3. Unity Molecular Formula Approach
3.4 Applying UMF to Ternary Diagrams
When plotting these glaze systems (series A through J) on a ternary diagram, the region of glass formation can semi-quantitatively be assessed. Figure 3.41 is a ternary plot of Series A glazes, which distinguishes between regions of pure glass (high gloss), regions of devitrification, and underfired glazes. The following samples were assigned as glass (gloss) or crystalline phase (devitrified or underfired) based on characterization of the glazes as discussed in previous sections. Samples labeled glass are assumed completely amorphous, but in some cases small crystals of quartz are evident in the SEM, but are assumed articles of underfiring. Devitrified and underfired glazes are separated based on how the crystalline phase was formed. The same can be said for the remaining series that were investigated (the Li2O group of glazes is not discussed due to the entire range tested being crystalline and showing no evidence of pure glass phase).
With Series B and C on the ternary (Figures 3.42 and 3.43 respectively), it can be seen more easily that the glass formation area increases in the direction of the underfired glazes, due to increasing alkali in the glaze composition causing more dissolution of quartz into the glass phase. Also the higher alkali systems result in lower CaO levels, which explains the absence of wollastonite. Compositions containing wollastonite in the underfired region are considered as forming under solid-state reactions.37 These compositions under the proper firing schedule, which achieved a complete melt, may also result in the recrystallization of wollastonite upon cooling as seen in Stull’s and
Thompson’s work on glazes.2,35
Al2O3
0 10 20 30 40 50 60
SiO2
40 50 60 70 80 90 100
0.3 KNaO:
0.7 CaO
0
10
20
30
40
50
60
Gloss/Glass Devitrified Underfired
Figure 3.41. Ternary plot of Series A glazes with samples labeled as gloss representing the glass formation area (Molecular percentage composition).
Al2O3
0 10 20 30 40 50 60
SiO2
40 50 60 70 80 90 100
0.5 KNaO:
0.5 CaO
0
10
20
30
40
50
60
Gloss/Glass Devitrified Underfired
Figure 3.42. Ternary plot of Series B glazes with samples labeled as gloss representing the glass formation area (Molecular percentage composition).
Al2O3
0 10 20 30 40 50 60
SiO2
40 50 60 70 80 90 100
0.7 KNaO:
0.3 CaO
0
10
20
30
40
50
60
Gloss/Glass Devitrified Underfired
Figure 3.43. Ternary plot of Series C glazes with samples labeled as gloss representing the glass formation area (Molecular percentage composition).
Similar to Series A through C the glass formation region is seen increasing with increasing alkali in Series D through F (Figures 3.44-3.46) where MgO is introduced into the glaze composition. However, the boundary between devitrified compositions and glass compositions change significantly with increasing alkali. At high alkaline earth levels (Series D) the devitrified region contains diopside indicating the compositional limits for glass formation for CaO and MgO have been met. At higher alkali levels the glass formation region becomes similar to what is seen in Series B and C. The boundary for devitrification in Series E is again determined by alkaline earth level (diopside as crystalline species) while Series F is determined by alkali level in the glaze composition with sodium aluminosilicate as the crystalline phase.
Significant changes in the glass formation region are seen when larger amounts of MgO are introduced into the system leaving low levels of CaO in the system. This is seen in the ternary plots of Series G through J found in Figures 3.47 through 3.49. Series D glaze composition consists of 1.3:1 ratio of MgO to CaO while Series G composition is 4:1 of MgO to CaO. Notice that the larger concentrations of MgO in these glaze systems
result in a significantly smaller glass formation region with a majority of the testing range dominated by products of devitrification. A high degree of melting behavior is seen in these glaze compositions, but the systems precipitate spinel and forsterite phases in the glaze compositions labeled as devitrified. The lower CaO levels experienced in Series G through J, when compared to the Series D through F, correlates with the absence of diopside.
Al2O3
0 10 20 30 40 50 60
SiO2
40 50 60 70 80 90 100
0.25 KNaO:
0.32 CaO:
0.43 MgO
0
10
20
30
40
50
60
Gloss/Glass Devitrified Underfired
Figure 3.44. Ternary plot of Series D glazes with samples labeled as gloss representing the glass formation area (Molecular percentage composition).
Al2O3
0 10 20 30 40 50 60
SiO2
40 50 60 70 80 90 100
0.40 KNaO:
0.34 MgO:
0.26 CaO
0
10
20
30
40
50
60
Gloss/Glass Devitrified Underfired
Figure 3.45. Ternary plot of Series E glazes with samples labeled as gloss representing the glass formation area (Molecular percentage composition).
Al2O3
0 10 20 30 40 50 60
SiO2
40 50 60 70 80 90 100
0.65 KNaO:
0.20 MgO:
0.15 CaO
0
10
20
30
40
50
60
Gloss/Glass Devitrified Underfired
Figure 3.46. Ternary plot of Series F glazes with samples labeled as gloss representing the glass formation area (Molecular percentage composition).
Al2O3
0 10 20 30 40 50 60
SiO2
40 50 60 70 80 90 100
0.25 KNaO:
0.13 CaO:
0.62 MgO
0
10
20
30
40
50
60
Gloss/Glass Devitrified Underfired
Figure 3.47. Ternary plot of Series G glazes with samples labeled as gloss representing the glass formation area (Molecular percentage composition).
Al2O3
0 10 20 30 40 50 60
SiO2
40 50 60 70 80 90 100
0.40 KNaO:
0.12 CaO:
0.48 MgO
0
10
20
30
40
50
60
Gloss/Glass Devitrified Underfired
Figure 3.48. Ternary plot of Series H glazes with samples labeled as gloss representing the glass formation area (Molecular percentage composition).
Al2O3
0 10 20 30 40 50 60
SiO2
40 50 60 70 80 90 100
0.59 KNaO:
0.33 MgO:
0.08 CaO
0
10
20
30
40
50
60
Gloss/Glass Devitrified Underfired
Figure 3.49. Ternary plot of Series J glazes with samples labeled as gloss representing the glass formation area (Molecular percentage composition).
Again, for the reasons stated previously, the trend seen with the glass formation region with increasing MgO is due to the structural implications of MgO. The literature suggests with increasing temperature the gloss/glass formation area will increase in both directions.2, 35, 46 This research argues this previous statement to be too general. The composition of the glaze determines whether a glass will form under industrial type heat treatments. Compositions located in devitrification regions will crystallize on cooling if heated above the melting temperature (Tm). The only way to avoid recrystallization from the melt is increasing the cooling rate so that devitrification is unable to take place. In most cases quenching is the only solution, which is unfavorable to industrial glaze applications. Compositions in the underfired region are the only compositions that could result in glass formation with increased temperature. Even then devitrification is not out of the question. Referring to Figures 3.47-3.48 the underfired glaze composition will more than likely recrystallize on cooling even when heated to higher temperatures. These
underfired regions will more than likely require alumina additions to enhance glass- forming ability and counteract the devitrification process.
Further tests that need to be conducted are the temperature and cooling rate dependencies of the glass formation region. Due to the glass formation area being highly dependent upon temperature and multiple temperature ranges used to mature glazes it would be important to conduct these tests in the future. The glass formation boundary location, as seen in Figures 3.41-3.49, is not precise so new measures need to be diploid to accurately define the boundary. This facilitates the need for a procedure for further investigation into this area, which leads us into the next section that deals with a proposed technique to accurately define the glass formation boundary for glaze compositions.