“Porphyry” literally means “bright red tending to purple”, from the Greek “porphyreos”, the same root as “purple”. The Romans used the formula nomen omen to express the idea that the destiny of the bearer is in their name. “Nomen omen” applies perfectly to porphyry, but with something more: in the field of ornamental and building stones, the name porphyry is also a promise of the genetic, aesthetic and, above all, high-performance nature of the rock. In fact, the term “porphyry” is reserved for compact rocks ranging in colour from reddish- brown to purplish, to grey-green, generated by the cooling of magma on the surface or at shallow depth, very resistant to compression and bending, to wear and tear, to the aggressive action of freeze/thaw cycles, of de-icing salts, and chemical attacks in general. Starting with Egyptian porphyry, also known as imperial porphyry, with its intense and vivid colours in shades of purple – e.g. the Ancient Red Porphyry - or of green – e.g. the Ancient Green Porphyry -, very hard rocks that can be polished by means of costly processing and very long-lasting, the name “porphyry” has become inextricably linked to
the meaning of strong stone par excellence. Proof of this is the fact that, from the Egyptian Pharaohs onwards, porphyry artefacts have been reused countless times over the centuries, from the late Roman Empire to the Most Serene Republic of Venice through the Byzantine Empire, not only to embellish floors but also for sculptures and decorative elements (columns, statues, busts, sarcophagi, etc.), in a process of ante litteram circular economy for which the art historian Salvatore Settis recently coined the expression “recycling beauty”.

"Porphyry layering"

The prerogative of Trentino Porphyry is its immediately recognisable layered appearance, even in the rock mass, due to the presence of sub-vertical planar cracks with centimetric to decimetric to metric spacing. This very particular structural feature due to the contraction of the volcanic deposit when cooling, is defined “layering” or, with a term that porphyry borrows from sedimentary rocks, “stratification”. However, it should be remembered that with layering/stratification we do not refer to layers intended as levels of successive deposition, but to a typical phenomenon of the cooling of the lava and which can take on slab-like or, more often columnar, forms, as for example seen in the very famous Giant’s Causeway on the coast of Ireland. The raw material features a very articulated stratification in varying thicknesses, which on one hand conditions the extraction system, with quarry fronts that are completely different from those of the so called “block quarries”, such as marble and granite, and on the other hand allows the material to be divided into polygonal slabs that are extremely irregular in plan dimensions, but have natural planar and parallel planes, with a degree of roughness that does not wear out.

Colour

Colour is one of the aesthetic requirements particularly sought after to enhance and complete the design and architectural solution. In porphyry, “layering/stratification” is the main cause of the great structural mutability of the material in terms of colour. For this reason, it is unthinkable to attribute the colouring of products to the typical shades of a pantone or in any case a rigid classification. On the contrary, within its range, Trentino Porphyry allows for different levels of interpretation, with a variety that is difficult to find in other stones and shades ranging from grey to grey/brown, from grey-violet to purple, from mixed red to bright rust. In addition to the circulation of hydrothermal fluids, the presence of uneven layering at the various heights affected by infiltrations and cracks of complex nature, together with any contaminations that in any case originated from the natural environment, with concretions and oxides penetrating in different forms and quantities, has made any uniformity of colour in porphyry difficult. This circumstance can be detected not only at a general level considering the quarry area in its entirety, but even within individual quarry areas where diversity and inhomogeneity can distinguish one step from another, up to the point of being able to find shades of several colours on the surface of a single slab.

Most Trentino Porphyry production is based on exploiting the potential of the natural quarry plane, with variable thickness, which allows for the finished product to be obtained by processing the sides (or edges), mainly with mechanical splitting operations, but also by sawing in the laboratory using diamond
discs. The versatility of quarry plane products is completed by artefacts and processed products obtained in the laboratory from rough slabs or small blocks in sizes smaller than those that normally identify the actual blocks, but big enough to allow most of the traditional valuable processing typical of the stone sector (e.g. flaming, bush-hammering, brushing, iron blasting, polishing, etc.) with a prevalent indoor use.

Standard cubes

Cubes are without doubt the main product in the history of Trentino Porphyry. And in the social imaginary, the cube identifies the split product par excellence. In fact, stone cubes and, in particular, those made of porphyry have been used in every corner of the globe for more than a century. From Corso Vittorio Emanuele in Milan to Piazza del Viminale in Rome, from Les Halles in Paris to the National History Museum in London, from Castille Square in Malta to Piazza Bra in Verona, the porphyry cube, in different sizes and patterns, has shown all its personality by characterising the urban landscape and making it recognisable even to the least attentive tourist. The cubes are obtained by means of the splitting action of the manually operated guillotines, which compresses the four sides perpendicular to the two natural planes of the rough slabs obtained during the sorting phase. There are two major product categories: standard cubes with variable plan dimensions and thickness, and square top cubes with regular and fixed plan dimensions, varying in thickness only. For both categories, the cornerstone that influences all production operations is
the thickness of the material, to which the operator adapts to make the plan dimensions in order to obtain the cubic shape. Therefore, by way of example, with a rough slab approximately 70 mm thick, the operator can obtain a 60/80 mm type cube, where each of the three dimensions is between 60 and 80 mm. From this point of view, it must be pointed out once again that the natural “stratification” of the material does not allow for rough slabs of fixed thickness. Thickness deviations are attributed to specific assortments that identify the dimensional and usage categories, each of which is characterised in turn by an equal number of tolerance ranges, also because all elements are made by hand.

While all ornamental and/or building stones are rocks, not all rocks can be ornamental and/or building stones. In fact, for a rock to also be an ornamental and/or building stone, the fact that it has undisputed, albeit subjective, aesthetically pleasing colours and design, is not enough. It is essential for it to be available in exploitable deposits, accessible and with volumes that can meet the demand, have a good predisposition to texturing and good durability, in other words a good aptitude to maintain its aesthetic and morphological characteristics unaltered over time once laid. Moreover, availability and durability are decisive factors both for the economic value of one ornamental stone over another, and for the economic value that the ornamental stone brings to the artistic or architectural artefact.

Why is it important to have technical Standards?

Determining the technical properties of a stone in the laboratory, although derived from conventional procedures that are a necessary compromise to reproduce in a laboratory what happens on site, is usually predictive of its behaviour when in use; it is therefore advisable to carry out these measurements according to operating protocols that make them accurate and precise, i.e. repeatable. Only in this way can results, obtained from technical tests on different stones, in different places and at different laboratories, be compared with each other, and only in this way can a comparative evaluation of the stones be made during the design phase in relation to the stresses to which they will be subjected once laid.

Current Standards for ornamental stones

The current Standards for ornamental stones include European Standards (EN) and US
Standards (ASTM). In 2020, an ISO Standards committee was also formed and is working on a single set
of Standards applicable worldwide, which will be available in the coming decades.
The European regulations include 3 types of Standards: terminology Standards, which contain the vocabulary of the sector, test methods, which describe the operating procedures for carrying out laboratory tests to determine the technical characteristics of materials and products, and product Standards, with mandatory or voluntary implementation, which define the minimum performance requirements for natural stone products such as slabs, cubes and kerbstones for vehicular paving, modular tiles, cladding slabs and staircases.

The old UNI 7998 Standard defined stone paving as a «partial subsystem whose main function is to allow or improve transit and resistance to loads in certain conditions of use. Its surface finishing layer gives the paving predetermined mechanical, chemical, physical, comfort and safety performances», which in turn allow for the pursuit of durability and maintainability objectives that today must also be inspired by and commensurate with precise sustainability and environmental quality parameters. The merit of UNI 11714-1 Standard is that it has standardised techniques and operating methods which, until the last century, might have had significant differences determined by specific locations and territorial cultures. At the same time, the Standard has introduced and regulated procedural criteria for the use of new-generation construction materials for the durability of stone floors even in the presence of the highest stress classes. However, it cannot offer a complete and exhaustive picture of the possible stratigraphic combinations linked to the different design situations, the different types of materials and the equally different stress exposure classes.

Porphyry pavings made of cubes

Ai Paragraph 4.2 and 4.2.1 to which reference should be made, identifies the types of cubes distinguishing between standard production and production with a selected square top, and paragraph 7.3 illustrates the corresponding intended use and the relative sizing of the system. The picture of the options for using cubes is now completed with indications concerning the different bedding and sealing systems specifically suitable for the different sizes of the elements and consequently for the different intended uses, on the basis of what is illustrated in general in paragraph 7.4..

Section 5.6 of UNI 11714-1:2018 Standard introduces, for the first time in a technical Standard concerning stone, the principle of sustainability and environmental impact that can be referred to the entire life cycle of the products, and therefore suggests the general parameters that need to be taken into account when designing and building stone claddings. In fact, the standard indicates sustinability as one of the essential requirements for natural stone. Starting from the assumption that stone is an exhaustible non-renewable
resource, the designer must pay due attention to the choice of products and materials in order to guarantee performance in line with the main parameters of sustainability and environmental quality. Among the indicators that help achieve these objectives are: the recovery and recycling of natural minerals; the use of regional resources; the reduction of CO2 emissions into the air; the reduction of water consumption; the reduced use of substances harmful to health and the environment.

Sustainability is circular economy

In addition to being durable, the residues of Trentino Porphyry can also be used to make by-products for the containment of cliffs and embankments, sea defence blocks or retaining walls. Of equal importance is the range of aggregates (see paragraph 4.10) obtained by transforming waste and scrap material into crushed stone in the most varied grain sizes for use in construction, roads and railways, but also for paving as bedding material (3-6 mm or 4-8 mm crushed rock) and sealing material (2-4 mm crushed rock, washed and dried). Paragraph 4.11 illustrates actual products obtained from processing waste and tumbling processes: flat cobblestone and “Teres”. However, what further distinguishes Trentino Porphyry from many other stones concerns materials that, by means of a selection, washing and tumbling process, can be recovered from worn pavings. These are mainly cubes in different sizes (40/60; 60/80; 80/100; 100/120 mm) which can therefore be placed back on the market, fully selected and regenerated for a second, and even third, life.

Special aspects of laying in overlapping arches: intersections

When the paving must be connected at crossings and/or changes in direction, or when changes in slope require the laying direction to be reversed, the arches are connected as shown in the picture below.