Five porcelain and porcelain stoneware bodies were investigated to compare sintering mechanisms and kinetics, phase and microstructure evolution, and high temperature stability. All batches were designed with the same raw materials and processing conditions, and characterized by optical dilatometry, XRF, XRPD-Rietveld, FEG-SEM and technological properties. Porcelain and porcelain stoneware behave distinctly during sintering, with the convolution of completely different phase evolution and melt composition/structure. The firing behavior of porcelain is essentially controlled by microstructural features. Changes in mullitization create conditions for a relatively fast densification rate at lower temperature (depolymerized melt, lower solid load) then to contrast deformations at high temperature (enhanced effective viscosity by increasing solid load, mullite aspect ratio, and melt polymerization). In porcelain stoneware, the sintering behavior is basically governed by physical and chemical properties of the melt, which depend on the stability of quartz and mullite at high temperature. A buffering effect ensures adequate effective viscosity to counteract deformation, either by preserving a sufficient skeleton or by increasing melt viscosity if quartz is melted. When a large amount of soda-lime glass is used, no buffering effect occurs with melting of feldspars, as both solid load and melt viscosity decrease. In this batch, the persistence of a feldspathic skeleton plays a key role to control pyroplasticity.

During the various stages of ceramic tile production, sintering kinetics, phase transformations and variation of the main properties of non-crystalline matrix are considered the major parameters to be kept under control. Particularly, during the sintering process a complex evolution of both phase composition and chemistry of the liquid phase occurs, according to the dynamic equilibrium established between the residual minerals and the new crystalline phases formed during firing. This contribution aimed at comparing the evolution of phase composition and of non-crystalline matrix properties during the vitrification path of four representative industrial ceramic formulations (soft porcelain, vitreous china; two different batches of porcelain stoneware, including a glass-bearing one). These batches were designed and prepared at the laboratory scale, simulating the industrial ceramic process. The sintering kinetics of each sample was determined under isothermal conditions through an industrial-like firing schedule by optical thermo-dilatometric analysis. Samples were investigated between the temperature at which the viscous flow sintering starts (around 1000°C) up to the onset of deformation (up to 1400°C for porcelain), upon increasing dwell time. The phase composition was assessed by the Rietveld refinement and the chemical composition of the vitreous phase was obtained by subtracting the contribution of each mineralogical phase, considering its stoichiometric ideal formula. The melt properties were estimated by predictive models based on the chemical composition of the liquid phase. An increasingly faster sintering kinetics was observed in the order: soft porcelain, vitreous china, porcelain stoneware, glass-bearing stoneware. Different vitrification paths were observed with a correlation between the dissolution kinetics of feldspar and quartz. Remarkable differences were observed in those samples where mullite occurred as primary or secondary mullite. Those differences clearly reflected a distinctive evolution of chemical features and glass network connectivity parameters of the non-crystalline matrix. The porcelain stoneware sintered by fast cycles thanks to a sort of buffering effect played by quartz and primary mullite melting rates. In contrast, vitreous china and soft porcelain needed higher temperature and/or prolonged time to activate both the growth of secondary mullite and the contemporaneous quartz dissolution, and the variation of properties of the noncrystalline matrix.

The aim of the relearn was to study the phase changes during crystallization of some compositions of biocompatible glasses obtained in the RO-CaF2-P2O5-Al2O3-SiO2 (R=Zn, Mg) system. For this purpose, crystallization of glasses in the range of 700-1000 °C was carried out and X-ray analysis was carried out. A change in the phase composition of crystallized glass with a change in crystallization temperature and composition has been established. During crystallization, the phases of whitlockite, zinc-substituted fluorapatite, fluorapatite, anorthite, and gahnite were identified.

Porcelain stoneware is sintered by partial vitrification, through viscous flow of an abundant liquid phase formed at high temperature. The present contribution will overview the evolution of phase composition of porcelain stoneware during firing at different temperatures and soaking times. The firings were conducted in two distinct ways: dynamic (i.e. with a ramp simulating the industrial heating cycle in a roller oven) and static, by inserting the sample into the chamber furnace directly at the maximum temperature. Each mixture was characterized from the chemical point of view and, once fired, its phase composition was determined by quantitative XRPD (Rietveld method). The transformations affecting the minerals of the starting mixture determine a continuous variation of the phase composition during the heating treatment: feldspars melt quickly (K-feldspar>plagioclase) - largely melted at 1100°C - while quartz is only partially dissolved at the highest temperature. The liquid phase changes its chemical composition according to the dynamic equilibrium established with both the residual minerals (quartz, feldspar) and the new crystalline phases formed during the firing (mullite). Variations of the chemical composition of the liquid phase reflected on its physical properties, particularly on viscosity and surface tension, which define the densification kinetics in the sintering process.

In the latest years, the addition of glass-ceramics systems to porcelain stoneware bodies has been experimented to assess their expected technological benefits and aesthetic advantages. The aim of this study is to investigate the influence of glass-ceramics on mechanical properties appraising the intimate changes occurring as a consequence of the addition and particularly on microstructural characteristics and technological features. For this purpose a very deep mechanical characterisation has been carried out on industrial and laboratory products obtained by mixing up to 20% glass-ceramic systems to current industrial bodies, in particular elastic constants, Vickers hardness, toughness and failure strength have been measured. The glass-ceramic systems taken into account were BaO-Al2O3-SiO2 (BAS), Na2O- Al2O3-SiO2 (NAS), K2O-CaO-MgO- Al2O3-SiO2 (KCMAS) and ZrO2-CaO-SiO2 (ZCS). Information about material quality have been got from fractographic analysis, such as the observation of fracture-generated surfaces in terms of nature, size, shape, orientation and place of fracture origin. Moreover, microstructure and texture of the products have been studied and density, open and close porosity and phase composition, have been determined for this purpose. Industrial bodies resulted characterised by very low water absorption (0.07-0.12%) and closed porosity (1.6-4.1%), their bulk density ranging from 2.33 to 2.43 g.cm-3, flexural strength 30-40 MPa, Young modulus 63-71 GPa, and Poisson coefficient 0.18. The addition of NAS and KCMAS glass-ceramics precursors led to 3.7-4.3% of residual porosity and to bulk density of about 2.43 g.cm-3, with flexural strength 39-43 MPa and Young modulus 69-73 GPa, equal or higher than the industrial product ones. The sintering in standard conditions of bodies containing BAS and ZCS glass-ceramics was not complete, leaving significant residual porosity (12-14%) and the resulting flexural strength and Young modulus were rather low (26-29 MPa and 51 GPa, respectively). However, an appropriate mixture of BAS and ZCS systems allowed to achieve excellent mechanical performances (flexural strength 40 MPa, Young modulus 74 GPa, Poisson coefficient 0.26) notwithstanding the remarkable residual porosity (7.3%).