Changes to physical and mechanical properties of pyroclastic flow deposits occur during welding process, resulting from overburden pressure and temperature. A great deal of research has been carried out to identify which physical parameters can enable quantification of the degree of welding within pyroclastic deposits. Material properties, such as density, porosity, hardness, abrasion, and compressive and flexural strength, are critical in assessing the suitability of natural stone as a building material, but they have not been sufficiently integrated. Therefore, an attempt has been made to develop a welding classification which integrates measurements of material properties with petrographic and textural observations. The proposed classification covers physical, index, and strength parameters for pyroclastic flow deposits. The most important aspect of this investigation has been the quantification of material parameters and definition of a welding classification applicable to a wide range of ignimbrites, from nonwelded to densely welded, that are used as natural building stone. This welding classification is proposed to ensure a consistent approach in characterizing key aspects of welded facies in pyroclastic deposits and in the identification of natural stone reserves.