3.1. Melting of frites
The XRD patterns of the obtained frites were presented in Fig.5.
The obtained densities, the melting temperatures of six series of the frites with two different ZnO powder size and the calculated values of molar mass and (Coefficient of Thermal Expansion) CTEs were presented in Table 3.
It can be seen that by the increasing of the S ratio (from 0.24 to 0.37) the melting temperatures values of frites were decreased (from 1550 to 1450ºC respectively). In frites, CaO and K2O normally function as network modifiers (ZnO could be has two roles intermediate or modifier) and induce a decrease of the Tm .
The variation of molar volumes and S ratios with respect to the ZnO particle sizes (PSA) as a raw material used in frites batches were presented in Fig.6.
The Thermal Expansion Coefficient (CTE) values with various S ratios can be predicted using Appen’s equation as follows 
Where pi is the mole fraction of individual oxides and fi is a characteristic factor for each oxide. fi for ZnO is 5.0 × 10−6, B2O3 is −5.0× 10−6, SiO2 is 3.8 × 10−6 , Al2O3 is −3.0× 10−6, CaO is 13.0 × 10-6 and K2O is 42.0 × 10−6/K. It can be seen that by decreasing the S ratio, the CTE values of the frites decreased from 6.28×10-6/K to 6.96×10-6/K. In order to compare the resulted CTE of the obtained glazes and the tile's body, the expansion of the used substrate Tile was measured in Fig. 7.
The difference of thermal expansions of the two different tile and glaze compositions linked to each other gives rise to stress in these materials.
3.2. The DTA results
The thermal analysis of the obtained frites were compared as well in Fig.8 and Table 4. The Tg values of frites were not observed sharply and they were recorded in broad temperature range. Also, the crystallization peaks of the samples were not sharp corresponded to the surface crystallization. (However by differentiating of the heat flow graph the onset of Tg were obtained )
The compositions (N1-F1, N2-F2, N3-F3) were the same but the characteristic temperatures were altered (the melting temperatures were changed as well).
The variation of characteristic temperatures Tp, Tg, Tp-Tg by the S ratio with two different ZnO sizes were presented in Figs. 8.
The higher the value of glass transition temperature of a frit, the greater the stability of its elastic properties [17,18]. In order to compare the glass stability of the frites with different ZnO particles sizes and S ratios, the TP /Tm and Tp -Tg/Tm values vs. frit compounds were represented in Fig.8c and Fig.8d.
This figure shows that the stability of the frites increases as the network modifier contents increase.
In Fig.9, the infrared absorption spectra of the experimental frites with different compositions are shown. The absorbance of the spectra were compared to the values given in the literature [19,20,21]. In spectrums, the IR bands are identified as follows: the Si-O(s) stretching mode is located in the range 10000-1200 cm-1, the Si-O (b) bending mode is found around 800cm-1. Band at 650 cm -1is assigned to the symmetric stretching vibration of Si-O- (Si, Al) between the tetrahedral in N3 and F2 samples.
As the alkali oxide (K2O) content increased (in F1 sample), the position of the maximum absorbance of the Si-O(s) band shifts towards lower values, until 1044cm-1.
3.4. Sintering of resulted glazes
Series of sintering conditions at slow and high cooling as well as heating rates were done on F, N glazes that were applied on the fired tile. Keeping the firing temperatures in such a way that the surface of the glazes would not be phase separated, colored changed or warped by the naked eye. The firing temperatures were increased up to 1180ºC, to obtain the gloss appearance on glazes surfaces.
The heat treatments conditions used and the resulted glaze's appearance were shown in Table 5.
By decreasing the heating rates, the diffusivity increases in the frit and it will promote phase separation. On the other hand, the fast heating rate (90 minutes heating time) led to the higher crystallization temperature, compared to the slow heating rates (150 minutes).
On the other hand, fast heating rates (60 mins, heating.) led to warping of glazed samples due to the thermal gradient in samples. This damage occurs mainly during the cooling, because the bisque tend to shrinks after the fast expanding that may not well-matched with the glaze's shrinkage.
3.5 Phase Evaluation
Fig. 10 shows the phase evaluations of heat treated F2 , F1 and F3 frites at 1000ºC -30 min. in 90 minutes heating time .
The low crystallinity (low peak's intensity) of F1 sample was seen in Fig.10. In F3 sample, (with higher Trg values) the relative intensities of characteristic XRD peaks of wollastonite phase were 3 orders higher than the F1 one.
The phase’s evaluations of the resulted sintered glazes at 1060ºC (3 min holding time and 90 minutes heating) on the tile's body were presented in Fig.11..
The Wollastonite phase remains in the F1 sample by the temperature (ΔT=+60), but in the case of F2, F3 samples the crystalline phases (Wollastonite ) change to the Calcium Aluminum Silicate(Anorthite)phase which could be related to the low glass stability(GS) of these system. Calcium Aluminum Silicate (Anorthite) was confirmed by the Si-O-Al vibrations at 650 cm -1 by the FT-IR results in F2,F3 samples.
The Calcium Silicate and Wollastonite phases remained in N1, F1 and N3 samples by increasing the temperature but the Anorthite phase was appeared in F3 sample.
The XRD peak's intensities (and hence the crystallization ability) were higher in F2 and N2 samples compared to the other sample.
Although F1, N1 compositions have more ZnO content in composition, but the Willemite (Zinc Silicate) phase was not detected in F1, N1,F3,N3 systems, whereas the Zinc Silicate phase was developed in F2 glaze. It can be related to the ZnO4 unites formation which was demonstrated by FT-IR located at 566.82 cm-1 wavelength.
3.6 Micro hardness
The resulted Vickers micro hardness values, crystalline phases of the glazes are also compared in Table 6.
3.7 Optical Properties
The CIE Lab parameters gloss values of the F1 to F3 glazes are given in Table 7. As can be seen, The F1 and F2 glazes present the high gloss values but the F3,N3 glazes show the low GU values corresponded to the higher calcium silicate crystallization . Also the a* value in F1 sample describe the light red color in the glaze. Since the mentioned glazes had been applied onto the Tile without using an engobe layer , utilizing a suitable engobe would enhance their whiteness.
L values and traces amounts of a and b reveal the tendency of both glass ceramic glazes to the white color. The whiteness index of both were obtained around 65%.
3.8 Microstructural Evaluation
The etched and polished surfaces of the samples were presented in Figs.12. All samples show the prismatic morphology which is the characteristic form of pyroxene minerals (Parawollastonite, Anorthite, Calcium Silicate). These crystalline phases have 7.3×10-6/K,4.9×10-6/K,5.4×10-6/K Thermal Expansion Coefficients,(CTE) respectively. The lower CTE of these crystals led to the micro cracks formation around the crystalline phases in samples.
The needle like crystals of Willemites could be observed in F2 and N2 samples as well.
By comparing the XRD peak's intensities and the SEM micrographs of the F1 and N1 samples in Fig.12b it is clear that the crystalline volumes are lower than the N3,N2,N1 sample's .Likewise the F3 sample has almost the smaller prismatic like crystals than the N3 and N2 samples. The micrographs by high magnification are presented in Fig12 b.
The relict morphology of Wollastonite in F1, N2, samples are clearly observed but the F3 and N3 samples shows rounded morphology that could be related to Anorthite and Parawollastonite respectively .