Several articles are published daily describing the use of glazes, but the technology used in their composition and application is complicated and diverse. Several variations are feasible within various generic types such as glossy, clear, colored, matt and textured.
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In general, glazes are predominantly formed of a glassy material. When it comes to matt and opaque glazes the glassy matrix includes crystalline matter. Glass-ceramic glazes contain crystalline matter in large amounts and are intentionally grown in-situ to create unique properties. Sometimes a phenomenon called glass-in-glass phase separation produces opacity.
Even though there are several glass-forming systems, ceramic glazes are mainly based on alumino-silicate glass systems. A range of other oxides are added to modify silica (SiO2, the main glass forming oxide) to change the physical, chemical, and thermal properties of the glaze (see Table 1).
Table 1. Common components of a ceramic glaze
Raw and Fritted Glazes
Irrespective of their color, final surface texture or product/process type, glazes are described as raw or fritted. Raw glazes are formed by blends of natural and synthetic materials including clays, feldspars, carbonates, quartz and oxides. By contrast, fritted glazes include a specific percentage of pre-melted glass or frit and are used when compositional or firing demands rule out the possibility of producing raw glazes.
In general, the glaze materials are applied as a water-based suspension by dipping or spraying methods. Although there are a number of manual and craft techniques, mechanization is common. Dry and electrostatic techniques are employed in some instances.
Glazes are applied to several different substrates, such as sanitaryware, tableware, giftware, electrical porcelain, tiles, refractories and engineering ceramics. In addition, they are applied to less common substrates such as graphite and cement.
The Firing of Ceramic Glazes
Based on the application under consideration, traditional glazes are not fired below the temperature of 950 °C and may be fired up to a temperature of 1430 °C. Although oxidizing conditions are applied in the majority of cases, some products require reducing conditions.
Since the composition and use of glazes are highly varied there are no fixed properties. Glaze may be applied for aesthetic or functional purposes, or both. Of the most frequent functional uses, one purpose is to offer an impermeable barrier to an otherwise porous ceramic body, such as the traditional tea cup, while also ensuring a visually aesthetic surface to the item.
Given below are properties regarded to be important based on the process/product types and application.
In general, it is necessary for glazes to be in compression in the fired state to prevent the fault of crazing; hence, a differential of 0.02% to 0.04% at about 550 °C is usually recommended. Excessive compression could result in a fault called peeling.
The chemistry of the base glaze, firing temperature and composition of the chosen color govern the colors available.
Physical and chemical durability may be of significance based on the application. Durability is assessed using various tests for different types of products.
It is essential for glazes to mature at a suitable temperature and time interval. There must also be an appropriate range over which maturity is realized to allow for process variables. This governs the choice of glaze, as well as the materials employed to create the glaze batch for a specific product.
Glazes are used in various applications, including:
- Tableware - crockery, dinner plates, mugs, ceramic cups
- Sanitaryware - basins, toilets, bathtubs
- Ornaments - figurines, giftware
- Tiles - floor and wall tiles
- Electrical porcelains