PRODUCTION OF NATURAL STONE
The world of construction and design owes much of its splendor to the timeless elegance of natural stone. From majestic facades to exquisite interior finishes, natural stone is a hallmark of sophistication. However, the journey of transforming raw stone into the magnificent facing materials we admire is a meticulous and fascinating process that marries artistry with precision.
Extracting the Essence: From Quarry to Block
Facing stone typically resides near the surface, awaiting its transformation into architectural marvels. These stones are extracted from quarries, usually in the form of large blocks. A stone block is a substantial block that is mined and then processed into facing materials that adhere to specific standards for physical and technical characteristics, dimensions, solidity, and shape.
The nature of these stone blocks can be categorized based on the extraction method and the precision of their geometric shape. They are divided into two main types: sawn and chipped blocks. The volume of a block is determined by the volume of a rectangular parallelepiped that can fit within it.
The process of developing facing stone deposits encompasses three primary phases:
- Opening the Deposit: This entails the initial steps of revealing the stone deposit’s location.
- Preparatory Work: Creating the necessary outcrops and ledges to facilitate extraction.
- Mining Operations: The actual extraction of stone blocks.
Extraction activities include the separation of monoliths or blocks from the massif, cutting monoliths into blocks, loading and transporting both blocks and waste. One crucial aspect of extracting stone blocks is the preservation of the stone’s natural qualities throughout the process.
Stone Cutting: Where Art Meets Technology
Cutting and shaping stone blocks require a blend of craftsmanship and advanced machinery. Various tools and techniques are employed, depending on the hardness of the rock:
Stone Cutting Machines: Equipped with cutting discs or ring mills, these machines are used for softer and medium-hard rock, such as marbles, limestones, shells, volcanic tuffs, and dolomites.
Rope Saws or Cutters: These tools are also used for extracting softer and medium-hard rocks.
Drilling Stone-Cutting Units: This method involves drilling holes to weaken the rock structure.
Explosion Destruction: For particularly hard rocks, drilling and blasting techniques are used to extract the stone.
The choice of method depends on the specific characteristics of the stone and the desired result. It’s crucial to ensure that the physical and mechanical properties of the stone remain intact during cutting. Any defects, such as fracturing, weathering, non-polishing areas, or dirty spots, can diminish the stone’s quality.
Rigorous checks are performed on the stone’s physical and mechanical properties as well as its appearance and surface quality. Blocks must be crafted from rocks with specific physical and mechanical attributes. The appearance of the blocks is assessed based on criteria like the deviation from the right angle of two adjacent faces, the number of repulsed angles, the length of the chipped corner, and the presence of diagonal cracks on adjacent faces.
The Artistry of Stone Processing
Stone processing takes the rough-hewn blocks and transforms them into works of art. This intricate process involves a series of technological operations to shape the stone, determine its dimensions, and create the desired surface texture. The scheme of stone processing includes three key phases:
Approximate Processing: This initial step shapes the stone roughly to the size and form of the intended product.
Precise Processing: Here, the stone is meticulously carved to meet the exact dimensions and shape required for the final product.
Textured Processing: This phase imbues the stone’s front surface with the desired texture, giving it the unique character that defines it.
In conclusion, the journey from raw stone to magnificent facing materials is a testament to human ingenuity and artistry. Natural stone, with its enduring beauty and versatility, continues to be a source of inspiration for architects and designers worldwide, enhancing the world’s architectural treasures with its timeless allure.
Stone Finish Types
Surface finishing is the treatment that brings out the esthetic features of the material. The ornamental function and also some technical characteristics (e.g. its resistance to wear and weather conditions or its slipperiness) are strongly influenced by the surface finishing applied to the product. Depending on the treatment, we can divide the finishing into mechanical, impact and chemical methods.
Mechanical Finishing
In mechanical finishing, the stone is put in contact with an abrasive to reduce the original surface roughness to some extent.
Rough – Though infrequent, sometimes the sawn material or even just-quarried material is ready for installation and needs only to be cut to size. The surface in this case is generally rough, with an uneven face. Rough stone is predominantly used outdoorswhere it is appreciated for its non-slip quality. It is often used with slate and with some kinds of sandstone.
Honed – This finishing aims to produce a smooth surface by using abrasives of ever finer grain on the surface, so there is not a single honing but a series of progressive degrees of it. Honed finish is not reflective and makes the color tones slightly dull, but the treatment preserves the material’s natural esthetic characteristics.
Honed The difference between this type and the preceding lies only in the polish of the surface, so that the same types of protective treatments recommended above can be used also in this case. We can also recommend an alternative wax treatment, whereby this type of surface can be treated with two applications of a matte-finish wax, which is then obviously not polished. We can recommend this type of treatment in cases in which the customer—having chosen a smooth but opaque surface finish—wishes to keep the finish unchanged. A similar result can be obtained using a water-and-oil-repelling product or a combination of water-and-oil-repelling products, which normally gives a better result in terms of proofing but does not guarantee dirt repellence, which is what waxes do.
For routine care, the same considerations mentioned previously apply. As for special maintenance, we must distinguish between the two cases. Crystallization is not an option, and staining is not a problem due to the inherently opaque surface of the material. In this case, we may see stains due to the color of the staining agent, whether it is coffee, the tomatoes in ketchup or red wine. In such cases, a special color stain remover must be used. For oily or greasy stains, stain remover sprays or poultice are both excellent solutions.
Polished – is the main and most frequently applied finish. It follows the finest honing and employs polishing abrasives that add brilliance with mirror effect to the stone surface.
Polishing Just as with marbles and polished limestone, travertine is normally only used for special applications such as bathroom and kitchen flooring. In this case, protection is assured by the use of special water-and-oil-repelling products which protect the surface without generally changing its look. After application of these products, a surface residue can crop up. This must be removed after drying, from 4 to 24 hours after treatment according to the type of solvent used in non-walkover areas such as: vanity tops, cook tops, thresholds, etc.
Another widely used protective treatment is waxing. In the case of home floors, we recommend the use of traditional polishable wax, which requires polishing with a cloth or polisher after application to give it a shine.
In the case of public installations, and hence with high foot traffic, we recommend the use of self-shining waxes, such as metallized waxes.
Unlike marble, which is normally waxed when its shine starts to show wear, waxing on travertine can be carried out anytime in order to protect its surface. Foot traffic can damage the original shine faster than in the case of marble or granite, which are harder materials.
The material must be maintained with a neutral detergent to avoid damaging the surface, its shine or protective treatment. It can be applied manually, with a bucket, mop or brush and rag normally in a 3-5% dilution, for traditional maintenance over a small surface area. It can alternatively be applied with a scrubber-drier machine for larger areas, in which case the dilution will normally be 1-2%. If stronger solutions (5-10%) are required for washing away more tenacious stains, a final rinse is required.
In the case of waxed floors, an alternative is 200-300 ml (8-10 fluid ounces) of a “wash” and “wax” product that is poured into the washing solution. If the floor is treated with metallized wax, the normal procedure for these waxes must be followed. This includes the above-mentioned washes as well as de-waxing every one to two years, using a special wax-stripping detergent at the recommended dilution, with a subsequent application of two to three coats of the same wax. A further maintenance treatment is crystallization, using a special crystallizing product. This treatment is used in cases in which the mirror finish of the travertine is moderately damaged and the user wishes to avoid waxing or re-polishing with a wire wool disk added to the weight of the professional polisher. When the damage is significant, however, we recommend re-polishing the floor mechanically in the traditional manner.
There is also special maintenance, which falls outside of the normal routine care schedule. Two of these operations have already been mentioned, crystallization and de-waxing. There is also a third procedure: stain removal. A polished travertine surface can be stained in a variety of ways, especially if not treated. Note that in many instances—as with marble and polished limestone in general—these defects will not be penetrating stains, but rather surface opacity, which looks like a stain at first glance. This can happen when an acidic liquid is spilled onto the limestone surface: coffee, wine, ketchup, tea, beer, soft drinks, lemon juice, etc. In general, this covers the majority of food stains.
In such cases, there is an aggressive chemical reaction with the surface. The staining agent dissolves the salt constituting the mirror surface and renders it opaque. Normal protective agents can slow down and hold back this reaction, but they cannot fully prevent it. Only by creating a significant surface layer can the material be protected against stains of this type, but such a protective layer would destroy the natural aspect of the surface, and this is not generally an attractive option. A surface which has been damaged in this way can be partially restored with crystallization or using a polish. Other typical stains are those due to grease or oil, which can be completely removed using a stain remover spray or with poultice.
Impact Finishing
Brushed – Brushed finish is obtained by applying hard plastic or metal brushes to the stone surface. The heavily action removes the softer part of the stone and wears out the surface, giving it a look similar to that of antique finishing.
Bush-hammered – Bush-hammering is done by hammering the stone with a series of steel points to break up the surface and create a pitted look. Similar to the flamed finish above, bush-hammering is a technique for creating highly slip-resistant surfaces. Like all the processes, it can be done to a greater or lesser degree. It can also be used to recreate the look of a medieval finish, as though the stone had been chiseled by hand centuries ago. A final gentle brushing with steel brushes then gives it the “time-worn” look. The image below is a French limestone, Vallangis, that has been bush-hammered and brushed.
Chemical Finishing
Special machinery that looks like industrial washing machines is used to obtain an antique finish. The pieces to be treated are put in the machine with abrasive elements and the cylinder revolves. In a short time the impact of the stone with the abrasives produces an effect similar to aging caused by use and wear. The impact method is not suitable for large pieces, for which brushing or acid washing is the method of choice.
Chemical finishes are applied to stone in order to produce reactions that transform the material surface, or they are employed together with other types of treatment in order to improve their characteristics. These finishes can also be applied to cut, or even installed, materials.
Acid Wash – Acid washing has a corrosive action on the stone. It can be used to obtain different effects depending on the material, the chemical, and finally, the processing time. Finishes can range from simple superficial cleaning of the material to a more definite ruggedness, similar to that achieved by water finishing. Acid washing is sometimes used to obtain an antique finish in place of the impact method. It is possible to acid wash already cut pieces or, with appropriate precautions, already installed ones. Some chemicals produce other results affecting the aspect of the stone but not its roughness. These are acids that remove oily or rust spots on the material. However, there are others that instead induce oxidation effects and are employed to change the material color.
Initial Wash – In the case of a rustic finish—whether it is tumbled or sandblasted material—and when removing installation residue (i.e. cements), disregard the above cautionary remarks and use an acid de-scaling agent, because this is the only way to thoroughly clean the surface of these residues.
In this case, we strongly recommend the use of buffered acid de-scaling products, which are free of strong agents such as muriatic acid, at the greatest dilution compatible with the amount of dirt to be removed. Note that the material must be professionally installed. It is important to leave the smallest possible amount of residue so that it can be cleaned without the use of powerful chemicals.
Do not use acid products. Since travertine is a calcium-based material, it reacts with acids and is dissolved by them. Use only neutral or alkaline detergents, depending on the type of residue to be removed.
In the case of polished travertine, do not use strong alkaline products because they could damage the mirror finish of the material. In this case, we recommend only the use of neutral detergents.
Sandblasted – Sandblasting is a technique that also gives a more slip-resistant surface suitable for outside areas or for wet areas. We have used this on shower trays and wet-rooms and also on patios, as in the image below – sandblasted Italian Carrara marble set on Buzon pedestals. It is created by spraying the surface with sand, ceramic beads or other abrasive components at a high pressure onto the surface of the stone. This process tends to lighten the look of the stone and can mask the character by hiding the veins and shell fossils.
Antiqued/Tumbled stone – One of the most widely used materials employed to create tumbled stone is travertine, in all its colors: Roman, Walnut, Red, Yellow, Peach, etc. The recommended treatment in this case consists of applying a base coat of a product, such as the usual non-filming, solvent-based water and oil proofing or the more recent analogous water-based type of product, to ensure uniform absorption. This must be followed by a couple of applications of one of the vast range of commercial waxes, depending on the intended use and the type of finish desired: from extremely brilliant, self-shining metallized waxes to opaque wax.
An alternative to this treatment, which is widely used especially for exterior surfaces, consists in applying two coats of a stone enhancer, which results in the so-called “wet effect”. This is normally an oil or resin of diverse origin.
As for maintenance, in this case the problem is rather more complicated. Tumbled travertine has open cavities at the surface, in contrast with polished or honed materials which are filled. These cavities are excellent accumulation points for dirt, and hence, stronger products must be used for cleaning the surface, combined with the mechanical action of the cleaning machine. Furthermore, since tumbled stone is generally used in exteriors, the dirt in question is generally much more tenacious than indoor floor dirt. Cleaning, therefore, requires the use of alkaline products in combination with a professional cleaner with an abrasive disk (up to green disk) or hard nylon brush. On the other hand, the surface to clean is not particularly delicate, and therefore, it can accept the use of more energetic means than those employed in the case of polished or honed travertine. In extreme cases, a water jet cleaner may be used, after application of a solution of the alkaline product mentioned above (at a suitable dilution), left to act for a few minutes.
Flamed – A flamed finish is created by passing an oxy-acetylene torch over the surface of the stone and then following it immediately with a cold pressurised jet of water to fracture the top surface of the stone. On some types of stone, such as limestone and sandstone (as in the photo below of the Italian Pietra Serena), it gives a non-slip surface which is ideal for terraces or public paved areas. This is a specialised technique and not all quarries have this equipment in their workshops.
