Several articles published every day describe the use of glazes, but the technology used in the composition and application is complicated and diverse. Several or nearly an infinite number of variations are feasible within various generic types such as glossy, clear, colored, matt, and textured.
In general, glazes are predominantly formed of a glassy material. When it comes to matt and opaque glazes, the glassy matrix includes crystalline matter. Some glazes that consist of crystalline matter in large amounts, intentionally grown in-situ to create unique properties, are called glass-ceramic glazes. Sometimes, a phenomenon called glass-in-glass phase separation produces opacity.
Even though there are a number of glass-forming systems, ceramic glazes are mainly based on alumino-silicate glass systems. A range of other oxides is added to modify silica (SiO2), which is the primary glass forming oxide. The modifiers act to change the physical, chemical, and thermal properties (see Table 1).
Table 1. Common components of a ceramic glaze
Raw and Fritted Glazes
Irrespective of their color, final surface texture, and product/process type, glazes are described as raw or fritted. Raw glazes are formed by blends of natural and synthetic materials like clays, feldspars, carbonates, quartz, and oxides of appropriate composition to develop the final glaze. By contrast, fritted glazes include a specific percentage of pre-melted glass or frit 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 uncommon substrates like 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 the temperature of 1430 °C. Although oxidizing conditions are applied in a majority of the 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 purposes, 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 or coffee 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, the firing temperature, and the composition of the chosen color govern the available colors.
Physical and chemical durability may be of significance based on the application. Durability is assessed by using various tests for the different types of products.
It is essential for the glazes to mature at a suitable temperature and time interval. In addition, there must 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, some of which include the following:
- Tableware - crockery, dinner plates, mugs, ceramic cups, and so on
- Sanitaryware - basins, toilets, bathtubs, and so on
- Ornaments - figurines, giftware, and so on
- Tiles - floor and wall tiles
- Electrical porcelains