STUDY OF THE FUSIBILITY OF SOME RAW GLAZE COMPOSITIONS
David Tetteh
Institute Of Industrial Research, (C.S.I.R), Ghana
Published in The Ghana Engineer, May 1999
Reprinted with GhIE permission by the African Technology Forum

ABSTRACT

 The fusibility characteristics of four glazes with one Seger formula but varying raw materials have been investigated using the button test technique (Takashima, 1983). The results confirm that different materials have different fusibility characteristics and that glazes composed of materials with active network-modifying oxides (which form complex mixture) have a wider maturing range (Kredl and Weyl, 1942).  Thus the glazes, which were composed predominantly of nepheline syenite, limestone and pottery stone had a wider maturing range than those composed predominantly of feldspar, limestone and pottery stone.

1.0          INTRODUCTION

Fusibility is one of the most important factors in the production of glazes on account of its influence on the surface of glazed ceramic wares. It largely governs the viscosity of glazes. Viscosity, at maturing temperature, determines the extent to which the glaze can flow over the body to form a uniform layer without running off inclined or vertical surfaces. Fusibility may be defined as ability of a substance to change from the solid to a liquid phase, usually through the agency of heat (though pressure may also be an influencing factor).

The fusing or sublimation behaviour is largely determined by the strength of the atomic bond. Weakly bonded alkali and monovalent ionic ceramics have low melting temperatures (Richerson, 1982). Thus in an attempt to select appro­priate locally available materials for the production medium temperature glazes, different materials have been used to compose four glazes based on the same formula. The aim is to characterize the compositions on the basis of their fusibility

2.0          EXPERIMENTAL PROCEDURE

2.1          Sample Preparation

1 kg. each of the glaze composition (raw materials) was placed in 1 kg. ball mill with 1 kg. porcelain balls, and 500 m1. water (water content 50%). These were milled until they could pass through 200 mesh sieve. Each glaze specimen produced was de-watered in a P.O.P. mould until the specimen became mouldable. All the specimens were shaped in cylindrical forms (100 mm diameter x 100 mm height). These were placed on flat unglazed porcelain body and fired in an electric laboratory kiln to the following temperatures: 1080, 1120, 1140, 1160, 1180, 1220, and 1250 deg. C. The firing schedule was 3.5 deg. C per minute. The variations in the diameter of the specimens were recorded even when the latter fused, flowed and expanded.

The compositions of the glaze specimens are based on the formula:

0,2 KNaO               |
0,6 CaO                  |  }        0,25 Al₂O₃  2,5 SiO₂       (x:y mole ratio = 1:10)
0,2 ZnO                  |

Using Table 1 and the formula show above, the glaze compositions were calculated based on the oxide contents of the materials. Table 1 shows the chemical analyses of the raw materials (Tetteh, et al, 1989) and Table 2 shows the glaze compositions. The main differences in the compositions are with the use of Mouri feldspar, Senchi nephiline sysnite, Ekon kaolin and Anfoega pottery stone.

TABLE 1: CHEMICAL ANALYSES OF RAW MATERIALS

Source: Study to classify the raw material base for local production of glazes (Tetteh et al., 1989)

TABLE 2: GLAZE COMPOSITIONS

3.0          RESULTS AND CONCLUSION

 Fig. 1 shows a graph expressing the conditions of the four specimens over the range of temperature. From the curves in Fig. 1, the following were observed: specimen No. 1 sintered between 1080 and 1160 deg. C, after which it started fusing very gradually up to temperature 1180 deg. C, when there was rapid fusing between 1180 and 1200 deg. C. From 1200 to 1250 deg. C the fusing was gradual. Like No. 1, specimen No. 2 sintered between 1080 and 1160 deg. C. However, from 1160 to 1250 deg. C, the fusing was higher than No. 1.

FIG. 1 FUSING PROCESS CURVES

Thus specimen No. 2 had a higher fluidity and consequently was less viscous than No. 1. Specimen No. 3 did not start fusing until temperature 1180 deg. C, recording a brief rapid fusing (1180 - 1200 deg. C), and then gradual fusing up to the final temperature. This specimen was the last to start fusing until it had the lowest fluidity of all the specimens. Specimen No. 4 started fusing fairly rapidly from temperature 1140 deg. C, becoming very slow between 1200 and 1220 deg. C and then very rapid up to the final temperature. This specimen exhibited the widest maturing range and the highest fluidity.

It was observed that even though all the specimens had the same formula, their fusibility characteristics were different. This could be attributed to the combination of the materials used, which tend to have varied influence on the glass structure due to the varied effectiveness of their network-modifying oxides. Consideration of the structure of glasses shows that in ceramic glazes, the basic glass former is silica, and that its properties are varied by the addition of other glass-forming oxides (i.e., B₂O₃ and P₂O₅), network-modifying oxides (i.e., Al₂O₃, ZnO, TiO₂, etc.) (Singer and Singer, 1963). Thus the presence in the materials of the alkalis and alkaline earths, as well as the intermediate oxides of the specimens (see Table 1) accounted for their fusibility characteristics. The large atomic radius of Na⁺ similar to that of K⁺ with which it is often found enables replacement of K⁺ by Na⁺, giving rise to the compound KNaO (a very powerful network modifier with SiO₂), which helps to cause the maturing temperature of a glaze to be lowered.

The ionic radii of K₂O and Na₂O are large enough to promote vitrification by creating irregularities due to the broadening of network by SiO₄. Also, since their bonding force with oxygen is weak, the heat energy heeded to break the bondage chain (for liquification) is relatively small. Thus, since the fusing of glass structure by the cut of silicate network due to heat energy', the larger portion of the element having large ionic radius and small or weak bonding force in the glass structure is likely to fuse at a lower temperature.

Looking at the alkali content of the raw materials, the order of their fluxing effectiveness may be assumed as follows: Senchi nepheline synite > Mouri feldspar > Anfoega pottery stone, as their KNaO mole content was 0,226, 0,164, and 0,112 respectively.  Specimen No. 4 was the first to start fusing (1140 deg. C) and it had the highest fluidity at 1250 deg. C because almost 80% of its composition was made up of nepheline synite, pottery stone and limestone which are major sources of alkali network-modifying oxides. These oxides also formed a complex mixture, giving the glaze specimen a wider maturing range.

Specimen No. 2 came next with respect to fluidity. It had over 50% total of nepheline synite and limestone with no pottery stone addition. Specimen No. 1 was made up of over 70% feldspar and limestone, also with no pottery stone addition, and yet its fusibility was lower than that of No. 2. This indicated that the nepheline synite might have very active network modifiers. Specimen No. 3, made up of over 80% feldspar, limestone and pottery stone (but predominantly feldspar and limestone at appx. 70%), was the last to start fusing and had the least fluidity. The result for this could be the same as that for specimen No. 1 (i.e. the fluxing power of feldspar is less than that of nepheline synite).

In conclusion, it may be reported that within the experiment range, glaze compositions made up made up predominantly of nepheline synite, limestone, and pottery stone have a higher fluidity and wider maturing range than glaze compo­sitions with predominantly feldspar and limestone.

REFERENCES:

 1.    Kreidl, N.J. and Weyl, W.A. "The development of low melting glasses on the basis of structural
       considerations",      The Glass Industry 23, (9, 10, 11, 12), (1942)

2.     Richerson, D.W., Modern Ceramic Engineering, 2nd Ed., Revised and Expanded, Marcel Dekker, Inc.,
        New York. (1992)

3.     Singer, F. and Singer, S.S., Industrial Ceramics, Chapman Hall Ltd. Ltd. London (1963).

4.   Takashimia, H. "Glaze and Color in Ceramics", Nagoya International Training Centre (J.I.C.A.),
        Nagoya (1983).

5.   Tetteh, D., Brenya, E.; Sackey, "Study to classify the raw material base for local glaze production of glazes",
        Technical  Report, Institute Of Industrial Research, (C.S.I.R), Ghana (l989).

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