LIKE an old drum skin, the rotten plaster on precious frescoes and murals can
vibrate and fall to pieces at the slightest touch. Unfortunately, the only way
conservators can detect weak plaster is to tap the wall carefully with their
fingers鈥攚hich risks causing further damage.
But now researchers in Germany and Switzerland have found a way to check the
condition of old murals without touching them. They use sound waves and lasers
to generate a map of the weaknesses lurking within a mural. The system is
helping restore Carolingian and Romanic frescoes from the 8th and 12th centuries
at the St Johann monastery in M眉stair, Switzerland. 鈥淥ur images show large
parts of the murals in serious danger of coming unstuck,鈥� says Klaus Hinsch, a
physicist at the University of Oldenburg.
Partially detached plaster in front of a rigid wall will oscillate when
disturbed鈥攍ike a miniature percussion instrument. So the new technique is
designed to cause tiny sound-induced mechanical vibrations that can be monitored
remotely with a laser. This means the technology can also be used without the
need for expensive scaffolding in the typically high-walled old buildings.
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The researchers beam sound waves at the section of the mural being examined,
which induces vibrations in any areas of loose plaster. At the same time, the
section is illuminated with a laser. The vibrations mean the waves of laser
light reflected from the mural鈥檚 loose regions are slightly out of phase with
those from the solid areas.
A video camera picks up the returning laser beam. To extract the information
it contains, the researchers superimpose another beam on it as a reference. The
interference between the returning beam and the reference beam causes the light
to gain intensity wherever the two waves add up, and lose brightness wherever
they cancel each other out鈥攖o a degree depending on the phase difference
between them.
But detecting these phase differences is not simple. Because the frequency of
the plaster vibrations is very small compared with that of the laser, the
differences between the brighter and darker parts of the interference pattern is
very slight. To tease out a more comprehensible picture, the researchers
modulate the phase of the reference beam by shining it through an optical fibre
wrapped around a piezoelectric crystal cylinder. Applying a voltage to the
crystal causes it to change its diameter, and this changes the length of the
path travelled by the beam inside the fibre.
So when the reference beam arrives to meet the beam returning from the mural,
the former has either travelled a longer or shorter path. This means that phase
difference between the reference beam and the beam from the mural is either
larger or smaller. As a result, the brightness of the beam fluctuates and is
seen as a flicker on a computer screen. The flickering parts represent the loose
regions of the mural.
The researchers then probe the same section of the mural using different
sound frequencies. A computer then 鈥渟tacks up鈥� the results across all the sound
frequencies to produce a colour-coded image showing the extent of damage in
different regions.
鈥淏y comparing maps of a mural taken at different times, we can check if the
damage is progressing,鈥� Hinsch says. Thalheim Spezial Optik, a German
manufacturing company, is now collaborating with the researchers to make the
technology commercially available.