Anna has previously talked us through archaeological sites and how conservators assess them, using Billingsgate Archaeological site as an example. But what next? How do you solve a problem like sites next to a marina? Or with a water table that is jusssst a little too high? UCL’s work at Chedworth Roman Villa is a nice example of seasonal conservation work that tackles two problems caused by water: salt damage and microbiological growth.
Built in the 4th Century BC, Chedworth Roman Villa is one of the largest Roman Villas discovered in Britain. The significant remains of three wings (or ranges) of the building survive above ground, the best preserved of which contain the dining rooms and the bathhouses. Originally discovered and excavated in the Victorian Period, the villa is now managed by the National Trust. It is famous, amongst other things, for having the only depiction of a British Native in mosaic form.While the site is stable and well cared for, the site is subject to problems associated with water movement. A relatively high water-table and run-off from the surrounding hills is partially combated by a modern French drain. The water that weasels its way through this defence has a habit of soaking into the foundations. This then allows water to spread to the other surfaces of the building, such as the mosaics. If this water dries, it leaves behind salts on these surfaces. If it doesn’t, it allows microbiological growth, namely algae, lichen, and cyanobacteria (think black sludge) to grow. This can damage and obscure the surfaces. UCL conservation students perform seasonal conservation work, removing salt and algae blooms to help National Trust staff maintain the site.
Sometimes, the cleaning of archaeological sites can involve delicate, painstaking work using specialised tools. Sometimes, as when removing efforvesence (salt blooms) from the surfaces of Chedworth Roman Villa, you do not.
It’s a simple process. Sponges are soaked in de-ionised water, which is then gently pressed against the surface of salt-encrusted tesserae. This dissolves the salt, and allows it to be lifted, wicking the encrustation away to reveal the surfaces underneath. By wearing thick socks, and laying down padded walkboards, we can move safely around the site.
Biological growth is slightly trickier. A fast way of killing and removing microbiological growth is through steam cleaning. These tools kill and remove the algae by applying high-heat steam to the organisms, where surfaces are stable enough to use it. You can see the progress and, as we’re essentially ‘boiling’ the algae off, you can also smell the over-cooked cabbage aroma of progress.
However, some areas (such as the mosaics) are too unstable to use steam cleaning. If attempted, we risk dislodging the tesserae en-mass, and beginning the world’s most stressful jigsaw puzzle. Therefore, microbiological growth is removed slowly, using these very special conservation tools:
But using tiny tools on huge mosaics is time consuming – even by conservation standards, which is saying something. Therefore, in this site we’re borrowing a trick that aquarium-owners use to keep their fish tank clean.
Many fish tanks use UV light to kill algae. By exposing the microbiological growth in Chedworth Roman Villa to specific UV light for four days, we can kill it. As we are able to completely killing the microbiological growth, this process will also slow down the rate that biological growth will colonise in this area. An added advantage is that it also vaporises the remains, leaving the mosaic surfaces largely clear of debris. Any residual remains can be cleaned off using toothbrushes and bamboo sticks much, much faster.
Like painting the Forth Bridge, in moving the UV light source from section to section, it takes some time for the whole surface of the mosaic floors to be treated. This means that the UV eradication takes place when Chedworth is open to visitors. To protect visitors from the UV light, the equipment is contained inside a light proof box , which make it perfectly safe for people to be in the same room.
Having carried out these cleaning techniques, our field trip to Chedworth has left the site cleaner, more stable, more accessible, and more visually appealing. Although it is not possible to totally prevent microbiological growth and salt blooms on the site, these techniques can be used to mitigate the damage that might otherwise result, if left untreated.