While visiting Milan recently, we naturally visited the magnificent Duomo Cathedral in the heart of this city. The renovation of the cathedral is ongoing. I thought about giving the City of Milan a bid to join in the restoration project, but was told that we did not meet the prerequisite of 100 years in the stone restoration business (34 years apparently is not enough!).
You can see from the photographs that this project has the potential of job security for life: the restoration is never ending. Once finished, it would be time to start the work over again. Naturally, the professionals who do handle this project have to consider many facets of the work, such as the stone itself. Fortunately, most of the marble on the exterior of the cathedral comes from the quarry of Candoglia, which is not far from Milan. Artisans therefore can study the characteristics of the marble and there are plenty of historical documents going back to 1386, when construction began on the cathedral (completed 1965). Restorers have to identify the various types of stains: pollution, animal droppings, mildew, graffiti, dirt, etc.
What restoration method to use considering the fragility, age and condition of the stone? Can the restoration be done wet or dry? Should chemicals and/or abrasives be used? If a wet process is used, how are the water and slurry collected and disposed of? If a dry method is used, how are the dust and particles of the abrasives collected and disposed of?
I did not find out the answers to all these questions; however, I was told that the dry abrasive method was chosen (low pressure sand blasting). Apparently, the sand used is calcium carbonate rather than silica. Silt size particles are used. This choice was made to avoid runoffs, which can damage the stone below, create disposal problems and most importantly, disturb the tourists.
Because of my profession, I look at stone slightly differently than other tourists. However, like other visitors, I cannot help but stand in awe of the beauty, the ingenuity, the artistry and the dedication of the hundreds of people who long ago designed and built such an astonishingly beautiful monument as the Duomo Cathedral of Milan.
President, International Stoneworks, Inc.