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Many articles are published on a daily basis illustrating the use of glazes, but the technology employed in their application and composition is quite complex and diverse. Many variations are possible within multiple generic types such as matt, glossy, colored, clear, and textured.
Typically, glazes are mainly formed from a glassy material. With respect to opaque and matt glazes, the glassy matrix contains crystalline matter. Large amounts of crystalline matter are present in glass-ceramic glazes, which are purposely grown in-situ to create exclusive properties. Occasionally, a phenomenon known as the glass-in-glass phase separation creates opacity.
Ceramic glazes are primarily based on alumino-silicate glass systems, although several glass-forming systems are also available. Silica (SiO2, the main glass-forming oxide) is modified by adding a wide range of other oxides. These oxides change the thermal, chemical, and physical properties of the glaze (see Table 1).
Table 1. Common components of a ceramic glaze
Raw and Fritted Glazes
Regardless of their color, product/process type, or final surface texture, glazes are described as either fritted or raw. Raw glazes are made by blending synthetic and natural materials such as quartz, clays, oxides, carbonates, and feldspars.
On the other hand, fritted glazes contain a specific percentage of pre-melted frit or glass. These are utilized when firing or compositional demands rule out the possibility of making raw glazes.
Generally, glaze materials are applied as a water-based suspension using spraying or dipping techniques. Mechanization is a common technique, although several manual and craft methods are also available. In some cases, dry and electrostatic methods are used.
Glazes are applied to a number of substrates, such as tableware, sanitaryware, giftware, refractories, tiles, electrical porcelain, and engineering ceramics. Additionally, they are applied to cement and graphite, that are less common substrates.
The Firing of Ceramic Glazes
Depending on the application being considered, conventional glazes are not fired at less than 950 °C but they can be fired up to 1430 °C. Even though oxidizing conditions are used in many cases, certain products still need reducing conditions.
Considering the use and composition of glazes, there are no permanent properties. Glazes can also be used for functional or aesthetic purposes, or both. Among the most common functional applications, one purpose is to provide an impervious barrier to an otherwise porous ceramic body, such as the conventional tea cup, while simultaneously ensuring a visually aesthetic surface to the item. The following properties are considered to be important, based on the application and the types of process/product.
Usually, glazes should be in a compressed state during the firing process to prevent crazing faults. Therefore, a differential of 0.02%–0.04% at about 550 °C is generally recommended. Excessive compression would lead to an error called peeling.
The firing temperature, the chemistry of the base glaze, and the composition of the selected color, control the colors available.
Depending on the application, the chemical and physical durability could be important. Numerous tests are performed to assess the durability of different types of products.
Glazes should mature at an appropriate temperature and time interval. A proper range is also required over which maturity can be achieved to enable process variables. This not only controls the choice of glaze, but also the materials used for creating the glaze batch, for a particular product.
Glazes are used in numerous applications, including:
- Electrical porcelains
- Tiles—Wall and floor tiles
- Tableware—Crockery, mugs, ceramic cups, and dinner plates
- Ornaments—Figurines and giftware
- Sanitaryware—Bathtubs, toilets, and basins