Candoglia, the thrilling experience of marble mining in the quarry is renewed once again
All the secrets of the extraction process
On 25 June 2019, with a majestic rumble that vibrated deep into the heart of the mountain, the third of three pink marble blocks was extracted from the Candoglia Quarries. Annually mined in spring, their dimensions are 50 cubic metres with an approximate weight of 135 tonnes. The Duomo’s annual need for marble is in the range of 150 cubic metres, of which only 35-40% will be actually used, since the remaining part will invariably become processing scrap. The extraction process was particularly interesting this year. The huge prisms formed in the quarry by the convergence of the detected natural cuts and the implemented artificial cuts present amazing shapes. The last block to be cut yielded a magnificent monolith. The “slice” just extracted fell on a bed of inert material that was ready to receive it in two phases: first a small fraction produced by a fracture inside the deposit, followed by a second single block of extremely precious marble for its unusual and remarkable dimensions. The Candoglia marble quarry was formed by precompression of a calcium carbonate lens that was processed through geological eras by the movements of tectonic plates, which induced greater tension inside the marble than the force applied by the mountain. A genuine mineral wonder!
But let’s step back a moment… where are we and what is marble mining?
The Candogliadistrict rises in the heart of Val d’Ossola, at the Piedmont and Lombardy border, facing the southern side of the Alps and surrounded by waterways (five lakes, on the banks of the river Toce). It has been famous since the 14thcentury for the mining of pink marble that was solely used for construction works of Duomo di Milano with a perpetual concession established by the city’s leading authority (Podestà) GianGaleazzo Visconti.
Generations of men have taken it in turns to transform this barren wild mountain into living energy for the Cathedral, placing manual skills, ancient know-how and perspiration at the service of history, of mankind and of nature.
The first news of extraction activities can be traced to the Roman period, when quarries were situated at the marble outcrops in the valley, which were easily accessible. But the vein of metamorphic rock was long and narrow; therefore, they had to rise in the course of time, relocating workers according to the resources, which were often hard to reach. Of all the quarries, traces of which we can still notice by climbing the green slopes, the “Parent Quarry” is still active at a height of 610 m a.s.l., where there is a lot of material available and where extraction conditions are better: the lesser inclination and wide descent are safer than the “via di lizza” to handle and transport marble blocks. Early reinforcements, consolidation and monitoring of rock movements have been implemented here, at the parent deposit, already since 1968. These are the necessary premises for the development of marble mining, both vertical and longitudinal. The amount required for restoration and replacement of damaged pieces at the Duomo di Milano is yearly mined.
The marble extraction process currently comprises 3 phases: perforation, cutting and tilting. The first phase entails perforation of 4 holes (each requires about 1 hour), first horizontally and then vertically, following the inclination of the schistosity in order to produce less discard. The scope is to create a continuous circuit into which a diamond wire with 20 cm diameter is inserted. It is made up of a series of pearls placed at regular intervals of 3 cm, impregnated with syntherised diamonds and plasticised to avoid sudden and violent scattering of the pearls if it happens to break. Once the wire is introduced into the mining holes having a diameter 90 mm, it is closed to form a ring on a pulley handled by an electric machine that moves backwards on a rack rail. The wire is then loaded, which means that it is repeatedly twisted on itself to prevent the pearls from being deformed during the cutting process. Cutting speed is in the range of 12 m2/h but varies based on the minerals present in the rock.
When the base, back and side have been cut, the marble block is isolated from the remaining mass. At this point hydrodynamic cushions inflated by introducing pressurised water are introduced into the vertical cuts. The spaces are increased until an oleodynamic jack can be inserted to further increase the distance between the cutting lips, thus causing either the whole block or parts of it to tilt. These pieces are conveyed outside the tunnel by a winch and then to the sawmill for squaring.
This process eliminates any “defects” presented by the block, such as pyrite or quartzite inserts, irregular areas or levels characterised by undesired mineral deposits. The “discards” are then ground with calcium carbonate in a jaw crusher to obtain material that can be reutilised for the road base or for building foundations. The water used during the cutting process is purified and filtered, yielding extremely pure calcium carbonate and crystalline water that runs to the valley. Conclusion, there are no unused discards.