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.
I
n 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.
