SUBMITTED TO THE FACULTY OF ALFRED UNIVERSITY

It is important to predict the limit of the glass formation because it makes it possible to control the quality of the glaze. The proposed glass formation boundary technique also allows reliable location of the boundary through quantitative XRD.

Introduction

Images were taken of the glossy surface using SEM (Philips Electron Microscope, Alfred University, Alfred, NY). Phase identification for KNaO group glaze compositions (Series A to J) of the alumina and silica mat regions. Phase identification for Li2O group glaze compositions (series L to Q) from the alumina and silica mat regions.

In the high alkali compositions (series F), alkali-rich (mostly Na) aluminosilicate phases crystallize from the melt (Figure 3.25a). EDS analysis of the diopside (CaO·MgO·2SiO2) phase found in glaze composition in the silica (low silica end) and alumina matte regions seen in Series P and Q. A schematic diagram of the glass formation region and experimental approach can be found is shown in Figure 4.1.

XRD pattern of the crystalline phase of anorthite (CaO·Al2O3·SiO2) found in A and B series alumina frosted glazes (JCPDS # 41-1486). XRD pattern of the crystalline phase of diopside (CaO·MgO·2SiO2) found in D & E series alumina matte glazes (JCPDS # 41-1370). XRD pattern of the wollastonite (CaO·SiO2) crystalline phase found in quartz matte glazes from the A, B series, and all Li2O glaze groups (JCPDS # 31-0300).

Table II-1. UMF Limits for Cone 04 Glazes. 17,19

Literature Survey

Introduction

Unity Molecular Formula Concept/Origin

The importance of the constant flux alumina:silica ratio is demonstrated by Seger in his development of the standard cone method for measuring high temperatures.8 The silica to alumina ratio is critical in providing insight into the temperature range of glaze maturation. The boundaries of the UMF are oversimplified in the context of the development of matte surfaces.

Role of Various Oxides in the Glaze

They increase the fluidity of the molten glaze and have an important influence on the development of color. Na2O (soda soda) is very active chemically and acts in glaze as one of the strongest fluxes.1 The primary disadvantage of using soda ash is the very high thermal expansion coefficient it imparts to the glaze.

Glaze Structure and Properties

The conditions are that there must be two alkali ions in the spaces near the Mg2+. Because the oxygen provided by the R2O and RO components is consumed in the formation of the alumina tetrahedrons, they are not available for NBO formation.27 Thus, it can be assumed that for each Al3+ ion, two NBOs are removed from the structure, as shown in Figure 2.2.

Figure 2.1. Schematic drawing of a two-dimensional structure for soda- soda-lime-silicate glass

Crystallization/Devitrification

Therefore, controlling the crystallization and dissolution of crystals in the glaze melt is crucial for controlling the surface and structure of the glaze. They are the number of nuclei formed, the rate of crystal growth and the viscosity of the melt (kinetic model).

Figure 2.3. Schematic of the relation between temperature, nucleation, crystal growth, and viscosity

Defining Glass Formation Boundary

The results of the XRD tests for each series can be seen in Table III-5 (KNaO group) and Table III-6 (Li2O group). XRD pattern of the pseudo-wollastonite (CaO·SiO2) crystalline phase found in the silica mat glazes of the Li2O Group glazes (JCPDS.

Figure 2.4. Region of glass formation in the Li 2 O-Al 2 O 3 -SiO 2 system (molecular percentage composition)

Unity Molecular Formula Approach

Introduction

The approach of Stull (1912), seen in Figure 3.1 provides an excellent road map to the development of good glaze control using the UMF approach.2 The rectangular box labeled KNaO Group is the compositional range tested for the glazes made only from KNaO exists as the alkali. Phase equilibrium diagrams will also be used to predict crystalline phase present in the glaze microstructures.

Figure 3.1. This diagram taken from Stull’s work shows the effect of changing silica and alumina levels in glazes with a constant flux ratio of 0.3 K 2 O: 0.7 CaO (normalized to 1.0)

Experimental Procedure

Approach
Glaze Characterization

Glossmeter Measurements
X-Ray Diffraction
Scanning Electron Microscopy

Gloss Meter measures the specular reflection of a light source from the surface of the glaze (figure 3.3). Schematic diagram of the gloss meter and measurement of specular reflection from the glaze surface.

Table III-1. Normalized Flux Ratios of the KNaO Glaze Compositions.*

Results and Discussion

Characterization of Glaze Compositions
Comparison to Previous Results
Influence of MgO on Glaze Characteristics
Influence of Li 2 O on Glaze Characteristics
Effect of Alumina and Silica on Glaze Characteristics
General Observations

SEM Photomicrographs of the glaze surface microstructure for Series D (normalized flux ratio of 0.25 KNaO: 0.43 MgO: 0.33 CaO): a) Matte region of alumina containing diopside as crystalline species, b) Matte region of aluminum which also contains a diopside species change c matte aluminum range, c) Matte silica with undissolved cristobalite and quartz. The matte aluminum composition contains anorthite as a crystallizing species as seen in the previous two figures (not shown here). This trend is seen in Figures 3.5-3.6 and Table III-4, where the matte silica gloss boundary line can be seen shifting in the direction of high silica and low alumina (higher gloss area).

In the high silica area of the matte silica area, cristobalite with undissolved quartz is found for all intermediate levels (Table III-5). In the case of alumina mats and light glazes, seen in the figures, MgO results in devitrification of fine, well-dispersed crystalline phases (Table III-5 provides phase identification). SEM images of the diopside phases can be seen in Figures 3.33a-b and 3.34a-c, showing the presence of the phase in the alumina matte region, which is now expanded compared to series L to O alumina matte regions.

Increasing the level of silica when the glaze composition is in the alumina matte region will help prevent recrystallization of the melt during cooling (indicated by Composition A).

Figure 3.4. Gloss of the glazes for 0.3 KNaO: 0.7 CaO (Series A) were tested by means of glossmeter

Applying UMF to Ternary Diagrams

Tripartite plot of Series E glazes with gloss-labeled samples representing the zone of glass formation (Molecular Percent Composition). Tripartite plot of F-Series glazes with gloss-labeled samples representing the glass-forming zone (Molecular Percent Composition). Ternary scheme of G Series glazes with gloss labeled samples representing the glass forming zone (Molecular Percent Composition).

Ternary plot of Series H glazes with samples labeled as gloss representing the vitrification range (percent molecular composition). Ternary plot of Series J glazes with samples labeled as gloss representing the vitrification range (percent molecular composition).

Figure 3.41. Ternary plot of Series A glazes with samples labeled as gloss representing the glass formation area (Molecular percentage composition)

Summary

Further tests to be performed are the temperature and cooling rate dependencies of the glass forming region. Due to the importance of the boundary location of the glass formation, a technique is proposed to more accurately define the boundary based on the glass phase compositions. This quantitative XRD technique uses the internal standard method, which allows the phase quantities in the glaze samples to be determined.

Using the calibration curve for the individual phases, the wt% of the phase present in the sample is found. The anorthite present does not allow proper representation, because the subtraction of CaO, found in the anorthite, from the glass phase (required by the calculations), without KNaO, would place the glass phase composition in another triaxial system, giving the reason is for point 5 is represented on a separate tripartite (A) in.

Determining the Glass Formation Boundary

Introduction

Predicting the glass formation limit in a glaze system is important because it enables glaze quality control. The composition of the glass phase in the glaze system lies at the glass formation boundary within this particular system, as shown in previous work on the evolution of the glass phase in porcelain material.47 Experimental data show that the glass formation boundary is an intrinsic link. Provided that the glaze system is exposed to industrial firing processes, i.e. is not quenched (50–35 K/s) or subjected to prolonged cooling rates (geological), the glass limit will be independent of temperature, assuming complete melting is achieved.

This section deals with a new technique that may improve the process for defining the glass formation boundary. Using this information together with the chemical analysis, the glass formation boundary and the crystallization products can then be determined and simplified into a triaxial system, consisting of flux, alumina and silica.