Gloss, Light Reﬂection and Iridescence in Ceramic Tile Enamels Containing ZrO 2 and ZnO

: Ceramic claddings on building facades not only present functional qualities and good resistance; they also add value to the architecture due to their qualities of light reﬂection, gloss and iridescence. The colour ranges produced by some enamel application techniques can vary widely. They change depending on one’s angle of vision and movement, colours in the surroundings, sunlight and their angle of incidence. In addition, the iridescent-pearl e ﬀ ect produced by light di ﬀ raction can lead to beautiful goniochromatic colours. This study analyses the production of square tiles of stoneware manufactured by extrusion, and their application to the Faculty of Education of the University of Alicante (FEUA) (Spain). Applying an enamel containing zirconium silicate ZrSiO 4 and other metals such as Zn and Al produces iridescence-like e ﬀ ects. The physical-chemical properties of enamel and gloss values were characterised. A colorimetric characterisation was conducted by evaluating goniochromatic or iridescent colours, measuring the light’s spectral radiance factor, and comparing these results with other ceramic tiles of marked iridescent e ﬀ ects, with the presence of a ﬁnal layer of anatase TiO 2 enamel. iridescence tiles.


Introduction
In recent decades, the ceramics industry has undergone a renewal, regaining its innovative and technological nature. After a period of being applied almost exclusively to bathrooms and kitchens, porcelain stoneware and enamelled ceramics are now also being used for cladding, especially on building facades [1]. This recovery has led to further developments of construction solutions that use ceramics to meet new functional, aesthetic and economic requirements in architecture [2]. Ceramics have been consolidated as a versatile material [3] thanks to research in both manufacturing and post-manufacturing processes.
A permanent field of research in ceramics is the improvement of mechanical and resistance properties. With porcelain stoneware, ceramic building claddings have achieved remarkable outcomes both in press and extrusion manufacturing. Today, the emergence of graphene is producing promising results regarding mechanical resistance. Ahmad et al. reinforce ceramics with carbon nanostructures (CNTs and graphene). They have successfully enhanced the toughness and other properties of brittle ceramics and converted them into useful materials for next generation applications [4]. This is certainly a priority line of exploration for building applications. Chen et al. applied nanoparticles coated with SiO 2 to improve Al 2 O 3 /TiC self-lubricating ceramic composites, increasing anti-friction and mechanical strength by more than 15% [5]. Ceramics are also experiencing important advances as a refractory material, with possible applications in construction. Giuranno et al. provide refractory composites based on zirconium disilicide ZrSi 2 [6], and Villalba et al. refractory bricks containing mullite-zirconia composites [7].

Objectives
Previous research has allowed to determine the factors that produce iridescent effects in ceramic tiles laid out on building facades. In the case of the Auditorium of Algueña (Casa de la Música y Auditorio de la Algueña, or MUCA) in Alicante (Spain), several enamels were applied over four firing and manufacturing phases. The result allowed to conclude that distributing dispersed zirconium on the surface favours iridescence. A final application of anatase in the third firing phase produced microscopic cracks, multiplying the effect of iridescence [8]. In the case of the headquarters building of the Botín Foundation in Santander (Spain), the spherical cap-shaped ceramic pieces were fired twice during the manufacturing: first with a white enamel base, and second with depositions of zirconium oxide and zinc oxide metals. The last layer of anatase-present in the ceramic tiles of the MUCA-was not applied, resulting in lesser iridescence [46].
This study focused on the comparative analysis of the ceramic tiles used in the renovation of the FEUA, and their relationship with light. To this end, the physical-chemical composition of the enamels was analysed, and some colorimetric and gloss parameters were quantified, with the following objectives: To determine whether they have iridescent properties, due to light diffraction, or, conversely, whether they only present gloss effects and the reflection of light with elements in the surroundings.

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To determine the factors that produce the tiles' high capacity for light reflection, reproduction and colour variety.

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To compare the colorimetric and gloss analysis to that of the ceramic pieces found on the façade of the Algueña Auditorium (MUCA).

Materials and Methods
The objectives set forth in this study required a rigorous analysis of the different layers of enamel on the ceramic pieces. The various phenomena of solar irradiation, reflection and diffraction of light also needed to be analysed. To this end, a four-phase structured analysis methodology was designed: 1.
Physico-chemical analysis of the bases and various enamel layers of the ceramic pieces. The goal was to determine the factors that produce the phenomena of gloss, reflection, iridescence and light diffraction. It was essential to characterise the layers' various metals, distribution and possible microcracks. The physical-chemical characterisation of the analysed ceramic materials was performed using X-ray diffraction (XRD, Bruker D8-Advance, Alicante, Valencian State, Spain) and scanning electron microscopy (SEM, Hitachi S3000N, Alicante, Valencian State, Spain).
1.1. X-ray diffraction (XRD) is based on optical interference that occurs when monochrome radiation passes through a slit that has a thickness similar to that of the radiation wavelength. When irradiated over the sample to be analysed, the X-rays diffract at various angles depending on interatomic distances, thus allowing to identify the mineralogical composition of the crystalline samples. The equipment used in Alicante university's technical services is a Bruker D8-Advance with a high temperature camera (up to 900 • C), and a KRISTALLOFLEX K 760-80F X-ray generator, equipped with an RX tube with a copper anode. The analysis was conducted in two phases, the first for a 2θ angle scanning between 25 • -113 • , and the second between 3 • -70 • , and a step of 0.05 • .

1.2.
Scanning electron microscopy (SEM) allows to identify the elements present in the sample and establish their concentration through the X-rays generated after electronic bombardment. The equipment used at the University of Alicante is a Hitachi S3000N model scanning electron microscope, which features a Bruker XFlash 3001 X-ray detector for microanalysis (EDS) and mapping. This SEM is equipped with an energy-dispersive X-ray spectrometry (EDX), a dispersive energy detector that allows you to collect the X-rays generated by the sample and perform various analyses. This allows obtaining images of the element distribution on polished surfaces.

2.
Colorimetric characterisation. The objective was to analyse the goniochromatic colours and the various colour ranges on the enamelled faces of the ceramic tiles under study when subjected to light. The characterisation was performed based on observer perceptions according to viewing position and angle. We thus aimed at assessing the results of the gloss and iridescence effect pursued during the manufacturing of the ceramic pieces.
The colorimetric study was mainly performed using a multi-angle spectrophotometer, of the brand/model BYK-mac i 23 mm (BYK-Gardner GmbH, Alicante, Valencian State, Spain) ( Figure 1). It consisted of a configuration of the measurement of light reflected in various directions of the light's incidence and reception, or measuring geometries, in accordance with the ASTM E2194 standard [47], typically used in goniochromatic colours [48] for the automotive sector [49], as well as in cosmetics and other industrial sectors. To measure the colour, the light is set at a 45 • angle of incidence on the sample, and the reflected light is determined in six angular positions ( Figure 1). This is known as six different measurement geometries. 3. Characterisation of the behaviour of gloss before the incidence of light, observers' visual perception produced during the different daily and annual phases. The behaviour of the enamelled sides had to be measured according to the light coming from various angles, the quality of gloss of these enamels, the ability to reproduce the colour of elements or objects in the immediate surroundings of the enamel façade. For this purpose, the Minolta Multi-Gloss 268 equipment was made available by the University Institute of Physics Applied to Sciences and Technologies (IUFACyT) of the University of Alicante. The spectral radiance factor, or reflectance, was determined for each measurement geometry, and the chromatic coordinates in the CIE-L*a*b* space under D65 illumination (daylight) were calculated based on it. The reflectance of the ceramic samples could thus be displayed for the 6 different measurement geometries. Spectral measurements of characteristic colours [50] were analysed by tilting around the spectral profile and geometries closest to the direction of the gloss. 4. A comparative analysis of the results obtained in the previous test stages were made. It was important to evaluate the above parameters in each phase according to the manufacturing and enamel process of the ceramic tiles of both buildings. This way, it was possible to determine the factors that produce the effects of reflection, diffraction and iridescence of light making it possible to meet the study's proposed objectives.

Faculty of Education of the University of Alicante: Ceramics and Reflection of Light
The Faculty of Education of the University of Alicante (FEUA) has recently been renovated. The building is located on the campus of the university of San Vicente del Raspeig (Alicante). It has a built surface area of 1820 m 2 , and it consists of twin volumes, a ground floor plus a first floor. Its construction is based on masonry walls clad with single-layer mortar, a reinforced concrete structure and a hip roof with Spanish roof tiles. During the renovation, two small new volumes were added to the two building entrances, in the south and north facades. They are covered with ceramic pieces presenting a high level of light reflection, with iridescent-like effects.
These two units resemble two cubes, their sides measuring approximately 6.80 m. The cube on the south façade is lined with 20 × 20 cm soft grey glazed ceramic pieces ( Figure 2). Their function is that of the building's entrance hall, and heat pumps for the building's air conditioning are housed in their rooves. They are hidden by the parapet.

3.
Characterisation of the behaviour of gloss before the incidence of light, observers' visual perception produced during the different daily and annual phases. The behaviour of the enamelled sides had to be measured according to the light coming from various angles, the quality of gloss of these enamels, the ability to reproduce the colour of elements or objects in the immediate surroundings of the enamel façade. For this purpose, the Minolta Multi-Gloss 268 equipment was made available by the University Institute of Physics Applied to Sciences and Technologies (IUFACyT) of the University of Alicante. The spectral radiance factor, or reflectance, was determined for each measurement geometry, and the chromatic coordinates in the CIE-L* a*b* space under D65 illumination (daylight) were calculated based on it. The reflectance of the ceramic samples could thus be displayed for the 6 different measurement geometries. Spectral measurements of characteristic colours [50] were analysed by tilting around the spectral profile and geometries closest to the direction of the gloss.

4.
A comparative analysis of the results obtained in the previous test stages were made. It was important to evaluate the above parameters in each phase according to the manufacturing and enamel process of the ceramic tiles of both buildings. This way, it was possible to determine the factors that produce the effects of reflection, diffraction and iridescence of light making it possible to meet the study's proposed objectives.

Faculty of Education of the University of Alicante: Ceramics and Reflection of Light
The Faculty of Education of the University of Alicante (FEUA) has recently been renovated. The building is located on the campus of the university of San Vicente del Raspeig (Alicante). It has a built surface area of 1820 m 2 , and it consists of twin volumes, a ground floor plus a first floor. Its construction is based on masonry walls clad with single-layer mortar, a reinforced concrete structure and a hip roof with Spanish roof tiles. During the renovation, two small new volumes were added to the two building entrances, in the south and north facades. They are covered with ceramic pieces presenting a high level of light reflection, with iridescent-like effects.
These two units resemble two cubes, their sides measuring approximately 6.80 m. The cube on the south façade is lined with 20 × 20 cm soft grey glazed ceramic pieces ( Figure 2). Their function is that of the building's entrance hall, and heat pumps for the building's air conditioning are housed in their rooves. They are hidden by the parapet.
The cube provides a touch of colour to the whole, on the façade's white monolayer, and the ceramic generates a wide variety of light effects via the highly reflective enamels applied to the visible faces ( Figure 3). Depending on the time of year and day, the façade faces can reflect a broad range of colours coming from elements in the surroundings. The ceramic tiles lack flatness because the grey enamel layer was applied using a brush. Thanks to this, the effect of light vibration is enhanced at The cube provides a touch of colour to the whole, on the façade's white monolayer, and the ceramic generates a wide variety of light effects via the highly reflective enamels applied to the visible faces ( Figure 3). Depending on the time of year and day, the façade faces can reflect a broad range of colours coming from elements in the surroundings. The ceramic tiles lack flatness because the grey enamel layer was applied using a brush. Thanks to this, the effect of light vibration is enhanced at certain times during the day, leading to visual perceptions of iridescent-like effects. Solar radiation, common in Alicante's climate, reinforces these lighting effects.   The cube provides a touch of colour to the whole, on the façade's white monolayer, and the ceramic generates a wide variety of light effects via the highly reflective enamels applied to the visible faces ( Figure 3). Depending on the time of year and day, the façade faces can reflect a broad range of colours coming from elements in the surroundings. The ceramic tiles lack flatness because the grey enamel layer was applied using a brush. Thanks to this, the effect of light vibration is enhanced at certain times during the day, leading to visual perceptions of iridescent-like effects. Solar radiation, common in Alicante's climate, reinforces these lighting effects.

Description of the Manufacturing Process
The ceramic tiles used in the FEUA building were developed on ceramic pastes stoned at high temperatures, of approximately 1000 • C. Using the techniques described below, it was possible to Coatings 2020, 10, 854 7 of 22 achieve the technical qualities, associated with an effect of light reflection resembling iridescence, and the quality required in the field of architecture for claddings particularly resistant to weather.
The volumes with an exterior ceramic coating corresponded to a widely used coating validated by numerous previous architectural works. The ceramic tiles were 20 × 20 cm square-shaped pieces 10 mm thick. They are red paste stoneware, with a water absorption of 3-3.5%, produced by extrusion. Their enamel layer over a slip layer beneath makes them waterproof. The joining material ensures low rainwater absorption on the edges of the tiles.
These ceramic enamelled stoneware tiles can be manufactured by press or extrusion. The press manufacturing technique was not applied to avoid an excessively flat finishing surface. Indeed, the use of 25 Tm hydraulic presses, with an applied pressure of 62 kg/cm 2 , followed by a polishing treatment to remove the atomised dust particles that remain on the surface before applying the slip and enamels leads to a remarkably flat finish. The subsequent light reflection effect on the building's cladding is usually homogeneous. This effect was not the one that was sought.
Instead, the extrusion technique was used to make the biscuit. This means the paste contains higher levels of moisture, and its linear geometry can be shaped when passing through the nozzle. Square parts, measuring the same width as the extrusion exit bandwidth, are cut according to a template. They are then placed in a continuous dryer to reduce their humidity, thus doubling or tripling their mechanical strength and allowing them to undergo subsequent processing. The enamel is applied at a later stage, allowing to reach a level of gloss and smooth texture that generate the targeted light reflection effects. To start with, these applied enamels undergo a biscuit phase that enables removing certain organic matter, if there is any, or simply facilitating the enamel's handling processes. This latter stage applies in particular in the case of special parts, in terms both of structure fragility and size. After manually applying silica-based enamels, a less homogeneous and above all less flat surface is achieved than during press manufacturing. During the firing process at 1000 • C, which lasts about 45 min, the silica melts into the varnish and is perfectly associated with the base of the biscuit, which presents irregularities in its flatness.
To summarise, these square ceramic tiles with iridescent-like gloss effects are achieved over a five-phase process: Phase 1: Geometric production of the semi-craftsmanship tile by extrusion ( Figure 4). Phase 2; Introduction into a continuous dryer to reduce the moisture in the paste. Phase 3: Application of a slip layer to generate an interface between the biscuit and the enamel, which minimises the tension between the two layers due to differences in dilation coefficients and post-cooling contraction.
Phase 4: Application of enamel using any of the usual methods: bell-shaped system, airbrush, disc glazing, spraying.
Phase 5: Firing. Biscuits of ceramic 20 × 20 cm stoneware tiles, with mono-firing system at 950 • C to vitrify the biscuit and the ceramic piece's protective enamel.
In this way, the reflection of light on the tiles produces vibration effects as well as formal and geometric variations as users vary their angles of vision, or simply approach and circulate through the building ( Figure 5). Visual perception thus varies in space and over time, sometimes producing iridescence-like effects.
The process differs significantly, a priori, from that employed to make other porcelain stoneware with iridescent effects, such as those used in the MUCA. The ceramic is also produced based on ceramic pastes stoned at high temperatures, via semi-craftsmanship production. The iridescent effect is achieved through three successive firings: Firing 1: Application of white-based enamel to vitrify the biscuit. Firing at 1180 • C. Initial ceramic pieces with white-based enamel are obtained for later phases.
Firing 2: Application of enamel with a titanium oxide, zirconium oxide and zinc oxide base. Manually applied with a brush. Firing at 1180 • C. Initial iridescence and pearl results are produced. In this way, the reflection of light on the tiles produces vibration effects as well as formal and geometric variations as users vary their angles of vision, or simply approach and circulate through the building ( Figure 5). Visual perception thus varies in space and over time, sometimes producing iridescence-like effects. The process differs significantly, a priori, from that employed to make other porcelain stoneware with iridescent effects, such as those used in the MUCA. The ceramic is also produced based on ceramic pastes stoned at high temperatures, via semi-craftsmanship production. The iridescent effect is achieved through three successive firings:  In this way, the reflection of light on the tiles produces vibration effects as well as formal and geometric variations as users vary their angles of vision, or simply approach and circulate through the building ( Figure 5). Visual perception thus varies in space and over time, sometimes producing iridescence-like effects. The process differs significantly, a priori, from that employed to make other porcelain stoneware with iridescent effects, such as those used in the MUCA. The ceramic is also produced based on ceramic pastes stoned at high temperatures, via semi-craftsmanship production. The iridescent effect is achieved through three successive firings: It is precisely before the third firing that the balm formed by solutions of metal salts in resins or other vehicles is applied to the vitrified enamels. Following the guidelines set out, some recent research carried out various eutectic combinations [51], which allow the obtaining of satisfactory structural results [52]. A coating manually applied with a brush based on titanium oxide, zirconium oxide and zinc oxide consists of particles with different dilation coefficients, and sizes between 50 and It is precisely before the third firing that the balm formed by solutions of metal salts in resins or other vehicles is applied to the vitrified enamels. Following the guidelines set out, some recent research carried out various eutectic combinations [51], which allow the obtaining of satisfactory structural results [52]. A coating manually applied with a brush based on titanium oxide, zirconium oxide and zinc oxide consists of particles with different dilation coefficients, and sizes between 50 and 350 microns. The last vitrified layer is cracked, which multiplies the iridescent effects of the reflection of light, with colours and shades of colours ( Figure 6).

Physico-Chemical Analysis of the Ceramic Tiles
The outer and inner enamels of the FEUA ceramic pieces, as well as a section or profile of the FEUA, were analysed in order to determine the various enamel layers applied. The results of the SEM analysis of the outer enamel, combined with its energy-dispersive X-ray spectrometry EDX, determined the presence of Al, Zn, and Zr (Table 1), in average proportions of 5.42%, 1.87%, and 1.43%, respectively. Microscopy images at 350 magnifications show that the distribution of the Al and Zn atoms was homogeneous throughout the surface, while the Zr was concentrated at various isolated points in a quasi-circular shape measuring approximately 75 mm in diameter (Figures 7 and 8). This phenomenon resembles the results obtained for the MUCA, with similar metals (Al, Zn, and Zr), in

Physico-Chemical Analysis of the Ceramic Tiles
The outer and inner enamels of the FEUA ceramic pieces, as well as a section or profile of the FEUA, were analysed in order to determine the various enamel layers applied. The results of the SEM analysis of the outer enamel, combined with its energy-dispersive X-ray spectrometry EDX, determined the presence of Al, Zn, and Zr (Table 1), in average proportions of 5.42%, 1.87%, and 1.43%, respectively. Microscopy images at 350 magnifications show that the distribution of the Al and Zn atoms was homogeneous throughout the surface, while the Zr was concentrated at various isolated points in a quasi-circular shape measuring approximately 75 mm in diameter (Figures 7 and 8). This phenomenon resembles the results obtained for the MUCA, with similar metals (Al, Zn, and Zr), in which the Zr was also concentrated in isolated points, although with a smaller surface area (about 15 m in diameter), and with amorphous shapes other than circles [8]. The proportions of Zn and Zr were twice as high as in the case studied here. This point concentration causes a diffraction of light, which, depending on the incidence of sunlight and observation point, produces iridescent effects. In the case of a surface area that is 25 times larger, the ratio between the magnitude and the light wavelengths is different, producing much weaker iridescent effects, and significantly increasing the enamel's level of gloss. Figure 9 shows the X-ray spectrometry EDX of the outer enamel.
in diameter), and with amorphous shapes other than circles [8]. The proportions of Zn and Zr were twice as high as in the case studied here. This point concentration causes a diffraction of light, which, depending on the incidence of sunlight and observation point, produces iridescent effects. In the case of a surface area that is 25 times larger, the ratio between the magnitude and the light wavelengths is different, producing much weaker iridescent effects, and significantly increasing the enamel's level of gloss. Figure 9 shows the X-ray spectrometry EDX of the outer enamel.  in diameter), and with amorphous shapes other than circles [8]. The proportions of Zn and Zr were twice as high as in the case studied here. This point concentration causes a diffraction of light, which, depending on the incidence of sunlight and observation point, produces iridescent effects. In the case of a surface area that is 25 times larger, the ratio between the magnitude and the light wavelengths is different, producing much weaker iridescent effects, and significantly increasing the enamel's level of gloss. Figure 9 shows the X-ray spectrometry EDX of the outer enamel.  The XRD performed on the sample of ceramic outer enamel indicated the presence of ZrSiO4 zirconium silicate, coinciding with the SEM analysis ( Figure 10). These results are similar to those obtained for the ceramic pieces of the Auditorium of the Algueña (MUCA). This zirconium dispersion produces the phenomenon of light diffraction by influencing zirconium and zinc. The physicaloptical phenomenon is mainly caused by the lack of homogeneity in the distribution of Zr, in the outer enamel layer. Light, in its path, encounters different refractive indexes between the Zr and Zn metals, diffracts in the crystallisation of zirconium silicate and also due to the its wavelength ratio to the dispersion of Zr [53]; it also induces the white light to separate into all the colours of the spectrum. The wavelengths produced interfere with each other causing iridescent effects. To analyse the final reason explaining differences in iridescence between the two buildings, the various layers of enamel of both ceramic tiles were examined. To do this, a section or profile of the FEUA ceramic tile was analysed. As seen through microscopy images (Figure 11), and EDX analysis of the profile, only one enamel application was performed, with a single firing. Presence of silicabased slip was also observed. The same was observed again in the surface sample of dispersion of The XRD performed on the sample of ceramic outer enamel indicated the presence of ZrSiO 4 zirconium silicate, coinciding with the SEM analysis ( Figure 10). These results are similar to those obtained for the ceramic pieces of the Auditorium of the Algueña (MUCA). This zirconium dispersion produces the phenomenon of light diffraction by influencing zirconium and zinc. The physical-optical phenomenon is mainly caused by the lack of homogeneity in the distribution of Zr, in the outer enamel layer. Light, in its path, encounters different refractive indexes between the Zr and Zn metals, diffracts in the crystallisation of zirconium silicate and also due to the its wavelength ratio to the dispersion of Zr [53]; it also induces the white light to separate into all the colours of the spectrum. The wavelengths produced interfere with each other causing iridescent effects. The XRD performed on the sample of ceramic outer enamel indicated the presence of ZrSiO4 zirconium silicate, coinciding with the SEM analysis ( Figure 10). These results are similar to those obtained for the ceramic pieces of the Auditorium of the Algueña (MUCA). This zirconium dispersion produces the phenomenon of light diffraction by influencing zirconium and zinc. The physicaloptical phenomenon is mainly caused by the lack of homogeneity in the distribution of Zr, in the outer enamel layer. Light, in its path, encounters different refractive indexes between the Zr and Zn metals, diffracts in the crystallisation of zirconium silicate and also due to the its wavelength ratio to the dispersion of Zr [53]; it also induces the white light to separate into all the colours of the spectrum. The wavelengths produced interfere with each other causing iridescent effects. To analyse the final reason explaining differences in iridescence between the two buildings, the various layers of enamel of both ceramic tiles were examined. To do this, a section or profile of the FEUA ceramic tile was analysed. As seen through microscopy images (Figure 11), and EDX analysis of the profile, only one enamel application was performed, with a single firing. Presence of silicabased slip was also observed. The same was observed again in the surface sample of dispersion of To analyse the final reason explaining differences in iridescence between the two buildings, the various layers of enamel of both ceramic tiles were examined. To do this, a section or profile of the FEUA ceramic tile was analysed. As seen through microscopy images (Figure 11), and EDX analysis of the profile, only one enamel application was performed, with a single firing. Presence of silica-based slip was also observed. The same was observed again in the surface sample of dispersion of zirconium silicate ZrSiO 4 . Zn and Al display a homogeneous distribution ( Figure 11). The EDX makes it possible to quantify the presence of metals, including Zr, Zn, and Al. The proportions are shown later in Table 2.
Coatings 2020, 10, x FOR PEER REVIEW 12 of 22 zirconium silicate ZrSiO4. Zn and Al display a homogeneous distribution ( Figure 11). The EDX makes it possible to quantify the presence of metals, including Zr, Zn, and Al. The proportions are shown later in Table 2. In the case of the MUCA, three enamel layers were applied in the process, with three successive firings. As demonstrated in previous research, the final layer of TiO2 or anatase titanium oxide enamel, following the firing process at 750-780 °C, undergoes microcracking in the cooling phase ( Figure 12) [8]. Magnifying 3000 times, a cracking of this last layer of anatase is observed, with a groove thickness between 0.5 and 1 micron. This micro-breathing is due to the differences in coefficients of thermal expansion between titanium and zirconium, 1.3 and 0.3 × 10 −5 in/in/°C. The anatase, in its final application, greatly reinforces the effect of light reflection iridescence both for its physical qualities and for its microcrack effect in relation to the wavelengths of light and its refraction [54]. The end result of the manufacturing process is porcelain stoneware tiles of well-known construction characteristics, but which incorporate a vitrified enamel without microcracks, making the ceramic pieces resistant to moisture, frost and chemical action, while enhancing the goniochromatic or iridescent colour properties. In the case of the MUCA, three enamel layers were applied in the process, with three successive firings. As demonstrated in previous research, the final layer of TiO 2 or anatase titanium oxide enamel, following the firing process at 750-780 • C, undergoes microcracking in the cooling phase ( Figure 12) [8]. Magnifying 3000 times, a cracking of this last layer of anatase is observed, with a groove thickness between 0.5 and 1 micron. This micro-breathing is due to the differences in coefficients of thermal expansion between titanium and zirconium, 1.3 and 0.3 × 10 −5 in/in/ • C. The anatase, in its final application, greatly reinforces the effect of light reflection iridescence both for its physical qualities and for its microcrack effect in relation to the wavelengths of light and its refraction [54]. The end result of the manufacturing process is porcelain stoneware tiles of well-known construction characteristics, but which incorporate a vitrified enamel without microcracks, making the ceramic pieces resistant to moisture, frost and chemical action, while enhancing the goniochromatic or iridescent colour properties.  We can conclude that the iridescent effect of the ceramic tiles of the FEUA is very weak due to, on the one hand, the lesser presence of zirconium silicate, which is about half that in the case of the MUCA, but above all, due to the absence of the final layer of anatase (TiO2). The first tiles were made with a single-firing system versus the three firings of the second building. The targeted effect was to enhance the gloss against iridescence, as we will see in the colorimetric and gloss characterisation. The cost of the ceramic pieces is also much lower, which was one of the requirements of the FEUA project competition.
With regard to the inner face of the ceramic tile, without enamel ( Figure 13), it is possible to review, through the XRD analysis, the presence of silica SiO2, Cristobalite SiO2, orthoclase K(Al,Fe)Si2O8-widely used as a flux material in the manufacturing of ceramics and porcelain-, calcite magnesian (CM)-Mg0.1Ca0.9CO3, Dolomite CaMg(CO3)2 and rutile TiO2 (Figure 14). The presence of these elements and the metals Al, Fe and Ba were also found via the EDX of SEM microscopy. The presence of barium Ba was also encountered, although the XRD analysis did not specify the presence of any compound including it. Figure 13 shows its dispersed distribution, marked in red and magnified 700 times under the microscope.
The presence of barium is due to the application of barium carbonate (BaCO3), also known as witherite, which is added to clays to precipitate soluble salts (calcium and magnesium sulfate), which cause efflorescence in the stoneware. Barium oxide BaO is also frequently used in the application of We can conclude that the iridescent effect of the ceramic tiles of the FEUA is very weak due to, on the one hand, the lesser presence of zirconium silicate, which is about half that in the case of the MUCA, but above all, due to the absence of the final layer of anatase (TiO 2) . The first tiles were made with a single-firing system versus the three firings of the second building. The targeted effect was to enhance the gloss against iridescence, as we will see in the colorimetric and gloss characterisation. The cost of the ceramic pieces is also much lower, which was one of the requirements of the FEUA project competition.
With regard to the inner face of the ceramic tile, without enamel ( Figure 13), it is possible to review, through the XRD analysis, the presence of silica SiO 2 , Cristobalite SiO 2 , orthoclase K(Al,Fe)Si 2 O 8 -widely used as a flux material in the manufacturing of ceramics and porcelain-, calcite magnesian (CM)-Mg 0.1 Ca 0.9 CO 3 , Dolomite CaMg(CO 3 ) 2 and rutile TiO 2 ( Figure 14). The presence of these elements and the metals Al, Fe and Ba were also found via the EDX of SEM microscopy. The presence of barium Ba was also encountered, although the XRD analysis did not specify the presence of any compound including it. Figure 13 shows its dispersed distribution, marked in red and magnified 700 times under the microscope.
microscopy. The presence of barium Ba was also encountered, although the XRD analysis did not specify the presence of any compound including it. Figure 13 shows its dispersed distribution, marked in red and magnified 700 times under the microscope.
The presence of barium is due to the application of barium carbonate (BaCO3), also known as witherite, which is added to clays to precipitate soluble salts (calcium and magnesium sulfate), which cause efflorescence in the stoneware. Barium oxide BaO is also frequently used in the application of enamels, acting as a flux, protection and crystallisation agent. It is combined with certain colouring oxides to produce unique colours not easily achievable by other means. This research has shown that it was not used in enamels applied in the FEUA.   The presence of barium is due to the application of barium carbonate (BaCO 3 ), also known as witherite, which is added to clays to precipitate soluble salts (calcium and magnesium sulfate), which cause efflorescence in the stoneware. Barium oxide BaO is also frequently used in the application of enamels, acting as a flux, protection and crystallisation agent. It is combined with certain colouring oxides to produce unique colours not easily achievable by other means. This research has shown that it was not used in enamels applied in the FEUA.
The composition of the ceramic biscuit that serves to support the enamels differs substantially from the ceramic pieces of the Auditorium of the Algueña (MUCA), since in this case it is porcelain stoneware, made with other raw materials and with three firing materials, one of them at 1180 • C. Table 2 shows the results of the element analysis or chemical characterisation of the energy-dispersive X-ray spectrometry (EDX) obtained for the outer face, the inner face and the profile or section of the ceramic sample analysed. Results of two measurements performed in two different areas of SEM microscopy are shown for both the outer face or enamel, and inside the ceramic tile. Only the "norm" value is displayed. C (wt.%), which is the normalised concentration in weight percent of each element. The presence of Zr, Zn, Fe, and Pb in the enamel is observed, and the absence of Ti, which only appears on the inner face or biscuit, although the Fe does not appear in the second measurement of the enamel. The presence of Al aluminium is notable in all parts of the analysed tile.

Colorimetric and Gloss Analysis
The colorimetric analysis of the ceramic tiles of the Faculty of Education of the University of Alicante (FEUA), measured with the multi-angle spectrophotometer BYK-mac i 23 mm (D65 illuminant), results in a high level of grey-colour homogeneity. Figure 15 shows the reflectance spectra curves for the characteristic grey colour, in which the colour curves according to observational geometries are very close (slight variation in luminosity) and share a similar form of spectrum (no shade variation). Unlike the MUCA ceramic pieces, they present a low degree of individual goniochromatism, allowing us to conclude that there is a very slight iridescence effect. Table 3 and Figure 16 show that the colour variation by observation geometries is very reduced, according to CIE L* a*b* units.      Figure 15. Spectral reflectance curves of the characteristic colours of FEUA ceramic tiles for the 6 geometries 45as110 to 45as15.   In the MUCA, by means of the group analysis of the characteristic colours, and grouping the reflectance measurements according to the six geometries in each characteristic colour, it was possible to verify that following the thermal curing process, these colours presented a certain degree of individual goniochromatism ( Figure 17), but it was not so high compared to the goniochromatic colours [30,39] currently used in automotive paints [40], cosmetics, etc. With regard to gloss measurements, as set out in the methodology, the glossmeter was applied to the ceramic tiles of the FEUA and the MUCA (Figure 18). Tables 4 and 5 show sample gloss measurements in GU (Gloss Units), based on the 20-degree light angle. They were measured up to 10 times at the same point to check measurement repeatability and to validate the results, which were satisfactory. They were then measured at three different points to observe the levels of gloss homogeneity throughout the material. Although the results are displayed according to the 20° light angle, measurements were made, as required, at 20°, 60°, and 85°. Since values at 60° exceeded the value "70", the measurement value selected was 20°.   With regard to gloss measurements, as set out in the methodology, the glossmeter was applied to the ceramic tiles of the FEUA and the MUCA (Figure 18). Tables 4 and 5 show sample gloss measurements in GU (Gloss Units), based on the 20-degree light angle. They were measured up to 10 times at the same point to check measurement repeatability and to validate the results, which were satisfactory. They were then measured at three different points to observe the levels of gloss homogeneity throughout the material. Although the results are displayed according to the 20 • light angle, measurements were made, as required, at 20 • , 60 • , and 85 • . Since values at 60 • exceeded the value "70", the measurement value selected was 20 • . With regard to gloss measurements, as set out in the methodology, the glossmeter was applied to the ceramic tiles of the FEUA and the MUCA (Figure 18). Tables 4 and 5 show sample gloss measurements in GU (Gloss Units), based on the 20-degree light angle. They were measured up to 10 times at the same point to check measurement repeatability and to validate the results, which were satisfactory. They were then measured at three different points to observe the levels of gloss homogeneity throughout the material. Although the results are displayed according to the 20° light angle, measurements were made, as required, at 20°, 60°, and 85°. Since values at 60° exceeded the value "70", the measurement value selected was 20°.    The value, at 20 • , 60 • , and 85 • , is high. The comparison with the MUCA ceramic pieces produced significant results. At 20 • and 60 • angles, the gloss is null, and the appearance is matte. This may be due to the presence of microcracks in the last anatase layer, which prevents light reflection at low incidence angles [55]. When the angle increases to 85 • , the gloss values are very high, around 97.2 ( Table 6). This value exceeds that of the ceramic tiles of the FEUA. We can conclude that when an observer is roughly facing the front of the FEUA building, the gloss value is high; the façade reflects its surroundings relatively faithfully as far as colours are concerned, whereas in the case of the MUCA, it is mainly iridescence that reaches the observer's eye, and colours in the surroundings are only reflected at high observation angles.
The characteristic colours of the FEUA environment-reddish, greenish and blue-are reflected on the uneven surface of the ceramic tiles. Due to the enamel's high gloss index and dark grey base, notable light vibrations are produced for observers. When an observer crosses the campus, a visual phenomenon resembling iridescence occurs, although it differs from iridescence regarding its ultimate causes. The ZrSiO 4 zirconium silicate present in the enamel layer [56], coupled with its surface irregularities, increases the reflection of sunlight [57] and produces light diffraction values, with an aesthetically attractive result. The phenomenon is intensified at night, under artificial halide-lamp lighting.

Conclusions
When applied to building façade claddings, ceramic tile enamels based on metal deposition of ZrO 2 zirconium oxide combined with additional techniques can trigger singular reflection and light refraction effects for observers. Sometimes one encounters marked iridescence and on other occasions this effect is combined with the reflection of the range of colours in the surroundings due to high levels of gloss.
In the present study, we examined the application of these enamels to intense grey-based tiles manufactured by extrusion. The irregularities of the surface, contrary to that of a press manufacturing system, together with the semi-craftsmanship application of enamel, cause the FEUA's pieces to generate an iridescent-like visual effect for observers. But the colorimetric characterisation analysis, compared to the ceramic pieces of the MUCA, with a marked iridescent effect and an attractive range of goniochromatic colours, has shown that the iridescent effect is very weak. The characteristic colours of the FEUA's surroundings-reddish, greenish and blue-are reflected by the uneven surface of the ceramic tiles. Due to the high gloss index and dark-grey base of its enamel, significant light vibrations are produced for observers, as well as a varied range of colours. When circulating through the campus, a visual phenomenon occurs resembling that of iridescence, although for different reasons.
Despite the presence of zirconium silicate (ZrSiO 4 ) in the applied enamel, which is distributed non-homogeneously on the surface, and of other metals such as Zn and Al, similar to those observed in the ceramic pieces of the MUCA in a physical-chemical analysis by microscopy, the ceramic tiles of the FEUA do not contain a finishing layer based on anatase TiO 2 . This layer, present in the MUCA, underwent microcracking based on the two underlying ZrO 2 and ZnO based enamels applied with a brush. The microcracks are responsible for multiplying iridescent effects, producing a marked and varied diffraction of light across the surface of the pieces.
The gloss measurement values of both tiles were very different. In the case of the FEUA, the values at 20 • produced on average 64.5 gloss units, while at 60 • and 85 • , they produced values of 81.2 and 82.7, respectively. In the case of the MUCA, there were no gloss values at 20 • and 60 • . The behaviour was thus almost matte, while at 85 • , the value was very high: 97.2 gloss units.
The characteristic colours of the FEUA ceramic tiles, given their spectral and colorimetric characterisation for various representative measurement geometries, can provide a much broader range of colours, which could be observed in this building under artificial lighting in specific positions and directions.