Cleaning & Restoring of historic buildings and monument

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.

Inspite ofthe complexity of the cleaning task, listed buildings and monuments must not be detrimentally affected in any way. Traces of the original stone finishing process, for example, should remain completely intact. Even with the most painstaking working methods, this is not always possible.

St Peter’s Square

One of the most spectacular of these was the cleaning of the colonnades of St Peter’s Square in Rome in 1998/1999. Two curving corridors of columns, which give St Peter’s Square its elliptical shape and define its outer boundary.

The colonnades are among the most significant structures of the baroque era: They were built in 1656-67 by Gianlorenzo Bernini. The sheer size of the two corridors impress visitors: 284 columns and 88 pillars of the Tuscan order enclose St Peter’s Square in four rows. Each column is 12.80m high and measures 1.42m at its widest point. The columns are made of travertine, a material used in thousands of Roman buildings.

The contamination was of the kind normally found in large cities: A tough layer containing oil which had built up on the stone surface due to the heavy road traffic in the immediate vicinity of the colonnades. Industry and households probably had less impact: Industry is traditionally not as prevalent in Rome, the administrative and civil service center, and homes are not heated to the same extent as in our climate zone. Other contamination included pigeon droppings and some graffiti, but the latter were very limited because St Peter’s Square is closed and patrolled at night.

After performing several preliminary tests, the responsible Vatican museums and restoration departments decided in favour of the particle blasting method. The abrasive used was a powder made from calcium carbonate (grain size 40-90 µm). It is very soft (hardness 2.5 on the Mohs scale) and safe from an environment point of view. The small quantity of water that was added (50°C, 450 l/h for three blasting guns) served to suppress dust and to soak and soften the dirt. Following the blasting process, the columns were rinsed with clear water to remove any residue of the abrasive used. The team, consisting of three experts and six assistants, cleaned the entire surface area of 25,000sqm in nine months.

The result was stunning. The layer of dirt had been sooty black, while the stone surface it was covering was very light. This meant the contrast between the columns that had been cleaned and those that had not been cleaned was eye-catching. Cleaning had completely changed the character of the structure: A really gloomy and oppressive passageway had been transformed once again into a graceful, light-filled structure as its builder would have seen it.

Colossi of Memnon, Luxor, Egypt

The most complicated task to date had to be solved in 2002 on the Colossi of Memnon in Luxor, Upper Egypt. The two 3,300-year-old stone figures that each weigh more than 800 tonnes, once guarded the entrance to the mortuary temple of Amenophis III, of which only a few relics remain. As part of various conservation studies and measures, these seated figures were freed from the coating of grime that had attacked the stone’s surface. The two quartzite figures, each over 14 metres tall, are effigies of Pharaoh Amenophis III and once stood outside the first pylon of what may have been Egypt’s largest temple complex. They are thought to have been built by Amenophis’ son, Hapu, who had the Colossi transported by barge from the Red Mountain quarry north-east of Cairo 700km up the River Nile to Thebes. Scientists are still debating today what techniques were used to erect them. In Ancient Roman times the Colossi were a popular destination for travellers, who identified the northern statue with Memnon, a hero of Greek mythology. In the early mornings it used to emit plaintive-seeming sounds that were attributed to the hero, who fell in battle outside Troy. However, they are more likely to have been caused by a crack that split the statue during an earthquake in 27BC. One of the numerous visitors who recorded their presence by means of inscriptions in Greek or Latin was Emperor Hadrian. The “singing pillar of Memnon” must have collapsed partially in around 200AD. Roman Emperor Septimus Severus ordered its torso to be rebuilt with sandstone blocks from the Gebel Gulab quarry near Aswan. However, when the emperor died, work came to a halt and the northern statue remained unfinished.

Both monuments have been severely damaged in a variety of ways. The face of the southern Colossus was shattered in the 18th century by Mameluke catapults. Drastic fluctuations between daytime and night temperatures have led to the formation of tiny cracks in the arm and leg sections. High concentrations of airborne pollutants accumulate on the stone surface in the morning dew. Over time, they form stable grime encrustations beneath which saline fissures occur in the rock.

The primary objective of the cleaning operation performed by Kärcher with professional assistance from expert restorer Jens Linke of Mellingen, was to remove the layer of grime that was damaging the stone, thereby stopping the monuments from deteriorating any further. The dirt crust was removed layer by layer with a blasting gun that permits air pressure and abrasive flow to be finely regulated from the handle. This enabled operators to react very flexibly to the noticeable differences in the resilience of the Colossi’s stone surfaces. Surprisingly, remains of the original colour scheme were discovered. Afterwards, they were carefully exposed without any further damage, and are now being analysed with the aim of ensuring their long-term preservation.

Dirt particles in the stone’s pores were left there so as to make it harder for particles laden with aggressive airborne pollutants to settle again. As a precautionary measure badly eroded places, primarily in the head and back areas of the northern colossus, were not treated. They will be cleaned at a later date, after they have been stabilised. The bases of the two statues were not cleaned either, because they are the only ancient Egyptian monuments that still bear traces of Nile flooding in former times.

Monuments in Athens

In the run-up to the 2004 Olympic Games, Kärcher cleaned a whole range of listed buildings and monuments in Athens and Piraeus. The first was “The Runner”, an ingenious work of art made of glass panels by Greek sculptor Costas Varotsos in 1988. Its surface was covered in a very stubborn layer of black dirt consisting of soot, oily substances and lime dust, which came mostly from vehicle exhaust gases and resisted conventional attempts at cleaning, especially as the sharp-edged and fragile glass panels had to be cleaned without being touched, i.e. without using either brushes or cloths.

In a two-week test phase in the laboratories of the Kärcher research and development centre, a cleaning process was found which achieved very good results: First of all the alkaline RM 31 cleaning agent was applied to the surfaces with the FS 2000 prespraying unit and kept moist for about four hours. Then an HDS 1000 BE hot-water pressure washer a mobile, petrol-powered unit was used to rinse the “The Runner” down at a pressure between 80 and 100 bar and a water temperature of 60°C. The osmosis water used in this process was produced by one of Kärcher’s WTC 600 water treatment plants. In this way it was possible to avoid the problem of spots forming on the glass. Rinsing was performed for more than 20 hours, spread over a period of three working days, to remove all residue of cleaning agent and dirt from between the glass panels.

Frank Schad

Alfred Kärcher GmbH & Co. KG