Ceramic glaze
Most pottery produced in recent centuries has been glazed, other than pieces in bisque porcelain, terracotta, and some other types.
[3] Glazed brick goes back to the Elamite Temple at Chogha Zanbil, dated to the 13th century BC.
[4] Lead glazed earthenware was probably made in China during the Warring States period (475 – 221 BC), and its production increased during the Han dynasty.
[5] During the Kofun period of Japan, Sue ware was decorated with greenish natural ash glazes.
In the 13th century, flower designs were painted with red, blue, green, yellow and black overglazes.
The first Islamic opaque glazes can be found as blue-painted ware in Basra, dating to around the 8th century.
[6][full citation needed] Other places for innovative pottery in the Islamic world included Fustat (from 975 to 1075), Damascus (from 1100 to around 1600) and Tabriz (from 1470 to 1550).
Fluxes lower the high melting point of the glass forms silica, and sometimes boron trioxide.
Various metal oxides, such as those of sodium, potassium and calcium, act as flux and therefore lower the melting temperature.
Dry-dusting a mixture over the surface of the clay body or inserting salt or soda into the kiln at high temperatures creates an atmosphere rich in sodium vapor.
The best known type of underglaze decoration is the blue and white porcelain first produced in China, and then copied in other countries.
[13] However many of the imitative types, such as Delftware, have off-white or even brown earthenware bodies, which are given a white tin-glaze and either inglaze or overglaze decoration.
This works well with tin-glazed pottery, such as maiolica, but the range of colours was limited to those that could withstand a glost firing, as with underglaze.
Many historical styles, for example Japanese Imari ware, Chinese doucai and wucai, combine the different types of decoration.
[11] Glaze components are more likely to be leached into the environment when non-recycled ceramic products are exposed to warm or acidic water.
[14] Lead and chromium are two heavy metals which can be used in ceramic glazes that are heavily monitored by government agencies due to their toxicity and ability to bioaccumulate.
Ceramic manufacturers primarily use lead(II) oxide (PbO) as a flux for its low melting range, wide firing range, low surface tension, high index of refraction, and resistance to devitrification.
[16] Lead used in the manufacture of commercial glazes are molecularly bound to silica in a 1:1 ratio, or included in frit form, to ensure stabilization and reduce the risk of leaching.
[20] To reduce the likelihood of leaching, barium carbonate is used in frit form and bound to silica in a 1:1 ratio.
