Resistance to abrasion
This measurement is based on the loss of thickness, expressed in mm, demonstrated by 2 samples with 7.1 x 7.1 cm square base which must endure a certain number of laps of a rotating abrasive track against which they are pressed with a pressure of 0.03 N/mm^2 (the Amster triometer test).
According to article 11 of R.D. 2232 of 16.11.1939 and the norm CNR of 1953, the result is the relative coefficient of abrasion, i.e. the ratio between the thickness of a sample of S.Fedelino granite after abrasion and that of the sample under examination after undergoing the test described above carried out simultaneously. The relative coefficient of abrasion is therefore a number which is directly proportional to the resistance to wear of the material. In fact rocks which are more resistant than S.Fedelino granite give a result of more than 1, while those less resistant give a result of less than 1.
This characteristic is extremely important in interior and, in particular, exterior public places where stone used for paving is subjected to particular wear, and above all in road surfaces where there is wear due to sliding friction which, over the course of time, tends to alter the original surface of the material.
The different hardness of the minerals of which it is composed gives paving of porphyry the characteristic roughness which it maintains over time thanks to its vitreous structure. Porphyry, in fact, unlike other volcanic rocks like granite, tends not to disintegrate (lose individual crystals) but remains rough, and so has optimum adherence even in the rain.
Porphyry - resistance to abrasion, relative value with reference to the S.Fedelino granite: coefficient 1.51.
Other properties
Other properties should be mentioned even though they are not measured precisely or classified on the basis of regulation physical - mechanical tests. They nevertheless help to define the characteristics which are often important for the end use.
Durability
Any rock material situated outside is subjected to a process of ageing due to chemical and physical agents produced by atmospheric factors (rain, sun, temperature etc.) and aggravated by localized conditions of pollution (Carbon monoxide and other chemicals present in the atmosphere, pollen etc.).
As nothing can be done to reduce ageing (neither varnish, nor treatments have proved effective) it is necessary to use rocks which have the highest level of durability like igneous rocks, and in particular porphyry.
This should be remembered as one of the most important qualities of this material, its high level of resistance to chemical attack which makes it particularly suitable as paving material in special circumstances (chemical, mechanical) or in the presence of lubricating oils, where the hardness and roughness of the surface are of particular importance.
Workability
This is the property of the rock which means that it can be transformed into the desired shape. It is closely related to the fabric and structure of the rock and also its hardness. The workability can be defined as behavior to:
• breaking, which usually depends on the fabric of the rock. A specific characteristic of porphyry, thanks to its "stratified" structure, is that it splits along natural, parallel planes. As it behaves like glass from a mechanical point of view, porphyry can easily be split along planes which are perpendicular to stratification;
• sculpting or reaction to being shaped with sculpting tools;
• sawing or reaction to being sawn; a widely used technique with porphyry both for finishing edges and for obtaining slabs or tiles with all their faces sawn;
• polishing or the reaction to being treated to produce a polished or shiny surface. The porphyry used normally for interiors is highly polished, mirror finish.
