## 1 Introduction  
In recent years, metal-free dental ceramics with high strength have gained popularity due to the fact that current restorative dentistry focuses on treatments that are conservative, functional, and long-lasting, always seeking to achieve optimal prosthetic-periodontal integration [1, 2, 3]. This has led to a constant search for restorative materials that are biocompatible with soft tissue and possess high aesthetic value [4, 5]. Two major categories of such materials are feldspathic ceramics with crystalline reinforcement [6] and zirconium oxide materials [7]. Chemically, silicate-reinforced crystalline ceramics consist of a crystalline phase (lithium disilicate and lithium orthophosphate). The mechanical strength of lithium silicate ceramics is increased due to the homogeneous dispersion of round and submicrometer elongated metasilicate grains. Usually, tetragonal zirconia fillers are added to them [8], with the aim to increase the strength values. This structural typology has been developed in order to combine favorable optical properties with improved mechanical characteristics [9, 10]. This set of ceramics possesses exceptional and highly beneficial mechanical properties, making them primarily ideal materials for plural fixed prostheses (PFPs) in both natural teeth and implants [11]. Zirconium dioxide, a polycrystalline ceramic endowed with attributes of polymorphism and allotropy, exhibits an unstable nature as it, under thermal influence, undergoes three distinct phases. Up to 1170 \({}^{\circ}\)C, it is in the monoclinic phase; between 1170 \({}^{\circ}\)C and 2370 \({}^{\circ}\)C, it adopts the tetragonal phase; and above 2370 \({}^{\circ}\)C up to its melting point, it transforms into the cubic phase [12]. There are different generations of zirconium according to the amount of additives or dopants incorporated [13]. The most common variant is that partially stabilized with yttrium oxide (Y\({}_{2}\)O\({}_{3}\)), characterized by low porosity and high density [14]. Restorations fabricated with zirconium dioxide are distinguished by their remarkable mechanical properties, which, depending on the generation (e.g., 1 and 2), significantly outperform silica-based ceramics [15]. The flexural strength of conventional yttria-stabilized polycrystalline tetragonal zirconia (3Y-TZP) ranges from 1000 to 1500 MPa [16]. Early versions of zirconia were opaque, intended for use as copings and frameworks that required feldspathic ceramic veneering to enhance translucency. However, these experienced low success rates due to ceramic detachment, a situation also observed in metal-ceramic restorations. Hence, current trends favor monolithic restorations [17; 18], which enhance translucency through slight adjustments in the Y\({}_{2}\)O\({}_{3}\) content (between 4 and 5 mol%) instead of the standard 3 mol%, resulting in a higher presence of cubic-phase particles [19]. Zirconia with a more cubic phase offers greater light transmission but exhibits lower bending strength values than conventional zirconia, ranging from 550 to 800 MPa, limiting its widespread application for PFPs [14; 20; 21]. Both silicates and zirconium oxides can undergo additional heat treatment (AHT), a decisive and crucial procedure that not only determines their final geometry but also enhances their mechanical and optical characteristics. The lack of control over sintering or crystallization parameters can compromise the microstructure and inherent properties of the material. Achieving a fine-grained and homogeneous geometry, along with high density, ensures optimal biomechanical performance of the material [22; 23]. Additionally, AHT plays a key role in repairing microcracks originating during milling processes and facilitates the transformation of a pre-crystallized metasilicate, which is intrinsically weak, into a strong and tough lithium disilicate [24; 25]. Due to the importance of AHT in CAD/CAM ceramic materials, companies have developed a variety of furnaces with specific protocols adapted to different thermal units.