| The use of glazes is familiar in many everyday articles, however, the technology involved in composition and application is complex and diverse. Many, almost an infinite, number of variations are possible within many generic types e.g. clear, glossy, matt, coloured, textured. Glaze Systems Glazes are usually mostly comprised of a glassy material. In the case of opaque and matt glazes the glassy matrix contains crystalline matter. Certain glazes which contain large amounts of crystalline matter, deliberately grown in-situ to develop special properties, are described as glass-ceramic glazes. Opacity is occasionally produced by a phenomenon described as glass in glass phase separation. Glaze Components Although several glass-forming systems exist, ceramic glazes are almost exclusively based on alumino-silicate glass systems. The main glass forming oxide, silica (SiO2) is modified by the addition of a range of other oxides. The modifiers act to alter thermal, chemical, and physical properties, (table 1). Table 1. Common components of a ceramic glaze. | | | | | SiO2 | Al2O3 ZrO2 TiO2 | BaO CaO SrO ZnO PbO Na2O K2O Li2O Bi2O3 B2O3 | Raw and Fritted Glazes Glazes are (regardless of colour, process/product type, and final surface texture) described as raw or fritted. Raw glazes are combinations of natural and synthetic materials such as feldspars, clays, quartz, carbonates and oxides of suitable composition to produce the final glaze. Fritted glazes contain a proportion of pre-melted glass or frit used when firing or compositional demands dictate the raw glazes cannot be made. Glaze Application The glaze materials are usually applied as a water-based suspension by spraying or dipping methods. Mechanisation is common though many craft and manual techniques persist. In some cases, dry and electrostatic methods are used. Glazes are applied to many varied substrates including table and giftware, sanitaryware, tiles, electrical porcelain, engineering ceramics and refractories. They are also applied to unusual substrates such as cement and graphite. Firing of Ceramic Glazes Conventional glazes are not fired below 950°C and may be fired as high as 1430°C depending on the application in question. In most cases, oxidising conditions are used but for certain products reducing conditions are required. | 
