Six white opaque commercial sanitaryware articles from major producers were acquired in the market, one of which was manufactured in Europe (E1), two in North America (NA1, NA2), and three in South America (SA1, SA2, SA3♠). Characterization of the glazes was performed to provide a reference point typical of the sanitaryware industry for the development of the “high-quality” porcelain glazes in this work. Since the evaluation of the quality of a glaze has both subjective and objective components, pieces were characterized using both qualitative and quantitative approaches.
3.2.1 Qualitative Characterization
Qualitative characterization was addressed in terms of the general appearance of the glazes by assigning a ranking on a relative scale ranging from 1 to 5, where 1 corresponds to glazes exhibiting surface flaws and with little visual appeal and 5 corresponds to glazes which have maximum visual appeal. Presence of surface defects such as crazing, pinholes, waviness, eggshell finish, and crawling were considered when assigning the ranking to the samples. The characterization was performed in accordance with the Colombian Technical Standard 920-1 (NTC 920-1, equivalent to ASME/ANSI A 11.19.2M),27 issued by ICONTEC (Colombian Institute of Technical Standards and Certification), an institute affiliated with the International Organization for Standardization (ISO). Crazing was included in this characterization as an undesirable surface defect.
3.2.1.1 X-Ray Diffraction Analysis
Evidence of crystalline species was examined by X-ray diffraction (XRD). The glazes were analyzed by XRD (Philips XRG 3100 X-ray Generator CuKα1 radiation, Philips Electronics Instruments Inc., Mahwah, NJ) using the following parameters:
♠Samples SA2 and SA3 correspond to pieces manufactured in Colombia.
starting angle = 5° 2θ; ending angle = 80° 2θ; stepsize = 0.05° 2θ, and dwell time = 3.0 s.
A commercial-software routine (Jade v 6.0, Materials Data Inc., Livermore, CA) was used to identify the phases present in the glazes. Flat sections of the samples were cut using diamond-type cutting tools to fit the sample chamber of the X-ray diffractometer.
Preliminary data showed a penetration depth of the CuKα1 radiation less than the typical thickness of the fired glaze, i.e., between 450 µm and 550 µm, and phase identification using software routines showed no evidence of any phase known to be present in the ceramic substrate. Therefore, there was no need to grind a glaze sample into a powder to perform XRD.
3.2.1.2 Microstructure
Both glaze microstructure and glaze flaws, e.g., blisters, bubbles, pinholes, and possible devitrification region were analyzed for this study by Scanning Electron Microscopy (SEM) (Philips 515 Electron Microscope, Philips, Eindhoven, Netherlands).
Microstructures of the opacified glazes, in particular opacifying crystals, were examined by this technique.
Samples were cut using a diamond saw and mounted using epoxy resin. SEM coupled with Energy-Dispersive X-Ray Spectrometry (EDS) (Evex v 2.0.653, Evex Analytical, Princeton, NJ) was used to generate images and to perform qualitative compositional analysis of the glazes. Cross sections of the glazes were analyzed to determine the thickness of the coating and to observe the distribution of possible defects, e.g., bubbles.
3.2.2 Quantitative Characterization
Typical parameters affecting the visual appearance of a glazed surface are gloss, color, and surface roughness. Therefore, quantitative characterization of the glazes was addressed by measuring gloss, color, and surface roughness by using a glossmeter, a spectrophotometer, and an optical interferometer respectively.
3.2.2.1 Gloss Measurements
The gloss of glazed-ceramic articles is often only assessed visually by comparing different samples against a reference i.e., the samples are ranked in terms of their ability to produce sharp and exact reflections of an image, thus giving semi-quantitative, and often, irreproducible results. Gloss consists of an impression formed in the mind of the observer of the reflected light distribution from the surface of the sample, thus making it difficult to define quantitatively. Gloss has been found to be related most closely to the sharpness and perfection of a reflected image, and thus to the intensity of the specular reflection (Figure 3.1).28
Figure 3.1. Schematic diagram showing diffuse (light scattering) and specular reflection from a glaze. Note that the incident beam of light also undergoes refraction, internal reflection and scattering when it penetrates the surface.
Incidence angle Incident light
Specular reflection Diffuse
reflection
Ceramic substrate
Glaze
Factors that affect the intensity of specular reflectance include refractive index of the glaze, surface roughness, and concentration of any internal surfaces caused by crystals, bubbles, or phase separation.29 Quantitative measurements of gloss can be performed based on the measurement of the specular reflection of a light source off the glaze surface by using a glossmeter. A commercial glossmeter (Photovolt G-3 Gloss, ASTM 2457, UMM Electronics Inc., Indianapolis, IN) was used in this study to measure the gloss of the glazes.
This instrument offers the option of performing the measurements at an incidence angle of 20°, 60°, and 85° from the perpendicular to the specimen surface. Disagreement was found in the literature regarding the best angle to measure the gloss in ceramic glazes.29,30 Preliminary results showed that when measuring either opaque or glossy glazes, the lowest standard deviation is obtained when an incidence angle of 60° is used.
This agreed with the guidelines shown in ASTM C 584–81.30 Therefore a 60° incidence angle was selected to run the experiments in this work. The glossmeter reports the results of the measurements in Gloss Units (GU), where 100 GU corresponds to the light reflected from a black reference crystal with refractive index of 1.567. A change in gloss greater than three gloss units is visible to the naked eye.31
This method is limited by the necessity to carry out gloss measurements on an absolutely-flat sample which is virtually free of any prevailing gloss defect. Curved or warped sample surfaces and certain defects such as waviness (which distorts the smoothness of the glaze surface and gives rise to diffuse-light reflection) can cause the meter to under-report the gloss value by reducing the amount of light reaching the detector at the specular angle, i.e., there are slight variations in the angle of the incident light relative to the surface.
3.2.2.2 Color Measurement
Color is also a parameter affecting the visual appearance of a glaze surface. The CIELAB• system of color is currently used in the whiteware industry,32 thus it was used in this research to assess color parameters of the glazes. This system, also know as the
• CIELAB stands for CIE L*a*b*. CIE: Commission Internationale de l'Eclairage
Hunter color system, uses a three dimensional color space arrangement for representing all possible colors. This color space is defined by the color scales L*, a*, and b*, where scale L* is a measure of lightness, a* is a measure of redness (+a*) or greenness (-a*), and b* is a measure of yellowness (+b*) or blueness (-b*).1 L* is also considered a valid way to measure the whiteness of a surface since it only accounts for the difference between black and white (L*=0 is black; L*=100 is white). Only L* parameter is used in this work rather than the entire three-coordinate system since only the degree of whiteness may be related with the presence of opacifier in the glaze.
A GretagMacbeth spectrophotometer (Color-Eye® XTH Spectrophotometer, New Windsor, NY) was used to take the whiteness measurements. The instrument uses CIELAB1976 L*, a*, and b* color equation with a D65 illuminant, 10° observer, and specular component included.
3.2.2.3 Surface Roughness
Roughness of the fired glaze surfaces was quantified using an optical interferometer (NewView 5032, Zygo Corporation, Middlefield, CT). The instrument uses a white light source which is passed through a partially silvered mirror splitting the beam. One half of the light intensity is reflected from the sample surface while the other half is reflected from a reference mirror. The light is then recombined at the partially silvered mirror creating either constructive or destructive interference depending on whether the light is in or out of phase.
The MetroPro™ software (v 7.3.4, Zygo Corporation, Middlefield, CT) was used to remove background noise prior to analyzing the roughness of the surface. For a scan length of 100 µm in the z-direction, the time required to collect the data was 11 seconds.
A 5x Michelson objective was used in the interferometer. Five scans of 1.44 mm by 1.08 mm area (each corresponding to 76,800 data points) were taken for each sample and the surface roughness results were averaged.
Surface roughness results were reported as root-mean-square roughness (RMS), which is an area weighted statistic. It is an indication of the standard deviation of all the measured data points on the surface, i.e., 95% of the surface roughness falls within ± the RMS roughness from the reference plane.