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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.