The Candoglia marble extraction process today, yesterday and always…
Insights into the negatives of the Duomo di Milano
On 3 September 2018 a huge block of pink marble was extracted from the Candoglia marble quarries with a majestic roar, whose vibrations shook the very core of the mountain. The block was 60 m3 with weight 2.8 tons, height 5 m, width ca. 8 m and thickness diminishing from 1.45 m to 0.90 m. Given the scarce presence of amphiboles or pyritised layers, ca. 40% of the material will be used for the Duomo di Milano, marking an excellent year.
But let’s step back for a moment… where are we and what do we mean by marble cultivation?
In the heart of Val d’Ossola, at the border between Piedmont and Lombardy, overlooking the southern slopes of the Alps and surrounded by waterways (five lakes, on the banks of the river Toce), we find the Candoglia district. It has been famous since the 14th century for the cultivation of pink marble that is only used for construction works at the Duomo di Milano, with a perpetual concession granted by the Mayor Gian Galeazzo Visconti.
Generations of men have taken it in turns to transform this barren and wild mountain land into vital energy for the Cathedral by placing their manual skills, ancient know-how and the sweat produced by hard work at the service of history, of mankind and of nature.
The first news of extraction activities can be traced to the Roman age, when the quarries were situated where marble surfaced in the valley bottom and was more easily accessible. But the vein of metamorphic rock was long and narrow, generating the need to reach higher altitudes in the course of time. Hence the workers were transferred based on the resources, which were often hard to reach. Of the many caves, which can still be seen today by climbing the green slopes, the “Mother Cave” is still active at a height of 610 m a.s.l., where there is plenty of material available with ideal extraction conditions, namely a lesser incline and a broader descent that is safer than via di “lizza” for handling and transporting marble blocks. The first reinforcement and consolidation actions, and monitoring of rock movements have been implemented here, in the main marble quarry, since 1968. These are the necessary premises for the development of marble cultivation, both vertical and longitudinal. To date the yearly excavated marble suffices to meet the needs of the Duomo’s restoration activities and to replace its deteriorated pieces.
The marble extraction process currently comprises 3 phases: drilling, cutting and tilting. Four holes are initially bored (each takes about 1 hour to perform), first horizontally and then vertically, following the incline of the schistosity to produce less waste. The scope is to create a continuous path for the insertion of a 20 cm diameter diamond wire, made up of a series of pearls placed at an equal mutual distance of 3 cm, impregnated with syntherised diamonds, entirely lined with plastic to avoid sudden and violent dispersion of the pearls in case of breakage. Once the wire is inserted into the 90 mm diameter cultivation holes, its ends are tied to form a ring on one of the pulleys operated by an electric device that moves backwards on a rack. The wire is then loaded, which means that it is repeatedly twisted to prevent the pearls from deforming during the cutting process. Cutting speed is about 12 m2/h but varies based on the minerals present in the rock.
After basic cutting of the back and sides is completed, the marble block is isolated from the rest of the mass. Then, hydrodynamic cushions are inserted into the vertical cuts and inflated by introducing pressurised water. Hence, the cracks are widened until an oleodynamic jack can be inserted to further increase the gap, causing either the entire block or parts of it to tilt. These parts are carried outside the cave with a winch and then conveyed to the sawmill, where they are subjected to final coping.
This process eliminates the “defects” of the block, such as inserts of pyrite or quartzite, uneven surfaces or mineralised layers that are not appreciated. The “scrap” is ground in the jaw crusher with calcium carbonate to obtain a material that can be reutilised as sub-base for roads or for building foundations. The water used during the cutting process is purified and filtered to yield pure calcium carbonate and crystal clear water that flows to the valley. No scrap is left unused at the end of the process.