 MARBLE: BEHING THE SCENESMARBLE: BEHING THE SCENES
MARBLE: BEHING THE SCENES
MARBLE: BEHING THE SCENES
MARBLE: BEHING THE SCENES
By Paola Blasi
The possible applications and uses of natural stone are not only a function of its various properties, but are also affected by the bedding of the rock and methods of quarrying, processing, installation and maintenance.
The history of any product made of natural stone, whether modular tile, façade panel or kitchen worktop, begins in the quarry, or even earlier, when the rock strata are identified and described in the field.
In the past new beds of rock were generally discovered by chance, or by reopening ancient quarries.
Today geology, experience and modern technology are used, particularly in countries or regions where stone has not previously been quarried, in order to develop mining inventories to assess the presence and extent of deposits of ornamental stone.
These surveys mean that the nature and potential of the deposit are known even before quarrying begins and so preclude unnecessary expenditure.
Quarrying has to be carefully planned.
Already in the quarry the material reveals its distinctive features.
The degree of fracturing of the rock determines the maximum size of the blocks that can be extracted.
The arrangement of the crystals is an important mechanical and aesthetic factor.
So even in the case of more homogeneous rocks, it is essential to carefully study the orientation of the quarry face to make the most of the material and avoid waste. Non-rational quarrying, as in the case of deposits of erratics (boulders deposited by glaciers), can entail lengthy and therefore expensive selection of the material.
Different cutting planes through the stone will lay bare different kinds of texture and color (and even patterns).
Other important decisions are the kind of machinery and quarrying techniques to be used.
These will depend on the type of material (calcareous or siliceous), the characteristics of the deposit and the location of the quarry, above all the presence of water.
The wrong choice of mining technique can cause serious damage.
For example, using explosives indiscriminately or misjudging the quantities of explosive needed (especially in the case of granite) can cause micro-lesions in the material and so diminish its physical and mechanical properties.
Many important deposits have been rendered virtually useless by the use of heavy explosives.
An example is the famous white marble from the Makrana deposit used in the Taj Mahal.
The effects of this technique can be very subtle.
A block may look healthy and be shipped for sawing, often to places remote from the quarry entailing high transport costs.
Once in the sawmill, the block may crumble during the principal cutting, or the slabs carved from the block may break later, during polishing.
To avoid these problems specific technologies have been developed in recent years to strengthen both blocks and slabs of the materials most sensitive and susceptible to breakage during processing.
The characteristics of the quarry are also very important to designers, especially if they intend to use large quantities of material or special formats.
The availability of natural stone, both the quantity and the maximum size of the slabs that can be supplied, is closely dependent on the nature of the deposit and the technologies used to extract it.
In nature there exist very rich and uniform reserves, like those of the Apuan Alps, Roman and Turkish travertine or the many granites.
But there are also many other materials that are sometimes very valuable but are much more limited and less uniform.
The designer then has to decide which materials will be most appropriate, bearing in mind that nature can supply everything necessary, both in terms of the quantity and characteristics of the material.
Once a block has been extracted from the quarry it is moved to the workshop for fabrication into the finished product.
The technologies used and the degree of experience and professionalism of the experts engaged in dressing the stone are naturally crucial.
Over the last thirty years fabrication technologies have made important advances.
Among the most significant improvements are block cutters for extracting useful material even from shapeless blocks, those which have not been squared.
This means significant savings in the quarry because it recovers material that would otherwise be sent for landfill.
Out of respect for the environment, these technologies have been developed to reclaim blocks and above all slabs.
Excellent results have also been obtained in the fabrication of thin slabs and the use of diamond wire for making curved cuts.
Also in the fabrication stage, the materials are subjected to different processes depending on their properties.
Particular attention is devoted to the most delicate materials, such as colored marbles and those of particular value.
Fabrication techniques, especially for surface finishes, can be extremely important to the designer who wants to obtain specific aesthetic and functional results.
In recent years companies have concentrated on the finishes, creating a very wide range of surface effects that have expanded the choice of the designer and helped renew the image of natural stones.
The visual qualities of natural stone, though very important, are only one of the factors that interest the designer.
The identity of a material includes its physical and mechanical properties and these can be decisive in the choice of one material over another.
In recent years there has been an EU directive which regulates all construction materials.
It requires manufacturers to CE mark finished products.
The directive applies to the stone industry.
Hence outdoor paving stone in the form of slabs and cubes, pavement curbs, stone for cladding walls, stone for flooring and stairs and modular stone tiles all have to bear the CE mark, which includes the product’s principal physical and mechanical properties.
To provide further guidance in the choice of different materials there are numerous catalogs that list the specifications, such as the recent one completed by the Regione Toscana for Tuscan marble and stone.
This catalog can be found online at www.marmiditoscana.it.
Fabrication is completed with the packaging of the material.
This phase is preceded by a careful review of the properties of the finished articles.
In the case of serial products (like tiles, steps, etc.) and those in which uniformity of color is essential, the aesthetic qualities of the material are carefully assessed (to evaluate color and veining where present) and so divide the end product into distinct batches.
This ensures standards of uniformity are maintained, despite the natural range of aesthetic variation in ornamental stone.
As mentioned above, some materials are naturally more homogeneous than others, and in these cases the sorting will be much faster and above all the amount of material discarded will be significantly lower, making for considerable savings.
The concept of uniformity in stone always needs to be understood flexibly, as the natural variability of these products is their greatest value.
Even the ceramic industry, always very careful to reproduce the beauty of nature in various ways, has launched products that seek to reconstruct the diversity of marble and the beauty that every single stone product possesses: the fact that it is unique!
Finally, in the case of cut-to-size products, it is important to do the pre-laying and marking of the pieces before delivery to the building site so as to facilitate laying.
To make the most of the product it is all-important to ensure correct techniques of laying and maintenance.
The success of the work and the complete satisfaction of the customer or user call for the proper selection and sizing of the stone and also of the techniques used for laying it.
Inappropriate techniques can damage both the material and the whole work.
For this reason it is important to consult specialists in the laying of stone products, who use specific products and follow the recommendations contained in the national or international codes of practice.
The same applies to maintenance and cleaning, which can damage the materials in various ways.
For example, the use of aggressive or non-specific products might alter the appearance of the stone by changing both the coloring and its composition.
Here, too, it is important to use the right products for ordinary and extraordinary maintenance, taking into account the different kinds of stone, with particular reference to its chemical composition (calcareous or siliceous).
All this shows the importance of the whole range of professional skills involved in the production cycle and the need to choose and apply stone materials with respect for this resource, which is non-renewable but, if used correctly, can last practically forever.
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