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.
In Roman antiquity, the Colossi were a popular destination, as the northern of the two was linked to Memnon, a hero of Greek mythology. During the morning it would make seemingly doleful noises which were attributed to the hero who was slain in Troy. However, it is more likely that these noises were the result of a crack that shattered the statue during an earthquake in 27BC. The Roman Emperor Hadrian was amongst the visitors who documented their attendance with Greek or Latin inscriptions on the legs of the Colossus. Around 200AD, the “vocal Memnon pillar” must have partially collapsed. The Roman Emperor Septimius Severus had the upper tiers reconstructed using sandstone from the quarries of the Gebel Gulab near Aswan. When the emperor died, however, the work stopped and the northern statue remained unfinished.
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.
Removing the layer of dirt that was damaging the stone, and hence preventing the monument from further decaying or at least slowing the decay down, was the main aim of the cleaning work which was overseen from a technical point of view by Grad. Jens Linke, a restorer from Mellingen. The encrusted dirt was removed layer by layer with a Kärcher blasting gun with which the air pressure and amount of spray agent can be carefully controlled using the handle. This afforded the team flexibility in the way that they cleaned the Colossi as there were huge variations in the resilience of different sections of the stone’s surface. Surprisingly, remnants of the original paintwork were discovered and subsequently cautiously exposed without damaging them.
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.