Throughout history the stone has been used several times, with different methodologies, as external or internal coating to the surface of walls.
In antiquity the lining of a stone building was built from thick slabs that made up nearly a second wall. The chromatic effect that you obtained was of excellent quality and embellished the walls texture of the building. An example that you can cite is the lateral scale wall of the Church of San Ciriaco in Ancona, here alternating two types of stone material (pink marble of Verona and the white stone of the Conero) recounts a long history of careful selection and processing of coating to produce a schema to alternate courses. Also the chromatic effect is testimony to centuries of restorations, additions and restorations of wall surface. (Figure 1)
In the Romanesque period the custom of covering stone buildings was little widespread, using this technique only for fine buildings, taking advantage of the properties of stone materials to give greater hardness, a different coloring and a longer duration of the entire Cabinet.
Spread to cover buildings, including brick, occurred in later times. The lining was now used to protect the most exposed parts of the parietal surface such as the ground floor (subject to the phenomena of capillarity bounce or rain water and high humidity), the cantons and the skirting boards.
In the 20th century, right in the Fascist period, there was a reminder of the ancient tradition, using the stone as the lining of the entire building. The architect Terragni, emblem of the regime, was one of the masters who managed to combine the use of antique material like stone to new architectural trends that were emerging international at that time in different parts of Europe. This capability also is observed in successful determination to forge the link between progressive and conservative aspects of the fascist regime.
A significant example of his work is the Casa del Fascio of Como, used until the late 1930s as the local fascist party headquarters. Here the facade has a strict and linear appearance, also the entire building was covered with white marble and finely worked (with new cutting techniques tested in those years) which prevents any reference to mechanistic processing but suggests instead a solemn and celebratory character at work. (Figure 2)
Architects saw a number of issues arise during the construction of the stone wet coatings with the use of lime and cement.
Dynamic tensions created by a continuous thermal displacement between the different hours of the day, caused by direct radiation during the day and the rapid cooling at night, formed cracks and cracks in the material because the latter, having decreasing thickness, could not deploy dynamic tensions on adequate thickness and had a very high thermal storage.
The first system used to resolve this problem, use copper or stainless steel nails to fasten the cover to the wall, was not successful because increased dynamic tensions of the material and then the consequent breaking of the lining.
Only after decades of attempts you arrived at a solution that responds satisfactorily to the phenomenon of dynamic tension crack. The criterion used was to detach the stone plate from the wall using metal clamps that allow a certain freedom and the ability to increase or decrease its size due to thermal variations.
The techniques used for lining the walls today with stone material are mainly classified into two categories:
- Ventilated facade system: is to predict, at design and implementation, a special ventilated space, for the so-called "chimney" effect, on the back of the outer surface of the wall. It solves simultaneously problems related to excessive heating and condensation of the vapor coming from inside. (Figure 3)
- Courtain wall system: consists of an outer frame construction of the building to support the insulators, the cladding panels and glazed surfaces. Has an analogy with the system described above, given by the fact that even ventilation of large glass surfaces because it forms a continuous air Chamber for which can be adopted innovative technologies such as active, interactive facade, shaded or bioclimatic.
Interesting applications of external facing stone are today at experimentation and implementation, as evidenced by the Bilbao Museum by Richard Grey where for each slab mats were studied, using computer models, appropriate bends and dimensions to provide the entire support system a single plasticity. (Figure 4)
Despite being one of the most durable materials present in the broad field of the building industry, the stone is subject, albeit slowly, to phenomena of degradation caused by biological agents (bacteria or molds that ruin the material), chemicals (like on the surface crystallization of soluble salts from pollution and resulting acid rain), physical (freeze-thaw cycles that cause fractures in the material due to the expansion of the volume of the water and the subsequent withdrawal of its volume).
To perform proper maintenance of external surfaces of stone you proceed through two phases:
- the first consists in cleaning the surface with blowing or brushing using powerful jets of water or sand;
- the second phase is to direct intervention on the consolidation of the structure by incorporating special resins which may be organic or inorganic.