Laser cleaning a stone capital from Lewes Priory, Sussex
This capital (the top of a column) is originally from
Lewes Priory in Sussex, England. This capital is made from Caen
stone, which is a creamy-yellow limestone quarried in the northwest
of France. This type of limestone is able to take fine carved
detail as on this capital and is a hardwearing stone.
The capital has four carved faces showing
biblical scenes from the life of St Peter. The carving is very
detailed and generally in a good condition, but a few areas have
suffered from weathering and a thick layer of grey/black dirt has
accumulated on the surface over time.
To remove unwanted dirt from stone
conservators usually carry out small cleaning tests on an
inconspicuous part of an object to determine the most appropriate
method of cleaning. Depending on the type of stone, and the
condition of the stone surface, established conservation cleaning
methods can range from steam cleaning and solvent cleaning, to
poultice treatments and dry cleaning, which avoids solvents or
water.
After carrying out unsuccessful cleaning tests
on the Lewes capital using traditional cleaning methods it was
discovered that laser cleaning using an Nd:YAG laser was a
considerably more successful treatment for removing the black
pollution crust on the capital to reveal the natural colour of the
limestone. The laser is a non-contact method of cleaning which can
accurately remove the types of ingrained dirt layers similar to
that on the capital without removing any original material on the
underlying surface. Another advantage of using laser cleaning was
that the beam of the laser could be precisely directed to clean
into the deepest recessed areas of the carving.