There are three main types of ceramic glazes – tiles, tableware and sanitary ware. The greatest amount of borate is used in tile glazes including wall, floor and clay roof tiles. Tableware including china, porcelain, stoneware and earthenware includes some borate. None is used in vitreous china and porcelain sanitary ware.
Frit and glaze formulations can contain concentrations as high as 25% borates and the entire market accounts for more than 13% of the global demand. Borates improve glazes in a number of ways: facilitating the production process; ensuring a good fit between the glaze and the item it covers; and enhancing the chemical and mechanical strength.
Ceramic glaze is the glassy surface on a tile used to strengthen, waterproof, color and decorate a ceramic body. Borates are used to initiate glass formation and reduce glass viscosity, helping to form a smooth surface. They also help to reduce thermal expansion, thus facilitating a good fit between the glaze and the clay. Borates also increase the refractive index, or luster, of glazes and can enhance their resistance to chemicals. When used in glazes, borates (boric oxide) also help to reduce the melting temperature.
Many glaze ingredients including borax are soluble in water. If water-soluble ingredients were applied wet to the surface of a clay body, they would be absorbed into the clay. This would limit their ability to provide a glassy outer surface. The process of fritting, or fusing the solubles with silica, renders these ingredients insoluble. The resulting frit is ground and applied to the ceramic body prior to firing.
Fritting also starts the glass formation process well before the glaze is applied to the ware, significantly lowering the glaze firing temperature. Advanced ceramic components are increasingly being used in diesel and automotive engines where their light weight, high temperature and wear resistance result in more efficient combustion and significant fuel savings.
Boric oxide is a network former, but allows more fluxing oxides to be introduced without destroying the silicate lattice. The oxides of only four elements (boron, silicon, germanium and phosphorus) are able to form glasses alone. [Return to Top]
Adding borates to tile bodies can make the tiles themselves stronger, and reduce the energy used and the waste formed in the process. Including them in the batch also allows manufacturers to use a broader range of clays. Borates may also reduce furnace emissions at ceramic tile plants. These environmental benefits are associated with borates’ dual role in the tile bodies.
Borates act as a flux and as a powerful inorganic binder in ceramic tile body compositions . . . none more effectively than boric acid, in particular. A small amount of boric acid used in a ceramic body can have a significant effect during the firing process by promoting the formation of a glassy phase with low viscosity. In the pressed body, the dry mechanical strength is increased, typically by about 40%.
Ceramic firing has moved to single firing which favors the non-sodium borates. Traditional double firing of ceramic tiles has essentially disappeared from most producing regions due to higher energy and labor costs and has been replaced by single firing cycles. In double firing cycles sodium oxide brings benefits as a powerful flux, in single firing cycles sodium oxide is not desirable and glazes for this production method are normally formulated with a non-sodium borate material. [Return to Top]
For the porcelain floor tile composition, the main benefits of adding borates include:
- Reducing the firing cycle typically by 10-20%, which translates the increased throughput in the furnace.
- Reducing tile thickness due to a substantial increase (30-80%) in the dry mechanical strength of the unfired tiles. For example, an increase of 25% in dry mechanical strength allows the thickness to be reduced by around 10%.
- Reducing the body formulation cost by substituting up to 20% feldspar and balancing with lower cost silica to maintain the peak temperature and cycle length. Alternatively, the binding effect of the boric acid can be used to substitute some high quality clay with lower quality, less plastic clay to further increase cost savings.
- Reducing firing temperature by more than 25°C, keeping the same cycle length, and thereby producing energy savings.
Furthermore, addition of boric oxide to porcelain stoneware bodies raises the vitrification temperature and improves the densification properties, as well as increasing the modulus of rupture. Boric acid is added in the milling stage, mixing and distributing homogeneously in the composition. Its strong fluxing power—1 % boric acid has the same fluxing power as 10-20% feldspar in the body composition—optimizes the fluxing system wherever feldspar, talc, spodumene or other fluxing agents are used. [Return to Top]
Ceramics are also used to contain oil spills and encapsulate nuclear wastes. Despite established procedures and limits, industries and governments are under pressure to find alternative ingredients to Lead (Pb) in ceramic glazes, particularly those used to serve food. Two of the most viable alternatives to Lead-based glazes are Bismuth and advanced Borosilicates (glass containing high levels of borates), or combinations of both. Therefore, Borates may serve as a viable alternative to enhancing safety while maintaining quality. [Return to Top]