CRYSTALLIZATION AND STRUCTURE FORMATION OF GLASS ARTICLES WITH THE COMPOSITION OF A STEATITE BODY DURING FIRING

P. G. Usov, Yu. I. Nesterov, N. V. Sobora

(Presented by the scientific seminar of the Department of Silicate Technology)

Crystallization and structure formation were studied on articles fabricated by pressing a powder from glass of the following composition: SiO₂—56.70; Al₂O₃—2.06; Fe₂O₃—0.24%; MgO—28.86%; BaO—11.95%; K₂O—0.18%.

The glass was melted in a kryptol furnace in corundum crucibles at a temperature of 1560–1580°C, with holding at the final temperature for about 30 minutes. The glass was ground until it passed completely through a No. 100 sieve. From this powder, using a paraffin binder, articles were formed. After a biscuit firing, the formed articles were fired at different temperatures, up to complete sintering, according to standard regimes. Examination with a microscope showed the following change in structure.

A thin section from a specimen fired at a temperature of 1100°C shows, under the microscope, a fragmental structure. The fragments are rounded, ovoid, and, more rarely, irregular in shape. Between the fragments there are pores with an average size of 0.01–0.02 mm. The fragments are crystallized and are composed of fibrous aggregates in which the individual individuals have dimensions of 15 × fractions of a micron; as a rule, they are located along the periphery and perpendicular to the contours of the fragments. As a result of such crystallization, in the central part of the fragment there form clarified, isotropic areas resembling “pores.” The uncrystallized, clarified area is always located in the center of the fragment—grain—and in its outlines repeats the shape of the fragment. The fibrous crystals have a gray interference color, a positive sign of the zone, and a mean refractive index \(n = 1.618\), corresponding to protoenstatite. The amount of crystallized substance constitutes 70–75% of the thin section. The cement of the fragments is an isotropic mass consisting of glass with small anisotropic areas. The latter are probably composed of fine aggregates of crystalline substance and glass, the amount of which constitutes 25–30% of the thin section. The glass has a refractive index of about 1.544. Individual grains up to \(0.02 \times 0.03\) mm in size are identified as quartz and a carbonate-like mineral with high birefringence and pseudoabsorption.

A thin section from a specimen fired at a temperature of 1200°C, under the microscope, differs hardly at all from the preceding one. The fragmental structure is distinctly preserved. Clarified areas inside the fragments and pores between the fragments are retained. The crystallized substance

in the predominant part of the fragments it has a fibrous structure. Alongside them, however, grains represented by fine-grained aggregates are encountered.

In some fragments the fibrous aggregates split along the long axis into separate fine grains, as a result of which the finely fibrous aggregates are replaced by fine-grained ones. Crushing of the crystal fibers proceeds from the periphery toward the center of the grain. According to the refractive index, the crystalline phase has a protoenstatite composition. Quartz and a carbonate-like mineral are recorded as individual grains. The cementing material is glass; its amount is 25–30% of the thin section. Most of the glass has a refractive index \(n = 1.544\). However, small areas of glass are observed (about one percent of the thin section) with a reduced refractive index, about \(n = 1.486\).

A thin section from a specimen fired at a temperature of 1230°C shows, under the microscope, a heterogeneous structure. About 1/3 of the fragments retain crystals of fibrous structure. The remaining fragments are made up of fine-grained aggregates with individual crystalline individuals 0.001–0.02 mm in size. The total refractive index of the fine microcrystalline accumulations is \(n = 1.610\). Quantitatively, they predominate in the thin section. There is considerably less crystalline substance with a refractive index \(n = 1.618\), corresponding to protoenstatite. The amount of glass is 30–35% of the thin section. It fills all the interstices between the fragments. The predominant refractive index of the glass lies within the range 1.544–1.554. There are few areas of glass with \(n < 1.544\). Crystals of quartz and of a carbonate-like mineral are preserved.

A thin section from a specimen fired at 1280°C retains a fragmental structure. In some grains fibrous aggregates are preserved, oriented perpendicular to the outer surface of the fragments. But in general the specimen is composed of fine-grained aggregates of crystalline substance and glass. Individual crystalline individuals have dimensions of 0.001–0.002 mm. These finest grains are uniformly bordered by a film of glass. The total refractive index of the fine-grained aggregates is \(n = 1.608 \pm 0.003\) mm.

Slightly elongated grains up to 0.003–0.005 mm in size, with a refractive index \(n = 1.658\), corresponding to clinoenstatite, are recorded only rarely. The amount of glass is 30–35% of the thin section. Accumulations of glass alone are not observed; it is uniformly distributed and cements the crystalline substance. The refractive index of the glass is \(n = 1.545–1.530\). Individual grains of quartz and of a carbonate-like mineral are preserved.

A thin section from a specimen fired at 1350°C has, under the microscope, a fine-grained structure. The fragmental structure of the thin section becomes barely perceptible; the structure is homogeneous.

In crossed nicols, separate patches stand out in which the grains are oriented in one direction, and therefore appear light. The thin section is composed of fine-grained aggregates of magnesium metasilicate and glass. Individual crystalline-substance individuals have dimensions of 0.001–0.002 mm. The total refractive index of the fine-grained formations, surrounded by a film of glass, is \(n = 1.610\). Separate elongated grains 0.003–0.005 mm in size with a refractive index \(n = 1.660\) are observed only rarely.

The amount of glass is 30–35% of the thin section. It envelops all crystalline formations with a thin film and is a film cement in relation to the crystalline substance. The refractive in-

the refractive index of the glass changes from 1.545 to 1.530. A carbonate-like mineral is encountered.

X-ray studies of specimens fired at a temperature of 1050°C show (Table 1) that the crystals that have separated have major structural defects, as a result of which strong scattering of X-rays is observed. In the composition of the crystallization products

Table 1

Indices of X-ray diffraction patterns of glass articles of steatite composition after firing
at different temperatures (taken from a disk)

$d$ — interplanar spacing, Å; $I$ — absolute intensity

the predominant phase is protoenstatite with its principal indices $d = 3.15$–$3.19$; $1.97$–$1.99$; $1.46$–$1.49$, and clinoenstatite is present in subordinate amount, $d = 2.96$–$3.00$; $1.60$–$1.61$. With increasing firing temperature, the clinoenstatite phase disappears completely. From a temperature of 1280°C, it is not fixed by X-ray analysis in the firing products.

is not recorded at all. As the firing temperature is increased, the structure of the crystals becomes more perfect. A larger number of planes participate in the reflection of X-rays, and the X-ray patterns become sharper. The crystalline phase is composed of protoenstatite.

Discussion of Results

During crystallization of glass having the composition of a steatite body at low temperatures, the primary crystalline phase that separates is a mineral of fibrous structure. The fibers, with their elongated axes, are arranged perpendicular to the grain surface. Crystallization and growth of the crystals proceed in the direction from the surface toward the center of the grain. At the same time, internal diffusion of the substance takes place in the opposite direction, from the center toward the periphery, ensuring crystal growth; as a result, the central part of the fragment remains uncrystallized, “clarified.” The refractive index of the glass inside the fragments (the clarified regions) is \(n = 1.532\).

The fibrous formations have a refractive index \(n = 1.618\). When firing at higher temperatures, the fibrous form of the crystals changes into a fine-grained one, which becomes predominant on firing at \(1230^\circ\mathrm{C}\) and above. The granular form of the crystals is produced by rupture of the fibers along the long axis into small grains. The rupture sites are filled with an isotropic mass, forming fine-grained aggregates with a total refractive index \(n = 1.608\text{—}1.610\). The process of transformation of fibers into grains also proceeds from the periphery of the fragment toward the center. The grain size of the fine-grained aggregates is very small. Only rare individuals reach a size of \(0.001\text{—}0.002\) mm. After firing at \(1350^\circ\mathrm{C}\), approximately \(100^\circ\mathrm{C}\) above the optimum temperature, their dimensions do not increase. Individual crystals of clinoenstatite, with refractive index \(n = 1.658\text{—}1.660\), form only when firing at \(1280^\circ\mathrm{C}\) and above. Clinoenstatite is always recorded in crystals of larger size, not less than \(0.003\text{—}0.005\) mm. The fibrous formations with refractive index \(n = 1.618\) and the fine-grained aggregates with \(n = 1.608\text{—}1.610\) mm are assigned by us to protoenstatite. We explain the decrease in the refractive index of protoenstatite upon transition from the fibrous to the granular form by the influence of the glass in which they are immersed.

An article obtained from ground glass of the composition of a steatite body, after firing to complete sintering, is composed of a fine-grained crystalline phase—protoenstatite—and glass, uniformly enveloping the crystalline formations in the form of a film.

Protoenstatite is stabilized to such an extent that, when the article is crushed to \(0.063\) mm, no transition of it into clinoenstatite is observed.

When articles made of glass are fired even at a temperature approximately \(100^\circ\mathrm{C}\) above the optimum, no bloating phenomenon is observed.

The crystallization capacity of glass of the composition of a steatite body is high. At a temperature of about \(1000^\circ\mathrm{C}\), quantitatively all the magnesium metasilicate crystallizes completely. When firing at higher temperatures, only growth and structural perfection of the crystals occur. This conclusion is confirmed by data on the change in the refractive index of the glassy phase in the firing products. The refractive index of the glassy phase of the investigated steatite ceramic, manufactured by the ceramic method, is \(n = 1.544\).

The refractive index of the glassy phase of a glass article of steatite composition after firing at the following temperatures:

\[ \begin{aligned} 1100^\circ\mathrm{C} \quad & n = 1.544,\\ 1200^\circ\mathrm{C} \quad & n = 1.544,\\ 1230^\circ\mathrm{C} \quad & n = 1.544\text{--}1.550,\\ 1280^\circ\mathrm{C} \quad & n = 1.529\text{--}1.550,\\ 1350^\circ\mathrm{C} \quad & n = 1.525\text{--}1.550. \end{aligned} \]

The amount of amorphous phase, determined with the aid of a microscope in glass articles during firing, is characterized by the following data:

\[ \begin{aligned} 1100^\circ\mathrm{C} \quad & 25\text{--}30\%,\\ 1200^\circ\mathrm{C} \quad & 25\text{--}30\%,\\ 1230^\circ\mathrm{C} \quad & 30\text{--}35\%,\\ 1280^\circ\mathrm{C} \quad & 30\text{--}35\%,\\ 1350^\circ\mathrm{C} \quad & 30\text{--}35\%, \end{aligned} \]

which closely corresponds to the theoretical content.