At present, it is more and more important to accurately and finely decorate fine art ceramics. The use of crystalline glaze is a method of decorating ceramic products. A crystalline glaze is a fusible glass applied to a ceramic substrate that partially crystallizes during calcination to form crystals of different compounds, different sizes and shapes. Microcrystalline glazes contain tiny crystals that are invisible to the naked eye and appear opaque (emulsion). In megacrystal glazes, the crystal size is large, visible to the naked eye, and the residual glassy phase is either transparent or opaque.
Crystal glaze is regarded as one of the most luxurious decoration types, it can give the ceramic products an elegant and non-repeating picture. The uniqueness of this effect is achieved by crystal forms of various shapes and shades precipitated in the transparent base glaze. The size of the formed material is several millimeters, or even a few centimeters. The zinc-containing crystal glaze has a very promising future. Spherical and inflorescence wurtzite 2ZnO.SiO2 formations formed as individual crystals or aggregates of different colors and sizes can be precipitated.
2 When the chemical composition of the glaze is ternary, many components belong to the Si02-Zn0-K20 system.
The main component of this glaze crystal is zinc oxide. In addition, ZnO can also reduce the viscosity and surface stress of glaze melts. This promotes the formation of a fluidized layer and a high-quality glaze on the substrate. The introduction of ZnO also has a good effect on the gloss and chemical stability of the glaze.
When the content of eight 123 is high, it has a negative effect on the crystal formation process, so its weight content in the glaze should not exceed 10%. The content of CaO, especially MgO, in the glaze composition is strictly limited because they will increase the surface of the melt. stress. In view of this, it has been proposed to completely remove MgO. The alkali components in the crystalline glaze are sodium oxide and potassium oxide. It has been determined that Na20 can significantly reduce the viscosity of zinc-containing glazes and K20 can guarantee lower surface stress. Therefore, in many cases it is better to incorporate K20 into the glaze.
In order to reduce the viscosity and surface stress, B203 can be introduced in the glaze composition, which is often used as a flux in various glazes and enamels. The total amount of alkali in the high boron enamel can be reduced. The total content of alkali components and boron oxide and its ratio determine the firing humidity and other process properties (viscosity, surface stress, linear expansion coefficient, etc.) of the glaze.
3 Glazing Crystallization Mechanism Crystalline glazes are formed on the basis of complex physical and chemical transformations.
In the firing at higher temperature conditions, the process taking place in the frit layer of the ceramic body first causes the appearance of a glass layer of a thin glaze, and secondarily partially crystallizes the layer.
The formation of the glaze layer is the result of the flow of the glass melt on the surface of the ceramic substrate and the physicochemical interaction with the substrate. The interaction process between glaze and ceramics includes cation diffusion, mass transfer of viscous flow through glaze melts and ceramic softened glass, chemical interactions between melt and ceramic crystalline phases, and formation of novel compounds in the transition layer. This process occurs not only in the formation of transparent glazes (non-crystalline glazes) but also in the formation of crystalline glazes.
The crystals containing zinc glaze are the special crystallization conditions of the glass-forming melt, subject to the general crystallization laws of the solution and the glass. It is well known that the crystallization ability of a melt depends on two factors: the natural or directed formation (nucleation) of the crystallization center and the crystallization speed, ie, the average growth rate of the crystal. Both of these factors are constrained by the melt temperature (or degree of supercooling).
The crystal formation and development kinetics were studied in detail. The number and maximum size of precipitated crystals under different firing conditions were determined, and the relationship between the nucleation speed and the test temperature of crystal growth rate was established. He emphasized that in order to avoid glaze turbidity and crystal formation of trapped spheres, it must first be heated to a high temperature sufficient to dissolve a large number of crystallization centers participating in the melt (the spread temperature, then the temperature is lowered to the crystal development temperature and incubated for 0.51 hours). .
Later researchers pointed out the role of the firing temperature system in the production of crystalline glaze. Indeed, by changing the firing and cooling regimes, it is possible to form conditions in which a small amount of crystals (a opaque glaze) or a limited amount of large crystals (a crystalline glaze) are formed in the glaze layer. WanieW listed experimental evidence in many papers on crystalline glaze. He pointed out that the formation mechanism of crystalline glaze crystals is similar to the opacifying mechanism except that the number of crystals that appear is small, and the crystal size is much larger than that of opaque glazes.
Zinc-containing crystal glaze is the most important in terms of forming various decorative effects by changing the firing temperature and time conditions. In the melt, zinc oxide forms compounds with silicic acid and other constituents and precipitates as a beautiful formation of pine crystals resembling certain rosettes when cooled. At this point, the concentration of ZnO plays an important role because it can be an opacifier on the one hand to form an opaque opaque glaze, and on the other hand, it can also continuously crystallize the citrate phase when the concentration is too high.
According to the results of minerals such as X-ray analysis, two types of crystal formers are precipitated in zinc-bearing glazes: microcrystalline zinc aluminate-zinc spinel ZnOA1203, which constitutes glaze opacifier, and coarse-grained silicic acid, which imparts glaze special decorativeness Zinc-climatic zinc 2Zn0Si03. If the crystal size of zinc spinel is about lgm, the crystal length of silicon ore reaches 30-8xm, and in some cases, it is much larger than this. Sometimes other phases such as rutile TiO2 also crystallize in small amounts.
The crystalline form of wurtzite varies greatly depending on the development temperature of the crystal. At different firing temperatures, the glaze can form independent columnar crystals (1160-1190), bilobal granules (1100-1160) or coarse cassia spheres (980-1100T). It is known that these crystal formations are genetically related to each other. For example, the bifoliate spherules are produced on the basis of single crystal division, and are in the form of spherical particles with insufficient development. The different crystal forms in the glaze are often due to different development conditions (different overcooling and different viscosities of the glaze melt). At high temperatures, the glaze melt is too cold and has a low viscosity.
Such conditions are not conducive to the splitting of single crystals, thus producing columnar conjugates. As the temperature decreases, the degree of supercooling and viscosity of the glaze melt increase, and the single crystal splits and forms a double-lobed body. In the low temperature range, due to the higher degree of supercooling of the shaft melt and the greater viscosity, the single crystal splits more completely and spherical particles appear.
The structure of the Polyester and the art silica-zinc ore pellets is a complex polycrystalline combination with a radial structure composed of some distinct linear crystals. One of the characteristics of this pellet is its two-dimensional nature (flat spherules), which is due to the formation of spherulites in the thin glaze layer (200300 pm). The crystal at the edge of the pellet is much shorter than that near the center, and the arrangement is more disordered. The crystals near the center may be slightly elongated, ordered, and transformed into more stable and stable shapes during the development of the spherulites.
It can be seen that the above-mentioned characteristics of the silsteinite granules are determined by the acicular fibrous structure of the crystal.
4 Glaze firing system The zinc-containing crystalline glaze is a high-temperature glaze that is fired in the 11501300T range. The firing temperature and time regime plays an extremely important role in this type of glaze process and determines the crystalline protein of the glaze. The establishment of firing regimes and the optimization of firing parameters are based on the theoretical concepts described above with respect to the glaze formation process and crystal formation mechanism.
The practical firing system should guarantee the progress of each stage of glaze formation: the formation of glassy glaze layers and nucleus, and the controlled development of the main phase crystals. Because the temperature range of these stages is not consistent, the firing regime should be phased. The most commonly used is a two-stage firing system that heats up to the spread temperature of the glaze and at that temperature is maintained until the glaze has completely flown out; it is cooled to crystallographic humidity and incubated at this humidity until the crystal develops to the desired size.
Under this firing regime, the role of the crystallization center is the undissolved fine particles and microscopic fluctuations of the glaze melting.
Sometimes, a three-stage system is used in the glaze firing process, that is, it is cooled to the nucleation temperature after the stage of glaze flow, and is maintained at this humidity to form a crystallization center. In general, the nucleation temperature is close to the vitrification humidity of the glaze. At the end of this period, the glaze is heated to crystallisation humidity and suitably kept warm.
SunDuhai and OrlovaLA comprehensively studied the effects of firing regimes and temperature-time parameters on the crystallization and decoration of Zinc-containing Zinc-Siliceudite. Their findings (Table 2) demonstrate that changing the firing regime of glazes can control the crystallization process of glazes and can produce glazes with a variety of decorative effects—from opaque glazes to I-form glazes that form neat pellets. However, even if the temperature time parameter is slightly changed within a narrow range, a significant change in the number and size of the crystal formation can be caused.
Table 2 Effect of firing schedule and temperature and time parameters on the crystallization and decoration of zinc-containing crystal glaze firing system glaze flow development (first stage) nucleation (second stage) crystal (third stage) development glaze visual inspection Crystallization features 3040 spheres of 2 to 2 minutes in diameter per square metre. Spherule formations in microcrystalline structures Spherical aggregates in individual spheres 5 The coloring of the glaze is done by coloring. The addition of the colorant does not have a large effect on the shape, size, and number of crystals precipitated during firing, but rather imparts rich expressiveness to the product.
The degree of saturation of the glaze depends on the weight content of the colorant and it varies from 0.5% to 3.0%. The color tone of the glass substrate and the precipitated crystal is constrained by the type of colorant and its coordination valence state. If the states of the ion chromophores in the glass and crystalline phases are the same, the color of the crystal and its surrounding glassy phase differs only in color intensity (co-blue, CuO-green, MnO-brown, Fe203-brown). The glaze is most unique in its ability to change its own coordinated state of ion coloring. For example, the wurtzite that participates in NiO can be colored differently: the spheres are bright blue by the tetracoordinated cation Ni2+ embedded in the wurtzite crystal structure, and the surrounding glassy phase is cationized by a hexacoordinated NP. Yellow.
When firing a colored glaze, oxidizing or neutral conditions should be maintained and the fel colorant will be partially or fully reduced to metal in a reducing atmosphere.
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