Antique Underwater Treasures Endangered

Stereo Microscope Reveals Destructive Work of Marine Sponge

December 15, 2009

In the 2nd century BC, Baiae in the Gulf of Naples was a notorious bathing resort and spa for wealthy Romans. Today, part of the town is submerged under the sea and can be visited as an 80,000 square metre archaeological underwater park. The magnificent mosaics from the underwater ruins are analysed at the Istituto Superiore per la Conservazione e il Restauro (ISCR) of the Ministry for Cultural Heritage and Activities in Rome: Using a stereo microscope, scientists track down the activities of an endolithic sponge that is threatening to gradually destroy the antique limestone tesserae.

Disappeared in the sea

Baiae’s rise to fame as a luxury spa of the Roman élite began in the 2nd century BC. The volcanic soil not only provided excellent growing conditions for olives, chestnuts, fruit and wine, but also hot mineral springs and sulphur vapours in abundance – and since Hippocrates, these were well known for their power to heal all kinds of ailments. Wealthy Roman citizens, senators and emperors came to Baiae, built splendid villas and thermal baths, enjoyed bathing in the hot springs and indulged in a sensuous life, as some antique authors criticised. However, the same volcanic activity that brought fame to Baiae was also responsible for its demise. The geological phenomenon of bradyseism (the uplift and subsidence of ground in a volcanic area) caused the entire coastal region at the Gulf of Naples to sink six metres in the year 300 AD: Baiae and its palaces disappeared in the sea. Today, the sea is giving back some of the antique ruins, as the sea bed is rising several millimetres a day.

Fig.1, 2: Stereomicroscopic images of tesserae sections showing the system of holes and tunnels made by the endolithic sponge.

Animals bore tunnels and holes

Gianfranco Priori, member of the "Nucleo per gli Interventi di Archeologia Subacquea" of the ISCR, studies the mosaics the wealthy villa owners used to decorate their floors and walls together with a multidisciplinary team of archaeologists, chemists, architects, conservators and biologists. Made of limestone tesserae, the ancient works of art are obscured by thick colonies of animal and vegetable organisms that are gradually destroying the material. The sponge Cliona celata is causing particularly invasive bioerosion. This sponge uses acid secretions to bore tunnels in calcium carbonate in which the animal then lives.

Fig. 3: Mosaic floor in the underwater park of Baiae: The limestone tesserae show distinct signs of destruction by Cliona celata.

Microscopic vision reveals destruction

"On the surface, the bore holes are almost imperceptible", says Priori. "Only when we examine them under a microscope in the lab can we see the real state of decay." To document the visual state of the mosaics and take samples of the tesserae, the scientists have to view them in situ – that means under water. First, they take photos at a depth of five to seven metres with the underwater camera. The samples taken there of the antique limestone mosaics are then examined in the laboratory with an electron microscope, a research microscope and a stereo microscope with integrated camera. The stereo microscope visualises both the bore holes at the surface and the interior penetration pattern of the sponge.

A digital imaging software is used to count the borings and measure their diameters. "It was the stereoscopic examination that showed that the state of the mosaic plates is worse than was to be expected from looking at the surface", Priori sums up. Morphological analysis of the tesserae reveals a system of interconnected holes and tunnels whose dimensions are far greater than the bore holes visible on the outside of the sample. Curiously, the number of interior cavities is in no direct relation to the number of bore holes on the surface.

"In our investigations, we not only benefited from the optical qualities and high resolution of the stereo microscope, but particularly from the perfect interface between the stere omicroscope and the imaging software", says Priori, who is responsible for microscopy and photomicrography in the laboratory. "In addition, the software with the Multifocus module extends the microscope’s depth of field many times over."

Fig. 4: Gianfranco Priori uses the stereomicroscope to examine the bore holes on the surface and also the interior penetration pattern of the sponge.

New method for retarding bioerosion

The study of the ISCR is one of the first works on the impact of marine endolithic organisms on historical works of art located under water. The examinations are now to be extended to other types of stone such as white or multicoloured marble. "The most significant result of the work we have done so far is the development of a method for retarding bioerosion", comments Priori. In the winter months, when fewer tourists come to Baiae, the mosaics will be covered with a geofabric that limits the water flow and sun radiation on their surface and restricts the settlement and growth of many animal and vegetable species.

Fig. 5: In the winter months the endangered mosaics are protected by a geofabric.

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