction time, the dissolved dirt is washed away using high pressure. Foam and gel cleaners considerably increase the reaction time on smooth surfaces. In the past 25 years, Kärcher has used the methods described above in its cleaning projects on about 90 monuments all over the world as part of its culture sponsoring activities.