Borates have been an essential ingredient in ceramic glazes and porcelain (vitreous) enamels for centuries. Glazes are the thin, glassy coatings fused onto the surfaces of ceramic substrates such as wall and floor tiles, tableware (e.g. bone china, porcelain), and ceramic sanitaryware.
Porcelain enamels are similar in nature but are used on metals substrates such as pots and pans, household appliances, metal bathtubs, storage tanks and silos, architectural panels and signs.
Before use in glazes and enamels, borates are incorporated into frits to render them insoluble. Frits are materials of a glassy nature rich in silica (SiO2), obtained by fusing different crystalline materials at high temperatures (up to 1550°C), and rapidly cooling the melt. The resulting material is then mixed with other materials including water, and finely ground to make a suspension that can be applied to the surface of the desired substrate. After application, the substrate is dried and fired to fuse the glaze or enamel onto the surface.
In glazes and enamels, borates are used to initiate glass formation and reduce glass viscosity, helping to form a smooth surface and reduce thermal expansion. This facilitates a good fit between the glaze or enamel and the item it covers. Borates also increase the refractive index (or luster), enhance mechanical durability and resistance to chemicals, and help to dissolve coloring agents.
More recently, borates have gained acceptance as an important ingredient in ceramic tile bodies where they act as powerful binders, allow manufacturers to reduce tile thickness, use a wider range of clays, heighten productivity and decrease energy usage.