The origins
In the following paragraphs, after mentioning the relevant legislation, we will recapitulate the physical-geological origins, the processes which determined the formation and technical data of the following construction stones, typical of the Belluno province and used in the past, too in the Veneto region:
- Castellavazzo stone, in Castellavazzo;
- Rosso Secca, in Ponte nelle Alpi;
- Cugnan stone in Ponte nelle Alpi;
- Latreghe stone in Ponte nelle Alpi;
- Cansiglio stone in Tambre;
- Dolomite stone in San Tomaso Agordino.
They are small activities for limestone extraction, in relation to current regulations (Regional Law, September 7th 1982, n. 44. (Quarry activity regulation, Art. 3 – Quarry material classification, group “B” consisting in material whose extraction involves a lesser degree of use of the territory; cutting… limestone…).
GENERAL CHARACTERISTICS OF STONE MATERIALS
Construction stone use is subject to national (UNI) and European (EN) specific technical rules, continually updated: about 30 rules, from terminology and classification to requirements and test methodology. Therefore, technical data – such as those in the following paragraphs, on the basis of information made available by the firms – might refer to old rules, but give main technical characteristics of each material.
Commercially, stone materials are grouped under the following categories (UNI 8458, substituted by UNI EN 12670:2003 – Natural Stone – Terminology):
- Marble: compact and polishable crystalline stone, used in decoration and building, mainly consisting of minerals with hardness betweeen 3 and 4 on the Mohs scale (such as calcite, dolomite or serpentine). E.g. marbles as per the scientific definition recrystallized metamorphic limestone, calciphyres, cipollinos); limestone, dolomites, and calcareous breccias, provided that they are capable of taking a mirror polish; calcareous alabasters, serpentines and ophicalcites.
- Granite: alkaline stone, compact and polishable, used in decoration and building, mainly consisting of minerals with hardness between 6 and 7 on the Mohs scale (such as quartz and feldspars). E.g. granites per the scientific definition (intrusive alkaline magmatic stones, made of quartz, potassium-sodium feldspar and micas); other intrusive magmatic stones (diorites, granodiorites, syenites, gabbro, etc.); correspondent extrusive magmatic stones with pore-structure, some metamorphic stones with analogous composition, like gneiss and serizzo.
- Travertine: sedimentary limestone formed by chemical deposit, with porous fabric, used in decoration and building; some varieties are polishable.
- Stone: used for decoration and/or construction, generally non-polishable. This category includes stones of various mineral composition, which cannot be included in any of the preceding groups. These stones go under two categories: soft and/or not very compact stones and hard and/or compact stones. Examples for the first group are: sedimentary stones (calcarenites, limestone-cement sandstones, etc.), pyroclastic stones (peperinos, tuffs, etcc.): Examples for the second group are: naturally split stones (quartzites, mica shists, slab gneiss, slate, etc.) and some vulcanites (basalts, trachytes, leucitites, etc.).
The mineralogical-petrographic and paleontological characteristics can be determined with macroscopic and microscopic observation: the former by directly observing the just cut surface, and aims to highlight the general characteristics of the stone, such as structure, grain, colour and possible presence of minerals and fossils recognisable with the naked eye; the latter with a polarizing microscope, by examining a significant fragment of stone, in thin section, so that most mineralogical and micro-paleontological elements are penetrated by light and observed in transparency. Polarized light makes the minerals assume characteristic colours – called interference colours – thanks to which stones are unequivocally recognized.
A material’s technical characteristics are determined through physical-technical tests on carefully prepared stone cubes and slabs and are important for deciding their use. In the Belluno province, above all for external usage, the most used tests are the imbibition coefficient and the ultimate tensile strength by simple compression (indicating how compact the stone is), freezing and the thermal expansion coefficient (indicating whether a stone can be used in cold, damp climates).
