Green courtil stone
A UNIQUE PIECE OF THE AOSTA VALLEY EXPORTED ALL OVER THE WORLD
DESCRIPTION OF THE MATERIAL
The aestetic value of Courtil stone lies both in its Arabesque design (which could also be defined “fractal”) and in the combination of colours ranging from white to green, with warmer shades and swirls tending towards yellow.
Its technical value consists of an exceptional tenacity that enables large pieces to be worked to a minimum thickness, also due to its particular natural fissility. We shall now describe the origins of these qualities on the basis of the studies conducted up to now.
The Arabesque design is the fruit of two series of stress forces to which the rock was exposed during the course of geological time. Initially, subsidence (which gives rise to every chain of mountains, 90 million years ago for the Alps) created a rapid succession of small and large folds that deformed the original components of the rock (granite? shale?) to an extent that made them unrecognizable. Subsequently, the rock mass returned upwards at an angle, sliding slowly on itself like a stringy paste for several kilometres. This further deformed the rock into an infinite succession of thin layers all parallel to the same sliding plane, which is now almost vertical in the mountain. This deformation redistributed some minerals inside the rock thus making it easy to split into sheets along foliation planes and favouring its strength in a direction perpendicular to them. In fact, the Courtil quarry was known above all for the unrivalled quality of its “lose” (slate), the traditional stone tiles used to cover the roofs of houses in the Aosta valley.
The colour is due to the natural distribution of coloured minerals in its rock paste. The white veins, which appear flat on the surface of the sheets, reflect a mixture of quartz and albite (albite is a silicate of sodium and aluminium, frequently found in many continental surface rocks). The green streaks in these Alpine rocks indicate the presence of amphibole and chlorite, minerals that contain crystallized iron and magnesium, elements that come from the depths of the Earth’s crust. Finally, the swirls of yellow to pistachio green reveal the presence of epidote, a silicate of calcium and iron that are often found on the surface in place of minerals that have crystallized at depth, and that take the place of jadeite here.
The geological history of Courtil stone is thus identical to that of Silver Green Granite, with one extra stage: the almost complete transformation of the rock during its movement up to the surface, a transformation that is likely to be favoured by the presence of a greater amount of water in the subsoil.
History and Geography of the product
Fort of Bard
The Courtil Stone quarry is situated on the left side of the lower Valley of Champorcher, at an altitude of over 1500 m a.s.l. just below the peak of Mount Charvatton, at the border between the municipalities of Hône and Pontboset, 1 km West of the village of Courtil, as the crow flies.
The stone extracted from the quarry has received numerous historical recognitions for its material and its form. The former include its use in the roofs of the Fort of Bard (shown in the photograph) and several castles of the Aosta Valley, including Fénis, Introd and the Passerin d’Entrèves Castle in Châtillon. The latter include the gold medal awarded at the 1913 Milan Show. Although there is little documentation of these recognitions, there is much evidence of the importance attributed to the road link between the quarry and the bottom of the valley, a problem that was only solved in 1997 with the paving of the road to Courtil with asphalt. In fact, in ancient times, materials were transported using hand-drawn sledges, which was dangerous as well as extremely time-consuming and energy-consuming. The material was brought down a mule track built for the purpose, subjected to repeated and documented maintenance operations, and along part of which transit is possible even today. In particular, in 1890 when the road for Champorcher was built, as its route coincided in part with the Courtil quarry mule track, some crossings were set up and the quarry company was paid compensation for the interruption of transport during the road works.
The blocks of stone are now transported by road from the quarry to the factory in Verrès where they are sawn into sheets following their foliation, and washed with high-pressure water jets. For applications such as ornamental stone and/or interior use, the sheets are subsequently brushed with diamond wire cutters that enhance their velvety appearance, fine grain and soft touch. This refined material has been and is still widely used in interior and exterior floors and coverings in Switzerland, France, Austria and Germany as well as in Italy, especially in the Aosta Valley, often in combination with rustic and precious wood
SLATE ROOFING: AN INNOVATIVE METHOD AND CALIBRATED THICKNESS
Green Courtil Stone is used to cover the roofs of houses. Due to an innovative sawing system, the slating weighs just 90 kg per m2 (compared to 200/300 kg per m2 of other products). With respect to naturally split slating with an irregular thickness, Green Courtil Stone has an unrivalled advantage in terms of both weight and cost of laying.
Using the frame and steel granules, the sheets are sawn along their original veins. The surface remains rough and then the split effect is obtained by subjecting it to a vigorous wash with water at a high pressure (400 bars). After this initial process, the sheets may be used as roof slating. They have a uniform thickness of 20 mm, a size of 1 x 1 mt and a weight of 60 kg/m2. Once laid with the appropriate overlap, they reach a final weight of 90 kg/m2.
Courtil Stone slating has a characteristic colour closely resembling that of the roofs built over the past centuries and, despite their reduced thickness, they have an excellent inhibition coefficient and thus are particularly appreciated in our mountainous region of the Alps, even on mountain huts.
TRADITIONAL FLAT NATURAL SPLIT METHOD
On request, green Courtil stone is supplied to cover the roofs of house with “natural split slating”. When worked in this way, Courtil slating entitles customer to the benefits granted by the law known as “the Aosta Valley regional contribution”.